# The high-resolution imaging science experiment (HiRISE) in the MRO extended science phases (2009–2023)

A.S. Mcewen, S. Byrne, C. Hansen, I.J. Daubar, S. Sutton, C.M. Dundas, N.

Bardabelias, N. Baugh, J. Bergstrom, R. Beyer, et al.

► To cite this version:

A.S. Mcewen, S. Byrne, C. Hansen, I.J. Daubar, S. Sutton, et al.. The high-resolution imaging science experiment (HiRISE) in the MRO extended science phases (2009–2023). Icarus, 2024, pp.115795. 10.1016/j.icarus.2023.115795. hal-04262337

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# The high-resolution imaging science experiment (HiRISE) in the MRO extended science phases (2009–2023)

A.S. McEwen<sup>a, *,</sup> S. Byrne<sup>a</sup>, C. Hansen<sup>b</sup>, I.J. Daubar<sup>c</sup>, S. Sutton<sup>a</sup>, C.M. Dundas<sup>d</sup>, N. Bardabelias<sup>a</sup>, N. Baugh<sup>a</sup>, J. Bergstrom<sup>e</sup>, R. Beyer<sup>f, k</sup>, K.M. Block<sup>a</sup>, V.J. Bray<sup>a</sup>, J.C. Bridges<sup>g</sup>, M. Chojnacki<sup>b</sup>, S.J. Conway<sup>h</sup>, W.A. Delamere<sup>i</sup>, T. Ebben<sup>e</sup>, A. Espinosa<sup>a</sup>, A. Fennema<sup>a</sup>, J. Grant<sup>j</sup>, V.C. Gulick<sup>a, f, k</sup>, K.E. Herkenhoff<sup>d</sup>, R. Heyd<sup>a</sup>, R. Leis<sup>a</sup>, L. Ojha<sup>l</sup>, S. Papendick<sup>a</sup>, C. Schaller<sup>a</sup>, N. Thomas<sup>m</sup>, L.L. Tornabene<sup>n</sup>, C. Weitz<sup>b</sup>, S.A. Wilson<sup>j</sup>

## ARTICLE INFO

### Keywords:

Mars, surface

Image processing

Instrumentation

Geological processes

Data reduction techniques

## ABSTRACT

The Mars Reconnaissance Orbiter has been orbiting Mars since 2006 and has acquired >80,000 HiRISE images with sub-meter resolution, contributing to over 2000 peer-reviewed publications, and has provided the data needed to enable safe surface landings in key locations by several rovers or landers. This paper describes the changes to science planning, data processing, and analysis tools since the initial Primary Science Phase in 2006–2008. These changes affect the data used or requested by the community and how they should interpret the data. There have been a variety of complications to the dataset over the years, such as gaps in monitoring due to spacecraft and instrument issues and special events like the arrival of new landers or rovers on Mars or global dust storms. The HiRISE optics have performed well except for a period when temperature uniformity was perturbed, reducing the resolution of some images. The focal plane system now has 12 rather than 14 operational detectors. The first failure (2011) was a unit at the edge of the swath width, reducing image width by 10% rather than creating a gap. The recent (2023) failure was in the middle of the swath. An unusual problem with the analog-to-digital conversion of the signal (resulting in erroneous data) has worsened over time; mitigation steps so far have preserved full-resolution imaging over all functional detectors. Soon, full-resolution imaging will be narrowed to a subset of the detectors and there will be more 2 × 2 binned data. We describe lessons received for future very high-resolution orbital imaging. We continue to invite all interested people to suggest HiRISE targets on Mars via HiWish, and to explore the easy-to-use publicly available images.

<footnote><sup>a</sup> Lunar and Planetary Lab, University of Arizona, Tucson, AZ 85721, USA</footnote>

<footnote><sup>b</sup> Planetary Science Institute, Tucson, AZ, USA</footnote>

<footnote><sup>c</sup> Brown University, Department of Earth, Environmental, and Planetary Sciences, Providence, RI 02912, USA</footnote>

<footnote><sup>d</sup> U.S. Geological Survey, Astrogeology Science Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001, USA</footnote>

<footnote><sup>e</sup> Retired from Ball Aerospace and Technology Corp., Boulder, CO 80301, USA</footnote>

<footnote><sup>f</sup> SETI Institute, Mountain View, CA 94043, USA</footnote>

<footnote><sup>g</sup> Space Park Leicester, School of Physics and Astronomy, University of Leicester, LE17RH, UK</footnote>

<footnote><sup>h</sup> Nantes Université, Université d'Angers, Le Mans Université, CNRS UMR 6112 Laboratoire de Planétologie et Géosciences, France</footnote>

<footnote><sup>i</sup> Delamere Support Systems, Boulder, CO 80304, USA</footnote>

<footnote><sup>j</sup> CEPS, Smithsonian Institution, Washington, DC 20560, USA</footnote>

<footnote><sup>k</sup> NASA Ames Research Center, Moffett Field, CA 94035, USA</footnote>

<footnote><sup>l</sup> Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA</footnote>

<footnote><sup>m</sup> University of Bern, Sidlerstrasse 5, CH-3012 Bern, CH, Switzerland</footnote>

<footnote><sup>n</sup> Institute for Earth and Space Exploration, Western University, Dept. of Earth Sciences, 1151 Richmond Street, London, ON N6A 5B7, Canada</footnote>

<footnote>* Corresponding author at: 1541 E. University Blvd., Tucson, AZ 85721, USA.
E-mail address: mcewen@pirl.lpl.arizona.edu (A.S. McEwen).</footnote>
# 1. Introduction

## 1.1. Purpose and organization of this paper

This paper was written to provide information useful to the HiRISE data user community, updating the information in McEwen et al. (2010) about the Primary Science Phase (PSP) of Mars Reconnaissance Orbiter (MRO). There are 12 major sections:

1. Introduction

2. Science Publications and Team

3. Operations History

4. Summary of HiRISE Archived Dataset

5. Special Data Products and Analysis Tools

6. Imaging for Lander and Rover Missions

7. Science Planning and Uplink Tools and Techniques

8. Instrument Issues and Anomalies

9. Calibrations

10. Citizen Science and Machine Learning

11. Education and Public Outreach

12. The Future of Very High-Resolution Orbital Imaging of Mars

## 1.2. MRO and HiRISE overview

The Mars Reconnaissance Orbiter (MRO) was launched on August 12, 2005, carrying six scientific instruments (Zurek and Smrekar, 2007). In addition to the HiRISE camera (McEwen et al., 2007), MRO carries the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM; Murchie et al., 2007), the Shallow Radar (SHARAD; Seu et al., 2007), the Context Camera (CTX; Malin et al., 2007), the Mars Color Imager (MARCI; Malin et al., 2001), and the Mars Climate Sounder (MCS; McCleese et al., 2007). The Primary Science Phase (PSP) covered slightly more than one Mars year, from November 2006 until December 2008, with MRO in its 255 × 320 km altitude mapping orbit. It was followed by the Extended Science Phase (ESP) for two Earth years, followed by Extended Missions (EM) 1–6. The first four EMs each lasted two years (a little more than one Mars Year (MY; Piqueux et al., 2015a)), and EM5 was three years long. We find it useful to think in terms of MY rather than EM (Table 1). Results from the PSP were described by McEwen et al. (2010); this paper updates that information for mission extensions from 2008 through 2022 and describes future expectations. HiRISE image names from the PSP begin with “PSP”, while those from the ESP and all EMs begin with “ESP” (a small number of pre-PSP images in the transition phase (TRA), and aerobraking phase (AEB) also exist).

The HiRISE camera features a 0.5-m diameter primary mirror, 12-m effective focal length, and a focal-plane subsystem (FPS) that can acquire images containing up to 28 Gb (gigabits) of data in as little as 6 s (McEwen et al., 2007). However, the maximum image length has shrunk in the EMs due to thermal limits (Section 7.14). HiRISE images are acquired via 14 charge-coupled device (CCD) detectors, each with two

output channels, and with multiple choices for pixel binning and number of Time Delay and Integration (TDI) lines. The 10 CCDs that cover the full swath width (1.14° field of view, 6 km wide swath of Mars' surface at a typical range of 300 km) are covered by broadband red filters (RED) and four extra CCDs in the middle are covered by blue-green (BG) and near-infrared (IR) filters to provide 3-color coverage in the center 20% of the image swath. With the failure of the electronics for RED9 in 2011, HiRISE has used the remaining nine RED CCDs covering a swath of 1.03°. RED4 failed in 2023 but the gap can be filled by the IR10 or BG12 image. Fig. 1 shows the layout of CCDs in the HiRISE FPS. The very short orbital time separation between the BG, RED, and IR CCD image data is desirable to minimize parallax for good color registration, but has nevertheless enabled feature tracking and velocity measurements for rapid-moving avalanche clouds and dust devils (Russell et al., 2008; Choi and Dundas, 2011).

MRO's orbit altitude ranges from about 255 to 320 km above Mars (resulting in HiRISE nadir-pointed images at 0.255 to 0.32 m/pixel), and with a Local Mean Solar Time (LMST) ranging from 2:45 to 3:15 PM, although this shifted to somewhat later between 2020 and 21 to support entry, descent, and landing (EDL) for the Perseverance rover (Farley et al., 2020). Periapse is near the southernmost point of the orbit. MRO can routinely roll and point up to 30° to the left or right of the orbit groundtrack, allowing imaging of anywhere on Mars within one to two weeks and enabling stereo coverage. However, there are limits to off-nadir pointing (see Section 7.5), and there may be competition from other targets, so in practice, it may take months for repeat imaging of a location.

## 1.3. The People's Camera

HiRISE has been called “The People’s Camera” because we aspire to make HiRISE as useful and accessible as possible to the science community and general public. This philosophy includes accepting public image suggestions (www.uahirise.org/hiwish) and releasing useful data products as quickly as possible. HiRISE is the only experiment within NASA’s Planetary Science Division that releases calibrated and map-projected products to the Planetary Data System (PDS) within just one to two months of data acquisition from the spacecraft. Data products associated with imaging of candidate landing sites and captioned image releases (prepared by science team members) may be released even sooner. There are 2249 captions as of June 2023 that are written for a public audience and explain the significance of the features in the image in layman’s terms. Furthermore, we have designed these data products for ease of use, including making tools available to analyze the data. The success of this approach is demonstrated by the existence of >1800 peer-reviewed science publications using HiRISE data, ~80% from lead authors who are not part of the HiRISE team, and by extensive use of HiRISE images by the general public. We describe these aspects of the experiment in greater detail below.

<caption>**Table 1**</caption>

Mars Years and MRO Science Phases. $L_s$ stands for solar longitude, which describes the time of year, or season with $L_s = 0$ being the northern hemisphere's spring equinox.

<table><thead><tr><th>Mars Year (MY)</th><th>L<sub>s</sub> range of HiRISE data</th><th>Earth Dates of MRO images (m/d/yr)</th><th>MRO Phase</th><th>MRO Orbits</th><th># HiRISE Mars Images Returned</th></tr></thead><tbody><tr><td>28</td><td>132-355</td><td>11/08/2006-11/29/2007</td><td>TRA, PSP</td><td>1330-6294</td><td>3790</td></tr><tr><td>29</td><td>2-328</td><td>12/13/2007-08/26/2009</td><td>PSP, ESP</td><td>6463-14,447</td><td>7882</td></tr><tr><td>30</td><td>25-360</td><td>12/16/2009-09/13/2011</td><td>ESP, EM1</td><td>15,894-24,051</td><td>8179</td></tr><tr><td>31</td><td>0-360</td><td>09/13/2011-07/31/2013</td><td>EM1, EM2</td><td>24,054-32,865</td><td>9800</td></tr><tr><td>32</td><td>0-351</td><td>07/31/2013-05/30/2015</td><td>EM2, EM3</td><td>32,870-41,445</td><td>9339</td></tr><tr><td>33</td><td>7-360</td><td>07/02/2015-05/05/2017</td><td>EM3, EM4</td><td>41,858-50,500</td><td>10,307</td></tr><tr><td>34</td><td>0-360</td><td>05/05/2017-03/23/2019</td><td>EM4, EM5</td><td>50,501-59,317</td><td>11,438</td></tr><tr><td>35</td><td>0-360</td><td>03/23/2019-02/07/2021</td><td>EM5</td><td>59,318-68,134</td><td>8420</td></tr><tr><td>36</td><td>0-360</td><td>02/07/2021-12/26/2022</td><td>EM5, EM6</td><td>68,135-76,952</td><td>8804</td></tr><tr><td>37</td><td>0-111</td><td>12/26/2022-8/28/2023</td><td>EM6</td><td>76,953-80,096</td><td>5404</td></tr><tr><td></td><td></td><td></td><td>Total:</td><td></td><td>83,363</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** HiRISE CCD layout in the FPA, with spacecraft motion up as shown here. Each CCD is 2048 pixels wide, and adjacent CCDs overlap by 48 pixels. Colors indicate spectral bandpasses (blue for BG (12,13), red for broadband RED (0–9), and brown for IR (10,11)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

## 2. Science publications and team

### 2.1. Publications

HiRISE data have played a part in thousands of peer-reviewed papers since the beginning of operations in 2006. The MRO project maintains a database of team publications that has 294 peer-reviewed HiRISE papers in the 15-year period from 2007 to 2021 (Fig. 2; left). Assessing the number of publications enabled by HiRISE data from the wider community is more difficult as these papers have not been tracked systematically. As a proxy, a search of the NASA Astrophysics Data System (ADS) for peer-reviewed publications containing the words HiRISE and Mars yields 1817 results between 2006 and 2021 (Fig. 2; right), with over 2000 results as of June 2023. These results likely include some papers where HiRISE use is minimal, but also miss some publications that are not tracked by the ADS or do not have full text search available. Even with these uncertainties, the search result is useful to track the relative number of publications over time and provide a first-order estimate of the impact of the HiRISE experiment.

HiRISE team publications (Fig. 2; left) increased from the beginning of the mission and peaked in 2010 (likely due to special issues delaying some publications, so this peak is better understood as being split between 2009 and 2010). As extended mission funding decreased for the team and the MRO participating scientist program ended, team

publications decreased throughout the 2010s. Note that this count includes papers that team members carried out via other funding sources; the funding in the later extended missions was insufficient to support robust scientific investigations. A recent publications uptick lacks a simple explanation but is likely the result of other Mars mission publications using HiRISE data, the fruition of long-term monitoring campaigns, review papers, and HiRISE/CaSSIS collaborations. CaSSIS is the Colour and Stereo Surface Imaging System (Thomas et al., 2017) on ExoMars Trace Gas Orbiter (TGO). Total community papers (Fig. 2; right) show a similar increase from the beginning of the mission to a peak in 2010. Community paper production using HiRISE data was relatively constant from 2011 to 2015 and steadily increased since then. The number of papers in 2021 exceeded even the 2010 peak. This increase is likely due to the same reasons as the recent increase in team-led papers. Many of these papers are led by scientists funded by countries other than the United States.

### 2.2. Science highlights

Here we try to summarize the most significant results from HiRISE. Most of these results depend on other datasets in addition to HiRISE.

* HiRISE full-resolution stereo images enabled identification of small-scale hazards aiding successful landings by the Phoenix, MSL,

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (left) Number of peer-reviewed team-led papers over time through 2021 that are associated with HiRISE. Data are from the MRO publication database at: https://mropd.psu.edu/. (right) Number of search results on NASA's ADS at https://ui.adsabs.harvard.edu/ for all peer-reviewed articles that contain the text Mars and HiRISE, including those led by team members and other scientists.</figcaption></figure>
InSight, and Mars2020 missions (Golombek et al., 2012b, 2017, and others).

* Many current and candidate landing and traverse sites have been mapped in color and stereo, leading to regional geologic studies (Sun and Stack, 2020; Quantin-Nataf et al., 2021), enabling complementary surface results and traverse planning for rovers (see Section 6 and citations).

* New impact craters have constrained current impact rates and assisted the interpretation of seismic signals (Daubar et al., 2022; Posiolova et al., 2022).

* New craters also show that shallow water ice is present at lower latitudes than previously known, plus mid-latitude scarps have revealed thick bodies of massive ice (Dundas et al., 2021b, 2023).

* Mapping of shallow valleys, alluvial fans, channels, and deltas has shown that water flowed on Mars relatively recently, within the past 2 billion years (Grant and Wilson, 2019; Kite et al., 2019; Wilson et al., 2021).

* Mapping of sediments and ancient bedrock with stereo-derived topography combined with mineralogic information from MRO's CRISM has revealed that minerals containing water are widespread on early Mars (reviewed by Ehlmann and Edwards, 2014) and formed more recently than previously known, again within the past 2 billion years (e.g., Weitz et al., 2011, 2022).

* Pristine gullies on Mars have been shown to be forming at a high rate today from the action of carbon dioxide frost, rather than being remnants of a recent wetter climate (reviewed by Diniega et al., 2021 and Dundas et al., 2021a).

* Recurring Slope Lineae (McEwen et al., 2011) look like water seeps but are probably strange dry flows of dust and sand, showing that Mars today is very different than Earth (Dundas et al., 2017; McEwen et al., 2021).

* Sand dunes and other wind-driven bedforms are now known to be actively migrating across much of Mars (reviewed by Diniega et al., 2021). Another class of widespread bright ripple features is much less active, indicating that they were formed under different conditions than present-day Mars (Silvestro et al., 2020; Chojnacki et al., 2021).

* Some polar landforms like "spiders" or furrows are also actively forming on Mars today from seasonal processes involving carbon dioxide ice and gas (Hansen et al., 2011; Portyankina et al., 2017).

* HiRISE has revealed the remarkable diversity of volcanic landforms and processes on Mars, including mud volcanism (Okubo, 2016; Broz et al., 2022), flood lavas (Dundas et al., 2019b), and distinguishing aqueous from volcanic geomorphology (Hamilton et al., 2018; Sutton et al., 2022b).

* Sedimentary rocks have been studied in detail (McLennan et al., 2019), revealing periodicities in the layering that may relate to orbital cycles (Lewis and Aharonson, 2014).

* The north- and south-polar layered deposits and their outliers have periodic climate signals in the topography of their exposures stretching back millions of years (Becerra et al., 2016, 2019; Sori et al., 2022).

* Accumulation and erosion of the residual polar ice caps can be tied to the current climate and its variability from year to year (Byrne, 2008; Wilcoski and Hayne, 2020).

* The recognition of many glacial and periglacial landforms on Mars has only been possible using the detailed images of Mars (e.g., Butcher et al., 2020; Soare et al., 2014; Gallagher et al., 2011) and the story they tell us about Mars recent climate and hydrosphere has been instrumental in understanding the distribution of shallow ground ice that will underpin future human exploration (Morgan et al., 2021).

## 2.3. Science and operations teams and roles

The original HiRISE team had 12 Co-Investigators (Co-Is), which has

expanded over the years to 20 Co-Is. New Co-Is approved by the MRO project and NASA were chosen to add a range of expertise, institutions, and demographics. Each Co-I typically has both a relevant science specialty plus a skill essential to the successful outcome of a flight instrument investigation.

The high data volume represented by a single HiRISE image (typically gigapixel size) dictated from the outset that extensive spatial coverage would not be possible. Ideally, every site imaged by HiRISE should have a specific investigation or hypothesis in mind. A target database and catalog (HiCat) was built to be a repository of all the requests for HiRISE images. HiRISE images are broadly useful for Mars science, so HiRISE Co-Is and affiliates take responsibility for a particular science theme, becoming a Science Theme Lead (STL). The current list of STLs is given in Table 2. When someone requests a HiRISE image they must also designate an appropriate science theme, and the STL for that theme then assigns the target an initial priority. This is the time-independent part of the planning process. The STL is also responsible for interacting with peers in their theme area in the general Mars community to ensure that science targets are being identified and entered into the database.

HiRISE science planning is accomplished via a collaboration between a Co-I of the Pay Period (CIPP) science team member and a HiRISE Targeting Specialist (HiTS) uplink operations engineer. The HiRISE science team members serve as CIPP for typically 1–3 planning cycles per year. The CIPP role is open to all science team members and not limited to official Co-Is, thus giving early-career scientists the opportunity to gain experience in the planning and operations aspects of the mission. MRO commands are planned in 2-week cycles, starting ~4 weeks before the commands are sent to the spacecraft. At this point, the downlink capacity and orbit groundtracks are known to a high enough degree of accuracy to predict which targets will be observable. This time-dependent part of the planning process is tied to the uplink schedule with rigorous deadlines and requirements for command files to be sent to the Jet Propulsion Laboratory (JPL).

The HiTS creates a HiRISE Target List (HiTList) of seasonally appropriate targets viewable by the instrument considering roll limits, spacecraft exclusion zones, project input, scene requirements (like local illumination angle), and other factors, while the CIPP takes on the task of prioritizing these targets based on their initial priority from the STL, illumination, the state of the atmosphere, time criticality, frost coverage, etc. These factors change, thus the inclusion of CIPP scientific understanding into the decision-making process optimizes the science value of images acquired. Team members also propose a short list of important

<table><caption>**Table 2**
Science theme leads.</caption><thead><tr><td>HiRISE Science Theme</td><td>Science Theme Lead</td></tr></thead><tbody><tr><td>Climate Change</td><td>Margaret Landis, Patricio Becerra</td></tr><tr><td>Composition & Photometry</td><td>James Wray</td></tr><tr><td>Aeolian Processes</td><td>Matt Chojnacki</td></tr><tr><td>Fluvial Processes</td><td>Virginia Gulick</td></tr><tr><td>Future Exploration / Landing sites</td><td>John Grant, Alfred McEwen</td></tr><tr><td>Geologic Contacts / Stratigraphy</td><td>Dan Berman</td></tr><tr><td>Glacial/Periglacial Processes</td><td>Michael Mellon</td></tr><tr><td>Hydrothermal Processes</td><td>Virginia Gulick</td></tr><tr><td>Impact Processes</td><td>Ingrid Daubar</td></tr><tr><td>Landscape Evolution</td><td>John Grant</td></tr><tr><td>Mass Wasting Processes</td><td>Colin Dundas</td></tr><tr><td>Other</td><td>Shane Byrne</td></tr><tr><td>Polar Geology</td><td>Margaret Landis, Shane Byrne, Kenneth Herkenhoff</td></tr><tr><td>Rocks & Regolith</td><td>Mike Mellon</td></tr><tr><td>Seasonal Processes</td><td>Candice Hansen</td></tr><tr><td>Sedimentary / Layering processes</td><td>Cathy Weitz</td></tr><tr><td>Tectonic Processes</td><td>Chris Okubo</td></tr><tr><td>Volcanic Processes</td><td>Laszlo Kestay</td></tr></tbody></table>
<table><caption>**Table 3**
Ph.D. Dissertations where a HiRISE Team Member Played a Significant Role.</caption><thead><tr><th>Name</th><th>Initial</th><th>Institution</th><th>Year</th><th>Dissertation Title</th></tr></thead><tbody><tr><td>Atwood-Stone</td><td>C.</td><td>University of Arizona</td><td>2018</td><td>Planetary Granular Topography: Slope Angles and Crater Concentric Ridges</td></tr><tr><td>Banks</td><td>M.E.</td><td>University of Arizona</td><td>2009</td><td>Glacial processes and morphologies in the southern hemisphere of Mars</td></tr><tr><td>Bapst</td><td>J.</td><td>University of Washington</td><td>2018</td><td>Mars' Water Cycle Seen Through an Ice Lens</td></tr><tr><td>Becerra</td><td>P.</td><td>University of Arizona</td><td>2016</td><td>The poles of Mars, past and present: A high-resolution observational study of the martian polar regions and their connection to climate</td></tr><tr><td>Bramson</td><td>A.M.</td><td>University of Arizona</td><td>2018</td><td>Radar Analysis and Theoretical Modeling of the Presence and Preservation of Ice on Mars</td></tr><tr><td>Brooker</td><td>L.M.</td><td>Open University</td><td>2019</td><td>The History of Water at Lyot crater, Mars: Possible Surface Manifestations of Ancient Groundwater and/or Recent Climate Change</td></tr><tr><td>Butcher</td><td>F.E.<br>G.</td><td>Open University</td><td>2019</td><td>Wet-Based Glaciation on Mars</td></tr><tr><td>Caudill</td><td>C.</td><td>University of Western<br>Ontario</td><td>2020</td><td>Characterization of Impactite Clay Minerals with Implications for Mars Geologic Context and Mars Sample Return</td></tr><tr><td>Dapremont</td><td>A.M.</td><td>Georgia Inst of<br>Technology</td><td>2021</td><td>Subsurface Sediment Mobilization on Mars: Insights Provided by Orbital Remote Sensing Datasets</td></tr><tr><td>Daubar</td><td>I.J.</td><td>University of Arizona</td><td>2014</td><td>New Dated Craters on Mars and the Moon: Studies of the Freshest Craters in the Solar System</td></tr><tr><td>Diniega</td><td>S.M.</td><td>University of Arizona</td><td>2010</td><td>Modeling Aeolian Dune and Dune Field Evolution</td></tr><tr><td>Dundas</td><td>C.M.</td><td>University of Arizona</td><td>2009</td><td>Investigations of the Martian Mid-Latitudes: Implications for Ground Ice</td></tr><tr><td>Eggers</td><td>G.L.</td><td>Georgia Inst of<br>Technology</td><td>2021</td><td>Modeling and Observations of High-silica Magmatic Systems on Earth and Mars</td></tr><tr><td>Guimpier</td><td>A.</td><td>Nantes Université</td><td>2022</td><td>La géomorphologie des instabilités gravitaires sur Mars</td></tr><tr><td>Harrison</td><td>T.</td><td>University of Western<br>Ontario</td><td>2016</td><td>Martian Gully Formation and Evolution: Studies From the Local to Global Scale</td></tr><tr><td>Kolb</td><td>K.</td><td>University of Arizona</td><td>2009</td><td>Investigating evidence of geologically recent liquid water on Mars</td></tr><tr><td>Landis</td><td>M.E.</td><td>University of Arizona</td><td>2018</td><td>Icy Craters on Mars and Ceres</td></tr><tr><td>McKeown</td><td>L.E.</td><td>Trinity College Dublin</td><td>2019</td><td>The Role of Contemporary CO<sub>2</sub> Sublimation as a Geomorphic Agent on Mars</td></tr><tr><td>Ojha</td><td>L.</td><td>Georgia Inst of<br>Technology</td><td>2016</td><td>Geophysical and remote sensing study of terrestrial planets</td></tr><tr><td>Wilson (Purdy)</td><td>S.A.</td><td>University of Virginia</td><td>2017</td><td>Fluvial landforms and the post-Noachian environment of Mars</td></tr><tr><td>Runyon</td><td>K.D.</td><td>Johns Hopkins University</td><td>2017</td><td>Agents of Planetary Geomorphic Change: Martian Aeolian Morphodynamics and the Emplacement of Crater Ejecta</td></tr><tr><td>Schaefer</td><td>E.I.</td><td>University of Arizona</td><td>2018</td><td>Quantifying geomorphic features: Relative albedos, skeletonization, and multifractality</td></tr><tr><td>Sutton</td><td>S.S.</td><td>University of Arizona</td><td>2022</td><td>Fissure-fed Volcanism on Mars and Earth</td></tr><tr><td>Viola</td><td>D.</td><td>University of Arizona</td><td>2017</td><td>Expanded Craters on Mars: Implications for Shallow, Mid-latitude Excess Ice</td></tr><tr><td>Wray</td><td>J.J.</td><td>Cornell University</td><td>2011</td><td>High-Resolution Studies of Aqueous Environments on Ancient Mars</td></tr></tbody></table>

suggestions each cycle that the CIPP evaluates and usually awards higher priority relative to other targets.

MRO mission management at JPL receives prioritized observation requests from several teams and determines MRO's activities through an automatic procedure, including enforcement of the MCS rules which limit the number and clustering of large rolls (>9°) that cause gaps in the

<table><caption>**Table 4**
Major MRO<fnref n="1" /> flight mission or Mars events.</caption><thead><tr><th>Date (m/d/yr)</th><th>Mission or Mars Event</th></tr></thead><tbody><tr><td>08/12/2005</td><td>Launch</td></tr><tr><td>03/10/2006</td><td>Orbit insertion</td></tr><tr><td>09/29/2006</td><td>Transition Phase imaging (1 week)</td></tr><tr><td>07/01/2007</td><td>Start of GDE of 2007</td></tr><tr><td>05/25/2008</td><td>Phoenix EDL</td></tr><tr><td>08/26/2009</td><td>Start extended spacecraft safe mode</td></tr><tr><td>12/16/2009</td><td>Resumed science operations</td></tr><tr><td>08/06/2012</td><td>Mars Science Laboratory (MSL) EDL</td></tr><tr><td>08/12/2013</td><td>MRO switches to inertial measurement unit (IMU)-2</td></tr><tr><td>10/02/2013</td><td>Comet ISON closest approach to Mars</td></tr><tr><td>10/19/2014</td><td>Comet Siding Spring closest approach to Mars</td></tr><tr><td>10/19/2016</td><td>Schiaparelli module crash</td></tr><tr><td>04/26/2017</td><td>First in-flight demo of All-Stellar mode</td></tr><tr><td>09/13/2017</td><td>Orbit Trim Maneuver (OTM)-49 pointed solar arrays off-point angle in sunlight, then entered eclipse, battery voltage dropped rapidly, all instruments powered off</td></tr><tr><td>06/02/2018</td><td>Start of GDE of 2018</td></tr><tr><td>11/26/2018</td><td>InSight EDL</td></tr><tr><td>06/10/2019</td><td>Completed battery reconditioning campaign</td></tr><tr><td>03/01/2020</td><td>Begin COVID-19 remote operations</td></tr><tr><td>02/18/2021</td><td>Mars 2020 Perseverance rover EDL</td></tr><tr><td>05/14/2021</td><td>Zhurong EDL</td></tr><tr><td>05/08/2022</td><td>CRISM no longer planning regular observations</td></tr></tbody></table>

MCS monitoring. Thus, only a minority of the prioritized HiTList is scheduled for observation. Once this subset is known, the CIPP may provide input for the image length and observation mode in accordance with data volume and other constraints. The HiTS has final responsibility for instrument settings, scheduling ride-along observations (adding HiRISE images to observations planned by CRISM or CTX), calibrations, and non-interactive observations, and creating a plan such that temperatures and spacecraft data storage use remain within an acceptable range. The HiTS turns the final sequence of observations into commands that are later radiated to the spacecraft. Parent suggestions for the planned observations are “retired” so they will not appear in later HiTLists.

The final responsibility of the CIPP is to look at the new images and decide whether any of their parent suggestions need to be un-retired (for example, if hazy atmospheric conditions lead to a poor image of the surface). The CIPP also writes several captions for images to be released on the public HiRISE site. Images are checked by student validators trained to evaluate image quality for rapid feedback to the CIPPs and HiTS. STLs routinely receive updates about images acquired in their science theme and check that each image satisfies the needs of its parent suggestion. Images with distinctive topography or layering may have their parent suggestion changed to stereo so that another image of the same target but at a different roll angle will be acquired.

## 2.4. Benefits to graduate students

Multiple generations of graduate students have matriculated and graduated while HiRISE has operated. The “People’s Camera” model, encouragement of target suggestions from everyone, and HiRISE’s rapid data release timeline significantly benefits graduate students, who typically work on several publishable projects within a few years.

Graduate students also benefit from the involvement of HiRISE team

<footnote n="1">¹ See Acronyms list at end of this paper.</footnote>
<table><caption>**Table 5**
Major HiRISE flight events.</caption><thead><tr><th>Date (m/d/ yr)</th><th>HiRISE event</th></tr></thead><tbody><tr><td>09/01/2005</td><td>First lunar images; IR10_1 bit-flip anomaly</td></tr><tr><td>12/01/2006</td><td>RED9_1 became very noisy due to bit flips</td></tr><tr><td>12/01/2006</td><td>Strategy of taking fewer but larger images to reduce number of thermal cycles</td></tr><tr><td>01/01/2007</td><td>RED0_1 and RED3_1 show minor bit-flips</td></tr><tr><td>02/03/2007</td><td>Reverse-clock warmup increased from 30 to 60 s</td></tr><tr><td>02/05/2007</td><td>Lowering FPE from 18 to 15 °C revealed bit-flips in RED1_1 and RED8_1</td></tr><tr><td>02/20/2007</td><td>Increase warmup to 71 s</td></tr><tr><td>04/01/2007</td><td>Began 200-s warmup, eliminated problems (temporarily) except IR10_1</td></tr><tr><td>05/18/2007</td><td>Engineering model aging test results show changes similar to those seen in-flight</td></tr><tr><td>09/01/2007</td><td>Passive heating of engineering model ADCs for 48 h at 85 °C, improved margin</td></tr><tr><td>12/20/2007</td><td>Passively-heated engineering model ADCs showed rapid decrease of margin when back in life test</td></tr><tr><td>05/13/2008</td><td>Higher-T EM life test showed improved margins, annealing in-flight seemed possible</td></tr><tr><td>07/01/2008</td><td>Stereo anaglyph pipeline production begins</td></tr><tr><td>07/01/2008</td><td>300-s warmup time, max possible without changing flight software</td></tr><tr><td>11/01/2008</td><td>300-s warmup insufficient, begin taking long nighttime warmup images</td></tr><tr><td>01/01/2009</td><td>8 of 14 CCDs show bit-flips in cold nighttime stim images</td></tr><tr><td>05/06/2009</td><td>HiRISE started monthly PDS releases</td></tr><tr><td>06/05/2009</td><td>MRO reinstated pausing solar array gimbals for high-stability observations (had been off since 1/2008) after demonstration that jitter was reduced.</td></tr><tr><td>07/01/2009</td><td>New product with color merged with full RED observation</td></tr><tr><td>07/01/2009</td><td>HiSEAS implemented using shadow tip algorithm</td></tr><tr><td>01/20/2010</td><td>HiWish launched</td></tr><tr><td>06/05/2010</td><td>SPORCs initiated</td></tr><tr><td>07/01/2010</td><td>Engineering model unit failed, no further life testing at Ball</td></tr><tr><td>07/21/2010</td><td>HiRISE safed because warmup images filled raw space on solid-state recorder (SSR)</td></tr><tr><td>10/01/2010</td><td>increased bit-flips when CRISM cryocooler off, cooling environment</td></tr><tr><td>12/20/2010</td><td>Inflight annealing test-no benefit apparent</td></tr><tr><td>06/01/2011</td><td>HiView implemented</td></tr><tr><td>06/13/2011</td><td>Lunar observations for calibration</td></tr><tr><td>08/01/2011</td><td>Majority of CCD channels show bit-flips when FPE at 18 °C for stim images</td></tr><tr><td>08/31/2011</td><td>RED9 electronics failed</td></tr><tr><td>10/24/2011</td><td>HiPrecision pipeline</td></tr><tr><td>10/24/2011</td><td>Implementation of CIPP-lite</td></tr><tr><td>01/23/2012</td><td>Raised FPE setpoint to 25 °C (heater almost always on), precludes cold stim images to monitor performance</td></tr><tr><td>04/10/2012</td><td>Fixed systematic crosstrack pointing error due to leap-second tracking</td></tr><tr><td>02/24/2013</td><td>Mars 3 candidate found in PSP_006154_1345</td></tr><tr><td>10/01/2013</td><td>Color orthoimages released to PDS</td></tr><tr><td>10/31/2013</td><td>"This is Mars" coffee table book published</td></tr><tr><td>06/29/2014</td><td>Beagle-2 located in ESP_037145_1915</td></tr><tr><td>01/12/2015</td><td>Corrected software error resulting in 1800-line downtrack targeting errors.</td></tr><tr><td>10/01/2015</td><td>Found bug is color registration; new color stretch parameters</td></tr><tr><td>06/01/2016</td><td>Revisit of procedures to swap sides of block-redundant electronics</td></tr><tr><td>11/20/2016</td><td>Lunar observations for calibration</td></tr><tr><td>11/21/2016</td><td>HiRISE safed-instrument electronics box over temperature limit due to use of multiple solar array motors for lunar observations</td></tr><tr><td>05/11/2017</td><td>"Mars: The Pristine Beauty of the Red Planet" coffee table book published</td></tr><tr><td>06/11/2017</td><td>HiRISE warmup images moved out of eclipse to reduce load on batteries</td></tr><tr><td>10/20/2017</td><td>HiRISE images in All-Stellar mode (also at apoapsis) often blurred</td></tr><tr><td>12/09/2017</td><td>HiKERS implemented</td></tr><tr><td>08/25/2018</td><td>Enabled optics heaters during imaging to eliminate blur</td></tr><tr><td>11/13/2018</td><td>HiRISE decision to adapt stand down during some cycles to work on simplifying operations</td></tr><tr><td>10/01/2019</td><td>HiRISE decision to stop raising FPE-T, target max 50 °C (AFT at 65 °C)</td></tr><tr><td>12/03/2019</td><td>Begin ADC setting test campaigns</td></tr><tr><td>08/02/2020</td><td>Change RED1-3 to ADC 74_74</td></tr><tr><td>03/01/2021</td><td>Change Bin1-2A mode so RED3 is binned</td></tr><tr><td>08/03/2021</td><td>Updated HiRISE LUTs for range of possible new ADC settings</td></tr><tr><td>01/30/2022</td><td>New procedures developed; rapid testing of many ADC settings</td></tr><tr><td>07/03/2022</td><td>New ADC settings implemented for RED0-3, 7-8</td></tr><tr><td>07/14/2023</td><td>HiRISE safing due to RED4 electronics failure</td></tr></tbody></table>

members on their committees and as PhD advisors. Twenty-five such students (Table 3) are known to have graduated over the course of the MRO mission from 2009 to present day. Many of these students attended HiRISE team meetings and remain active on the project. Some of these graduates have gone on to advise their own students who are also now involved with HiRISE. Many other graduate students have utilized HiRISE data in their research, but without direct team involvement.

### 3. Operations history

Although an orbiter in a nearly circular orbit and with a consistent local mean solar time (LMST) may sound ideal for stable operations, there have been many complications due to gaps in observing, roll

restrictions, dust storms (including the Global Dust Events, GDEs), special events (like supporting entry, descent, and landing (EDL) activities of other Mars missions), spacecraft or instrument anomalies, and other orbit or hardware changes. MRO or Mars events relevant to HiRISE are summarized in Table 4; HiRISE events are summarized in Table 5. Many of these events are explained in subsequent sections of this paper.

There have been multiple extended periods (at least 2 weeks) with no HiRISE observations (Table 6). Solar conjunctions (roughly every other year) pauses imaging for 4 to 5 weeks. There was an extended spacecraft "safe" (or contingency) mode in 2009, plus a 2-week cycle in 2020 with no imaging during a spacecraft non-volatile memory rewrite. On top of these interruptions, in 2019–2020 HiRISE needed to "stand down" and not participate in some cycles to free up resources to develop
<table><caption>**Table 6**

Periods when HiRISE did not image for 2 weeks or longer.</caption><thead><tr><th>DATE</th><th>CYCLE</th><th>ORBIT</th><th>L<sub>S</sub></th><th>EVENT</th></tr></thead><tbody><tr><td>2008-Nov-22-2008-Dec-19</td><td>54-55</td><td>10,902-11,260</td><td>162° - 177°</td><td>Solar conjunction</td></tr><tr><td>2009-Oct-10-2009-Dec-04</td><td>77-80</td><td>15,034-15,752</td><td>352° - 19°</td><td>S/c safe mode</td></tr><tr><td>2011-Jan-15-2011-Feb-11</td><td>110-111</td><td>20,964-21,322</td><td>218° - 235°</td><td>Solar conjunction</td></tr><tr><td>2013-Apr-06-2013-May-03</td><td>168-169</td><td>31,384-31,743</td><td>296° - 312°</td><td>Solar conjunction</td></tr><tr><td>2015-May-30-2015-Jun-26</td><td>224-225</td><td>41,446-41,805</td><td>350° - 004°</td><td>Solar conjunction</td></tr><tr><td>2017-Jul-15-2017-Aug-08</td><td>280</td><td>51,418-51,738</td><td>34° - 45°</td><td>Solar conjunction</td></tr><tr><td>2019-Mar-02-2019-Mar-15</td><td>322</td><td>59,054-59,232</td><td>349° - 356°</td><td>Stand down cycle</td></tr><tr><td>2019-Apr-27-2019-May-10</td><td>326</td><td>59,773-59,952</td><td>17° - 23°</td><td>Stand down cycle</td></tr><tr><td>2019-Jun-22-2019-Jul-05</td><td>330</td><td>60,492-60,670</td><td>42° - 49°</td><td>Stand down cycle</td></tr><tr><td>2019-Aug-17-2019-Aug-30</td><td>334</td><td>61,210-61,569</td><td>67° - 79°</td><td>Stand down cycle</td></tr><tr><td>2019-Aug-31-2019-Sep-13</td><td>335</td><td>61,390-61,569</td><td>73° - 79°</td><td>Solar conjunction</td></tr><tr><td>2020-Jan-04-2020-Jan-17</td><td>344</td><td>63,007-63,186</td><td>130° - 137°</td><td>Stand down cycle</td></tr><tr><td>2020-Feb-152,020-Feb-28</td><td>347</td><td>63,546-63,725</td><td>151° - 158°</td><td>Non-volatile memory rewrite</td></tr><tr><td>2020-Jul-18-2020-Jul-31</td><td>358</td><td>65,522-65,700</td><td>241° - 250°</td><td>Stand down cycle</td></tr><tr><td>2020-Sep-12-2020-Sept-25</td><td>362</td><td>66,241-66,420</td><td>276° - 285°</td><td>Stand down cycle</td></tr><tr><td>2021-Sep-25-2021-Oct-22</td><td>389-390</td><td>71,094-71,453</td><td>104° - 117°</td><td>Solar conjunction</td></tr><tr><td>2022-Mar-12-2022-Mar-25</td><td>401</td><td>73,250-73,429</td><td>189° - 197°</td><td>Stand down cycle</td></tr><tr><td>2022-Jun-18-2022-Jul-01</td><td>408</td><td>74,508-74,687</td><td>249° - 258°</td><td>Stand down cycle</td></tr></tbody></table>

streamlined operations and to continue HiRISE with reduced funding. In 2022, there were additional stand down cycles due to lack of available personnel.

# 4. Summary of HiRISE archived datasets

## 4.1. Image products

The HiRISE data archive is a NASA Planetary Data System (PDS) version 3 (PDS-3) archive that can be directly accessed at http://hiri.se-pds.lpl.arizona.edu/PDS/. The data archive can be broken down into two classes of products: standard products and various extra products (Table 7). The standard products consist of the raw data or Experimental Data Record (EDR) products, calibrated and map-projected Reduced Data Record (RDR) products, and Digital Terrain Models (DTMs), which include sets of orthorectified images derived from each DTM. The standard products can also be accessed at doi:10.17189/1520303 for RDRs, doi:10.17189/1520179 for EDRs, and doi:10.17189/1520227 for DTMs. The extra products are derived from the standard products, and various cosmetic corrections, color stretching, size reductions, and lossy compression may have been applied that may make these products unsuitable for quantitative scientific analysis (McEwen et al., 2010). The NOMAP products consist of CCD mosaics in the raw geometry, without any pixel resampling, thus preserving the full resolution. As of August 2023, the archive contains >2.2 million standard products with a total data volume of 116 terabytes, and 6.3 million extra products with a total data volume of 126 terabytes. Browse images are available with and without annotations that include a scale bar and elevation range (for DTMs).

## 4.2. Imaging modes

Because HiRISE can be commanded with up to 14 CCDs, each with its own binning and TDI setting, there are many possible imaging configurations. Standard imaging modes were established to simplify the application of these configurations (McEwen et al., 2007). In the years that HiRISE has been operating, some standard imaging modes have been applied much more frequently than others. Modes with red CCDs in 4 × 4 binning are often saturated over illuminated terrain and are currently exclusively used for very low light scenes. Binning modes 8 × 8 and 16 × 16 are not useful for imaging Mars, and we have chosen to not use 3 × 3 binning to simplify the data processing. Modes that enable only a subset of the 14 CCDs have also gone largely unused in favor of full swath width modes, with a handful of exceptions. Narrow images with only the center two or four RED CCDs, and all four BG and IR CCDs have utility over potential landing sites, where full monochrome

coverage exists but additional color coverage is desired, and over small targets during periods with low downlink rates. Several additional modes restricted to only those CCDs less affected by bit-flips due to ADC issues (Section 8.9) at low temperatures were defined in 2017. Additionally, changing susceptibility to bit-flip degradation in certain CCDs has resulted in occasional updates to some mode definitions; for example, Bin 1-2A historically specified RED3 as unbinned, but is now 2 × 2 binned to mitigate bit-flips.

Table 8 lists the HiRISE standard imaging modes and their definitions as currently in use (see McEwen et al., 2007, for other defined modes). Of these, the most frequently used are Bin 2A, Bin 1A, and Bin 1-2A. Bin 2A, with all CCDs binned 2 × 2, strikes a good balance between coverage extent, resolution, and high (>100:1) Signal-to-Noise Ratio (SNR) for most martian terrains under most lighting and frost conditions. Bin 1A, with un-binned red CCDs and 2 × 2 binning for the color CCDs provides the highest resolution (BG and IR CCDs require 2 × 2 binning for high SNR). Bin 1-2A, with RED3 (until recently) and REDs 4–6 at full resolution and all remaining CCDs binned 2 × 2, has proven useful to cover small features or samples of patterned terrain at full resolution, with additional binned coverage for local context. The mode can also serve as a compromise when operational or lighting constraints make full resolution over the full swath width unfeasible. Another new mode is called the “no-warmup” mode, consisting of a subset of CCDs that return good data when the FPE is relatively cold (below the minimum temperature for full images). That subset includes REDs 4–6, BG12–13, IR11, and IR10 channel 0 (we also acquire channel 1 with some erroneous data). We now also include REDs 7–8 with recently improved ADC settings (Section 8.9), although they may be removed in the future depending on ADC bit-flip degradation rates.

In response to the failure of RED4 in 2023, perhaps due to cycling to high temperatures, we expect to reduce bin-1 imaging in RED0–3 to enable operation at lower temperatures. This leads to a new mode that uses bin-1 for RED5–8 and bin-2 for RED0–3.

Almost all HiRISE images of Mars are acquired with 2 types of data compression, and a third type of compression is used on the ground. First, the input 14-bit data are converted to 8 bits via non-linear look-up tables (LUTs), then the lossless Fast and Efficient Image Compression System (FELICS; Howard and Vitter, 1993)) is applied in the spacecraft solid-state recorder. Returning uncompressed data is also possible but takes ~5 times more data volume per image. For data products on the ground we use JPEG2000 format (Taubman and Marcellin, 2012), typically just slightly lossy, for rapid browse and zoom over the images (Section 5.2).
<caption>**Table 7**</caption>

HiRISE archive product listing organized by type (Standard or Extra), the current PDS-3 storage format and the planned storage format after conversion into PDS-4.

<table><caption>**Table 8**</caption><thead><tr><th>Product</th><th>Type</th><th>PDS-3 Format</th><th>PDS-4 Format</th></tr></thead><tbody><tr><td>Experimental Data Records (EDR)</td><td>Standard</td><td>PDS-RAW (IMG) + attached PDS-3 label</td><td>PDS-RAW (IMG) + detached PDS-4 label</td></tr><tr><td>EDR browse and thumbnail products</td><td>Extra</td><td>JPEG</td><td>JPEG + PDS-4 label</td></tr><tr><td>Reduced Data Records (RDR) (Map- Projected Products)</td><td>Standard</td><td>Non-lossy compressed JPEG2000 image with detached PDS-3 label</td><td>Uncompressed PDS-RAW or geotiff product + detached PDS-4 label</td></tr><tr><td>RDR browse and thumbnail products</td><td>Extra</td><td>JPEG</td><td>JPEG + PDS-4 label</td></tr><tr><td>RDR "Quicklook" products</td><td>Extra</td><td>Lossy compressed JPEG2000</td><td>Lossy compressed JPEG2000 + PDS-4 label</td></tr><tr><td>Merged products (IRB and simulated RGB)</td><td>Extra</td><td>Lossy compressed JPEG2000</td><td>Lossy compressed JPEG2000 + PDS-4 label</td></tr><tr><td>RED, BG, IR, IRB, RGB NOMAP products</td><td>Extra</td><td>Lossy compressed JPEG2000</td><td>Lossy compressed JPEG2000 + PDS-4 label.</td></tr><tr><td>Stereo Anaglyphs</td><td>Extra</td><td>Lossy compressed JPEG2000, JPEG</td><td>JPEG2000, JPEG + PDS-4 label</td></tr><tr><td>Digital Terrain Model (DTM)</td><td>Standard</td><td>PDS-RAW (IMG) + attached PDS-3 label</td><td>PDS-RAW + detached PDS-4 label</td></tr><tr><td>DTM-derived Orthorectified images</td><td>Standard</td><td>Compressed JPEG2000 image + detached PDS-3 label</td><td>Uncompressed PDS-RAW or geotiff + PDS-4 label</td></tr><tr><td>DTM browse and thumbnail images</td><td>Extra</td><td>JPEG</td><td>JPEG + PDS-4 label</td></tr><tr><td>DTM Figure of Merit (FOM) map</td><td>Extra</td><td>JPEG2000</td><td>JPEG2000 + PDS 4 label</td></tr><tr><td>DTM Slope Maps (coming soon)</td><td>Extra</td><td>PNG</td><td>PNG + PDS 4 label</td></tr></tbody></table>

Common HiRISE Imaging Modes.

<table><thead><tr><th>Name</th><th>Description</th><th>RED 4-6*†</th><th>Other REDs†</th><th>NIR†</th><th>BG†</th><th>Size†</th></tr></thead><tbody><tr><td>Bin 1A</td><td>Highest res at >100:1 SNR; stereo</td><td>Bin 1<br>TDI 128</td><td>Bin 1<br>TDI 128</td><td>Bin 2<br>TDI 128</td><td>Bin 2<br>TDI 128</td><td>0.66</td></tr><tr><td>Bin 1</td><td>Bin 1 full FOV RED, 4 × 4 color</td><td>Bin 1<br>TDI 128</td><td>Bin 1<br>TDI 128</td><td>Bin 4<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>0.61</td></tr><tr><td>Bin 1-2A</td><td>Bin1 RED4-6; others bin 2</td><td>Bin 1<br>TDI 128</td><td>Bin 2<br>TDI 64</td><td>Bin 2<br>TDI 128</td><td>Bin 2<br>TDI 128</td><td>0.37</td></tr><tr><td>Bin 1-2</td><td>Bin1 RED4-6; others binned</td><td>Bin 1<br>TDI 128</td><td>Bin 2<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>0.32</td></tr><tr><td>Bin 2A</td><td>Bin 2 everything</td><td>Bin 2<br>TDI 64</td><td>Bin 2<br>TDI 64</td><td>Bin 2<br>TDI 128</td><td>Bin 2<br>TDI 128</td><td>0.22</td></tr><tr><td>Bin 2</td><td>Bin 2 full FOV RED, 4 × 4 color</td><td>Bin 2<br>TDI 64</td><td>Bin 2<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>0.17</td></tr><tr><td>Bin 4</td><td>Bin 4 everything</td><td>Bin 4<br>TDI 32</td><td>Bin 4<br>TDI 32</td><td>Bin 4<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>0.06</td></tr><tr><td>Bin 1A No Warmup</td><td>Bin 1 RED4-8, 2 × 2 color</td><td>Bin 1<br>TDI 128</td><td>—</td><td>Bin 2<br>TDI 128</td><td>Bin 2<br>TDI 128</td><td>0.40</td></tr><tr><td>Bin 2A No Warmup</td><td>Bin 2 RED4-8, color</td><td>Bin 2<br>TDI 64</td><td>—</td><td>Bin 2<br>TDI 64</td><td>Bin 2<br>TDI 128</td><td>0.15</td></tr><tr><td>Bin 1 No Warmup</td><td>Bin 1 RED4-8, 4 × 4 color</td><td>Bin 1<br>TDI 128</td><td>—</td><td>Bin 4<br>TDI 64</td><td>Bin 4<br>TDI 64</td><td>0.10</td></tr></tbody></table>

### 4.3. Coverage over Mars

HiRISE coverage of Mars is highly non-uniform; Fig. 3 shows how many HiRISE images are within 100 km of each point of the surface; some locations have hundreds of images. About 0.85% of Mars has no HiRISE coverage within 100 km, while the most-frequently imaged 1% of the planet's surface has at least 120 images within that distance. MRO's polar orbit ensures the scientifically interesting polar regions are well covered. Some non-polar locations stand out as programmatically important (e.g., potential or actual landing sites such as Meridiani Plenum, Jezero crater, or Gale crater) and/or scientifically important (e.g., Valles Marineris). Broad areas that are heavily imaged often have regions to their north and south with poor coverage due to limits on how closely HiRISE images can be spaced along MRO's south-to-north dayside orbit; isolated hot spots (Fig. 3) result from monitoring sites where many images are concentrated. Many such sites occur in the southern highlands between 30°-60°S as a result of monitoring gullies and RSL.

HiRISE coverage of Mars has increased in a quasi-linear fashion (Fig. 4) throughout the mission, with occasional hiatuses (Table 6). Fluctuations in the data return rate (with a period of approximately 26 months) are driven by the changing Earth-Mars distance. The most recent fluctuations are muted in amplitude as HiRISE is increasingly limited by thermal constraints rather than downlink during high data rate periods.

Stereo coverage has also increased quasi-linearly with additional slow periods corresponding to seasons with spacecraft roll restrictions. As of 19 June 2023, 7578 complete planned stereo pairs have been acquired. There are also unplanned stereo pairs, typically with poorer convergence angles or illumination angle matches.

### 4.4. How Mars affects the images

Mars and its atmosphere have numerous effects on images, both predictable and unpredictable. The most obvious of these is surface reflectivity which affects the signal received by HiRISE, thus driving the choice of the number of TDI lines and binning to receive adequate signal (McEwen et al., 2007).

Although MRO is in a Sun-synchronous orbit that results in a nearly fixed local imaging time (Zurek and Smrekar, 2007), the lighting at that local time varies with latitude and season. The incidence angle describes the angle between a vector to the Sun and the local vertical. Small incidence angles emphasize color and albedo variations, while topographic effects dominate the appearance of images at larger incidence angles. Shadows grow at larger incidence; their abundance depends on local topography, but they are usually not severe at incidence angles <65°, thus serving as a typical criterion for stereo targeting. SNR is also lower at large incidence angles. Sub-solar azimuth (the direction to the Sun) also varies with the season. The details of the lighting variation are latitude dependent. For the typical 3 PM MRO local time, equatorial sites

<footnote>* Includes RED3 for Center Bin 1A, REDS7-8 for No Warmup modes.</footnote>

<footnote>† TDI varies with lighting conditions.</footnote>

<footnote>‡ Gb/10 K bin-1 lines (compressed), varies with scene and atmospheric conditions.</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Global view of the number (on a log scale) of HiRISE observations (up to orbit 74,507) within 100 km. The color scale saturates at 50.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Area of all HiRISE images through time (units of % of Mars surface covered). Many targets are imaged multiple times for stereo or change-detection investigations so the unique area of Mars with HiRISE coverage is lower than the area of the images.</figcaption></figure>

have their smallest incidence angles near the equinoxes, but even at solstices, the surface is well-illuminated. The mid-latitudes are never in complete darkness but have severe shadows around the winter solstice, so imaging non-seasonal targets is concentrated in spring and summer. The poles are in darkness for half the year, but incidence angles approach 60° at the summer solstice.

Atmospheric dust also influences imaging and image quality. The globally averaged optical depth of dust in the atmosphere is typically ~2× higher in southern spring and summer (e.g., Smith, 2008), thereby reducing image quality. These seasons also include most dust storms (often between $L_s$ 240–320°), including the GDEs of MY28 and MY34. Those two GDEs resulted in severely degraded images and led to restricted target selection (favoring the south pole and high-elevation sites) for many weeks. Regional dust events sporadically degrade image quality as well. For example, the Hellas basin is often obscured by dust or condensate haze except for imaging windows that are usually best between $L_s$ 0–50° and 140–180°.

Another atmospheric effect is the formation of condensate clouds as well as diffuse hazes. The most significant cloud effect is the northern polar hood, which occurs through most of northern autumn and winter, starting near the pole but eventually extending into the mid-latitudes (Calvin et al., 2015). Images near the north pole commonly have degraded quality starting near $L_s$ = 160°, and the effects reach 35–40°N in northern winter.

Another significant influence that Mars has on imaging is the occurrence of seasonal frost mainly composed of $CO_2$ (Wood and Paige, 1992). Frost is of interest for some science objectives but a hindrance for others. This frost covering is a dynamic agent of surface change but

obscures the underlying materials and challenges the stereo matching algorithms. Seasonal frost has both a latitude and slope dependence. On level ground, it is present throughout fall and winter near the poles while appearing only briefly near the winter solstice at its lowest latitudes, around ±50° (Piqueux et al., 2015b). Local frost on pole-facing steep slopes extends to proportionately lower latitudes (Dundas et al., 2019a).

### 4.5. Statistics of each science theme

HiRISE suggestions are divided into 17 science themes and an additional “Other” category for planning purposes. Over time, some themes have proved less prevalent, and their use has evolved. For instance, the “Hydrothermal Processes” theme has had few suggestions; the “Climate Change” theme now concentrates on north polar avalanches and south polar residual cap changes (although neither necessarily indicates that any climate change is occurring); and the “Other” theme is now restricted to observations acquired with no prior suggestion, such as ride-along observations with other instruments.

Each suggestion is assigned one primary science theme (and an optional secondary theme), and the observation that retires the suggestion inherits that categorization. STLs (Table 2) prioritize these suggestions and verify the data acquired satisfies the needs of the suggestion. Table 9 summarizes the number of observations that stemmed from suggestions in each science theme and the number of remaining suggestions in each priority category.

### 4.6. Remaining observation suggestions

As of August 2022, 22,603 suggestions prioritized by STLs are still active (not yet retired). The spatial distribution of active HiRISE suggestions is extremely non-uniform (Fig. 5), as is the spatial distribution of acquired images. Poleward of about 60°, active suggestions are sparse as MRO's polar orbit provides many opportunities to image these areas. Extensive dust-covered areas (such as Tharsis) provide fewer high value targets at HiRISE scales and so have fewer suggestions. Also, suggestions that are entered in “unpopular” regions tend to be quickly planned and retired. Popular target areas (e.g., Valles Marineris, Syrtis Major) have many remaining high-priority suggestions as opportunities to image there are limited and are further reduced by competition with other MRO instruments. MRO orbits in the vicinity of recent landing sites have been heavily used for programmatic candidate landing site characterization and data relay (Section 7.16) and so many other suggestions in these areas remain to be acquired, particularly because such areas are often of high scientific interest. Over time the suggestion map has become increasingly non-uniform because the few high-priority targets in unpopular regions are soon acquired. We encourage new targets in
these unpopular regions.

## 4.7. Data biases

HiRISE observations have a variety of spatial and temporal biases. Seventeen years since the start of full operations, HiRISE images cover a cumulative area of ~4.5% of the martian surface; however, unique coverage (excluding repeats for stereo and monitoring) is <3%. That coverage is not random and is subject to both targeting preferences and operational constraints.

Because HiRISE is a targeted imager, coverage is inherently concentrated on sites thought to be of interest. Thus, images preferentially cover features such as fresh craters or outcrops of layered deposits, compared with featureless areas. This selection strategy is necessary to best use the limited HiRISE coverage; as a result, the images are not random samples. Another result of this strategy is that sub-polar latitudes (~60–80° in each hemisphere) and other regions have a relatively low density of images.

Various operational considerations also influence the HiRISE data set. Because MRO is in a polar orbit, the poles can be imaged most easily and are of high scientific interest, which is why they have the most coverage per unit area. MRO data-relay procedures for landed spacecraft result in restricted imaging on orbits that pass near landers and rovers (Section 7.16). A similar effect arises when any given target competes with nearby suggestions that cannot fit in the same field of view, and with observations along the same orbit track. These constraints mean that frequent monitoring of one site limits the ability to image nearby sites. Additionally, targets along the same orbit tracks as Valles Marineris, candidate landing sites, or other regions with many high-priority target suggestions are harder to acquire and may be underrepresented.

A final consideration is that some regions have limited intervals when high-quality images can be acquired, such as seasonal shifts of coverage and resolution due to frost, lighting considerations, and the polar hood (Section 4.4). The most pronounced effect is in the low-elevation Hellas basin due to the combination of atmospheric haze in summer and darkness in winter.

## 4.8. Surface change detection

Observations to characterize surface changes (also called monitoring) include both designated seasonal series and occasional (often yearly) repeat images testing for longer-term changes. These were a relatively small part of HiRISE operations in the early mission, focused

on seasonal defrosting processes. They have increased over time, driven particularly by the discovery of bedform migration (e.g., ripples, megaripples, dunes) (Silvestro et al., 2010, 2020; Bridges et al., 2011) and multiple types of slope activity (Diniega et al., 2010; Dundas et al., 2010; McEwen et al., 2011) and the opportunities afforded by multiple extended missions for change detection. During the PSP, 6.2% of acquired observations were in the Seasonal Processes science theme, 3.4% in Aeolian Processes, and 2.0% in Mass Wasting. In contrast, during MY35, 10.5% of acquired observations were in Seasonal Processes, 7.4% in Aeolian Processes, and 17.0% in Mass Wasting. While not every observation in those themes is a monitoring suggestion, a large majority are, indicating the growth in change detection imaging. This temporal coverage has resulted in some of the most significant HiRISE science results but, as a consequence, reduced non-monitoring observations of new, unique locations. More non-monitoring image suggestions are needed to help restore balance to future observations.

## 4.9. Coordination with other instrument experiments

Coordinated observations—the simultaneous observations of a common target by multiple instruments—are acquired at locations where the synergy between HiRISE, CRISM, CTX, and CaSSIS can address significant scientific questions. HiRISE/CRISM/CTX coordinated observations have enabled detailed geological and compositional characterization of many targets on Mars. For example, coordinated HiRISE observations have allowed researchers to correlate CRISM-based mineral detection with morphological features, stratigraphic relationships, and spatial distributions (e.g., Murchie et al., 2009; Wray et al., 2011; Weitz et al., 2022). HiRISE color ratios complement CRISM-based spectral characterization of geological units (Delamere et al., 2010). Coordinated CRISM and HiRISE observations have allowed a detailed characterization of the seasonal evolution of the composition of the seasonal ice on the polar caps (Pommerol et al., 2011). Limited spectral and morphological characterization of dynamic, seasonal features on Mars, such as recurring slope lineae and topographic slumps, have been possible with coordinated HiRISE-CRISM observations and MARCI daily global images (Ojha et al., 2017). SHARAD observations are also important for joint analyses (e.g., Morgan et al., 2021) but are typically acquired at night, so they are not simultaneous with optical datasets. MCS operates both day and night, and MCS plus HiRISE data have been used to understand seasonal processes and the evolution of polar volatiles (e.g., Wilcoski and Hayne, 2020).

In addition to the compositional and morphological characterization,

<table><caption>**Table 9**
Science Theme Statistics extracted in August 2022.</caption><thead><tr><th rowspan="2">Science Theme</th><th rowspan="2">Number of Observations</th><th colspan="5">Remaining Prioritized Suggestions with this priority</th></tr><tr><th>10</th><th>9</th><th>8</th><th>7</th><th>6</th></tr></thead><tbody><tr><td>Climate Change</td><td>1743</td><td>29</td><td>108</td><td>142</td><td>53</td><td>51</td></tr><tr><td>Composition &amp; Photometry</td><td>3615</td><td>21</td><td>105</td><td>149</td><td>119</td><td>39</td></tr><tr><td>Aeolian Processes</td><td>4311</td><td>184</td><td>431</td><td>542</td><td>591</td><td>92</td></tr><tr><td>Fluvial Processes</td><td>5327</td><td>32</td><td>510</td><td>612</td><td>181</td><td>3010</td></tr><tr><td>Future Exploration / Landing sites</td><td>2425</td><td>27</td><td>81</td><td>143</td><td>63</td><td>57</td></tr><tr><td>Geologic Contacts / Stratigraphy</td><td>5424</td><td>64</td><td>225</td><td>1052</td><td>996</td><td>52</td></tr><tr><td>Glacial/Periglacial Processes</td><td>6031</td><td>26</td><td>71</td><td>218</td><td>182</td><td>227</td></tr><tr><td>Hydrothermal Processes</td><td>384</td><td>4</td><td>55</td><td>33</td><td>12</td><td>4</td></tr><tr><td>Impact Processes</td><td>8720</td><td>36</td><td>159</td><td>698</td><td>517</td><td>179</td></tr><tr><td>Landscape Evolution</td><td>4809</td><td>10</td><td>53</td><td>354</td><td>860</td><td>790</td></tr><tr><td>Mass Wasting Processes</td><td>7978</td><td>513</td><td>1496</td><td>2176</td><td>585</td><td>79</td></tr><tr><td>Other</td><td>2478</td><td>0</td><td>2</td><td>7</td><td>6</td><td>24</td></tr><tr><td>Polar Geology</td><td>4225</td><td>17</td><td>128</td><td>129</td><td>31</td><td>29</td></tr><tr><td>Rocks &amp; Regolith</td><td>1501</td><td>0</td><td>5</td><td>34</td><td>31</td><td>25</td></tr><tr><td>Seasonal Processes</td><td>6930</td><td>162</td><td>235</td><td>309</td><td>112</td><td>74</td></tr><tr><td>Sedimentary / Layering processes</td><td>3124</td><td>40</td><td>169</td><td>264</td><td>100</td><td>22</td></tr><tr><td>Tectonic Processes</td><td>3087</td><td>29</td><td>142</td><td>264</td><td>58</td><td>132</td></tr><tr><td>Volcanic Processes</td><td>3752</td><td>8</td><td>92</td><td>360</td><td>225</td><td>35</td></tr><tr><td>Total</td><td>75,864</td><td>1202</td><td>4067</td><td>7486</td><td>4722</td><td>4921</td></tr><tr><td>Percent</td><td></td><td>6</td><td>18</td><td>33</td><td>21</td><td>22</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Global view of current active HiRISE suggestions. Colors are Science Theme Lead priority 10 (red), 9 (orange), 8 (teal), 7 (cyan), and 6 (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

coordinated HiRISE-CRISM observations enable the characterization of the dynamic behavior of active geological features (e.g., Reiss et al., 2014). The imaging technique employed by CRISM results in net-positive and negative time offsets from the center of the image, which causes a slight offset in the acquisition time between CRISM and HiRISE/CTX. The approximately ±1 min acquisition time differences can be utilized to track the motion of active geological features such as dust devils (Fig. 6; Reiss et al., 2014). These coordinated observations are an excellent complement to the faster changes due to dust devil rotation that can be investigated between HiRISE color channels (Choi and Dundas, 2011).

Despite their scientific utility, coordinated observations are only planned for a subset of all observations. The HiRISE team requests co-ordinated observations from CTX for all stereo images and when coordination is mentioned in the image suggestion. Additionally, CTX images are acquired for all targeted observations by both HiRISE and CRISM. By contrast, coordinated CRISM observations are requested by the science theme lead or if an image suggestion mentions composition as one of the targeted science outputs.

The HiRISE team strives to satisfy coordination requests from CTX or CRISM, but this is sometimes impossible due to thermal, data volume, time, or other constraints. Of particular importance, the CTX team alerts the HiRISE team of candidate new impact events for HiRISE follow-up to confirm and measure the craters, if present. >1200 new impact sites are now documented by before-and-after images, most from MRO (Daubar

et al., 2022). Targeting old CRISM data locations with interesting mineral identifications continues beyond the end of CRISM routine operations in 2022 (Seelos et al., 2023).

HiRISE also coordinates observations with CaSSIS on ExoMars TGO to view a location at two different times of day in the same season. Typically observations can be acquired within about two weeks, so seasonal changes are minimized. TGO plans observations weeks in advance of MRO for the same period, so the HiRISE team can target observations that correspond to already-planned CaSSIS observations.

### 4.10. Lunar observations during extended missions

HiRISE observations of Earth's moon acquired during cruise and PSP were not useful for absolute radiometric calibration due to poor observational geometry. Therefore, new HiRISE images of Earth and the Moon were acquired in 2011 and 2016 that are more suitable for absolute radiometric calibration. The martian moons were also observed in the PSP (McEwen et al., 2010).

### 4.11. Comet observations

In 2013 and 2014, two comets (ISON and Siding Spring) made close approaches to Mars and were observed by HiRISE (Table 10). For ISON (named after the International Scientific Optical Network), ten useful observations were made near the closest approach. Assuming an albedo

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Coordinated HiRISE observation ESP_021925_1650 (a) and CRISM observation FRT0001D8DC (b). The black arrow points to a ~50-m-diameter dust devil that moved ~400 m between the acquisition of the two sub-images (~34 s). The red outline shows the same crater in the HiRISE and CRISM view. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<table><caption>**Table 10**
HiRISE observation campaigns for comets ISON and Siding Spring.</caption><thead><tr><th></th><th>ISON C/2012 S1</th><th>Siding Spring C/2013 A1</th></tr></thead><tbody><tr><td>Perihelion</td><td>28 Nov 2013</td><td>25 Oct 2015</td></tr><tr><td>CA to Mars</td><td>13,000,000 km</td><td>138,000 km</td></tr><tr><td>Observation time</td><td>29 Sep to 02 Oct 2013</td><td>17 Oct to 20 Oct 2014</td></tr><tr><td>Phase angle (deg.)</td><td>47 to 89</td><td>88 to 107</td></tr><tr><td>IFOV</td><td></td><td>1 micro-radian</td></tr><tr><td># observations</td><td>10</td><td>54</td></tr><tr><td>Colors</td><td>RED, BG</td><td>RED, BG, IR</td></tr><tr><td>Exposure time</td><td></td><td>2.5 s max</td></tr></tbody></table>

of 0.03 and using a nucleus phase law, Delamere et al. (2013) estimated an upper limit to the nucleus diameter near 1 km. Early news reports had estimated a 5-km diameter nucleus of ISON. HiRISE obtained 54 observations as close as 138,000 km, for comet Siding Spring, showing the comet and associated jets. The nucleus was largely behind the comet, but Farnham et al. (2017) state that the nucleus is a ~ 1 km non-spherical body. Early news reports estimated a 50-km diameter nucleus for Siding Spring. HiRISE data proved critical to accurately constrain the cometary nuclei size and for deriving high-precision trajectories for the comets (e.g., Farnocchia et al., 2016).

### 4.12. How to access the HiRISE data

HiRISE data are archived in the PDS Cartography and Imaging Sciences Node (see Table 11 for links to all the resources in this section). HiRISE releases standard data products to the PDS on a monthly cadence. Data are typically released within two months of receipt of the raw data on the ground. The PDS is designed to be an archive from which various tools query the data for viewing, analysis, or visualization. Here we provide a partial list of current tools and websites that can be used to search, process, or download HiRISE images and DTMs.

The HiRISE website (https://hirise.lpl.arizona.edu/) serves as one way to browse and search the catalog. The “Map” page provides a Mars graphical interface showing HiRISE image footprints plus those of CaSSIS, CTX, MOC, and CRISM, as well as suggested targets for HiRISE and CaSSIS. It is searchable by keywords or observation IDs in the main search field or by region, season, theme, or other parameters available in the “Advanced Search” option. DTMs are searchable via the Advanced Search page or a global map linked to the DTM page. The HiRISE website provides a link to download the free HiView (Section 5.2) application (see “software” link at the bottom of the web page), which allows users to easily view the full-scale JPEG2000 image files (JP2 files) without needing to download them. The website also provides CTX images (https://hirise.lpl.arizona.edu/ctx) that have been map-projected using the same mapping definition and global shape model as HiRISE, ensuring accurate co-registration with simultaneously-acquired HiRISE products.

The PDS Geosciences node provides a website called Mars Orbiter Data Explorer (Mars ODE) that interfaces with multiple Mars missions, including MRO. Users can query by instrument and data type, by geographic coordinates, time ranges, or within a map. The Mars ODE

also includes a tool to search for coordinated observations between MRO instruments (e.g., HiRISE and CRISM). Selected files can be downloaded in raw or processed formats. The site provides instrument footprint files in shapefile and Keyhole Markup Language (KML) vector format, enabling users to perform their own spatial queries.

Other free, map-based viewers that serve the HiRISE images (but not DTMs) include the Google Earth app (when set to Mars), Arizona State University’s (ASU) HiRISE image map, and the USGS PDS Planetary Image Locator Tool (PILOT; Bailen et al., 2013). One caution to users of these sites is to be aware that they may not display all of the most current data available in the PDS. PILOT allows for the selection of images to be sent for further processing (i.e., map projection) via the USGS Map Projection on the Web (POW) service (Hare et al., 2013) and provides tools for identifying unplanned stereo acquisitions. Perspective views from projected orthoimages and DTMs can also be explored at Areo-Browsser (https://areobrowsser.com/). The online GIS application, MarsSI, is a searchable global map of Mars data that includes services to process HiRISE image data and DTMs (Quantin-Nataf et al., 2018).

HiRISE data (including DTMs) are available within the Java Mission-planning and Analysis for Remote Sensing (JMARS) application (Dickenshied et al., 2019). The program is installed locally, but data are served from ASU via the internet. The interface functions as a geospatial information system (GIS) in that it is map-based and can display various datasets in the same geographic coordinate system. JMARS is particularly useful for analysis of HiRISE data directly with other Mars datasets.

### 4.13. PDS 4 support

We are developing procedures to convert the PDS-3 archive into PDS-4 format (Raugh and Hughes, 2021). The updated PDS-4 standard requires standard science products to be archived without any special encoding (such as JPEG2000) to help ensure very long-term accessibility of the data (Planetary Data System Standards Reference v1.18.0). This restriction poses the greatest challenge to the conversion process, as the JPEG2000 compression can achieve reductions in the product data volume by as much as a factor of 5. Creating these uncompressed products thus presents a significant expansion in the amount of storage required to house the data archive. However, this restriction only applies to the standard science products. JPEG2000 is acceptable for the derived extra products, and it is anticipated that these products will not

<table><caption>**Table 11**
URLs and descriptions of various tools to query, visualize, and analyze HiRISE data.</caption><thead><tr><th>Name</th><th>URL</th><th>Description</th></tr></thead><tbody><tr><td>PDS</td><td>https://pds-imaging.jpl.nasa.gov/volumes/mro.html</td><td>MRO volumes on the PDS Cartography and Imaging Sciences Node</td></tr><tr><td>HiRISE</td><td>http://uahirise.org</td><td>HiRISE public website</td></tr><tr><td>HiView</td><td>https://www.uahirise.org/hiview/</td><td>JPEG2000 Viewer provided by HiRISE</td></tr><tr><td>HiRISE DTMs</td><td>https://www.uahirise.org/dtm/</td><td>DTM catalog on the HiRISE website</td></tr><tr><td>Mars ODE</td><td>https://ode.rsl.wustl.edu/mars/</td><td>Search and download Mars data including HiRISE images and DTMs</td></tr><tr><td>ASU HiRISE image map</td><td>http://global-data.mars.asu.edu/bin/hirise.pl</td><td>Global map of HiRISE image footprints that link to downloadable files</td></tr><tr><td>PILOT</td><td>https://pilot.wr.usgs.gov</td><td>Map query and send for processing via POW</td></tr><tr><td>POW</td><td>https://astrocloud.wr.usgs.gov</td><td>Map projection on the web of images selected in PILOT</td></tr><tr><td>JMARS</td><td>https://jmars.asu.edu</td><td>GIS for analysis of HiRISE images and DTMs with the Mars datasets.</td></tr><tr><td>HiMap</td><td>https://marsoweb.nas.nasa.gov/HiRISE/hiweb/marsbrowser/index.html</td><td>See section 5.3</td></tr><tr><td>MarsSI</td><td>https://marssi.univ-lyon1.fr/MarsSI/map/mars/</td><td>Web application for processing Mars image data, including HiRISE.</td></tr></tbody></table>
be redelivered in an uncompressed format. Table 7 lists all the different types of products in the HiRISE data archive, and how they will be changed for PDS-4 compatibility.

The initial delivery of PDS 4 compatible products is anticipated to occur in 2024. This initial delivery will only contain products derived from data acquired no earlier than October 2022, the start of EM6. All earlier HiRISE data are planned to be reprocessed into PDS-4 format over the next five years. These products will be delivered to a cloud service maintained by the PDS Imaging Node to allow easier management of the increased data volume required by the decompressed science products. All subsequent deliveries will continue to be delivered in the PDS 4 format. We intend to continue delivery in the PDS-3 format as well as PDS-4, because the JPEG2000 format greatly aids accessibility to the large (Giga-pixel) images using tools such as HiView.

### 4.14. Incomplete observations

When only part of an observation has been returned, it is incomplete. This data loss typically occurs when image packets are overwritten in the SSR by newer data, mostly when the DSN downlink is reduced, such as problems with DSN ground equipment or altered DSN pointing to address a spacecraft problem. Incomplete observations are concentrated in periods of high data rate because there is little time to replay the data before it is overwritten. The partial HiRISE observation is processed and released to PDS but contains gaps with no data. The gaps range from nearly all of the observation to just a single 20-line gap in one channel. Most gaps are at least 20 lines of a channel because that is the image size used for lossless compression in the SSR. Uncompressed images may have gaps of any size. 14.7% of HiRISE observations are incomplete.

## 5. Special data products and analysis tools

Basic PDS Data Products are summarized in Section 4.1 above. This section provides more details on how PDS products are produced, special data products, and publicly available analysis tools, updating the information in McEwen et al. (2010).

### 5.1. ISIS updates

HiRISE image processing pipelines utilize individual programs from the Integrated Software for Imagers and Spectrometers (ISIS; Becker et al., 2007). ISIS is frequently updated with new capabilities (Backer et al., 2018). Few such updates have been made specifically for HiRISE in recent years, but some of the updates do change the HiRISE data products in subtle ways, so products are made in different ways over time. In addition, the HiRISE team has made some changes to how images are processed, i.e., using different ISIS input parameters. As part of the PDS-4 conversion effort, we plan to reprocess all images to ensure consistency.

### 5.2. HiView

HiView is the best way to explore HiRISE images of the martian surface at full resolution, taking full advantage of the JPEG2000 format for rapid response in navigating large images. Once the application has been downloaded to your computer (https://www.uahirise.org/hiview/), all that is needed is a quick drag and drop of any of the links for JP2 files on the HiRISE website to the HiView application window. No downloading of multi-gigabyte files is required. HiView also provides dynamically generated statistics about the current area of an image being viewed, whether it is the entire original source at a low resolution, or a small area that has been expanded to full resolution. Multiband histograms can be used to enhance the color stretch and contrast of the currently viewed area and can be reapplied to other areas of an image or to different images. HiView can save the area of an image currently displayed as a TIFF, JPEG, PNG or other format for use in

different image editing applications for further enhancement of any particular region of interest at full resolution.

### 5.3. HiWeb and HiMap

Prior to the development of HiWish (Section 7.2) in 2013, the HiRISE science team exclusively used HiWeb in selecting imaging targets. The HiRISE team continues to use HiWeb for targeting and for prioritizing image suggestions. HiWeb is a web-launched Java application, and geospatial information system, that enables users to peruse the various Mars orbital data and image sets and enter their desired imaging targets in an image suggestion registry, as described previously (McEwen et al., 2007, 2010). HiWeb is not publicly available, but HiMap (Fig. 7) is a dynamic, multi-faceted web tool that is an updated front-end to the entry maps interface of HiWeb. HiMap was born out of the desire to make HiRISE, CRISM, and CTX footprints and images available on an interactive map interface within HiWeb without having to first access the image suggestion facility. In addition, HiMap allows HiRISE JPEG2000 images to be viewed at full resolution in a web browser environment and adds updated analysis tools such as the automated elevation profiler, distance tool, and interactive contrast image enhancer.

### 5.4. Color merge products

The official PDS-archived images include a full RED CCD and narrow 3-color products. In 2009 we added a new PDS "extras" product in which the color images (both RGB and IRB in 2 separate products, Table 7) are merged onto the RED image (Fig. 8). We produce this at a scale of 0.5 m/pixel even for bin-1 image modes to keep the data volume reasonable, so users need to be aware that this product may not preserve the full RED-image resolution. We also increase the color saturation to better show the color variations. The COLOR products preserve the correct radiometric balance between colors for photometry.

### 5.5. Stereo anaglyphs

Stereo anaglyphs have been produced since 2008 for latitudes 80°S-80°N (McEwen et al., 2010) as PDS extras, but initially omitted polar locations where the parallax is complicated because the spacecraft off-nadir pointing is more toward the pole than to the east or west. An update to the procedure was completed in 2014 to enable processing of anaglyphs for polar images. A projection rotation that optimizes parallax must be calculated using geometric and photometric information of the stereo pair.

### 5.6. HiPrecision

After radiometric calibration, precision geometric calibration is performed to remove distortion across the focal plane due to the optical curvature of the beam, remove the CCD offsets, and optionally correct for spacecraft jitter. All three of these corrections are accomplished by the HiPrecision subsystem, which has two branches. The HiRISE Jitter-analyzed CK (HiJACK) branch applies the jitter modeling described in Sutton et al. (2019) to correct geometric distortions. The NoProj branch performs geometric correction only, without modeling and correcting for jitter, as not all HiRISE images require jitter correction. While we apply HiPrecision to only small numbers of images, as requested by members of the science community, it has been applied to all of the images used to produce DTMs over landing sites (Section 6).

### 5.7. DTM production

Since the PSP, the number of HiRISE DTMs that have been produced has risen from ~60 to >1000 and counting (Fig. 9). The percentage of planned and acquired HiRISE stereo pairs made into PDS DTMs
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Example of HiMap's interface. HiRISE, CTX, and CRISM footprints can be displayed and accessed directly by clicking on the footprints. An elevation profiler can give a quick insight to a 3D perspective. Interactive latitude, longitude, and elevation is given by cursor location.</figcaption></figure>

represents ~12% of the >7660 planned stereo pairs acquired as of August 2023. (There are also unplanned stereo pairs due to overlap of image coverage on a target.) DTM s are now released to the PDS monthly as they are completed.

HiRISE DTM s in the PDS are produced using a combination of the ISIS software and the commercial photogrammetry software SOCET Set (BAE Systems, Inc.). Although details of the process have evolved, the basic procedure is consistent with what is described by Kirk et al. (2008) and described in more detail by Sutton et al. (2022b). HiRISE DTM s are typically produced at either 1 m/post for 25 cm/pixel (bin-1) stereo pairs or at 2 m/post for 50 cm/pixel (bin-2) stereo pairs. The accompanying orthoimages are produced at full resolution and the same scale as the DTM, with some exceptions. Files exported from SOCET Set are post-processed in ISIS to generate PDS (v. 3) standard files. DTM s produced using the ISIS/SOCET Set workflow conform to the formats and outputs that can be converted to standard PDS and PDS Extras products. Eight different institutions have contributed to the HiRISE DTM PDS archive.

Modifications to the ISIS/SOCET Set workflow have been developed to incorporate multiple monitoring images taken over a target where a DTM exists to build a time series of orthoimages (Sutton et al., 2022b). Such sequences have been critical for studying active surface processes. The refinement of techniques and analysis have made HiRISE DTM s essential to the characterization of candidate landing sites for robotic and human exploration (Kirk et al., 2008; Fergason et al., 2017, 2020).

HiRISE DTM s have also been produced (typically at a reduced spatial scale) with the Ames Stereo Pipeline (Beyer et al., 2018). Because this is an automated pipeline that can be re-run relatively easily, we do not attempt to archive these with the PDS.

## 5.8. DTM extras and visualization products

The HiRISE DTM PDS extras products include “browse” images of the DTM and accompanying orthoimages (Fig. 10). Browse images (JPEG format) of the DTM include a shaded relief map, colorized altimetry combined with shaded relief, and a grayscale image. Browse images are available with and without annotations, including a scale bar and elevation range. The browse images provide a rapid and accessible way to visualize the terrain type, roughness, elevation variability, and

elevation range. A Figure of Merit (FOM) map, provided in the JPEG2000 format, displays a classified map of the quality of each DTM post draped over the shaded relief and is provided at full scale.

A new (to be released by 2024) DTM extras product is a slope map (Sutton et al., 2023). Prior to calculating slope, the DTM is smoothed with three sequential 5 × 5 lowpass filters followed by a 3× reduction in scale. This results in a slope calculation over ~9 m baseline for a DTM produced at 1 m post spacing. The lowpass filters (smoothing) minimize the systematic texture and small-scale slope errors at the post-spacing scale that is characteristic of the SOCET Set NGATE product (Kirk et al., 2021; Sutton et al., 2022b). Slope is calculated on the lowpass-filtered, reduced DTM using the Horn algorithm (Horn, 1981), as implemented in the ISIS application *slpmap*, which calculates the gradient at each pixel from a weighted difference of the surrounding 8 pixels (Horn, 1981). The greyscale slope map is archived and classified into discrete slope bins for additional color products. The range is 0°-90°, with breaks at 5°, 15°, 25°, and 35°.

The color slope product is produced in two versions: one displays classified slope map without shaded relief; the other is the same classified slope map combined with a shaded relief image (Fig. 11). The version without the shaded relief serves two purposes. It can be used as a layer in a geographical information system (GIS) and allows for the visualization of the slope classes without shading affecting the color ramp. The version with the shaded relief aids in interpreting landforms and surface textures. A plain text world file is provided that allows any version of the slope map to be displayed in a GIS, along with the DTM and orthoimages, which already have geospatial encoding.

## 5.9. DTM flyovers

The ability to view terrain and images from non-nadir perspectives aids in the geologic interpretation of landforms (e.g., Banks et al., 2009; Okubo, 2010; Weitz et al., 2011, 2022; Chojnacki et al., 2016), and is effective way to present a more human-relatable landscape. Diverse visualization techniques are available to facilitate the interpretation of topography. HiRISE DTM s have been utilized by an enthusiastic public in a variety of applications from video games to 3D prints. Rendering tools have been developed to ingest and manipulate HiRISE DTM s in freely available software such as Blender (https://www.blender.org),
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Example of IRB color-merge image product that was targeted to enable interpretation of stratigraphy exposed in Terra Sirenum.</figcaption></figure>

which includes a community-maintained plugin for HiRISE DTM s, and
Areob browser (https://areob browser.org), which generates 3D perspec-
tive views on the fly in an internet browser. Flythrough animations
created by both professionals and enthusiasts have been used to great
effect engaging the public in Mars exploration and discovery.

5.10. Merged HiRISE-CaSSIS data product

Pan-sharpened images are valuable for detailed investigations of the geology of the martian surface and for landing site characterization (Tornabene et al., 2018). Pan-sharpening is a transformation technique that provides a sharpened color image by merging a higher spatial resolution monochromatic or panchromatic image with a lower resolution color image. Such transformations are desirable because the HiRISE color swath only covers ~20% of the full image.

There are several techniques for pan-sharpening a color image with a

higher-resolution greyscale image. The Gram-Schmidt spectral sharp-
ening transform (Laben and Brower, 2000) was used by Tornabene et al.
(2018) as it retains I/F information. I/F is image intensity divided by
intensity of solar illumination, and is the radiometrically calibrated
output value for each pixel. One of the major challenges with the
technique is that most martian datasets are not orthorectified, or are
orthorectified to low-resolution topographic data such as MOLA that
does not remove small-scale (<1 km) geometric effects on viewing ge-
ometry. As such, to pan-sharpen HiRISE with other datasets, a wide-
spread and dense network of 10s to 100 s of Ground Control Points
(GCPs) must be defined between the two to warp (i.e., “rubber-sheet”)
the lower resolution color dataset to overlay precisely with HiRISE prior
to pan-sharpening, with more GCPs being needed in rugged terrain.

Fig. 12 shows 25 cm/pixel pan-sharpened example based on a 4 m/
pixel CaSSIS 4-filter color infrared observation, which provides the
equivalent to HiRISE IRB colors, of Moni crater situated in the larger
Kaiser crater. The HiRISE-CaSSIS pan-sharpened image compares well
with the colors provided by the 50 cm/pixel HiRISE IRB color strip
despite differences in observation geometries and lighting conditions
between the two images.

5.11. Image processing pipelines

The HiRISE image processing pipelines are programs that are
responsible for a specific segment of image product processing from raw
data to various kinds of finished products. The programs represent logic
that mostly script together individual ISIS programs. For example, the
HiCal pipeline runs the ISIS program called hical (Becker et al., 2021)
and other ISIS programs (whose names are all lowercase). The pro-
cessing pipelines create various browse and PDS products. Table 12
describes the image processing activities for each of the HiRISE pipeline
programs that begin with PDS EDR data (there are earlier pipelines that
primarily deal with fetching data and assembling the raw PDS EDRs).

5.12. Reprocessing status

The HiRISE team has begun but not completed reprocessing of older
images several times, to incorporate a variety of small improvements.
Before completing one reprocessing, there has always been another
change, so the effort has not been completed. As part of the full PDS4
conversion effort over the next five years (Section 4.13) we plan to
reprocess the entire dataset.

5.13. Replacing erroneous data from bit-flips

During the analog-to-digital conversion of charge from the CCD
channels to a 16-bit data number (DN), if one (or more) of those sixteen
bits gets incorrectly recorded as a zero instead of a one, or vice-versa, we
call this a “bit-flip,” and this results in an incorrectly recorded DN value.
During cruise and at Mars, an ever increasing number of images with bit-
flip errors were observed, and also an increasing percentage of bit-
flipped pixels in an image, at a given FPE temperature (Section 8.9). If
the bit that gets “flipped” was a low-significance bit, then the difference
between the “real” DN value, and the “recorded” DN value is small (2, 4,
8 DN difference in images whose DN range can typically span a few
hundred DN), and this has a low impact on the resulting image. If the bit
that was flipped is of higher significance, it could change the DN value
by hundreds or thousands of DN, and this results in a pixel that is clearly
recording an incorrect value and is anomalously bright or dark
compared to its surrounding pixels. The largest bit-flips produce null DN
values.

Since many images in the archive contain this bit-flip noise pattern,
an improved algorithm for cosmetically improving the images was
devised. >10% of the sub-images from three of the 28 image channels
(channel 1 of IR10, RED1 and RED3) contain bit-flip noise. The bit-
flipped pixels appear as outlying “islands” in a DN histogram plot,
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Global map showing the distribution of >1000 HiRISE DTMs (orange circles) available in the PDS as of August 2023, shown on a basemap of the MOLA shaded relief topography, equirectangular projection. Symbol is much larger than the actual DTM footprint. Axes are latitude and east longitude.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Examples of PDS browse products of DTM “extras.” (a) Colorized shaded relief, annotated with DTM ID, scale bar (500 m as shown), color ramp legend, and institution image credit. (b) Grayscale image of the DTM, without annotation. (c) Shaded relief image of the DTM, without annotation. (d) Figure of Merit (FOM) map, produced as a full scale JP2 with separate legend. (e) Color orthoimage annotated browse product from one of the source stereo images. (f) Annotated browse product from one of the source stereo orthoimages. An annotated browse, unannotated browse, and unannotated thumbnail image is produced for each product type, except for the FOM map.</figcaption></figure>

away from the main bulk of pixels in an image. The bit-flip correction algorithm attempts to find the best pair of minima on either side of the “good” data so that the erroneous DN values can be masked away, resulting in a well-behaved image histogram, and an image with some null pixels. Isolated null pixels or single-column nulls are replaced by the average of surrounding valid DNs in later stages of the HiCal pipeline.

Data users interpreting small image areas should examine EDRs to be certain that interpolation of erroneous data (or the erroneous data itself) has not changed the result, for example hiding boulders that might be missed in boulder counts.
<figure><img src="image_13.png" /><figcaption>Fig. 11. Example of the new DTM extras slope map browse images. The slope map is produced in two versions: (Left) Slope classified into five bins color-coded as shown in the legend, and (Right) Classified slope map using the same color ramp, combined with the shaded relief image.</figcaption></figure>

5.14. HiRISE image sharpening

There are several techniques used to sharpen digital images,
including deconvolution of the point-spread function (PSF) or use of
multiple images to derive a sharper image. PSF deconvolution was used
on landing site images (Kirk et al., 2008; Golombek et al., 2012a) and on
the MSL parachute image (https://www.uahirise.org/releases/msl/).
The MSL parachute image was acquired via a special sequence for a
longer exposure time and higher SNR than is possible for almost all other
HiRISE images of Mars.

Tao and Muller (2016) describe stacking and matching HiRISE im-
ages of the same area taken from different angles on different orbits for
what they call super-resolution restoration. However, the HiRISE data-
set is far from ideal for this image restoration, which ideally requires:

1. High SNR (>100:1)

2. A well-sampled point-spread function

3. A set of images that match in resolution and illumination angles

4. Regularly spaced sub-pixel pointing offsets between images in a set of images over a scene.

5. An unchanging scene for the set of images

HiRISE often satisfies #1 above, but the typical SNR with respect to
the surface (not the top-of-atmosphere signal after scattering) is typi-
cally ~50:1. As for #2, most planetary mission images are aliased (few
pixels across the point-spread function) because data rate is limiting. In
other words, it is too expensive in data volume to “oversample” the
point-spread function as is done by Earth-based telescopic imaging. The
HiRISE image point-spread function is about 2 pixels wide at half max

including both optical effects and smear from spacecraft motion, which
is useful but not ideal for restoration. For #3, MRO's orbit enables
crudely identical resolution and illumination conditions via its sun-
synchronous orbit, although sun angles vary considerably with season.
The viewing angles vary from about -30° to +30° cross-track, which is
the only way to image a surface location multiple times per Mars year,
and the pixel scale varies by up to 15% with viewing angle. To co-
register the images, the geometry difference with viewing angle can
be overcome with sufficient processing. Given the changing viewing
angles plus pointing jitter, the sub-pixel offsets from image area to image
area (#4) are random and variable within the overlap footprint. This
means that the resolution improvement varies from place to place in the
final image. For #5, Mars' surface does change over time, especially in
places we favor for multiple imaging to monitor the changes.

HiRISE has captured enough (>4) repeat images most useful for
image resolution improvement of only about a dozen areas on Mars.
Areas with larger numbers of images are targeted repeatedly because of
surface changes over time, so a combined product would defeat the
purpose of understanding these changes. For large landing sites such as
for the ExoMars rover or InSight lander, it takes years to acquire even
single HiRISE coverage over the whole “3-sigma” landing error ellipses,
and twice as long to acquire full stereo coverage. It would not be wise to
sacrifice full coverage of landing ellipses for the sake of image restora-
tion processing on a just a small part of the ellipse. However, a future
Mars orbiter may have a much larger data rate and landing site ellipses
are shrinking due to advances in EDL technology, which makes resolu-
tion improvement more attractive, either onboard the spacecraft or on
the ground. For the highest-priority sites, resolution improvement may
provide an economical alternative to flying a more capable camera and
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Context for the inputs and results for pan-sharpening processing of 25 cm/pixel HiRISE RED (ESP_015918_1325) with a CaSSIS NIR-PAN-BLU (NPB) color infrared image (MY34_002286_310_0). Image area is 0.95 × 2.93 km. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

spacecraft, to achieve a modest improvement in resolution (probably no better than a factor of two).

# 6. Imaging for lander and rover missions

## 6.1. Pre-MRO landers and rovers

HiRISE has imaged surface hardware from all successful pre-MRO surface missions (Vikings 1 and 2, Pathfinder, Mars Exploration Rover (MER) Spirit and Opportunity; see McEwen et al., 2010). We have also

carried out campaigns to search for sites of the failed Mars Polar Lander (MPL) and ESA's Beagle-2. MPL was not located and would be very difficult to recognize because it landed within the region of seasonal frost + dust cover, which “repaints” the surface yearly, removing unusual color/albedo patterns. The Phoenix lander (Smith et al., 2008), at a similar latitude in the northern hemisphere, became very difficult to recognize after a polar winter. Beagle-2 was eventually located (Bridges et al., 2017). The USSR's Mars 3 lander, which did land successfully and transmitted some data or noise, may also have been located. A written description of this effort appeared in an on-line article for the Planetary
<table><caption>**Table 12**
HiRISE image processing pipelines.</caption><thead><tr><th>Pipeline name</th><th>Description</th></tr></thead><tbody><tr><td>EDR_Stats</td><td>Runs ISIS hi2isis and histat to gather statistics and information.</td></tr><tr><td>HiCal</td><td>Runs ISIS hical and performs a variety of noise filtering specific to the HiRISE CCDs (Section 5.13).</td></tr><tr><td>HiStitch</td><td>Joins two channel products to form a whole CCD product (ISIS histitch), sometimes bin-2 and bin-4 images need extra filtering after that stage. Channel 1 normalized to match brightness of channel 0.</td></tr><tr><td>HiccdStitch</td><td>Stitches together all of the same-filter CCD images to a single RED, IR, and/or BG nomap image. This step can optionally perform additional correction of vertical striping artifacts via cubenorm. It performs a "balance" operation so that each CCD is radiometrically matched to either RED5, IR11, or BG13 before running ISIS hiccdstitch.</td></tr><tr><td>HiColorInit</td><td>Prepares images for coregistration. Scales BG and IR products to match the central RED CCD pixel scale.</td></tr><tr><td>HiJitReg</td><td>Uses ISIS hijitreg to gather registration information for the BG and IR CCDs to the RED CCDs.</td></tr><tr><td>HiSlither</td><td>Uses ISIS slither to correct for the jitter of the IR and BG CCDs relative to the RED CCDs and creates a three-band "color" mosaic with ISIS hicubeit.</td></tr><tr><td>HiColorNorm</td><td>Normalizes the BG and IR color across the two-CCD width of the color mosaic.</td></tr><tr><td>HiBeautify</td><td>Creates a synthetic RGB product and an enhanced IRB product (Delamere et al., 2010).</td></tr><tr><td>RedGeom</td><td>Perform geometric processing for each CCD, then trigger RedMosaic pipeline</td></tr><tr><td>REDMosaic</td><td>Create geometrically projected RED and COLOR mosaics.</td></tr><tr><td>RDRgen</td><td>Create official PDS RED and COLOR RDR products and labels.</td></tr><tr><td>HiMosMerge</td><td>Merge the geometrically projected RED and COLOR IRB and RGB RDR products into MIRB and MRGB products.</td></tr><tr><td>HiPrecisionInit</td><td>Prepares images for HiNoProj or HiJACK processing.</td></tr><tr><td>HiNoProj</td><td>RED images are assembled into a noproj cube with ideal camera geometry (Kirk et al., 2008).</td></tr><tr><td>HiJACK</td><td>If sufficient jitter was identified, an algorithm (Sutton et al., 2019) uses the two central CCDs of each color to solve for the jitter, and then uses similar approaches to HiNoProj to create "noproj" jitter-corrected versions of the RED, IR, and BG channels.</td></tr></tbody></table>

Society (https://www.planetary.org/articles/0412-how-we-searched-for-mars-3).

## 6.2. New landing sites for MSL, InSight, and Mars2020

HiRISE fundamentally changed how candidates and selected landing sites on Mars are assessed and certified for both science and engineering merits and characteristics. The sub-meter resolution of the images (typically reprojected to 0.25 m/pixel scale) and DTMs processed using stereo image pairs enable a detailed evaluation of the geologic setting and engineering risks associated with landing. For science, HiRISE images enable unprecedented evaluation of geomorphic signatures diagnostic of processes (e.g., details of crater degradation styles, aeolian cross-stratification, fluvial channels, lava levees, etc.) and stratal relationships that are key to assessing geologic settings and scientific merits of a site (Golombek et al., 2018, 2020a; Grant et al., 2010, 2018, 2020; Sweeney et al., 2018; Weitz et al., 2020). For engineering, assessment of hazards to landing and operations using HiRISE data enables direct interrogation of the rock size-frequency distribution on the surface (Golombek et al., 2008, 2012a, 2020b, 2021a; Grant et al., 2022), surface slopes and relief at length scales of a meter or less (Kirk et al., 2008; Beyer, 2017), and surface characteristics that can help evaluate mantling or other undesirable surface materials and properties for rovers (Arvidson et al., 2011; Golombek et al., 2012b, 2017, 2020b, 2021a).

Here we present a summary of how HiRISE data assisted in the assessment and selection of the candidate landing sites for the Mars Science Laboratory Curiosity (MSL) and the Mars 2020 Perseverance (M2020) rover missions, plus the InSight lander. (The Phoenix landing site process was summarized in McEwen et al., 2010.) The interiors of expected landing ellipses were targeted the most by HiRISE, but science targets outside of the ellipses were also imaged. HiRISE acquired >650 images to support landing site selection activities related to these three missions. Additional images were acquired during mission entry, descent, and landing (EDL), and ongoing imaging of landed assets continues. An important note is that HiRISE images acquired to assess the scientific merits of landing sites were quickly released. This rapid release enabled the community to propose and participate in landing site selection activities. The papers cited above present more detailed information regarding the science and engineering processes for selecting landing sites.

## 6.3. ExoMars

An EDL demonstration module called Schiaparelli was launched to Mars in 2016, along with the ExoMars Trace Gas Orbiter (TGO). During the descent phase, the Schiaparelli module separated from the backshell earlier than expected, so it crashed (see ESP_048120_1780), but valuable engineering data was returned (Bonetti et al., 2018). HiRISE has also extensively imaged the intended landing regions for the ExoMars Rosalind Franklin rover, with over 300 images returned (Fig. 13). The selected region in Oxia Planum (Quantin-Nataf et al., 2021) is one of the largest areas on Mars with complete HiRISE coverage. In addition to the high spatial resolution and stereo from HiRISE, its color products have, together with CaSSIS, enabled detailed mapping and subdivision of the Oxia Planum clay unit (Parkes-Bowen et al., 2022) – the target terrain for the rover and its drill.

## 6.4. Tianwen-1 lander and Zhurong rover imaging

On 15 May 2021, the lander/rover portion of the Tianwen-1 mission from the Chinese National Space Administration (CNSA) successfully touched down on Mars, and on 22 May, the Zhurong rover drove onto the martian surface (Sun and Rao, 2022). CNSA requested a list of NASA remote sensing datasets covering a region in southern Utopia Planitia a few months before landing, indicating that this would be their preferred landing. HiRISE was able to plan and acquire sample images throughout this region, covering a few percent of the total area; all surfaces were similarly free of small-scale hazards. After the Tianwen-1 landing, with known coordinates, HiRISE acquired a stereo pair over the landing site (ESP_069665_2055 and ESP_069731_2055) and created a DTM. We acquired images to monitor the rover location and wheel tracks about nine months later (ESP_073225_2055) and again after another seven months (ESP_075559_2055). The most recent image (ESP_077511_2055) revealed that the rover had not changed its position between 8 September 2022 and 7 February 2023. Expanded image coverage was acquired because this region exhibits landforms proposed to be the result of mud volcanism, perhaps related to an ancient ocean (e.g., Mills et al., 2021; Huang et al., 2022; Chao et al., 2022).

## 6.5. Imaging in support of future human exploration

NASA and SpaceX have requested HiRISE images for planning future human exploration. NASA held a single human landing site/exploration zone workshop in 2015, which led to the acquisition of 43 HiRISE images scattered across Mars. There has not been an effort to prioritize
<figure><img src="image_15.png" /><figcaption>Fig. 13. Footprints of HiRISE images over the Oxia Planum landing region as of August 2022 for the ExoMars Rosalind Franklin rover, near 18.2°N, 24.3°E (map adapted from Parkes-Bowen et al. (2022)).</figcaption></figure>

these regions. SpaceX has sponsored several workshops about future human landings on Mars, focused on the northern middle latitudes where ice is known to exist near the surface down to ~35°N latitude (Morgan et al., 2021; Dundas et al., 2023). A workshop was held at the University of Arizona in January 2019, attended by members of the science community and SpaceX personnel to discuss regions that meet the latitude and elevation constraints and had evidence for substantial ice deposits. Areas of interest in Arcadia Planitia, Erebus and Phlegra Montes, Utopia Planitia and Deuteronilus Mensae were evaluated using HiRISE images to see if suitably smooth and boulder-free areas existed. Golombek et al. (2021b) described potentially safe sites in Arcadia Planitia, and Erebus and Phlegra Montes, but Utopia Planitia and Deuteronilus Mensae appeared too bouldery. At a virtual workshop in August 2020, with more HiRISE and other data acquired and more DTMs produced, the prospective landing sites were evaluated and four leading

candidate sites (one in Phlegra Montes, two in Arcadia Planitia, and one in Erebus Montes) were identified (Golombek et al., 2021b).

## 6.6. Active mission support

HiRISE images of mission hardware during and after EDL provide a unique opportunity to assess the spacecraft on arrival at Mars and an inventory of where hardware components became distributed near the landing point (Table 13). Over time, these data are supplemented by ongoing imaging of landed assets to monitor traverse progress and spacecraft appearance (Table 13). For example, for the solar-powered MER and InSight missions, monitoring the spacecraft enabled a calibration of dust accumulation over time. In the case of InSight, monitoring also helped to characterize the size and frequency of passing dust devils (Perrin et al., 2020). The InSight team hoped that one of these

<table><caption>**Table 13**
HiRISE imaging of spacecraft during EDL and soon after landing on Mars.</caption><thead><tr><th></th><th>Phoenix</th><th>MSL</th><th>InSight</th><th>Perseverance</th></tr></thead><tbody><tr><td>Date (SCET)</td><td>2008-May-25</td><td>2012-Aug-06</td><td>2018-Nov-26</td><td>2021-Feb-18</td></tr><tr><td>Range to MRO (km)</td><td>793</td><td>342</td><td>920</td><td>696</td></tr><tr><td>Image parameters</td><td>RED CCDs bin-1 TDI 8<br>BG and IR CCDs bin-1 TDI 32<br>uncompressed<br>126,670 lines</td><td>All CCDs bin-1 TDI 32<br>compressed<br>85,000 lines</td><td>RED CCDs bin-1 TDI 8<br>BG and IR CCDs bin-2 TDI 8<br>uncompressed<br>30,000 lines</td><td>All CCDs bin-1 TDI 32<br>compressed<br>62,300 lines</td></tr><tr><td>EDL observation ID</td><td>PSP_008579_9020</td><td>ESP_028256_9022</td><td>ESP_057820_9023*</td><td>ESP_068281_9024</td></tr><tr><td>Initial post-EDL landing site observations with hardware</td><td>+11 h: PSP_008585_2915 + 22 h:<br>PSP_008591_2485 + 6d:<br>PSP_008644_2485</td><td>+1d: ESP_028269_1755 + 6d:<br>ESP_028335_1755 + 12d:<br>ESP_028401_1755</td><td>+3d: ESP_057860_1845 + 10d:<br>ESP_057939_1845 + 15d:<br>ESP_058005_1845<br>*EDL imaging attempt<br>unsuccessful</td><td>+1d: ESP_068294_1985 + 7d:<br>ESP_068360_1985 + 12d:<br>ESP_068426_198</td></tr></tbody></table>
vortices might pass over the spacecraft, thereby lofting dust and cleaning the solar panels. Additionally, the MER and MSL teams used HiRISE images to identify and avoid loose aeolian bedforms, such as where the Opportunity rover became stuck for weeks (Arvidson et al., 2011), and to plan MSL rover paths to minimize damage to the wheels (Arvidson et al., 2017).

HiRISE for the first time allowed rover drivers and science teams to know where the rover was after each drive and to accurately navigate to key locations to visit. Prior to HiRISE, knowledge of where the MER rovers were after each drive was approximate, because Mars Orbital Camera (MOC) images (Malin et al., 2010) were not high enough in resolution to localize the rover. After HiRISE arrived, the rover team was able to determine to within 1 HiRISE pixel (~30 cm) where the rover was by taking rover stereo images, creating orthophotos of them and directly correlating and georeferencing them onto a HiRISE image. This process is now standard procedure for rover missions (Golombek, 2019).

# 7. Science planning and uplink tools and techniques

## 7.1. Introduction

Here we summarize the internal science planning processes and tools for those who may benefit from the lessons received by the HiRISE team over the years. Additionally, this information may be useful to those who submit image suggestions, enhancing the odds of acquiring the best images.

## 7.2. HiWish

HiWish public suggestion portal is the primary Web tool for entering suggestions for future HiRISE images (Beyer et al., 2010; Chojnacki et al., 2020). Some of our best image suggestions come from our colleagues in the scientific community actively pursuing research that would benefit from additional, specific HiRISE coverage. The general public also utilizes the HiWish portal, along with the HiRISE team, and other MRO instrument team members. HiWish was announced in early 2010, and through April 2022, a total of 10,140 people have registered and suggested 27,681 images (Fig. 14); of these suggestions, 13,407 have been satisfied. Our most prolific public contributor, James Secosky, has contributed >14,000 suggestions, resulting, so far, in >5000 image acquisitions.

To suggest a new target, one first creates an account using the HiWish page of the HiRISE website (https://www.uahirise.org/). A HiWish user can create a stand-alone, repeat, or stereo suggestion. Suggestions require a Title, Science Rationale, and an appropriate Science Theme. The Title will typically include a location and feature or landform to be imaged (e.g., Crater in Meridiani Planum). The Science Rationale requires a short description of the intended objective, which informs the HiRISE team why this image should be taken. Along with a user-designated priority (1–5; 5 being the highest priority for that user), this rationale communicates to the team the significance of the target. All suggestions in a given Science Theme will be given a team-designated priority (6–10) by the appropriate Science Theme Lead (STL; see Table 2). For more details, including advanced targeting tips for stereo or seasonal suggestions, see Chojnacki et al. (2020) and their E-Poster.

(https://www.hou.usra.edu/meetings/lpsc2020/eposter/2095.pdf).

## 7.3. "Must-have" procedures

The MRO project science planning process is complex. Aspects of this include the round-robin selection process: taking the highest priority target than can be acquired from CTX, CRISM, HiRISE, and SHARAD, then the second-highest priority from each experiment, etc. Other complexities include the MCS rules (Section 7.5), the stereo completion process, and the time-limited nature of certain science objectives. To ensure that the very highest-priority targets are acquired, there is a short list of five highest-priority requests from each science team, dubbed "must-have" (MH) targets, that are integrated by hand before the round-robin selection. MHs are allowed to violate MCS rules and are targeted before the stereo completion process (which happens before the round-robin target selection). For HiRISE, the CIPP will typically select MHs from among the special requests from the team (Section 7.13). Sometimes there are also MEP must-have targets (such as certification of landing sites), and these get the very top priority, above the science MHs.

## 7.4. The planning process; targeted vs. non-interactive observations

The planning process for each two-week cycle is like that described

<figure><img src="image_16.png" /><figcaption>**Fig. 14.** Global distribution of public HiRISE suggestions in light green, on a colorized MOLA (Smith et al., 2001) basemap, with red as high elevation and blue as low elevation. Simple Cylindrical projection, latitude -90° to 90°, longitude -180° to 180°. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
in McEwen et al. (2010) but has been compressed in the extended science phases. The entire planning period lasts four and a half weeks, instead of the five and a half weeks needed in the primary science phase. This period includes 1.5 weeks to plan Interactive Observations (IOs), two weeks to plan Non-Interactive Observations (NIOs), two weeks of sequence execution, and up to two weeks of image validation, with overlaps, so the total is four and a half weeks. IOs may point the spacecraft or require high-stability mode, whereas NIOs do not require any action outside the HiRISE team. The long planning period has logistical complications; cycles begin planning when the prior cycle has yet to start executing, and images from two cycles before have yet to be acquired.

### 7.5. Large roll restrictions

MRO must point the high-gain antenna at Earth and the solar arrays at the Sun for most of each orbit (except when Mars is relatively close to the Sun and the solar arrays may be parked). Although both the high-gain antenna and solar arrays are on gimbals, there are some periods in which the off-nadir roll angles must be restricted to maintain both requirements. The orbiter seasonally encounters periods during which it cannot take advantage of its full ±30° off-nadir swath. In roll-restricted cycles, HiRISE targeting focuses on non-stereo images that can be acquired as small rolls or nadirs; we do not start new stereo pairs unless the first image can be acquired on rolls greater than ±4°, allowing the second image to be acquired on small rolls (<9°) yet at acceptable convergence angles. Additionally, ongoing stereo completion during this time is limited to second halves for which low convergence angles are acceptable.

Rolling the spacecraft for off-nadir pointing is problematic for MCS observations, particularly large rolls (defined as those >9°) clustered spatially or temporally which produce gaps in MCS monitoring. MRO science teams have agreed to several rules limiting off-nadir observations in order to minimize the impact on MCS data. These rules are automatically enforced for all observations during IO scheduling; observations that do not conform to the MCS rules are removed from the plan. A small number of exceptions are possible but require special efforts. The current MCS rules concerning spacecraft rolls (in place since the PSP but not described in McEwen et al. (2010)) are:

* Maximum of 2 large rolls per orbit, where the second roll must start at least 28 min after the end of the first.

* Large rolls may be in adjacent orbits if, and only if, the end of the earlier roll (in its orbit) is at least 9 min 20 s before the start of the later roll (in its orbit).

* Maximum of 4 large rolls per 24 h period.

Additionally, HiRISE is allowed up to 22 large rolls per cycle, to be used for the team's highest priority science observations, first-look stereo, second-look stereo, and time-critical non-stereo observations.

### 7.6. CIPP-lite and CIPP-less cycles

A main strength of HiRISE's image acquisition is its diversity in science goals, accomplished by a rotating science lead for each 2-week cycle, called the Co—I of the Pay Period (CIPP; McEwen et al., 2010). Although originally envisioned as a role for a Co-Investigator, this role has also been fulfilled by team affiliates, postdoctoral researchers, and graduate students, providing early career opportunities for mission involvement directly related to scientific research activities.

As science funding declined in the extended missions, reducing the workload on the science and operations teams was necessary. HiRISE developed the option for a science team member to only participate in the first two weeks of the planning process, a role referred to as “CIPP-Lite.” This compromise maintains the valuable science input and diversity of interests the science team brings to the IO stage while reducing

their working time. The tradeoff is that the CIPP-Lite cannot select “ride-along” targets, add lower-priority NIOs, or help plan the detailed camera settings for every observation. The use of CIPP-Lite cycles has expanded over time. As instrument temperature management became more demanding (Section 7.14), it became increasingly necessary for the HiTS to override the CIPP-chosen ride-alongs and NIOs, so CIPP-lite became the most efficient work mode.

As HiRISE funding declines, eliminating science team participation from tactical planning may become necessary for some cycles. Cycles could be entirely planned by an uplink engineer, with automated scripts replacing work done by the CIPP. This personnel descope may provide the opportunity to compress the IO planning down to 1 week from 1.5, but it comes at the cost of less scientific input into each image. Cycles without a CIPP would pull suggestions according to specific set rules, guided by STL priorities, but without an opportunity for thoughtful prioritization among science themes and images.

### 7.7. HiRISE stereo effect analysis software (HiSEAS)

As a single-aperture, fixed-pointing instrument, HiRISE can acquire stereo pairs only on separate orbits, one for each image of the pair. Ideally, these stereo images would be acquired at similar illumination conditions and with minimal variation in surface features, seasonal or otherwise. The MRO near-polar orbit is such that, at the equator, HiRISE can theoretically view the same target every 66 orbits (~5.5 days) with a spacecraft cross-track roll angle of 20°. Closer to the poles, the repeat coverage frequency improves, resulting in a dozen or more possible stereo pairs for a single target over the course of a week. If the next observable orbit is unavailable for some reason, then the time between stereo halves grows. This increased time, in turn, results in increasingly complicated lighting and seasonal differences. In practice, it may be months or years before HiRISE has another chance to schedule a stereo completion.

HiSEAS evaluates whether a candidate stereo pair is reasonable and which resulting pair is the best when more than one candidate exists for a target (Becker et al., 2015; Sutton, 2022b; 2022a). It computes the viewing geometry of both halves of each candidate pair and applies user-configurable rules to evaluate the stereo separation angle, the lighting differences, the seasonal differences (in terms of sun position), and the amount of time that has passed since the first half was acquired. The rules can be configured by latitude range, so different criteria are used for polar and equatorial stereo images.

HiSEAS can work with any pair of HiRISE images, whether one or both are still in the planning stage or both are already acquired. We have adapted it to search through the entire HiRISE image catalog to identify candidate stereo pairs that may not have been planned as stereo pairs. In this form, called Pairendipity, HiSEAS identifies groups of mutually overlapping HiRISE images and applies the same rules used during operations planning. We have generated and released anaglyphs from these candidate pairs. We have also adapted it to work with the Context (CTX) camera's PDS-released images in the same manner, called Pairendipity CTX. Pairendipity results are posted at https://www.uahirise.org/stereo/, and updated periodically.

### 7.8. Stereo pair orbits restricted to a cycle (SPORCs)

HiRISE stereo pairs must be acquired with off-nadir spacecraft rolls on two separate orbits (Section 7.7). Initially, evaluating a second orbit for acceptable convergence depended on the geometry of an already-planned first image from a prior cycle. This limitation frequently introduced significant delays between the first and second halves of a stereo pair. Due to the potentially confounding effects of changing illumination angles, surface morphology, and surface albedo on the quality of DTMs, HiRISE developed a procedure to minimize the time between the two halves of a stereo pair. Called SPORCs, (Stereo Pair Orbits Restricted to a Cycle), locations viewable on two or more orbits in
a single planning cycle are considered with the same criteria HiSEAS applies to evaluating second-half stereo opportunities for existing images. This consideration of potential image pairs allows acquisition of stereo pairs within a single two-week cycle, limiting illumination angle difference and minimizing changes to dust or frost cover and potential for changes in surface morphology. However, the MRO planning process often results in the acquisition of just a single image (of a stereo pair) in a cycle, so there are many attempted but not completed SPORCs that are treated like the first of two stereo images.

Once a compatible pair of orbits is found for a given stereo target, those potential observations can be prioritized in that cycle's target list, maximizing the chance for timely acquisition. There is a total of 983 stereo pairs acquired within 14 days as of 8/5/2022. Some of these image pairs were acquired in adjacent planning cycles, so they are not actual SPORCs but are equivalent. A list of known SPORC images (or equivalent) is periodically posted at https://www.uahirise.org/stereo/.

## 7.9. HiRISE ChecK for exact repeats (HiKERs)

Monitoring the martian surface for changes has led to many new scientific results during the MRO mission (e.g., Diniega et al., 2021; Dundas et al., 2021a). Change detection is most straightforward using orthoimages (Section 4.1) and is also aided by acquiring images with similar incidence angles, subsolar azimuths, viewing directions, and pixel scales. Subtle changes may only be recognizable from closely matched pairs of images. A pilot effort to identify “exact” match opportunities to previous images was initiated in December 2016, focusing on some Mass Wasting targets and was expanded to be more comprehensive in January 2018. Such opportunities are informally dubbed “HiKERs” as the software tool used to identify them is named the HiRISE ChecK for Exact Repeats. This tool, similar to HiSEAS, identifies opportunities in each planning cycle that match previous-image incidence and viewing angles to within 5° and subsolar azimuth to within 7°. While there is no sharp threshold for match quality, matches within this range are generally high-quality and make change detection more robust. A list of known HiKER pairs is periodically posted on the HiRISE website.

## 7.10. HiPhoP

From 2006 to 2019, the HiRISE Photometry Program (HiPhoP) used a Hapke photometric function (Hapke, 1981), requiring manually-input values for surface and atmospheric parameters for each observation (McEwen et al., 2007). This procedure was one of the most time-consuming steps in uplink planning. A more automated approach was needed to predict scene brightness (radiance) and set image parameters for CaSSIS (Almeida et al., 2023), based initially on HiRISE data (before sufficient CaSSIS data was acquired). Subsequently, we modified this automated method for HiRISE. With a database of >60,000 HiRISE images, we determined that scene brightness could be predicted with 10% accuracy (sufficient for setting image parameters) for >95% of the images, by using a simple model:

$$P1 [DN/ms] = (m_1 \times \cos i + b_1) \times (m_2 \times \text{TES} + b_2) \times 1.5^2/AU^2, \quad (1)$$

where P1 is how many DN (Data Numbers) of signal accumulate in a ms of integration, cosi is cosine of the incidence angle, TES is the Thermal Emission Spectrometer albedo (Christensen et al., 2001), AU is the Sun-Mars distance in astronomical units, and m₁, b₁, m₂, and b₂ are linear coefficients that fit the average signal levels of acquired HiRISE images. This function ignores atmospheric scattering because the top of the atmosphere brightness (radiance) is insensitive to low to moderate atmospheric opacity. In other words, the signal portions coming directly from the surface, the atmosphere, and more complicated scattering paths result in about the same integrated top-of-atmosphere brightness. The prediction may be too low when the atmosphere is extra hazy or

dusty, especially over low-albedo surfaces, but these conditions do not produce useful images of the surface, so overexposing the image is not a concern. A phase angle correction is needed if the phase angle is <20°, but MRO can never image at such low phase angles unless there is a special spacecraft pointing maneuver (done once for testing/validation on ESP_022243_9050 with a phase angle of 2.6° to observe recurring slope lineae in Horowitz crater).

Different empirical coefficients (m₁, b₁, m₂, and b₂) are needed for frosted versus unfrosted ground and for the frost-free north- and south-polar residual ice caps. In addition, frosted ground needs a seasonal correction because the frost albedo is variable (Pommerol et al., 2011):

$$L_{sC} = ax^5 + bx^4 + cx^3 + dx^2 + ex + f, \quad (2)$$

where LsC is the correction factor as a function of Ls, x = Ls/100, and a, b, c, d, e, and f are polynomial coefficients that fit the HiRISE image brightness (signal levels) of frosted ground. LsC values range from about 0.7 to 1.5. To predict when and where frost is present, we created a coarse map based on past HiRISE images for frost presence as a function of Ls and latitude. However, there are local surface anomalies (related to thermal inertia and topography) and season-to-season variability, so predicting scene brightness for these marginal cases when frost may or may not be present is difficult. The conservative approach in such cases is to use Bin 1/TDI 128 or Bin 2/TDI 32 for the RED CCDs, along with Bin 2/TDI 64 for the IR images and Bin 2/TDI 128 for the BG images; these settings almost never saturate.

The largest errors for unfrosted terrain brightness predictions are due to large image areas of steep sunlit or shadowed slopes or to changes in albedo since the TES data was acquired. The largest predict errors for the residual ice terrains occurs in the polar gaps in TES coverage, which we filled with estimated values. Once the mean scene brightness has been predicted, the next steps are to choose TDI and binning and look-up table (Section 4.2). We usually strive to achieve at least 100:1 SNR at the top of the atmosphere, but exceptions are made for special cases.

Updates to the HiPlan suite of tools (Section 7.11) were implemented to extract the TES albedo value at given image center coordinates and to determine the appropriate coefficients for those coordinates at a given Ls. For TES albedo, we use the most complete (MY26) map available at the PDS Geosciences Node (Christensen et al., 2001; Putzig and Mellon, 2007). Predicted DN values for each combination of binning and TDI may then be calculated and compared against the user-set minimum and maximum threshold values, and appropriate HiRISE LUTs determined for those predicted DN values.

## 7.11. HiPlan

HiPlan, the HiRISE planning software suite, has received numerous updates during the extended science phases. The core tool is built on top of the MRO project's customized version of JMARS ((Dickenshied et al., 2019).). Many HiPlan updates were in response to anomalies and instrument degradation mitigation efforts. Other updates have been for improved efficiency in operations and to accommodate remote operations, such as improvements to the suite's thermal modeling software, HiTemp, and the addition of several bulk or batch operation tools.

To mitigate the degradation of HiRISE's performance over time, we have conducted extensive experiments in which we have adjusted the instrument's analog-to-digital converter (ADC) settings (Section 8.9). The HiPlan tool suite has been updated to visualize and modify those settings as part of the general planning process. Another large-scale update was in the HiRISE photometry predictor tool, HiPhoP (Section 7.10). The HiPlan suite also gained tools (like Data Slacker) to perform a first-order estimate of HiRISE's spacecraft data volume usage (Section 7.15) and to examine short- or long-term trends in spacecraft targeting. Our stereo planning software, HiSEAS, was updated to plan both halves of a stereo pair in a single cycle (Section 7.8). The MRO project's tool reports orbital geometry events such as eclipse entry and exit, which
HiPlan uses for automated bulk planning to limit activity (e.g., to avoid eclipses) or focus activity (e.g., to conduct automated calibration observations on the night side).

## 7.12. HiTList

The HiTList (HiRISE Target List) is a list of potential observations that can be imaged during a particular 2-week planning cycle. The suggested observations are drawn from a database of Region(s) of Interest (ROIs). The database hosts over 22,000 ROIs made by the science team and the public. These suggestions are filtered to retain only the ROIs suitable for imaging during that planning cycle. This process includes consideration of: 1) whether the spacecraft passes over the ROI, or close enough to view the ROI with a < 30° roll of the spacecraft, 2) whether or not, and on which orbits, there are suitable imaging conditions to complete a viable stereo pair, and 3) any specific requests from the suggester, e.g., a specific orbit for a HiKER target.

The HiRISE Targeting Specialist (HiTS) assigned to the planning cycle constructs the HiTList. They do so using a variety of planning tools and the most recent information on predicted spacecraft groundtracks. Once a file of suitable targets has been constructed, it is limited in length by priority while still providing multiple targets on as many orbits as possible. That list is then prioritized by a science team member (the CIPP), then sent to JPL for the process that resolves conflicts.

## 7.13. STaR

STaR (Special Target Requests) is an application backed by a web service that allows science team users to enter prioritized requests for acquisition in an upcoming cycle. These requests are validated and stored in the database. In addition, it allows the targeting specialists to manage the process using request categories and user limits. The requests are automatically entered into the HiTList for each planning cycle. Before the implementation of STaR in late 2022, users entered special target requests on the wiki, and incorporation into the HiTList was a manual process.

## 7.14. Temperature management

HiTemp is a tool used to model temperatures at the HiRISE FPE (focal plane electronics) box and FPA (focal plane assembly) baseplate for a sequence of observations. It allows the HiTS to plan observation

sequences within a range of temperatures that optimize image data quality while keeping within the instrument's allowable flight temperatures (AFTs). Given input HiRISE environment temperatures and initial conditions, HiTemp has been found to accurately predict peak temperatures within ~ ± 2 °C of actual onboard temperatures. However, unpredictable thermal changes onboard the spacecraft that alter the HiRISE environment temperature can negatively impact the accuracy of predictions. For example, one common event that changes the HiRISE thermal environment is when the MRO spacecraft unexpectedly transitions from All-Stellar (star tracking cameras) to full attitude determination mode. Full attitude determination mode uses inertial measurement units (IMUs) and star tracking cameras to determine spacecraft orientation. Due to spacecraft heaters being activated for IMU operations, the HiRISE environment temperature can increase by ~2° when in full attitude determination mode. Since these transitions are frequently unplanned, and there is generally not enough time to generate and uplink new observation commands to account for the increase in environment temperature, peak temperatures must be padded during planning to ensure instrument safety. Exceeding an AFT causes "safing" (powering off) the instrument and losing several days of data collection and downlink. By contrast, if temperatures are too low during imaging, there is a risk of producing major bit-flips, negatively impacting the quality of the image data (Section 8.9).

Achieving warm enough temperatures to mitigate bit-flips and remaining cool enough for instrument safety is challenging. To do so, we limit image size because heating continues while transferring data from the CCD Processing Memory Modules (CPMMs) to the spacecraft solid state recorder (SSR). The frequent changes to temperature limits and resulting approximate maximum image lengths (in Bin 1A mode) are described in Table 14. Bin-2 max image lengths are more than two times longer in the number of lines, which is usually sufficient to meet science objectives.

## 7.15. Data tracker and slacker

HiRISE uses a third-party tool (Data Tracker) and a less precise, faster, in-house tool (Data Slacker) to model SSR use. This modeling is important because a misunderstanding about how HiRISE operates in the spacecraft flight software results in HiRISE "safing" when the HiRISE raw partition of the SSR is full (this happened twice; see Section 8.2 and Table 15). HiRISE operations are complicated by the need to monitor the SSR partition. The outputs of these tools are used while creating,

<caption>Table 14</caption>

FPE minimum temperature goals for imaging Mars. The image labels tend to record FPE box temperatures about 1–3 °C higher than those values listed here, for comparison to Table 16.

<table><thead><tr><th>Date implemented (month/day/year)</th><th>First MRO orbit</th><th>FPE minimum T (°C)</th><th>FPE/FPA Flight Software Limit (°C)</th><th>Upper Planning limit (°C)</th><th>Maximum Bin 1A image lines</th></tr></thead><tbody><tr><td>03/10/2006</td><td>1</td><td>18</td><td>35</td><td>31</td><td>120 K</td></tr><tr><td>02/05/2008</td><td>7166</td><td>22.5</td><td>41</td><td>37</td><td>100 K</td></tr><tr><td>12/16/2009</td><td>15,893</td><td>22.5</td><td>43</td><td>37</td><td>100 K</td></tr><tr><td>10/09/2010</td><td>19,706</td><td>25</td><td>43</td><td>37</td><td>80 K</td></tr><tr><td>01/28/2012</td><td>25,815</td><td>25</td><td>45</td><td>39</td><td>110 K</td></tr><tr><td>02/22/2014</td><td>35,517</td><td>28</td><td>45</td><td>39</td><td>70 K</td></tr><tr><td>10/17/2015</td><td>43,243</td><td>32</td><td>47</td><td>39</td><td>40K</td></tr><tr><td>10/24/2015</td><td>43,333</td><td>32</td><td>47</td><td>41</td><td>60 K</td></tr><tr><td>10/01/2016</td><td>47,735</td><td>33</td><td>47</td><td>41</td><td>50 K</td></tr><tr><td>08/05/2017</td><td>51,687</td><td>34</td><td>47</td><td>42</td><td>50 K</td></tr><tr><td>04/07/2018</td><td>54,831</td><td>34</td><td>47</td><td>44</td><td>80 K</td></tr><tr><td>04/14/2018</td><td>54,912</td><td>36</td><td>65*</td><td>44</td><td>50 K</td></tr><tr><td>06/23/2018</td><td>55,820</td><td>36</td><td>65*</td><td>46</td><td>80 K</td></tr><tr><td>02/23/2019</td><td>58,964</td><td>38</td><td>65*</td><td>46</td><td>50 K</td></tr><tr><td>07/31/2020</td><td>65,702</td><td>36</td><td>65*</td><td>46</td><td>80 K</td></tr><tr><td>12/19/2020</td><td>67,499</td><td>37</td><td>65*</td><td>46</td><td>70 K</td></tr><tr><td>05/08/2021</td><td>69,297</td><td>39</td><td>65*</td><td>46</td><td>40K</td></tr><tr><td>07/02/2022</td><td>74,688</td><td>36</td><td>65*</td><td>46</td><td>80 K</td></tr><tr><td>02/25/2023</td><td>77,742</td><td>38</td><td>65*</td><td>46</td><td>55 K</td></tr></tbody></table>

<footnote>* Although Flight Software Limit raised to 65 °C, a hard upper limit, the planning limit was kept much lower, at 46 °C, potentially preserving instrument lifetime.</footnote>
updating, and safety-checking imaging sequences before commanding.

Lockheed Martin Corporation developed *Data Tracker*, and it models the SSR input and output of one or all instruments using cycle-specific planning files as input. Before sequence generation, the HiTS use Data Tracker to determine whether to increase or reduce data volume use (lengthen or shorten, add or remove observations, or change binning). Because an SSR overflow will trigger HiRISE safe mode and result in lost data, we allow for headroom in the model-predicted raw space, erring on the side of caution. Daily SSR prediction models are generated for monitoring purposes. These model 'auto-runs' determine whether on-board files should be re-generated and replaced after commanding to adjust data volume usage.

The HiRISE team developed Data Slacker; it models only the HiRISE SSR partition and executes much faster than Data Tracker but is not sufficiently accurate for the final command loads. It is integrated within HiPlan, allowing for concurrent decision-making while planning observations.

## 7.16. Relay shadow zones

MRO relay support for landed assets results in so-called "shadow zones" where the science instruments encounter difficulty scheduling observations. During the interactive portion of the MRO planning process, relay passes are treated as observation records to direct spacecraft pointing and avoid flight rule violations. As such, a relay pass prevents the other MRO science instruments from acquiring science targets near the landed asset on relay orbits. First encountered after the successful landing of the Phoenix surface platform, the Phoenix shadow zone was mitigated by a detailed plan for coordinated science observations between the lander and the MRO science instruments (Tamppari et al., 2010) and by the small spacing between orbit tracks at those higher latitudes, ensuring multiple opportunities to image the same area.

After the successful landing and deployment of the more equatorial MSL the shadow zone soon became a particular impediment for HiRISE science in the area. Relay support for MSL took top priority in each planning cycle; as MRO provided relay support on almost all overflights in the vicinity of the landing site, a shadow zone from roughly 60°S–40°N and 125°E–160°E was rendered largely unavailable for targeted science observation. MRO later determined acceptable roll angle ranges for relay overflights allowing limited science pointing during a relay. In addition, the two projects negotiated a limited number of relay overflights that could be relinquished in favor of high-priority science targets. These two compromises resulted in a more porous shadow zone, particularly at latitudes above 20°S.

Complicating things further, the InSight lander site is aligned with the Curiosity landing site along an MRO ground track, and there is a newer shadow zone near the Perseverance rover. Yet another shadow zone may exist if the ExoMars lander and rover are launched to Mars at an upcoming launch window.

## 7.17. Use of data volume in extended missions

The volume of data that can be downlinked from MRO fluctuates depending on the distance from the Earth to Mars, the availability of the Deep Space Network (DSN) receivers on different parts of the Earth, and the competition for their use by other spacecraft. The downlink available to the mission is divided between the various instruments on board and telecommunications required by the spacecraft. HiRISE was allotted 35% of the available data volume (ranging from ~150 to 1146 Gb per two-week cycle) prior to retirement of CRISM in May of 2022, and is now allotted 48%. However, thermal management in recent years (Section 7.14) has prevented acquisition of more than ~1000 Gb per cycle of compressed HiRISE data. Most HiRISE images are compressed prior to transmission to Earth, with ~5:1 compression via both DN

<table><caption>**Table 15**
Beginning dates and orbit ranges when HiRISE could not image due to safing events.</caption><thead><tr><th>Date (m/d/yr)</th><th>Begin Orbit</th><th>End Orbit</th><th>Source</th><th>Cause of Safing if HiRISE</th></tr></thead><tbody><tr><td>03/14/2007</td><td>2950</td><td>3061</td><td>Spacecraft</td><td></td></tr><tr><td>07/20/2007</td><td>4594</td><td>4642</td><td>Spacecraft</td><td></td></tr><tr><td>09/27/2007</td><td>5479</td><td>5503</td><td>HiRISE</td><td>SSR full</td></tr><tr><td>11/07/2007</td><td>6009</td><td>6124</td><td>Spacecraft</td><td></td></tr><tr><td>11/29/2007</td><td>6295</td><td>6462</td><td>Spacecraft</td><td></td></tr><tr><td>02/14/2008</td><td>7273</td><td>7337</td><td>Spacecraft</td><td></td></tr><tr><td>02/22/2009</td><td>12,083</td><td>12,194</td><td>Spacecraft</td><td></td></tr><tr><td>06/03/2009</td><td>13,373</td><td>13,451</td><td>Spacecraft</td><td></td></tr><tr><td>08/06/2009</td><td>14,191</td><td>14,254</td><td>Spacecraft</td><td></td></tr><tr><td>08/26/2009</td><td>14,448</td><td>15,892</td><td>Spacecraft</td><td></td></tr><tr><td>07/21/2010</td><td>18,669</td><td>18,700</td><td>HiRISE</td><td>SSR full</td></tr><tr><td>09/15/2010</td><td>19,396</td><td>19,436</td><td>Spacecraft</td><td></td></tr><tr><td>08/27/2011</td><td>23,830</td><td>23,904</td><td>HiRISE</td><td>CPMM 13 (RED9) failed</td></tr><tr><td>09/06/2011</td><td>23,964</td><td>23,992</td><td>HiRISE</td><td>Test to confirm CPMM 13 failed</td></tr><tr><td>09/20/2011</td><td>24,141</td><td>24,197</td><td>Spacecraft</td><td></td></tr><tr><td>11/06/2011</td><td>24,741</td><td>24,803</td><td>Spacecraft</td><td></td></tr><tr><td>10/14/2012</td><td>29,155</td><td>29,212</td><td>Spacecraft</td><td></td></tr><tr><td>08/12/2013</td><td>33,022</td><td>33,036</td><td>Spacecraft</td><td></td></tr><tr><td>03/09/2014</td><td>35,706</td><td>35,755</td><td>Spacecraft</td><td></td></tr><tr><td>08/26/2014</td><td>37,890</td><td>37,945</td><td>Spacecraft</td><td></td></tr><tr><td>03/31/2015</td><td>40,679</td><td>40,768</td><td>Spacecraft</td><td></td></tr><tr><td>01/18/2016</td><td>44,436</td><td>44,473</td><td>HiRISE</td><td>Detected power supply over-voltage</td></tr><tr><td>11/21/2016</td><td>48,385</td><td>48,408</td><td>HiRISE</td><td>Over-temperature due to redundant spacecraft solar array controllers being unintentionally commanded on</td></tr><tr><td>09/13/2017</td><td>52,182</td><td>52,203</td><td>All Instruments</td><td>Under-voltage on instrument switches during eclipse following orbit trim maneuver</td></tr><tr><td>02/15/2018</td><td>54,172</td><td>54,381</td><td>Spacecraft</td><td></td></tr><tr><td>12/28/2018</td><td>58,231</td><td>58,360</td><td>HiRISE</td><td>Detected power supply over-voltage</td></tr><tr><td>12/09/2019</td><td>62,667</td><td>62,701</td><td>HiRISE</td><td>Detected power supply over-voltage</td></tr><tr><td>01/27/2020</td><td>63,295</td><td>64,443</td><td>Spacecraft</td><td></td></tr><tr><td>02/15/2020</td><td>63,541</td><td>63,725</td><td>Spacecraft</td><td></td></tr><tr><td>06/12/2020</td><td>65,053</td><td>65,162</td><td>Spacecraft</td><td></td></tr><tr><td>10/13/2020</td><td>66,638</td><td>66,743</td><td>Spacecraft</td><td></td></tr><tr><td>03/24/2021</td><td>68,713</td><td>68,797</td><td>Spacecraft</td><td></td></tr><tr><td>04/13/2021</td><td>68,577</td><td>68,667</td><td>Spacecraft</td><td></td></tr><tr><td>08/12/2021</td><td>70,531</td><td>70,734</td><td>Spacecraft</td><td></td></tr><tr><td>7/14/2023</td><td>79,525</td><td>79,629</td><td>HiRISE</td><td>CPMM 5 (RED4) failed</td></tr></tbody></table>
conversion from 14 to 8 bits/pixel plus lossless compression in the spacecraft SSR.

## 7.18. Retiring and unretiring suggestions

Target suggestions submitted through HiWish or HiWeb are ingested into a target database for consideration during each planning cycle (McEwen et al., 2010). Once an image has been planned that satisfies the suggestion, it is “retired”. The initial implementation of the retirement process consisted only of a simple geometric test: If the center point of an acquired observation lay inside the region of interest of a submitted target, or if the center of the region of interest of a submitted target lay inside the footprint of an acquired observation, then the submitted target would be flagged as retired in the target database. In the Extended Science Phase (ESP), the retirement procedure was updated. The current retirement procedure also tests that the acquired observation has met the photometric and seasonal/solar longitude constraints submitted with the target. In addition, stereo and seasonal repeat checks are also applied so that requests for repeat series are not automatically retired.

There are circumstances where a target suggestion may be marked as retired but needs to be reset to active status (unretiring). These cases include observations that were partly or entirely lost due to Deep Space Network ground station issues, instrument problems such as bit-flips, or weather conditions on Mars resulting in a poor image being acquired. Targets retired by lost observations will usually be automatically reset, while others may require an evaluation by a science team member before requesting that the suggestion be unretired by operations staff.

# 8. Instrument issues and anomalies

## 8.1. Hi5

Hi5 is a graphical interface developed in Java that easily allows team members to monitor the health and safety of the HiRISE instrument; it displays various HiRISE engineering data such as temperatures, voltages, and currents. These data can be used to create plots within the interface and those plots can be saved as PNG images, or the data can be exported as tab-separated or comma-separated text files. Also included in Hi5 are various status metrics and counters, along with information on errors and commands issued. The input engineering data can be streamed from real-time telemetry, or archived data can be loaded into the application. Hi5 has been an indispensable tool for monitoring HiRISE health and anomaly investigations.

## 8.2. “Safing” events

When the spacecraft or an instrument experiences an anomaly, it may go into a “safe” mode. For HiRISE, this means powering it off. The HiRISE flight software is very conservative, so if any “red” (uppermost) limit is exceeded from a temperature, current, or another sensor, the “keep-alive” counter is stopped, which then leads to the spacecraft powering off the instrument. When the spacecraft goes into safe mode, it powers off the instruments. No new imaging data can be acquired until HiRISE is powered on, the optics are sufficiently warm, and the LUTS have been re-loaded, resulting in at least several days with no new images. Table 15 lists all such events affecting HiRISE.

HiRISE has safed itself for several reasons during operations. Ideally, a full SSR should never cause an instrument to safe, but a flight software misunderstanding about how HiRISE works produced a problematic scenario. When the HiRISE raw data partition is full, the HiRISE FPS has already been turned on, but the spacecraft never commands an image, so the FPS heats up until it reaches the red limit, then safes the instrument. The HiRISE team developed a method to shut down the CPMMs and prevent overheating, but HiRISE safed a second time because of power supply limits associated with this off-nominal sequence. The team has thus had to carefully monitor SSR data usage (Section 7.15) to prevent

filling of the HiRISE partition of the SSR.

The next two HiRISE safing events were due to the failure in 2011 of CPMM 13, which controls the RED9 CCD (Section 8.5). Three safing events were due to power supply over-voltages, likely either spurious or actual. One safing resulted from exceeding a temperature limit on the IEA due to redundant spacecraft solar array controllers being unintentionally commanded on; that limit was also raised to avoid future safing from overly conservative limits.

An event on orbit 52,182 was due to an unexpected drop of spacecraft battery voltage, triggering safing of all instruments. This was followed by a successful campaign over several years to recondition the spacecraft batteries. Responses to this event led to several other issues (Sections 8.7, 8.8, 12.2).

## 8.3. Spacecraft jitter

TDI imaging with a 1-microradian instantaneous field of view (iFOV) makes HiRISE very sensitive to image smear from spacecraft pointing jitter. Low-frequency jitter can be measured with HiRISE image data, and the geometric distortions of the images can be corrected (Sutton et al., 2019), but high-frequency jitter (shorter than the typical TDI integration time of 10–13 ms) smears the image, so we strive to minimize this high-frequency jitter. In-flight jitter mitigation has been focused on minimizing instrument and mechanism motions during high-stability imaging modes. The MRO project considered jitter requirements for choosing spacecraft components and subsystems, such as the reaction wheels (Graf et al., 2005). MCS is a known source of jitter that smears and distorts HiRISE images from movement or acceleration of the scanning mechanism. For all high-stability imaging modes, MCS motion is paused.

Additionally, early in the mission, the solar array gimbal motions were paused for off-nadir high-stability images. Between January 2008 and June 2009, pausing the solar array motions for high-stability off-nadir observations was eliminated. The HiRISE team demonstrated an increase in jitter during this period, and pausing the solar array motions for high-stability off-nadir images was reinstated.

We almost always prefer to image in high-stability mode, but this is impossible when conflicts with other instruments or the spacecraft prevent us from using this mode. These non-high-stability (or “normal” stability) images are usually planned in bin-2 or bin-4 modes that are not as sensitive to smear as bin-1 mode. Due to operational constraints, images planned to coincide with relay activities (for data uplink from rovers and landers) must be in normal stability mode, so there is a chance that MCS could be in motion during these images. Approximately 20% of images taken during relay activities overlap with MCS motion. A little over half of these images are negatively impacted by jitter (Fig. 15). We sometimes use bin-1 in normal-stability mode, accepting the risk of jitter-induced smear in parts of the image. The PDS image labels contain information about the image stability mode.

## 8.4. BG13 blooming

Most images acquired before launch by CCD BG13 with 128 or 64 TDI lines included partial or complete blooming over the image, with saturated data. This blooming was thought to be due to a broken wire trace affecting TDI 128 and 64. HiRISE began the PSP using BG13 with 32 TDI lines and typically bin-4 to achieve a high SNR. However, we experimented with TDI 128 and 64 and did not see any blooming in flight except very rarely for bright scenes, and even then, only near the end of the ramp, bleeding into just the initial few Mars image lines. (The image ramp is produced when the instrument switches from reverse- to forward-clocking of TDI, resulting in double exposure; see Fig. 11 of McEwen et al., 2007). As a result, except for the first two months of the PSP, we have used the full capability of BG13.
<figure><img src="image_17.png" /><figcaption>**Fig. 15.** Example of image smear caused by motions of the MCS. The smear is most obvious in color products from sets of lines with color fringing (see arrows) ESP_055135_2190, RBG color, Bin 1A (NOMAP extras product). North is down. Scene width 0.5 km.</figcaption></figure>

## 8.5. Failures of RED9 and RED4 CPMMs

In 2011 HiRISE safed (see Table 15) because the instrument flight software received no response from the CPMM for CCD RED9 during the power-up sequence for a commanded image. Analysis of earlier data revealed a temperature dependence that worsened over time and eventually led to complete failure, suggesting an intermittent connection. A later failure of a CPMM in the engineering model at Ball Aerospace suggested a failure of a particular electronic part, the electrically erasable programmable read-only memory (EEPROM) that held the field programmable gate array (FPGA) programming code. A major worry in 2011 was that all the CPMMs could be susceptible to failure in this manner, but the absence of any other failures in more than ten years suggests that there was something unique about the RED9 CPMM, not inherent in the board design. Our planning and commanding software now ensures we never image with this CPMM. Fortunately, RED9 is at the outer edge of the ten RED CCDs, so our swath width was reduced by 10% rather than creating a gap in the processed images.

In 2023 HiRISE safed (Table 15) because the instrument flight software received no response from the CPMM for CCD RED4 during the power-up sequence for a commanded image. Analysis of earlier data revealed a temperature dependence that worsened over time, similar to the history of RED9, but worsening at a slower rate. Perhaps the same part failed as for RED9, or a different part failed. Either way, the temperature cycling of the HiRISE FPE is suspected to contribute to such failures.

## 8.6. Preparation for swapping sides of block-redundant electronics

The remote electronics consists of two boxes: the instrument controller and the power supply (see Fig. 1 of McEwen et al., 2007); both are entirely redundant, and together, they are called side A and side B of the electronics. Since MRO launch, HiRISE has always operated on side A. These boxes are cross-strapped to the spacecraft so that either side can be operated from either side A or side B of the spacecraft, which is very

fortunate because the spacecraft often “decides on its own” to swap sides, which has caused many of the spacecraft safing events (Table 15). However, there is no cross-strapping to the solid-state recorder (SSR), nor is the Electra ultra-high frequency (UHF) relay cross-strapped to the SSR. This lack of cross strapping means that if side A of either HiRISE, the SSR, or Electra needs to swap to side B, the other two systems must also swap to side B to store data. We have developed plans to safely switch HiRISE for this possibility.

The HiRISE FPE contains 14 sets of relays on its backplane, five each per CPMM. Swapping redundant sides is accomplished by switching each set of relays on the backplane to the active side. The remote electronics are first booted to the new side by the spacecraft, then each set of relays is moved to the new side via an instrument command, one per CPMM. The most conservative procedure would be to switch the relays for a single CPMM and examine the received power supply telemetry to confirm proper relay operation. Optionally, we may command a confirmation image to verify that the operation was a success if the telemetry is inconclusive. Each such operation requires an uplink window followed by some amount of ground data system time to study the results; the optional confirmation image requires yet another uplink window and more ground time. Swapping sides could take several weeks or months if each CPMM was switched and verified individually. To minimize the total time required while still maintaining some localized knowledge of relay operation, we plan to bundle individual CPMM commands into groups, starting with a single CPMM, followed by two CPMMs, followed by four, followed by five, and concluding with a two single CPMMs at the end. The final CPMMs are those for CCDs RED9 and RED4, which no longer work. We do not expect that a side swap would restore RED9 or RED4 functionality, but we would likely try anyway.

## 8.7. Blurred or out-of-focus images

From early 2017 until 4 Sept 2018, some bin-1 images appeared blurred. Blurring can be challenging to recognize because aeolian processes often smooth the surface, and other factors can reduce image
<figure><img src="image_18.png" /><figcaption>**Fig. 16.** Comparison of bin-1 images covering the same patch of ground under similar photometric angles but showing reduced resolution in the more recent image (ESP_053392_2210, acquired 16 Dec 2017). Image width is 150 m wide on Mars, and north is down.</figcaption></figure>

quality. However, a comparison to earlier images over the same location makes it clear when an image is blurred (Fig. 16). Through 2017 the percentage of blurred images increased, rising to about 40% of full-resolution images at the end of the year. We initially suspected that

the cause of this blurring was high-frequency pointing jitter introduced by a change to the spacecraft attitude control system. The limited lifetimes of the spacecraft inertial measurement units (IMUs) led to the development of an “All-Stellar” navigation technique that did not utilize the IMUs, saving them for special events when needed. We initially noticed the blurred images during the first in-flight test of All-Stellar mode with HiRISE imaging (20 Oct 2017), so we suspected high-frequency jitter resulting from the test. However, further analysis revealed a temperature dependence to the blurring, and the All-Stellar test happened near martian apoapsis when the spacecraft is coldest. The temperatures of the HiRISE optical components, especially the secondary mirror (Fig. 17) and the “spider” structure holding that mirror were unusually cool in the blurred (or out-of-focus) images. Although such temperature changes, if uniform, were not believed capable of causing such blur, transient temperature gradients became the top suspect, perhaps by changing the primary to secondary mirror spacing.

The secondary mirror and spider were more frequently cold during imaging in 2017 than previously due to various spacecraft changes leading to a colder environment around HiRISE, our use of longer reverse-clock warmup times, and after a HiRISE change made on 11 June 2017 to help mitigate the spacecraft battery undervoltage. Rather than acquire throwaway warmup images during eclipse (simplest for HiRISE operations) to mitigate the ADC bit-flips (Section 8.9), we started acquiring such images immediately before starting the dayside images of Mars. The goal was to reduce power demand during eclipses. However, the Thermal Control System (TCS) was disabled during HiRISE imaging in case it caused minor image noise. This made little difference to the temperatures of optical elements early in the mission because the TCS was only disabled for less than one minute. However, with 300 s of warming up in reverse-clock mode (where the TDI is run in the opposite direction such that charge is removed from the CCD) and throwaway warmup images acquired for up to tens of minutes to further warm up the FPE, the temperatures of small optical elements dropped

<figure><img src="image_19.png" /><figcaption>**Fig. 17.** Secondary mirror temperatures near the start of HiRISE forward-clock imaging. The low temperatures in late 2017 and early 2018 are often associated with reduced-resolution images. The low temperatures in the prior Mars year, such as near the beginning of 2016, may also be associated with other such images, but this has not been investigated.</figcaption></figure>
significantly (Fig. 17). To test this theory, we changed the HiRISE instrument operations on 09/04/2018 so that the TCS remained on during imaging, eliminating the blurring and also not leading to a detectable increase of image noise.

<10% of HiRISE bin-1 images acquired in 2017 and early 2018 are blurred. The PDS image labels contain information about optical part temperatures, such as SEC_MIRROR_TEMPERATURE, which should be near 20 °C. If this temperature is <17 °C, and part of the image was acquired at full resolution (bin-1), then image blur is possible, but not certain. Blurring was rarely large enough to be noticeable in bin-2 images. Images acquired within ~30° of apoapsis (L₈ 71°) and with cold optical parts are especially likely to be blurred. No known images were blurred due to low optics temperatures since implementing the operational change on 9/04/2018 that keeps the TCS on during imaging sequences.

## 8.8. Lowering the setpoint temperature of HiRISE optics?

As part of the spacecraft battery mitigation studies (Section 8.2), the MRO project asked us to consider permanently lowering the optics setpoint temperatures from ~20 °C to some lower temperature requiring less battery power during eclipses. Ball Aerospace models indicated that this could be done by lowering the primary mirror temperature to near 0 °C and adjusting other component setpoints, with only a minor loss of resolution if the focus mechanism position was re-set. One challenge with this plan was that the MRO project has been reluctant to turn the spacecraft nadir deck away from Mars to observe stars, so verifying the best focus position may rely on repeat imaging of small-scale features on Mars. With the success of the battery reconditioning, this plan has been on hold, and the major optical components remain at 20 °C nominal temperature.

## 8.9. ADC bit-flips and mitigation

Soon after launch, when observing the Moon, we saw an unexpected problem in channel 1 of CCD IR10 (McEwen et al., 2010). At the coldest FPE and FPA temperatures, the raw image DNs were mostly zeroes. At somewhat warmer temperatures, there were still significant regions of zeroes or otherwise obviously erroneous data. There was a minor amount of erroneous data at still warmer temperatures, and the images contained all valid DNs at the highest temperatures. Further testing at even colder temperatures during the cruise to Mars revealed this problem in channel 1 of RED0 and RED3. IR10_1 soon had erroneous data even in Mars orbit with the FPE box starting at 18C, and by late 2006 RED9_1 had become very noisy at 18 °C. (The FPE box and the FPA

baseplate temperatures provide the best indicators of the temperatures of electronic components inside the CPMMs, which are not measured directly.) We began taking fewer but larger images in case that would slow the degradation; we abandoned that strategy after the PSP because there was no evidence to support that assumption. The erroneous-data problem has worsened over time and affected more and more image channels (each CCD has 2 readout channels).

Analysis of the image data revealed that DN values are in error by 2ⁿ DN values (Fig. 18), so there seem to be bit-flips (explained in Section 5.13). When the most significant bits flip, negative DN values are created that get encoded as zeroes. The very coldest images for the problematic channels tend to be nearly all zeroes or all erroneous data in some manner. Sometimes there are contrast-reversed images—where low signal areas have relatively high DNs and high signal areas have lower DNs, and the DN values are highly quantized.

We suspect that the bit-flipping happens during the analog-to-digital conversion (ADC), although the actual ADC parts are not necessarily to blame—it could also be the input signal to the ADCs. A study at Ball Aerospace concluded that there were two fundamental problems: 1. The Focal Plane Electronics Assembly (FPEA) design produced ringing in the ADC sampling waveforms plus an asymmetric video waveform, and 2. the ADC performance degrades over time. Initial testing showed the problematic ADC sampling waveform could be improved by warming the CPMM circuit components, most notably the ADC itself. A life test on the HiRISE engineering model reproduced a loss of ADC sampling margin as a function of time and temperature. Passive bakeout of the ADCs for 48 h at 85 °C resulted in substantial improvement in margin. This behavior is consistent with ionic contamination of the ADCs, which can become aligned and cause faults, and heating can break up the aligned contamination. Later testing at elevated temperatures for 4330 images also improved performance, creating hope that the degradation could be reversed in-flight. However, in-flight attempts to reproduce this result did not show improvement. These experiments included acquiring many small images of Mars near orbit 10,000.

Fortunately, we were able to mitigate this problem for many years by warming the focal plane electronics before imaging. We increased the FPE heater setpoint until the heaters were almost always on, but this only raised the FPE box from our initial 18 °C to about 22 °C (varying with sun-Mars distance). We also increased the reverse-clock imaging time to warm the FPE. A small amount of reverse-clock imaging time was always needed to remove signal to prepare for forward-clock TDI producing the returned images. We increased the reverse-clock time from 30 s incrementally up to the maximum possible of 300 s as the bit-flips worsened. When even the 300 s warmup was insufficient, we began taking nightside warmup images that were effectively thrown away (not

<figure><img src="image_20.png" /><figcaption>**Fig. 18.** Rather than having one histogram peak like channel 0 (left), channel 1 (right) has two major peaks that are offset by 1024 (2¹⁰) DN, with a peak in the middle offset by 512 (2⁹) DN from the largest peaks, consistent with bit-flips creating the spurious data.</figcaption></figure>
returned to Earth). We established a minimum FPE box temperature goal before starting Mars imaging to mitigate major bit-flips. Over time, we raised that limit by taking longer warmup images, up to a maximum of about 39 °C. See Table 14 for when each change was implemented. We had been raising the minimum FPE temperature to start imaging by roughly 2 °C/year until 2019. Concurrently we had to raise the upper-most AFT for the FPE and coupled FPA, or acquire shorter images that ensure we do not exceed the AFT (which triggers “safing” or powering off the instrument). The initial AFTs were set very conservatively, with margin on margin; Ball Aerospace engineers eventually concluded that 65 °C was the safe maximum for the FPE and FPA. Although images acquired in the PSP had up to 120,000 bin-1 lines at full resolution (no pixel binning), limited by the CPMM memory size, the maximum in recent years has been about 60,000 bin-1 lines or 200,000 2 × 2 binned lines. Note that the bit-flips are lessened in bin-2 and bin-4 modes, and high-quality images can be acquired at several degrees cooler temperatures than in full-resolution (bin-1) mode.

Our early strategy was to mitigate bit-flips in the RED channels but allow IR10_1 to have erroneous data when too cold. The image processing pipeline could identify and replace minor erroneous data, but when it became too extreme (>10% zeroes), we did not process IR10_1, so many IRB color products had only ¾ the width of the RGB color. Although IR10_1 was the worst channel for many years, the channels worsened over time at different rates, so by around MRO orbit 50,000 (27 March 2017), RED1_1 and RED3_1 (bin-1 mode) had worse bit-flips than IR10_1 (bin-2 mode); steps taken to mitigate bit-flips in these RED channels also resulted in higher-quality IR10_1 images that were almost always processed.

Despite warming the FPE, bit-flips still occur in some images, especially channel 1 of IR10 and RED 1, 3, and 9 (while in operation); later channel 1 of RED 0, 2, 7, and 8 revealed extra noise. These images will sometimes have columns with all zeroes, and minor bit-flip noise throughout the image (Fig. 19). When especially erroneous (more than some percentage of the data are zeroes), we do not process the data into higher-level products. We recently decided to set the processing limits at 50% zeroes for bin-1 images; 40% zeroes for bin-2 or bin-4 images. CCDs BG12, BG13, IR11, and REDs 4–6 have remained free of bit-flips except at very cold temperatures that we do not normally use for imaging Mars.

By October 2019, we paused raising the FPE and FPA temperatures out of concern that cycling to high temperatures could eventually cause the electronics to fail, and because it became operationally difficult to plan many large warmup images. Fortunately, we had another mitigation strategy. During instrument development, the speed of data processing in the FPS was our major challenge, and the FPGA code was updated right up to the last possible date before launch of MRO. The FPGA code included selectable parameters for where to sample the video

waveform. There are two measurement points (reference and sample), and the difference between those values determines the image DN level. Unfortunately, there is ringing and asymmetries in the video waveforms, so sampling in the wrong locations might create extra noise or erroneous DN values. The FPGA code allows eight measurement points for references and samples. In pre-flight, we determined that the location 5 reference and location 4 sample (which we refer to as setting 54) were the sampling values that resulted in the lowest read noise. However, in case it could help with unexpected future problems, we made those ADC parameters commandable in flight. In 2019 we experimented with different ADC sampling points to see if that affected the bit-flips. To summarize, the use of different ADC sampling values allows us to lower the FPE starting temperature by about 8 °C while maintaining acceptable images. This strategy buys us several extra years in which we can image in bin-1 mode over the full RED swath width with a given starting FPE temperature.

In the FPGA code, both the reference and sampling points move together, always separated from each other by a set period, and some of these combinations are not valid because no signal is measured. There were 33 potentially acceptable ADC settings for a channel. Early in this study, we found ADC setting 74 significantly improved over setting 54 for the worst channels, so we changed our standard setting for REDs 1–3 to 74 on 2 Aug 2020. This change enabled us to lower the minimum FPE temperature by 3 °C. Worsening over time has continued with the new settings, so by 2022 these images were about as noisy as they were in 2020, and we hoped to find better ADC settings. Although there were only 44 possible settings, given how the FPGA code worked, we discovered that the ADC setting for channel 0 significantly affects bit-flips for channel 1, and vice versa. For example, although setting both channels of a CCD to 74 improved channel 1 of several CCDs, it made channel 0 noisier, so we tried 54 for channel 0 combined with 74 for channel 1 (we abbreviate this 54_74), and the resulting images were worse for channel 1 than 74_74. Thus, there were 44² or 1936 possible ADC settings to test. However, we found that five of these settings had large offsets that zeroed out calibration pixel values, 11 others produced very low signal levels in bin-1 mode, and six settings produced consistently erroneous results even at the highest temperatures. Thus, we had 22² or 484 settings to test. Multiple attempts to predict which settings might be best proved incorrect when test images were acquired, so testing all viable settings was the best path forward. We needed an efficient method to test 484 potential settings to be sure we had found the optimal settings for each CCD.

The ADC sampling settings for HiRISE were not intended to be updated frequently; they were implemented as volatile flight software constants that were expected to be updated only via a patch sequence at instrument reboot, if at all. As part of the bit-flip mitigation effort, we

<figure><img src="image_21.png" /><figcaption>**Fig. 19.** Bland portion of RED3_1 of ESP_074443_1790 showing minor noise (mostly vertical pattern) due to minor bit-flips. Raw image geometry with north down, image width 0.5 km.</figcaption></figure>
updated our observation command block to include the ADC settings commands. From a ground operations point of view, it was desirable to simplify the experimental procedure and to allow for maximum future flexibility in implementation. We elected to include the ADC commands as part of every observation, even if we were “setting” them to the most recent values. Doing so has led to a larger command block file size, which in turn has led to ~10% longer radiation times, but it greatly simplifies the ground software and reduces the workload; furthermore, since every observation command block includes ADC settings, we can forego updating them in the HiRISE boot patch sequence after safing events. Operations engineers (HiTS) can now modify a particular observation's ADC settings via the HiRISE planning tool, HiPlan.

In 2022 we executed the ADC study in two basic steps. First, we acquired hundreds of nightside stim-lamp (light-emitting diode or LED) illuminated images with the FPE box in the 35–37 °C range, as a first pass to eliminate many obviously erroneous settings (many bit-flips even at these temperatures). The result was about 8 settings that appeared promising in Bin 1A mode. Next, we acquired sets of images in each of the 8 settings plus the current standard (54_54 except for RED1–3 at 74_74). This test required nadir-only orbits to ensure we could place the images in the exact same sequence for each orbit (and after 3 orbits with no HiRISE imaging). The resulting images have very consistent FPE and FPA temperatures. There are 7 images for each combination of ADC settings, with FPE starting temperatures near 21, 27, 33, 37, 40, 42, and 44 °C. The last image of each sequence is a nightside image illuminated only by stim lamps; these are best for measuring image SNR independent of bit-flips (except minor bit-flips for RED1_1 and RED3_1).

With these results, we could measure the percentage of erroneous pixels due to bit-flips, the signal levels, and the standard deviation of calibration reverse clock pixels (returned with each image) as a measure of image noise. Our primary goal was to eliminate obviously erroneous pixels (DN values outside the valid image range) resulting from bit-flips since this problem worsens over time. However, in many cases, there are effectively “ties” between channel settings, so then we strive to optimize the SNR (high signal and low noise). Seven settings give nearly the same results regarding erroneous pixel values, but two result in low SNRs. The five best settings (at least for the problematic RED CCDs 0–3 and 7–8) are 64_74, 65_74, 66_74, 75_74, and 75_85. After optimizing SNR for each CCD, the best choices are 64_74 for RED2 and RED7, 65_74 for RED0 and RED8, and 75_74 for RED1 and RED3. For the best-behaved

CCDs, there are new settings that result in fewer bit-flips at cold temperatures than setting 54_54, but we will image Mars at warmer temperatures to mitigate bit-flips in other CCDs, so staying with 54_54 is best for the near future. IR10 is close to the edge: currently 54_54 returns acceptable results, but in a few years as bit-flips worsen we may change to an alternate setting. Setting 44_52 looks like the best alternate setting for IR10 in bin-2 mode, but the bin-2 settings will be re-evaluated in future years.

## 8.10. Predicting future performance

The most significant risk to the future of HiRISE is the complete failure of additional CPMMs, as happened to RED9 in 2011 and RED4 in 2023. Continued image degradation due to bit-flips is less catastrophic but sure to happen. There have been no problems with the HiRISE telescope, thermal control system, focus mechanism, or remote electronics (for which there is also a redundant side B).

We have data characterizing how fast some of the worst channels are degrading over time. Plotting the FPE temperature at a certain level of bit-flip noise versus orbit number or time (Fig. 20) reveals how the FPE starting temperature varies over time. These data are mostly collected for setting 54_54, so we must assume the same rate of degradation at new settings. With the reduction in FPE temperature now possible, we can make predictions (Table 16). The 1 °C/yr trends are upper limits for CCDs 4–6 and 11–13, for which there is not enough data for a prediction. RED1_1 may become problematic even in bin-2 mode before 2026; the same is probably true for RED3_1. Since we can image at ~3 °C lower temperatures and replace some amounts of erroneous data (Section 5.12), full resolution over the entire swath width will likely be possible until about 2024, but with short image lengths. Sometime after 2024 we will need to either use Bin 1–2 mode instead of Bin 1 (Table 8), or acquire narrow images that exclude RED0–3, or in some cases, use a higher FPE starting temperature and accept the risk from ending temperatures greater than ~55 °C. With the AFT at 65 °C (and not likely to be raised), the highest starting FPE temperature in a few years could rise to near 50 °C so that the ending temperature is well below the AFT to avoid safing HiRISE. However, FPE starting temperatures higher than 45 °C might be considered problematic in terms of potential CPMM failures. When we can image at full resolution only in RED5–6 (2 contiguous CCDs), we need to acquire ~5 images (including overlap) to cover an

<figure><img src="image_22.png" /><figcaption>**Fig. 20.** Plot of FPE starting temperatures (vertical axis) resulting in >0% and <1% zero-DN pixels versus orbit number (horizontal axis) for RED1_1. The offset near orbit 65,800 marks the switch from ADC sampling 54_54 to 74_74, and the offset near orbit 74,700 is from the later switch to 75_74. A linear fit to the data with ADC 54_54 yields a slope of 0.00054C increase per orbit, or 2.5C/yr.</figcaption></figure>
<caption>**Table 16**</caption>

Minimum FPE temperatures (T) needed to avoid any obvious erroneous pixels with ADC settings in June 2022 and with new ADC settings, and predictions for minimum FPE temperatures needed in 2026 and 2030. Numbers in red may be unacceptably high.

<table><thead><tr><th>CCD</th><th>Bin</th><th>Min-T 2021</th><th>Min-T with new ADC</th><th>Deg C/yr worsening</th><th>Min-T needed in 2026</th><th>Min-T needed in 2030</th></tr></thead><tbody><tr><td>RED0</td><td>1</td><td>43</td><td>35.5</td><td>1.4</td><td>41.1</td><td>46.7</td></tr><tr><td>RED1</td><td>1</td><td>43</td><td>38.5</td><td>2.5</td><td>48.5</td><td>58.5</td></tr><tr><td>RED1</td><td>2</td><td>40</td><td>35.5</td><td>2.5</td><td>45.5</td><td>55.5</td></tr><tr><td>RED2</td><td>1</td><td>38</td><td>34</td><td>2.5</td><td>44</td><td>54</td></tr><tr><td>RED3</td><td>1</td><td>43</td><td>38.5</td><td>2.5</td><td>48.5</td><td>58.5</td></tr><tr><td>RED4</td><td>1</td><td>25</td><td>25</td><td>&lt;1</td><td>N/A</td><td>N/A</td></tr><tr><td>RED5</td><td>1</td><td>25</td><td>25</td><td>&lt;1</td><td>&lt;29</td><td>&lt;33</td></tr><tr><td>RED6</td><td>1</td><td>21</td><td>21</td><td>&lt;1</td><td>&lt;25</td><td>&lt;29</td></tr><tr><td>RED7</td><td>1</td><td>41</td><td>31.5</td><td>2.8</td><td>42.7</td><td>53.9</td></tr><tr><td>RED8</td><td>1</td><td>39</td><td>31.5</td><td>1.9</td><td>39.1</td><td>46.7</td></tr><tr><td>IR10 (54_54)</td><td>2</td><td>36</td><td>36</td><td>1.4</td><td>41.6</td><td>47.2</td></tr><tr><td>IR10 (44_52)</td><td>2</td><td>36</td><td>26</td><td>1.4</td><td>31.6</td><td>37.2</td></tr><tr><td>IR11</td><td>2</td><td>27</td><td>27</td><td>&lt;1</td><td>&lt;31</td><td>&lt;35</td></tr><tr><td>BG12</td><td>2</td><td>27</td><td>27</td><td>&lt;1</td><td>&lt;31</td><td>&lt;35</td></tr><tr><td>BG13</td><td>2</td><td>21</td><td>21</td><td>&lt;1</td><td>&lt;25</td><td>&lt;29</td></tr></tbody></table>

area at full resolution, such as a candidate landing site. The same coverage previously required just one image. However, the rate of degradation might change if we greatly reduce acquisition of bin-1 images. Also, there may be new ADC settings for bin-2 and bin-4 mode that allow lower starting FPE temperatures. These possibilities will be studied to find the best way to optimize the useful lifetime of HiRISE.

## 9. Calibrations

HiRISE calibrations are of two types: geometric and radiometric. Geometric calibrations such as focal length and optical distortions have been well characterized since early in the mission (McEwen et al., 2007; Kirk et al., 2008). Geometric distortions due to pointing jitter are also correctable, as described by Sutton et al. (2019) (see also Section 8.3). Most science results from HiRISE depend more on geometric corrections than photometric corrections.

Radiometric calibration has been less stable due to FPEA design (to minimize mass and power) and in-flight issues. HiRISE acquires Gigapixel images in just a few seconds, generating considerable heat in the FPE and FPA. Stable calibration requires precise detector temperature control. However, given mass and power constraints on the imager design, we decided to let the FPE/FPA temperatures rise and fall dramatically. These issues cause relative (pixel-to-pixel) radiometric calibration uncertainty of 1–2% and absolute radiometric calibration uncertainty of ~20% (Milazzo et al., 2015). When the ADC bit-flip issue arose, we mitigated the erroneous data by going to even higher temperatures beyond the range of initial calibrations.

The new ADC settings have complicated radiometric calibration. When we first switched to 74_74 for REDs 1, 2, and 3, the offset changed enough to turn some calibration pixel data into nulls after the Look-Up Table (LUT) compression from 14 to 8 bits per pixel. New LUTs were developed to enable a wide range of potential offset values as a function of ADC setting but were not implemented until 3 Aug 2021. Thus, a year’s worth of HiRISE images (mostly bin-1 images) may contain artifacts introduced by the HiCal pipeline; these may be repaired when we reprocess the entire dataset and convert it to PDS-4 format. New ADC settings also change the image gains, but those should be normalized to channel 0 of RED5 by the image processing pipelines HiStitch and HiccdStitch (Table 12), and RED5 has remained at ADC setting 54_54.

## 10. Citizen science and machine learning

### 10.1. Citizen science

HiRISE Clickworkers was a citizen science effort from ~2007 to 2010 (Gulick et al., 2010; McEwen et al., 2010). Our experiences with Clickworkers and our HiRISE Student Image Challenges (Section 11.5)

<figure><img src="image_23.png" /><figcaption>**Fig. 21.** Fans in HiRISE image ESP_011960_0925, acquired at 87.3S / 167.8E. RGB color, scene width 0.5 km.</figcaption></figure>

show that given carefully thought-out tasks combined with easy-to-use online tools, students, educators, and the general public are very interested and capable of participating.

A citizen science project known as “Planet Four” began in 2013 using the Zooniverse platform built by the Citizen Science Alliance. They provide the web portal and technical expertise for involving large numbers of volunteers in research projects online (described at http://www.zooniverse.org). The original Planet Four task asked volunteers to identify and outline fans and blotches in images of Mars’ south polar region that appear every southern spring. The model of Kieffer (2007) describes the seasonal activity: In the winter, under the right conditions, the seasonal CO₂ polar cap becomes a translucent slab of impermeable ice. Penetration of sunlight through the ice, which warms the ground below, results in basal sublimation of the ice. Trapped gas escapes through ruptures in the ice, eroding and entraining material from the surface below. When this dust-laden gas is expelled into the atmosphere the dust settles in fan-shaped deposits on the top of the ice in directions oriented by the ambient wind (Fig. 21). The number, time history, area covered, and changes of the fans provide a wealth of information on the spring sublimation process. The length, width, and direction are snapshots in time of the local wind direction. With the aid of Zooniverse staff, we developed statistical tools to identify the “best fit” outline. The measurements are assembled into a fan and blotch catalog (Aye et al., 2019). Over 150,000 people have generously donated their time to the Planet Four task.

Four peer-reviewed science publications have resulted so far from the Planet Four efforts (Schwamb et al., 2018; Aye et al., 2019; Portyankina et al., 2022; McDonell et al., 2023). Portyankina et al. (2022)
compared the wind direction indicated by fan orientation to the predicted wind direction of the Mars Regional Atmospheric Modeling System (MRAMS), a mesoscale atmospheric circulation model. Two other citizen science projects were developed under the “Planet Four” brand: Planet Four Terrains and Planet Four Ridges. The Planet Four Terrains task asked volunteers to identify specific features in CTX south polar images: “Spiders” (radially organized channels or araneiform terrain), “Swiss cheese” terrain (quasi-circular erosion), and craters. HiRISE images were targeted to follow up on promising areas. Results showed that araneiform terrain exists in favorable locations, not just on the unit mapped as South Polar Layered Deposits (Schwamb et al., 2018). Planet Four Ridges aims for citizen scientists to map polygonal ridge networks in Nili Fossae and parts of Nilosyrtis/northern Terra Sabaea (Khuller et al., 2022). The catalog is also being used as a means of testing machine learning (McDonell et al., 2023).

## 10.2. Machine learning

The volume of HiRISE data greatly exceeds what any individual researcher can analyze in traditional ways. Machine learning methods can more rapidly discover patterns or features of interest in large datasets and automate tedious or time-consuming tasks. There have been machine learning studies with HiRISE data to map linear bedforms (Foroutan and Zimbelman, 2017), detect new block falls in the north polar region (Fanara et al., 2020), detect new impact sites (Wagstaff et al., 2022), to detect recent rockfalls (Bickel et al., 2020), and to classify terrains (Barrett et al., 2022).

# 11. Education and public outreach (E/PO)

## 11.1. Mars books

The first full volume of HiRISE images appeared from French publisher Éditions Xavier Barral in 2013, “This is Mars,” featuring black and white images. In 2017, the University of Arizona Press (UA Press) published a large color book, “Mars: The Pristine Beauty of the Red Planet.” The book featured captions written by members of the HiRISE team for hundreds of images that highlighted the enhanced color capabilities of the camera. All captions appeared in English and one of 24 other languages. HiRISE has also produced an ebook about Mojave crater (available for free on the Apple Bookstore and in PDF format) in English and Spanish, with two more planned (see: https://www.uahirise.org/epo/ebooks/).

## 11.2. Captioned releases

Throughout HiRISE operations, we have provided captioned images on a regular schedule to post to our website and social media channels for the benefit of explaining HiRISE science to the public (see https://www.uahirise.org/catalog/captions/). As of June 2023, we have over 2249 fully captioned images that include extras such as wallpaper, slides, and flyers. Some products include an audio version of the caption for the visually impaired, and often a video clip. Producing these extras for fully captioned images has become standard procedure, often resulting in media outlets using them when a particular image comes to their attention. Additionally, video and audio clips are posted separately to HiRISE's YouTube and Soundcloud channels to broaden our reach for those who not utilizing our website. For many years, we have also captioned our video files (HiClips) when posted to our website and other channels.

## 11.3. 11.3 Social media presence

HiRISE has maintained several social media channels that promote our captioned images, our HiPOD images (HiRISE Picture of the Day) and Mars and science-related topics. Our verified and flagship Twitter

account has over 91,300 followers; our combined Twitter channels featuring other languages (see below) exceed over 102,000 followers. In total, our social media channels have over 230,000 followers. A full list of our social media accounts is here: https://www.uahirise.org/epo/map/

## 11.4. The beautiful Mars project (formerly HiTranslate)

From the beginning of the mission, HiRISE has adhered to a simple philosophy: we are the People's Camera. In 2010 we created “The Beautiful Mars Project” to solicit volunteers to help us translate our image titles and captions into various languages (see https://www.uahirise.org/epo/). With dozens of volunteers, HiRISE is represented in 24 languages, the most of any active NASA mission. Volunteers from all over the globe have translated over 5220 titles and captions of our complete catalog. Several volunteers have also provided audio files (Spanish, Greek, Catalan, Irish, and Russian) to accompany captioned images that we subsequently posted to various YouTube channels.

## 11.5. HiRISE image suggestion challenges

Our student image suggestion program, conducted in association with NASA Quest as HiRISE Image Challenges (https://marsoweb.nas.nasa.gov/HiRISE/quest/all/), was described in McEwen et al. (2010) and is updated here. Over 600 image suggestions were submitted during the challenges, with over 1164 participants from 45 states and 42 countries. Participants represented over 18,800 students in grades 2 through 14 and consisted primarily of teachers, parents of home-schoolers, student clubs, college students, and life-long learners.

Several participants continued submitting HiRISE image suggestions long after the challenges ended. One of our most active participants was a group of seventh and eighth graders from Evergreen Middle School in Cottonwood, CA, where over half the students were economically disadvantaged. These students, led by their enthusiastic and passionate Math teacher Mr. Dennis Mitchell, formed an after-school NASA Mars club. The Mars club participated in all the HiRISE Challenges and ASU's Mars Student Imaging program. When the challenges ended, Mitchell and his students continued working with HiRISE Co-I Gulick through Spring 2017, when Mitchell retired. Each Spring, the students would visit NASA Ames to present their research.

## 11.6. Other outreach

Members of the public utilized HiRISE data in unexpected ways. These efforts were highlighted with the “Made with HiRISE” page on our website: https://www.uahirise.org/epo/made-with-hirise/. These uses have ranged from visual artistry through flyovers (made from digital terrain models) to artistic interpretations of the intersection between art and technology (e.g., see http://hyperspace.beck.com/#content-mars).

The HiRISE team also engaged in hundreds of public talks as well as tours for classes at the university level focusing on remote sensing and the production of DTMs. We have also answered numerous media requests for our images and coordinated important releases with our counterparts at JPL Media.

# 12. The future of high-resolution orbital imaging of Mars

## 12.1. The future of HiRISE

The Mars Exploration Program has relied on HiRISE to characterize landing sites. HiRISE contributes to the analysis of science potential, along with many other remote-sensing experiments, and uniquely contributes to assessing landing site safety (Section 6). DTMs from HiRISE stereo coverage are used for detailed landing simulations, terrain relative navigation (Fergason et al., 2020) and boulder counts, slopes, and topography to assess small-scale hazards. However, the full resolution of