Gebhardt and Abuelgasim Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0276-z

REVIEW(2019) 6:31

Progress in Earth andPlanetary Science

Open Access

Scientific innovations from the Mars aerosol
optical depth based on satellite data with atemporal resolution of hours
Claus Gebhardt1*

and Abdelgadir Abuelgasim1,2

Abstract
The ExoMars Trace Gas Orbiter (TGO), Emirates Mars Mission (EMM), and possible future Mars stationary satellite(s)will provide innovative data products on the optical depth of dust and water ice clouds in the Martian atmospherewith a temporal resolution of hours. The science phase of TGO has begun in April 2018, and the EMM science
operation is scheduled to start in 2021. In this work, the relationship between TGO, EMM, and precursor missions/
instruments is analyzed based on the obtained data products. This is followed by discussing the foreseen
innovation from the latter in the fields of Martian climatology and numerical weather forecasting.
Keywords: ExoMars Trace Gas Orbiter (TGO), Emirates Mars Mission (EMM), Mars stationary satellites, Mars
atmosphere, Aerosol optical depth, Dust, Water ice clouds, Mars climatology, Mars numerical weather forecasting,
Data assimilation

Introduction
The optical depth is one of the basic characteristics ofMartian aerosols, such as dust and water ice clouds. Primarily, it is an input parameter widely required for themodeling and satellite retrieval of Mars atmosphericdata. Furthermore, the Mars optical depth is used forthe specification of engineering parameters and planningof entry, descent, and landing operations of spacecraft.
Montabone and Forget (2018a) and Montabone et al.
(2015, 2006) illustrate the use of reconstructed climatological data on the dust optical depth for supporting future Mars landings. In addition, such data canpotentially be used in the planning of aerocapture maneuvers in the future. Moreover, the dust optical depth
has implications for the solar panel management ofMars rovers and landers, whose power generation may
be compromised due to the down settling and obscuration caused by atmospheric dust (Mazumder et al.
2006). A most recent example is the loss of contact with
the Mars Exploration Rover “Opportunity” during the
global dust storm event that occurred in the middle of* Correspondence: claus.gebhardt@uaeu.ac.ae
1
National Space Science and Technology Center, United Arab Emirates
University, P.O. Box 15551, Al Ain, United Arab Emirates
Full list of author information is available at the end of the article

2018. At the onset of the event, the rover entered a safe
mode and did not recover later (at least, not until thetime of writing this paper).
Systematic mapping is required as the Martian atmosphere exhibits considerable temporal and spatial variability. Because of the Mars orbit eccentricity, a sharp
contrast exists between the perihelion season (corresponding to solar longitudes LS from 180° to 360°) andthe aphelion season (whose LS ranges from 0° to 180°).
The perihelion season is relatively warm and dusty fromthe southern hemisphere (where is summer) to thenorthern midlatitudes. The temperatures in the northern
polar night are as low as those in the southern polarnight (although the southern winter lasts longer than
the northern one). Dust is present in the Martian atmosphere even in the aphelion season (at a backgroundlevel), and local dust storms can always occur. The
presence of water ice clouds in the equatorial region ismore characteristic of the aphelion season. Moreover,they are observed during both the aphelion and perihelion seasons in the respective winter hemispheres. The
seasonal cycle of water ice clouds exhibits relatively low
interannual variability.

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
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Gebhardt and Abuelgasim Progress in Earth and Planetary Science

Dust has pronounced interannual variability becauseof the occurrence of dust storms. Although local and regional dust storms can be observed during any Martianyear, global dust storm events are irregular in time. On
average, they occur every few Martian years (Zurek andMartin 1993). The Martian dust and water cycles are described in detail by Smith (2008) and Haberle et al.
(2017) among other publications.
This article is a review written to prepare for the scientific exploitation of satellite data on the Mars aerosol
optical depth with a temporal resolution of hours. These
data will be provided by several upcoming satellite missions including the ExoMars Trace Gas Orbiter (TGO),
which has been in the phase of science operation sinceApril 2018. Another data source will be the Emirates
Mars Mission (EMM, also known as the Hope Mars
Mission), which is currently in the final stage of planningand will enter the science operation phase in 2021.
Moreover, mission concepts for future Mars stationary
satellite(s) monitoring dust and water ice clouds are being developed. The main goal of this article is to provide
an overview explaining how the optical depth data obtained by these missions may complement each other.
Also, it illustrates synergies with precursor missions
such as the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) and Mars Color
Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO). Particular attention is paid to highlighting
the scientific value of newly generated data with a temporal resolution of hours.
The article is organized as follows. Section 2 focuses onsatellite missions and their science instrumentation. A
comprehensive description of the mission timelines andorbits is provided in Section 2.1. In Section 2.2, the relevant science instrumentation of TGO is described, andthe relationship between the TGO instrument of the Atmospheric Chemistry Suite/Thermal InfraRed spectrometer (ACS/TIRVIM) and EMM instrument of the
Emirates Mars Infrared Spectrometer (EMIRS) is outlined.
In Section 2.3, characteristics of the EMM instruments of
EMIRS and Emirates Exploration Imager (EXI) are provided, and similarities and modifications with respect tothe precursor satellite instruments TES onboard MGS andMARCI onboard MRO are highlighted. In Section 3, possible scientific innovations based on the obtained dust andwater ice cloud optical depths with a temporal resolutionof hours are suggested. In particular, the significance of
these data for Martian climatology and numerical weather
forecasting is discussed in Subsections 3.1 and 3.2, respectively. Section 4 provides an outlook on the mutual
benefit of using data from TGO, EMM, and precursor
missions/instruments. Finally, scientific innovations basedon the aerosol optical depth data with a temporal resolution of hours are summarized.

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Review
Relevant missions/instruments
From MGS to possible future Mars stationary satellite(s)A comprehensive plot containing the timelines of thepast, current, and upcoming satellite Mars missions including MGS, MRO, TGO, and EMM is provided inFig. 1. The science operation of MGS ended in 2006. The
routine science operation of MRO began in 2006, and thatof TGO has started in April 2018. The science operation
phase of EMM is scheduled to begin in 2021. Thus, temporal overlaps are expected to occur between TGO,
EMM, and several other missions (including MRO).
The illustration of the mission orbits is provided inFig. 2. The science orbit of MGS was sun-synchronousat an average altitude of less than 400 km. The ascending node of MGS was at 2 pm local solar time (Albeeet al. 2001). Note that MGS followed a different orbit
during its aerobraking phase as described in Section
2.3.1. MRO has a near circular science orbit at altitudes
of around 300 km, which is sun-synchronous with an ascending node at 3 pm local solar time (Zurek and Smrekar 2007). The science orbit of TGO is circular at analtitude of 400 km (Vago et al. 2015). The orbit periodsof MGS, MRO, and TGO are close to 2 h. On the contrary, the EMM science orbit is considerably higher withan altitude varying between approximately 20,000 and
44,000 km (corresponding to the periapsis and apoapsis,
respectively). The orbital period will be about 55 h
(Sharaf et al. 2017).
Note that the timeline of possible future Mars stationary satellite(s) is still unspecified. Montabone et al.
(2018) proposed a baseline mission scenario that involved ridesharing with an unidentified future primary
orbiter mission, which explicitly focused on dust andwater ice clouds. The Mars stationary orbit altitude isaround 17,000 km above the equator (see also Fig. 2).
TGO and its relationship with EMM

The main goal of TGO is to inventory trace gases (suchas methane and others) in the Martian atmosphere andtheir spatial and temporal variations (Zurek et al. 2011).
As already mentioned in Section 2.1, the science mapping phase of TGO began in April 2018, and the nominal end of the mission is scheduled to occur in 2022.
The final science orbit of TGO is circular at an altitudeof 400 km with an inclination of 74°. This configuration
will allow performing solar occultation measurements
during each orbit.
The thermal infrared component of ACS (Korablev
et al. 2018) of TGO named TIRVIM covers the wavelength range from 1.7 to 17 μm. The primary goal of
TIRVIM is the temperature profiling and aerosol monitoring in the lower/middle atmosphere by nadir observations. Following the orbit of TGO, nadir measurementsGebhardt and Abuelgasim Progress in Earth and Planetary Science(2019) 6:31

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Fig. 1 Timelines of satellite Mars missions successfully completed to date (Mariner 9, Mars 2, Mars 3, Viking 1, Viking 2, Mars Global Surveyor);
currently in science operation (Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, Mars Atmosphere and Volatile Evolution (MAVEN), Mars
Orbiter Mission (Mangalyaan), ExoMars Trace Gas Orbiter); and upcoming in the near future (Emirates Mars Mission and others). The color coding
specifies the countries of origin. The time axis denotes both Martian (top) and Earth (bottom) years. Adapted from Navarro et al. (2017b)

are confined to latitudes of ± 74° (in contrast, TGO solar
occultation measurements will extend well over polar regions). Complete spatial coverage in the monitored latitude range will be achieved by TIRVIM nadir
observations in 7 sols. In addition, full daily coverage including the characterization of the atmospheric state forany hour of the day will be achieved every 2 months.
The wavelength range of TIRVIM overlaps with thatof the EMM instrument of EMIRS (~6–40 μm; see also
Section 2.3.1). Since both instruments are thermal infrared spectrometers, their measurement operation principles have a certain degree of similarity. Because of thehigher spectral resolution of TIRVIM, a finer verticaltemperature resolution of a few kilometers and higher
altitude coverage (from the surface up to 55–60 km) will
be achieved. For comparison, a resolution of 10 km andaltitude coverage of 0–50 km are expected for EMIRS.
Practically, full global and diurnal coverage will be provided by EMIRS in ~ 10 sols (see also Section 2.3.1), andpolar latitudes will be covered by EMIRS.

TGO also hosts the CASSIS imager, which, however,does not operate in the ultraviolet (UV) range (Thomas
et al. 2017). Hence, it is not compared with the EMM
instrument of EXI described in Section 2.3.2.
EMM and its precursors

The primary science phase of EMM is scheduled to
begin in 2021 and continue for two Earth years (which
are equivalent to approximately one Martian year). In
addition, EMM is designed for an extended science
phase that will last two more Earth years. Two instruments of the EMM scientific payload will characterize
the optical depth of dust and water ice clouds. This will
be based on measurements conducted in the thermal infrared spectral range by EMIRS and in the UV and visible range by EXI.
EMIRS and its precursor TES/MGS

EMIRS is an interferometric spectrometer operatingin the thermal infrared range (Altunaiji et al. 2017).

Gebhardt and Abuelgasim Progress in Earth and Planetary ScienceFig. 2 Illustration of the orbits of TGO, MGS, MRO (Mars
Reconnaissance Orbiter), MAVEN, Mars Express (MEX), EMM, andpossible future Mars stationary satellite(s). The orbit of EMM andMars stationary orbit are higher than those of the other missions. In
addition, the orbits of the Mars moons Deimos and Phobos areshown. Adapted from Sharaf et al. (2017)(2019) 6:31

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EMIRS is close to 5 cm−1. The instrument has an instantaneous field of view (IFOV) with a width of 6 mrad.
These values are slightly smaller than the corresponding
TES parameters (6–50 μm, 5 or 10 cm−1, 8.3 mrad; see
Smith et al. (2001)). Further, EMIRS utilizes a 3 × 3 array
detector, which corresponds to a 3 × 3 element instrumental footprint (illustrated in Fig. 3). The ground pixel size of
EMIRS varies between ca. 100 and 300 km, which are consistent with the IFOV width of several mrad and orbital altitudes between 20,000 and 44,000 km. Similar to the TES
data products, the parameters that will be retrieved fromEMIRS include those of dust, water ice clouds, water
vapor, three-dimensional temperature distribution in thelower and middle atmosphere, and surface temperature.
The retrieval of the lower atmospheric temperature fromthe EMIRS instrument will be based on CO2 absorption
features. Its vertical resolution will be close to 10 km for
altitudes ranging from 0 to 50 km (Altunaiji et al. 2017).
EMIRS will achieve nearly complete global and diurnal
coverage of all retrieved variables in about 10 days.

The instrument has been jointly developed by the
Mohammed Bin Rashid Space Centre (MBRSC),
Dubai, UAE and the Arizona State University (ASU),
AZ, USA. It represents an upgraded version of thethermal infrared spectrometers from the Mars Space
Flight Facility, ASU, which include TES onboard MGS(Smith et al. 2001; Christensen et al. 2001). The latterachieved unprecedented global mapping of the seasonal variations of the Martian atmosphere and surface and provided crucial insight into the interannual
variability primarily driven by dust storms. Overall,TES defined the present standard of global Martianclimatology (see the review written by Smith (2008)).
The main specifications of EMIRS are listed in Table 1.
It covers a spectral range of ~6–40 μm, which is equivalent to around 200–1600 cm−1 (in the units of the inverse wavelength). The default spectral resolution of
Table 1 Instrument specifications of EMIRS onboard EMM
Instantaneous field of view

6 mradSpectral resolution

5 cm−1

Spectral range~6–40 μm

Spatial resolution≲ 300 km

Observation capability

Observation of about one half of Mars
within a period between ca. a quarter tohalf an hour
~ 20 global-hemispheric observations
per orbit (~ 60 per week)

Fig. 3 Observation strategy of the EMIRS instrument, which has a 3 × 3
element footprint (corresponding to its 3 × 3 array detector). During
one observation cycle, a scan of the Martian disk with minimal gaps is
performed. The line-of-sight pointing of the instrument is controlled by
spacecraft slew and rotation, retracing, and pausing of the scan mirror.
Source: Altunaiji et al. (2017)

Gebhardt and Abuelgasim Progress in Earth and Planetary Science

EMIRS has a measurement strategy based on one observation cycle scanning over the full Martian disk withminimal gaps. Its line of sight is controlled by both
spacecraft attitude maneuvering and rotating the instrument’s scan mirror as shown in Fig. 3. During any observation cycle, the spacecraft attitude moves at a constantslew rate, whose value depends on the orbital altitude.
The EMIRS mirror performs scanning sequences in the
direction crosswise to the spacecraft slewing that is accompanied by retracing and, if necessary, pausing. A
complete observation cycle will take about 32 min at theperiapsis and around 15 min at the apoapsis. In total,
about 20 global-hemispheric observations per orbit (oraround 60 global-hemispheric observations per week)
will be conducted.
With the high-altitude EMM spacecraft orbit (fromapproximately 20,000 to 44,000 km; see the details in
Section 2.1), the planned operation of EMIRS is reminiscent of the apoapsis portion of the aerobraking orbit of
TES onboard MGS. The latter was highly elliptical withan initial apoapsis altitude of 58,000 km. In the apoapsis
part of its orbit, the spacecraft executed a roll motion.
Combined with the motion of the TES pointing mirror,
the near-full Martian disk could be imaged in around
100 min. The algorithms employed for retrieving thedust and water ice cloud optical depths are described bySmith et al. (2000) and Pearl et al. (2001), respectively.
First, the composite column integrated optical depth of
dust and water ice clouds is determined by numericallysolving the radiative transfer equation (the atmosphericand surface temperature are necessary parameters). Vertically resolved atmospheric temperatures are retrievedin advance from the 15-μm CO2 band. Subsequently, theoptical depths of dust and water ice clouds are disentangled by fitting predefined spectral shapes. A
data-filtering criterion is given by the surface
temperature being above 220 K to ensure sufficient thermal contrast between the surface and the atmosphere.
Smith (2004) described an enhanced retrieval algorithm
version, which included the simultaneous retrieval of thesurface temperature, improved spectral shape fitting fordust and water ice clouds, the use of a map of surface
emissivity parameters, and more realistic assumptions
regarding the vertical distribution of water ice clouds.
The uncertainty of the dust and water ice cloud optical depth is specified to be 0.05 or 10%, whicheveris larger.
EXI and its precursor MARCI/MRO

As part of EMM, the multi-band imager instrument EXI(AlShamsi et al. 2017) will be sensitive to UV and visible
wavelengths. In total, it comprises six different spectral
bands: three in the UV range and three in the visible
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a dual lens system. The subdivision into six spectral
bands is accomplished by bandpass filters centeredaround the UV wavelengths of 220, 260, and 320 nm
and visible wavelengths of 437 (blue), 546 (green), and635 nm (red). All bands share a single focal plane. The
detector consists of a radiation-tolerant CMOS sensor
with a resolution of 12 megapixels and ensuring highradiometric quality. Both the visible and UV channels arecapable of capturing the full Martian disk from the apoapsis to the periapsis. A single pixel of the UV (visible) channel has an angular dimension of 23″ (22″). The ground
resolution is a few kilometers per pixel and varies by a factor of about 2 between the apoapsis and the periapsis. The
specifications of the EXI instrument are listed in Table 2.
It has been jointly developed by MBRSC and theLaboratory for Atmospheric and Space Physics (LASP),
University of Colorado Boulder, CO, USA.
The retrieval of the water ice cloud optical depth from
EXI will be similar to that from MARCI (Bell III et al.
2009; Malin et al. 2008). The retrieval methodology for
MARCI is based on multiple-scattering radiative transfer
modeling and relationships between aerosol properties
derived by Wolff et al. (2010). The algorithm was reported in detail by Wolff et al. (2011). The estimated uncertainty of the resulting water ice cloud optical depths
was in the range of 0.02–0.03 with a detection limit
close to 0.05. An algorithm version with enhanced retrieval sensitivity particularly under low water ice cloud
conditions was described by Wolff et al. (2014). The input parameters for the retrieval include dust opticaldepth from the CRISM1 climatology and a surface reflectance database (also generated by MARCI). Note thatthe algorithm cannot separate between water ice clouds
and surface ice. Thus, the resultant data must be
quality-flagged using auxiliary information on the Mars
polar caps. Note that CO2 ice clouds will be also detectable by EXI, but it is not possible to unambiguously discriminate between water and CO2 ice clouds with EXI.
However, because of their high altitude and local isolation, mesospheric CO2 clouds are generally optically thinwhen viewed at the EXI resolution. Thus, they represent
a minor error source in the planned retrieval of the optical depth of water ice clouds from the EXI instrument.
EXI will also have an option to use simultaneously retrieved data on the dust optical depth as an input parameter for the water ice cloud optical depth and other
retrievals. The dust optical depth at 220 nm will be retrieved by combining measurement information fromthe EXI bands centered around 220 and 635 nm(AlShamsi et al. 2017).
Overall, the scientific operation concept of the EXI instrument is based on contemporaneous images from thebands centered around 220, 260, 320, and 635 nm andwill allow the retrieval of dust, water ice clouds, andGebhardt and Abuelgasim Progress in Earth and Planetary Science(2019) 6:31

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Table 2 Instrument specification of EXI onboard EMM. Where appropriate, a distinction between the UV and visible channels ismade. Adapted from AlShamsi et al. (2017)

ozone (AlShamsi et al. (2017)). A detailed formulation of
the spatiotemporal mission requirements for EXI is
provided in Table 3. The diurnal and geographic
requirements are fulfilled within 10 days and less than
72 h, respectively.

Table 3 Mission requirements for the spatial and temporal
coverage of the EXI instrument subdivided into the diurnal,
geographic, and seasonal parts. Adapted from AlShamsi et al. (2017)
EXI coverage requirements
Diurnalrequirements

In any given span of 10 days, the four 3-hintervals spanning from 6 am to 6 pm local time
are sampled with at least 80% longitude
coverage in:≥ 3 local time intervals for all latitudes
equatorward of ± 30°≥ 2 local time intervals for all latitudes
equatorward of ± 50°.
In any given span of 10 days, at least one of the four
3-h intervals spanning from 6 am to 6 pm local time issampled with at least 50% longitude coverage for all
latitudes equatorward of ± 80°.

Geographic
requirements≥ 80% of the Mars geographic area is sampled
more frequently than every 72 h.
Latitudes ≤ 80° are sampled more frequently
than every 72 h.

Seasonal
requirements

Observations over one full Martian year (goal: 20 outof the 24 LS intervals of 15° are included).

Possible scientific innovations
Refinement of climatological data

A review on TES as well as foregoing and ongoing instruments was published by Smith (2008), with their individual data records on the dust and water ice cloudoptical depth being an integral part. In addition, an
overview of the dust climatology from TES/THEMIS2
and recent single-instrument records of dust andwater ice cloud characteristics is provided by Haberle
et al. (2017).
Various Mars atmospheric and surface parameters are
publicly available on the website of the Mars Climate
Database (MCD; http://www-mars.lmd.jussieu.fr/). As
part of its version 5.3, MCD provides a multi-instrument
climatology of the column dust optical depth describedby Montabone et al. (2015). These data are based on the
optical depth from TES and THEMIS nadir measurements at ~ 9 μm and Mars Climate Sounder (MCS)3limb measurements at ~ 22 μm. Vertical integration andextrapolation as well as wavelength conversion are performed on the limb-measured data from MCS. To account for topography-related variations, all optical depth
values are normalized to a reference pressure of 610 Pa.
Two different products of daily maps exist. The first
product includes regularly distributed grids with missing data at some grid points and the latitude/longitude

Gebhardt and Abuelgasim Progress in Earth and Planetary Science

resolution being different between the Martian years
24–26 and afterwards, generated by iterative weighted
binning. The second product is given by uniformly
gridded maps constructed through spatial interpolationby the kriging method.
A climate database on the water ice cloud opticaldepth is publicly available from https://gemelli.spac
escience.org/twiki/bin/view/MarsObservations/MarciOb
servations/. Progress reports on the construction of thisdatabase were provided by Wolff et al. (2011, 2014).
As explicitly noted by Montabone et al. (2015), upcoming dust climatological data may account for diurnal
variations by combining data resulting from day andnight side observations. Taken in the most general sense,
we note that any time-resolved data record can potentially improve the existing dust climatology. This, inturn, implies that dust optical depths obtained at different times of the day (e.g., in the morning, noon, andevening) will be suitable for the refinement of existing
dust climatologies. In principle, this approach is also applicable to new data products on the water ice cloud optical depth and existing water ice cloud climatology.
In this work, it is assumed that the majority of new
data products will be based on daytime measurements.
Nighttime data on dust and water ice clouds may not be
available for different reasons such as the necessity ofsufficient thermal contrast between the surface and theatmosphere for successful data retrieval or the presenceof solar light. A surface temperature higher than 220 K
is required for the data retrieval from TES onboard
MGS as described in Section 2.3.1. Therefore, the same
will likely be true for the EMM instrument of EMIRS.
Improvement of Martian numerical weather forecast
Data assimilation

Data assimilation is the combination of all available dataon the state of a system obtained by a numerical modeland measurement-based observations. To this end, datafrom both are interpolated/extrapolated in space andtime by mathematical methods. As a result, the system
is characterized by data that are optimal in terms ofmodeling and observations. Data assimilation mainly
evolved from the need of numerical weather forecastingon Earth to be based on the best possible representationof the initial state. A basic description of the use of data
assimilation in different areas of Earth System Science isprovided in the book of Lahoz et al. (2010).
In the 1990s, it has been proposed to use data assimilation for the reanalysis of spacecraft data obtained from
Mars. The latter accounts for sparse data coverage andthe unfeasibility to directly observe certain parameters
by satellite remote sensing. Different types of data assimilation schemes have been developed including theAnalysis Correction scheme (Montabone et al. 2014)(2019) 6:31

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and ensemble Kalman filtering ones (Greybush et al.
2012; Navarro et al. 2017a, 2017b). The remaining partof this section focuses on the future possibility of data
assimilation preparing the initial state for the numerical
forecast of Mars dust storms.
In Montabone and Forget (2018a, 2018b), forecasting
dust storms is considered one of the biggest challengesof numerical weather forecasting on Mars. Note that it
is not possible yet mainly because of the shortcomingsof the model representation of the dust storm initiation
via surface and boundary layer processes and transitionof dust storms to the regional and global scale. From theobservational side, Mars stationary satellites are a promising source of data for improving the dust storm
forecast in the future. By the assimilation ofnear-hemispheric satellite observations of aerosol andtemperature into numerical forecast models, dust storms
may be predicted days in advance. While presenting a
concept for a future Mars stationary mission, Montabone et al. (2018) noted that EMM was the only othermission that had an orbit with similar altitude andnear-hemispheric geographical coverage. Hence, it canbe concluded that EMM is another valuable resource forachieving a better understanding of the Mars dust processes (especially dust storms) and advancing their numerical modeling. It should be noted that, in the case of
their concurrent operation, EMM and Mars stationary
satellite(s) will be highly complementary to each other(for example, the polar coverage by EMM may complement the orbiting of Mars stationary satellite(s) above
the equator).
Note that for TGO, it is rather its ability to provide
vertically resolved dust aerosol data which is currently
foreseen to improve models. The assimilation of MCS
dust vertical profiles into Mars general circulation
models (MGCMs) was shown to replicate features
known from observations such as detached dust layers(Ruan et al. 2017; Streeter et al. 2017).
Comparison between satellite and model data

The comparison of aerosol optical depth data from modeling and satellite is an important contribution to numerical weather forecasting. This is particularly the casefor data on the optical depth of water ice clouds. Insight
into the feasibility of the assimilation of water ice cloudoptical depth data into MGCMs was provided by
Navarro et al. (2017a, 2017b) and Steele et al. (2014).
Such assimilation is currently not performed on a regular basis. The most critical aspect of this approach is its
relationship with the temperature accuracy of the utilized models. As alternative procedure, cloud microphysical schemes of MGCMs are improved by comparisons
against independent satellite datasets on water ice clouds
and water vapor. These comparisons typically cover a

Gebhardt and Abuelgasim Progress in Earth and Planetary Science

time span of one selected Martian year without global
dust storm events. The agreement with the satellite dataserves as an indicator of the model performance (Pottier
et al. 2017; Navarro et al. 2014; Madeleine et al. 2012).
Using the above-mentioned principle, the quality ofMartian numerical weather forecast models may be evaluated by comparing diurnal variations of the aerosol optical depth against satellite data (pronounced diurnal
variations of the Mars optical depth are known from satellite observations (Kleinboehl et al. 2018), lander instruments (Colburn et al. 1989), and Earth-based telescopes(Akabane et al. 2002)). It should be noted that, in thecase of a systematic error, it might not be possible to determine whether the observed data differences result
from the model or the satellite. This follows from basic
differences in the numerical weather forecasting on
Mars and Earth outlined by Montabone and Forget(2018b). Nonetheless, diagnostic studies may also bebased on the qualitative agreement between the diurnal
variations obtained from model and satellite, i.e., analyzing whether diurnal cycles have the same phase andamplitude. Satellite data with a temporal resolution ofhours providing a highly detailed evidence on diurnal
variations will be invaluable for that purpose.
Finally, it is worthwhile noting that comparisons of diurnal variations of data from local mesoscale numericalmodels and the Rover Environmental Monitoring Station were performed by a number of recent studies(Fonseca et al. 2018; Richardson and Newman 2018;
Newman et al. 2017). That is because such comparisons
are a good basis for investigating local meteorologicalphenomena at the Gale Crater.

Conclusions
Summary and outlook

TGO, EMM, and future Mars stationary satellite(s) havea significant potential to provide highly innovative data
products on Mars atmospheric properties with a temporal resolution of hours. In particular, diurnal variationsin the Martian atmosphere will be characterized. One ofthese data products will be the optical depths of dust
and water ice clouds from various satellite instruments
operating in different spectral ranges.
The relationship between recent, ongoing, and upcoming satellite missions/instruments was analyzed based on
their dust and water ice cloud data. The science phase of
TGO began in April 2018. Before the nominal end of
the TGO mission in 2022, EMM will start its routine science phase. Complementary to TGO, EMM will provide
high spatial and temporal data coverage from a high altitude orbit, which may be a basis for data consolidation
through inter-instrumental comparisons of optical depth
and other data. Moreover, the EMM instrument of
EMIRS will allow revisiting the data record from its(2019) 6:31

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precursor TES (the same is true for the EMM instrument of EXI and MARCI). Note that TES and MARCI
are parts of the payloads of the MGS and MRO missions, respectively. Thus, both instruments are tied tosun-synchronous orbits with a fixed local solar time.
For different scientific applications, the innovations
based on the data products on the dust and water ice
cloud optical depth with a temporal resolution of hours
have been identified and discussed. They include the refinement of dust and water ice cloud climatologies by
the integration of time-resolved data. Another field ofinnovations is the improvement of the ability to predict
dust storm events based on the assimilation of the dataobtained from near-hemispheric observations of EMM
and possible future Mars stationary satellite(s) into numerical forecast models. Moreover, an indicator for thequality of Martian numerical weather forecast models is
given by comparing diurnal variations against the observational evidence provided by the satellites.

Endnotes
1
CRISM: Compact Reconnaissance Imaging Spectrometers for Mars onboard MRO; see Murchie et al. (2007)
2
THEMIS: Thermal Imaging System onboard Mars
Odyssey; see Christensen et al. (2004)
3
MCS: Mars Climate Sounder onboard MRO; see
Zurek and Smrekar (2007)
Abbreviations
ACS: Atmospheric Chemistry Suite; ASU: Arizona State University;
EMIRS: Emirates Mars Infrared Spectrometer; EMM: Emirates Mars Mission(also known as the Hope Mars Mission); EXI: Emirates Exploration Imager;
IFOV: Instantaneous field of view; LASP: Laboratory for Atmospheric andSpace Physics; MARCI: Mars Color Imager; MBRSC: Mohammed Bin RashidSpace Centre; MCD: Mars Climate Database; MCS: Mars Climate Sounder;
MGCMs: Mars general circulation models; MGS: Mars Global Surveyor;
MRO: Mars Reconnaissance Orbiter; TES: Thermal Emission Spectrometer;
TGO: ExoMars Trace Gas Orbiter; UV: Ultraviolet
Acknowledgements
We would like to thank the Manuscript Editing Services, UAEU Libraries,
United Arab Emirates University for their professional help with final text
editing prior to article publication. In particular, we are grateful to Mr.
Shehab Majud, Coordinater, and any other person involved.
Funding
This work was supported by the National Space Science and Technology
Center at the United Arab Emirates University through the project Earth andMars Atmosphere (grant no. 21R033-NSS Center 7-2017).
Availability of data and materials
Not applicable.
Authors’ contributions
AA proposed the topic, initiated the study, and collaborated with thecorresponding author in the preparation of the manuscript. CG performed
the literature search and case study using the Mars Climate Database v5.3
(web interface: www-mars.lmd.jussieu.fr/mcd_python/). Based on theobtained results, he prepared the manuscript. Both authors have read andapproved the final manuscript version.

Gebhardt and Abuelgasim Progress in Earth and Planetary Science

Authors’ information
CG received his Ph.D in Atmospheric Physics from the University of Bremen,
Germany, in 2014. The focus of his Ph.D work was the scientific explorationof Earth atmospheric data from the satellite mission ENVISAT/SCIAMACHY(2002–2012). In 2014–2017, he held a postdoc-equivalent position at theGerman Aerospace Center (DLR). His task was extracting geophysical andmeteorological parameters from Earth observation satellite images. He
conducted several peer-reviewed research studies on the planetaryatmospheric boundary layer. Currently, he is working on the research project
Earth and Mars Atmosphere at the United Arab Emirates University, Al Ain,
UAE. Note that CG has discovered his interest in Astro and Planetary Physics
long before this work. During his graduation from the university, he wrote a
diploma thesis in the field of gravity theory at the Bremen Drop Tower(ZARM) in 2010. In addition, he is an active member of an amateur
astronomy club in the greater Bremen area (AVL Lilienthal).
AA is Principal Investigator of the project Earth and Mars Atmosphere.
Competing interests
The authors declare that they have no competing interests. They note
further that they are not part of the EMM Science Team.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
National Space Science and Technology Center, United Arab Emirates
University, P.O. Box 15551, Al Ain, United Arab Emirates. 2Department ofGeography and Urban Planning, College of Humanities and Social Sciences,
United Arab Emirates University, P.O. Box 15551, Al Ain, United ArabEmirates.
Received: 12 December 2018 Accepted: 6 March 2019

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