# The Timing and Origin of Lobate Ejecta Forms at Craters in Mercury's South Polar Region

A R Lennox, A J Blance, D A Rothery, J. Wright, M. Balme, S J Conway

► To cite this version:

A R Lennox, A J Blance, D A Rothery, J. Wright, M. Balme, et al.. The Timing and Origin of Lobate Ejecta Forms at Craters in Mercury's South Polar Region. Journal of Geophysical Research. Planets, 2025, 130 (3), 10.1029/2024je008713. http://dx.doi.org/10.1029/2024je008713

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Distributed under a Creative Commons Attribution 4.0 International License
# JGR Planets

## RESEARCH ARTICLE

10.1029/2024JE008713

**Key Points:**

* We report six previously undocumented examples of lobate ejecta in Mercury's south polar quadrangle (H15)

* We made morphostratigraphic maps of Nairne and Magritte and used shadow measurements to constrain the heights of their ejecta lobes

* Impact melt on top of the lobate ejecta indicates formation syn-impact and thus cannot be the product of mass-wasting after crater formation

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

A. R. Lennox,
annie.lennox@open.ac.uk

**Citation:**

Lennox, A. R., Blance, A. J., Rothery, D. A., Wright, J., Balme, M., & Conway, S. J. (2025). The timing and origin of lobate ejecta forms at craters in Mercury's south polar region. *Journal of Geophysical Research: Planets*, 130, e2024JE008713. https://doi.org/10.1029/2024JE008713

Received 9 SEP 2024
Accepted 3 FEB 2025

**Author Contributions:**

**Conceptualization:** A. R. Lennox

**Formal analysis:** A. R. Lennox

**Investigation:** A. R. Lennox, A. J. Blance

**Methodology:** A. R. Lennox

**Software:** A. R. Lennox

**Supervision:** D. A. Rothery, J. Wright,

M. Balme, S. J. Conway

**Visualization:** A. J. Blance,

D. A. Rothery, J. Wright, M. Balme,

S. J. Conway

**Writing – original draft:** A. R. Lennox

**Writing – review & editing:**

A. R. Lennox, A. J. Blance, D. A. Rothery,

J. Wright, M. Balme, S. J. Conway

© 2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

# The Timing and Origin of Lobate Ejecta Forms at Craters in Mercury's South Polar Region

<figure><img src="image_2.png" /></figure>

A. R. Lennox¹ ID, A. J. Blance¹ ID, D. A. Rothery¹ ID, J. Wright¹ ID, M. Balme¹ ID, and S. J. Conway² ID

¹School of Physical Sciences, The Open University, Milton Keynes, UK, ²Nantes Université, University Angers, Université du Mans, CNRS UMR 6112 Laboratoire de Planétologie et Géosciences, Nantes, France

**Abstract** We present six newly identified examples of lobate ejecta in Mercury's south polar quadrangle (H15), providing the first evidence of syn-impact formation through our observation of perched impact melt at Nairne and Magritte craters. This finding challenges the idea that lobate forms could have developed post-impact via mass wasting or landslide processes, suggesting instead that many, if not all, lobate ejecta deposits formed contemporaneously with the impact event. We present detailed morphostratigraphic maps of two exemplary case studies: Nairne and Magritte, for which we used shadow measurements to better constrain the morphology of their lobes. Many examples globally, including Nairne, have been formed by deposition of the lobe material into a topographically lower antecedent crater. While we confirm that topography plays a significant role in the formation of lobate ejecta, it cannot be the sole factor, as similar impacts onto pre-existing craters do not always produce these features and not all lobate ejecta exhibit evidence for a topographic control. Our study also highlights the frequent association between lobate ejecta and characteristics of oblique impacts. Considering analogous features at Mars and the Moon in particular, lobate ejecta on Mercury is likely the result of either fluidized or dry granular flows, with minimal delay between impact and emplacement. Our findings suggest that lobate ejecta are more widespread on Mercury than previously recognized, and future studies and missions, such as a comprehensive global survey with higher resolution data from BepiColombo, will continue to constrain their formation mechanisms and prevalence.

**Plain Language Summary** We discovered six new examples of unusual landforms surrounding impact craters at Mercury, referred to as lobate forms/ejecta. We found these six examples in Mercury's south polar region, an area referred to as the Bach quadrangle or H15. We present evidence that these landforms likely developed during the impact events that formed the host crater, rather than developing later by processes similar to those producing landslides. This study almost doubles the known examples of lobate forms/ejecta on Mercury, presents maps to better constrain the shape and structure of the crater and ejecta, and calculates the heights of the lobes. We confirm that the shape of the land onto which the impactor hits strongly affects their formation, though it is not the only factor. We also suggest that these landforms are linked to oblique impacts, where the angle between the trajectory of the impactor is less than the typical 45°, causing radial symmetry to be lost, and that the lobate forms were produced at the same time as the impact occurred. The study concludes that craters with lobate ejecta are more common on Mercury than previously thought. Future missions will provide more detailed data to better understand them.

## 1. Introduction

Flyby and orbital images of Mercury's surface acquired by the Mariner 10, MESSENGER and BepiColombo missions confirm that, consistent with the other terrestrial planets and the Moon, impact cratering is a fundamental process that has shaped the planet's geological evolution. As a result, impact craters make up much of Mercury's visible surface stratigraphy. Understanding crater morphologies and the characteristics of crater deposits on Mercury can improve our understanding of impact crater formation and help elucidate the geological history of the planet.

A common characteristic of impact craters on airless bodies is a ballistically emplaced continuous ejecta deposit. This typically comprises an inner hummocky facies, termed proximal ejecta, and an outer radially textured facies, termed distal ejecta, with a gradational boundary between the two (Barlow et al., 2021; Xiao et al., 2014). The thickness of the ejecta deposit decreases with distance from the crater rim.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Six impact craters in H15 with lobate ejecta deposits. White arrows point to the fronts of the lobate forms. Each frame shows a south polar stereographic projection. (a) **Nairne** (case study 1; −70.36°, 1.65°); (b) **Magritte** (case study 2; −72.75, 121.60); (c) unnamed crater overlapping the rim of Chopin (−66.38, −199.71), with ejecta flowing into Chopin; (d) **MacNicol** (−68.25, −3.99); (e) **Lorde** (−69.12, 49.29); (f) unnamed crater approximately 125 km north of Lorde, with ejecta deposited into a contiguous, polygonal crater.</figcaption></figure>

As part of systematic mapping of Mercury's south polar “Bach” quadrangle (H15), we report six craters that differ from these typical ejecta characteristics in that they are the sources of adjacent lobate forms (Figures 1a–1f). The morphology of the lobate forms differs from purely ballistic ejecta, whereby (a) they can extend atypically far from the crater rim in some azimuthal directions, (b) they show considerable textural roughness, and (c) their thickness is maintained with radial distance, terminating in a non-gradational steep lobate margin.

On Mercury, impact craters can be classified according to their degradation state, thus allowing for geological events across the planet to be correlated. The classification of degradation states of craters can usually be used as a proxy for age. We classify the six craters in H15 that host lobate ejecta as c3 and c4 according to the five-class system of Kinczyk et al. (2020), in which c5 is the youngest and freshest and c1 the oldest and most degraded.

Craters with associated lobate forms were previously regarded as rare on Mercury. Xiao and Komatsu (2013) compiled a global database of only 7 examples on Mercury (therein referred to as “ejecta flows”), none of which occurred in H15. Hokusai crater has also been identified as surrounded by apparent ejecta flows (Barnouin et al., 2015). Our discovery of six more craters with lobate forms within H15 alone nearly matches the number of examples documented in Xiao and Komatsu (2013) found globally, yet this is just a small fraction of a far greater set. A global survey by Blance et al. (2024) demonstrates that the total number of craters with associated lobate forms on Mercury nears a hundred, making these considerably more abundant than formerly realized.

Craters with lobate forms are not unique to Mercury and exist on a multitude of planetary bodies including Mars and the Moon (discussed in 4.4), Venus (e.g., Asimow & Wood, 1992; Barnouin-Jha & Schultz, 1998; Basilevsky
& Head, 2003; Komatsu et al., 1991) and some icy moons (e.g., Boyce et al., 2010; Horner & Greeley, 1982; Hughson et al., 2019; Moore et al., 2001). Universally, the timing of formation and implications for the origin of such features is inconclusive. This ambiguity is the product of two contrasting interpretations of lobate forms as either part of the ejecta deposit and by extension formation syn-impact or a post-impact feature arising from mass-wasting. For the latter, constraining the temporal relationship between the impact and mass-wasting event is often impracticable.

Despite the prominence of impact craters across the terrestrial planets and the Moon, the factors governing crater and crater ejecta morphologies are poorly understood, and numerous factors have been implicated as controls. These include the density, velocity and angle of impact of the projectile (Gault et al., 1975; Gault & Wedekind, 1978; Kenkmann & Wulf, 2018); the topographic and lithological properties, degree of heterogeneity and physical state of the target surface (Barlow & Bradley, 1990; Barnouin-Jha et al., 2019; Cintala & Head, 1976; Prieur et al., 2018; Shuvalov, 2020); and the body's gravitational acceleration, atmospheric density and thermal history (Gault & Wedekind, 1977; Kenkmann & Wulf, 2018; Melosh & Ivanov, 1999). The quantity of proposed influencing factors may be a function of the difficultly in inferring pre-impact conditions of the target and impactor properties from the morphology of the resulting crater and its associated crater deposits. This is further complicated by the effect of post-impact modification processes such as erosion or volcanism. We aim to investigate the influence of topography and impact angle on the formation of lobate ejecta at Mercury, considering analogs on other planetary bodies, and the implications for emplacement scenarios.

Two case studies among the six candidates at H15 warrant close investigation: the lobate forms associated with Nairne and Magritte craters (Figure 1). These are exceptionally good examples with important implications for stratigraphic-time relationships and formation setting, respectively. We present morphostratigraphic maps of these craters mapped to document the crater morphology and to help uncover the sequence of events and the history of impact and formation. Nairne in particular provides unmistakable evidence for lobe emplacement contemporaneous with the impact, consequently affirming an impact origin. While other origins are feasible, for simplicity, we refer to all six lobate forms as “lobate ejecta” because of the correlation between formation and impact.

## 2. Methods

### 2.1. Data

Morphostratigraphic mapping involves the differentiation of units based on their appearance in remote sensing images, considering spectral or topographic distinctions in form, texture, and context. To map the crater materials and associated ejecta deposits, we primarily used the 166 m/pixel Mercury Dual Imaging System (MDIS; Hawkins et al., 2007) monochrome basemap (Denevi et al., 2018). We used mosaics with different solar incidence angles (low, and high from both east and west) to examine differences in relief and texture. We used the 665 m/pixel enhanced color mosaic to distinguish units by color, and the 665 m/pixel digital elevation model (DEM v2) for the mapping of topographic features and approximation of regional slope. MESSENGER's elliptical orbit was such that the Mercury Laser Altimeter (MLA) was unable to obtain data at high southern latitudes (Cavanaugh et al., 2007).

We used individual MDIS narrow-angle camera (NAC) monochrome images at higher spatial resolutions for the more accurate mapping of contacts and for use in the measurement calculations at Nairne and Magritte. A list of the NACs used is supplied in the Supporting Information S1. We downloaded NACs from the web-based search interface Planetary Image Locator Tool (PILOT) v4 using Projection on the Web. PILOT uses Integrated Software for Images and Spectrometers (ISIS) v3 to produce images that can be projected in a Geographic Information System (Hare et al., 2016). We downloaded geometrically and radiometrically corrected images in a south polar stereographic projection with standard parallel −90° and no clipping of the dynamic range. The accompanying NAC extracted metadata provided values for the incidence (i) and emission (e) values that we used in later shadow calculations. These terms and their use in determining height from shadow lengths are demonstrated in Figure 2.

### 2.2. The Mapping Process

As the case studies are found within H15, a region encompassing everything poleward of −65°, we used a south polar stereographic projection centered on 90°S for mapping and image display. We digitized the map using
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Cross-sectional schematic showing the shadow cast by the lobate ejecta front and the illumination geometry in a negative spacecraft emission. *L* (highlighted in red) is the shadow length, *n* is the surface normal, and *e* and *i* are the emission and incidence angles, respectively. For a full explanation and derivation of this expression, see Appendix A in Barnouin et al. (2012). MESSENGER render from NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.</figcaption></figure>

ArcGIS Pro version 3.1.3. We mapped features and boundaries at a scale of 1:30,000, a larger scale than that used for the geomorphological mapping of the entire area of H15 (e.g., Lennox et al., 2021) as appropriate for the increased detail in these localized crater-focused maps.

The mapping process involved digitization of the contacts between morphostratigraphic units, linear features, and surface features. We first mapped contacts as polylines, designating each as either certain (a sharp and clearly observable contact) or approximate (where the exact location of the boundary is indistinct). This was followed by placing a point to assign each morphostratigraphic unit, and the creation of polygons from the digitized outlines using the “Feature to Polygon” geoprocessing tool. Linear features, also mapped as polylines, denote the rims of craters. These include the rims of craters hosting lobate ejecta and the exposed rims of any partially buried craters. Other crater rims mapped include the modified rims of antecedent craters, and the small (>1 km and up to 25 km in diameter) rims of superposing craters. We mapped the crater- and ejecta-related units for the main host crater and, in keeping with quadrangle mapping practices, associated craters greater than 20 km in diameter. We digitized catenae and areas of lobate ejecta as polygons.

## 2.3. Calculations

We used shadow-derived topography analysis of the ejecta lobes over use of the DEM as the vertical resolution of the latter is insufficient for purpose; without MLA coverage, the DEM was produced from MDIS images, from which it is more challenging to obtain high-resolution topographic information. We calculated lobe thicknesses of Nairne and Magritte following the shadow measurement method of Barnouin et al. (2012), itself modified from Chappelow and Sharpton (2002). This method has been widely employed on Mercury from its original application to measure crater depths (Chappelow & Sharpton, 2002) to the recent determination of the heights of circum-Caloris modified ejecta blocks (Wright et al., 2020) and the depths of small grabens on shortening structures (Man et al., 2023). The approach uses the following trigonometric formula:

$$h = L \times \frac{1}{\tan i \pm \tan e} \qquad (1)$$

The height (*h*) of the topographic lobe front can be calculated by multiplying the length of the shadow cast (*L*) by a scaling factor that considers the average solar incidence angle from the surface normal to the Sun (*i*), and the average emission angle from the surface normal to the spacecraft (*e*; Figure 2). For both case studies, the Sun was located behind the spacecraft, resulting in a negative *e* and thus the denominator expression became:

$$\tan i - \tan e \qquad (2)$$
For the lobate ejecta fronts at Nairne and Magritte, we measured *L* along the ground in the same direction as the solar azimuths of the respective NAC images. The measurement was made at six equidistant points across the lobe fronts. We made shadow length measurements at an increased scale of approximately 1:1,500, whereby the grayscale value of each individual pixel could be differentiated. We then measured the distance between the first and last completely black pixels. As a result, the penumbral periphery of the shadow is excluded, and thus the calculated heights are systematic underestimates. In keeping with the database of Xiao and Komatsu (2013), *h* was reported as a minimum and maximum range calculated from the greatest and smallest *L* measurements.

We quantified the extent of the lobate ejecta using the ejecta mobility (EM) ratio of the H15 lobate ejecta candidates and Hokusai for comparison. EM describes the relationship between the radial extent of the ejecta and the crater radius and can be influenced by crater size, ejecta velocity, or the composition of the target surface (Weiss & Head, 2014; Xiao & Komatsu, 2013). Six measurements were taken at each target to define the rim to lobe-margin length and the crater radius measured in the direction of the lobe. This method, used by Xiao and Komatsu (2013), was originally developed by Mouginis-Mark (1979), referred to as “ER” and used to illustrate Martian ejecta mobility variability with latitude and altitude.

We calculated errors using the standard error of the mean for the diameter, radius in the direction of lobe flow, and ejecta extent measurements. Errors for the EM were calculated through error propagation of the radius and ejecta extent.

# 3. Results

## 3.1. Morphostratigraphic Maps

We identified nine crater- and ejecta-related morphostratigraphical units during the mapping of Nairne and Magritte. We separated morphostratigraphical units into core elements common to all craters in this size range (ejecta, crater walls and internal structures), and ancillary features that add to the singularity of a specific crater (catenae and lobate ejecta). The unit descriptions are listed in Table 1 and the mapping products are shown in Figure 3.

## 3.2. Integrating the H15 Candidates Into the Current Database of Craters With Lobate Forms

Table 2 compares the morphometrical parameters of the lobate ejecta at H15 with the previously reported candidates: Xiao and Komatsu's (2013) examples and Hokusai. Our deliberately conservative shadow lengths indicate lobe heights of 300–450 m for Nairne, within the typical range of the Xiao and Komatsu (2013) examples, and 760–1,660 m for Magritte, which is a considerably greater than any previously reported example. Nairne's EM value surpasses that of Xiao and Komatsu's (2013) candidates but is within their measurement error. Magritte, Hokusai and the other four lobate ejecta candidates at H15 have EM values at the lower end of, or far smaller than, those of Xiao and Komatsu (2013).

## 3.3. A Morphological Analysis of Nairne and Magritte

### 3.3.1. Case Study 1: Nairne

We classify the Nairne crater as degradation state c4 in accordance with the five-crater classification system (Kinczyk et al., 2020), hence of probable Mansurian age (~1.7 Ga–230 Ma; Banks et al., 2017). This is based on Nairne's sharp crater rim, the crisp appearance of its central peak material, but a lack of ejecta rays and an inconsistently distinct boundary between the crater walls and hummocky crater floor (Figure 4). Nairne is a typical mature complex crater (Baker et al., 2011; Pike, 1988) with a central peak cluster, areas of wall slumping, and developed terracing. Nairne partially overprints the rim of a much larger (~130 km diameter) and significantly more degraded (c1) crater (which we informally designate as *Nairne*<sub>a</sub>; see Figure 4). The rim of *Nairne*<sub>a</sub> is discontinuous and there is no preserved internal uplift or crater wall terraces. The floor of *Nairne*<sub>a</sub> is heavily overprinted and modified by superposing craters, crater chains originating from Nairne, and tectonic features.

Nairne exhibits a range of asymmetries associated with its crater morphology. The crater rim is slightly elongated along the northwest-southeast axis, its major axis measuring around 60 km in this direction compared to its minor axis of just 50 km. The central peak is slightly offset from the crater's center, being approximately 10 km closer to the steeper northwestern crater wall than to the more shallowly sloping, terraced southeast crater wall. The width
<table><caption>**Table 1**

Names and Descriptions of All the Map Contents, Seven Morphostratigraphic Units and Two Ancillary Features, With Some Key Interpretations</caption><thead><tr><th>Unit name</th><th>Description</th></tr></thead><tbody><tr><td>Proximal ejecta</td><td>Positive relief ejecta material that fully or partially surrounds the crater and is normally thickest near the crater rim. This unit exhibits ropey to hummocky texture. Secondary impact craters are sparse to non-existent. Completely overprints underlying topography. Lobate ejecta is a subcategory of proximal ejecta material.</td></tr><tr><td>Distal ejecta</td><td>Exhibits ridges and striations, small secondary impacts, and modified secondary crater chains that project radially from the crater. This unit becomes less distinct with distance from the crater. There is a diffuse, gradational boundary between this unit and the underlying geological plains, and underlying topography is modified, resulting in obscured or indistinct rims to underlying craters.</td></tr><tr><td>Pooled melt deposits</td><td>Very smooth, localized deposits completely bounded by high relief ejecta materials. Deposits appear ponded in topographic lows occupying areas of little to no slope. We interpret these as flat and smooth pools of impact melt: molten rock produced by the intense heat and shock conditions generated during an impact that coalesced downslope into lower elevations before solidifying.</td></tr><tr><td>Crater wall material</td><td>The inwardly sloping sides of the crater rim occupying the area between the rim crests and the crater floor. Includes terraced structures and occasional impact melt flows.</td></tr><tr><td>Hummocky crater floor material</td><td>Crater floor material with a rough texture and lumpy/rolling surfaces. May be original crater floor morphology or occasionally include crater wall debris.</td></tr><tr><td>Smooth crater floor material</td><td>Crater floor material with little to no texture and level surfaces. Interpretations differ for Nairne and Magritte, as discussed below. Either impact melt or post-impact volcanic plains material.</td></tr><tr><td>Central peak material</td><td>Internal uplift in the form of a central peak cluster (Nairne) or a partial peak-ring (Magritte).</td></tr><tr><th>Feature name</th><th>Description</th></tr><tr><td>Catenae</td><td>Chains of similarly sized craters distributed predominantly proximal, straight and radial to the impact crater. Septae between craters in the chain are, in places, modified such that the floor of the chain appears smooth. Following the USGS convention (Skinner et al., 2018), this is shown on map as an ancillary feature.</td></tr><tr><td>Lobate ejecta</td><td>Portion of abnormal proximal ejecta that is best differentiated from typical proximal ejecta by its greater extent, and steep distal margin allied to a lack of discernible gradational decrease in thickness with distance from the crater. The lobate ejecta emanates from near the crater rim and terminates in a well-defined, steep, lobate front. Its margins near the crater rim are approximate, as there is not a sharp boundary between the lobate ejecta and continuous proximal ejecta. Following conventional USGS symbology (Skinner et al., 2018), this is shown on map as an ancillary feature and are less common than catenae.</td></tr></tbody></table>

of Nairne's walls ranges from 17.5 km (at the southeast portion) to 8.7 km (at the northwest) and the walls show marked spectral variation with yellow terraces in the south and blue in the north in the enhanced color rendering (Figure 4a), though this effect could be augmented by illumination biases. Units of hummocky and smooth material occupy Nairne's crater floor, the former associated with a distinctive blue spectral signature at comparatively higher elevations and the latter occupying the topographically lowest points of the crater floor.

Nairne's ejecta units are distinguishable not only by their morphological characteristics (described in Table 1) but also by their spectral signatures. The differentiation in color emphasizes the contact between the ejecta and the blue surrounding terrain, highlighting further asymmetries associated with the nature and distribution of the ejecta, and implies that the excavated material is distinct in composition from the surrounding terrain.

In the south-southwest region, Nairne's proximal ejecta includes two distinct lobes that propagate into the floors of pre-existing craters that we designate Nairne<sub>b</sub> and Nairne<sub>d</sub> (Figure 4). The lobes extend ~0.7–1.2 Nairne crater radii from its rim. The nearby arc of Nairne's rim is the topographically lowest elevation portion of Nairne's entire circumference. The highest rim elevations can be found in the northeast and southeast of Nairne, the latter corresponding with the most extensively terraced crater walls. Shadow measurements show that the lobe fronts have substantial thickness that varies across its front (530–780 m). The dominant texture of the lobes consists of small hummocks, hundreds of meters in scale that create the rough appearance, and lineations parallel to the inferred direction of flow of the lobe that stretch ~2–6 km. On the lower, southernmost lobe, surface texture is increased by what we interpret as small grabens (Figure 4c, likely having formed in response to instability near the frontal scarp inducing lobe front collapse. On the higher southwest lobe, there is a patch of perched very smooth material midway between the crater rim and steep lobe terminus. This smooth material is scarcely textured by fewer than 10 small (<1 km in diameter) impact craters. The material has a lower reflectance than the surrounding spectrally red ejecta materials. We interpret this smooth material as perched pools of impact melt. Nairne exhibits
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Morphostratigraphic maps of Nairne (a) and Magritte (b).</figcaption></figure>
<table><caption>**Table 2**
Diameter, Ejecta Extent, Lobate Form Height and Ejecta Mobility of Previously Published Lobate Forms and the New H15 Candidates of This Study</caption><thead><tr><th></th><th>ID</th><th>Diameter (km)</th><th>Ejecta extent (km)</th><th>Height (m)</th><th>Ejecta mobility (EM)</th></tr></thead><tbody><tr><td rowspan="7">Reported and calculated by Xiao and Komatsu (2013)</td><td>2A</td><td>88 ± 1.1</td><td>35 ± 0.7</td><td>390–470</td><td>0.80 ± 0.01</td></tr><tr><td>2B</td><td>60 ± 0.8</td><td>34.8 ± 0.5</td><td>—</td><td>1.16 ± 0.02</td></tr><tr><td>2C</td><td>77 ± 0.6</td><td>35.6 ± 0.6</td><td>450–520</td><td>0.93 ± 0.02</td></tr><tr><td>3A</td><td>37.5 ± 1.5</td><td>24.2 ± 0.5</td><td>320–430</td><td>1.29 ± 0.27</td></tr><tr><td>3B</td><td>39.5 ± 2.2</td><td>24.3 ± 0.2</td><td>—</td><td>1.23 ± 0.18</td></tr><tr><td>3C</td><td>83.7 ± 3.7</td><td>44.8 ± 0.6</td><td>590</td><td>1.15 ± 0.32</td></tr><tr><td>3D</td><td>40 ± 1.1</td><td>22.9 ± 0.2</td><td>350–550</td><td>1.15 ± 0.31</td></tr><tr><td>Barnouin et al. (2015) and this study</td><td>Hokusai</td><td>95.7 ± 2.0</td><td>31.7 ± 2.1</td><td>~500</td><td>0.66 ± 0.05</td></tr><tr><td rowspan="5">Reported and calculated in this study (Figures 1a–1f)</td><td><strong>Nairne</strong></td><td><strong>56.7 ± 0.6</strong></td><td><strong>29.8 ± 0.9</strong></td><td><strong>300–450</strong></td><td><strong>1.32 ± 0.05</strong></td></tr><tr><td><strong>Magritte</strong></td><td><strong>153 ± 1.7</strong></td><td><strong>41.2 ± 1.4</strong></td><td><strong>760–1,660</strong></td><td><strong>0.49 ± 0.03</strong></td></tr><tr><td>Unnamed (Figure 1c)</td><td>56.4 ± 0.6</td><td>19.2 ± 0.8</td><td>—</td><td>0.68 ± 0.03</td></tr><tr><td>MacNicol</td><td>43.4 ± 0.3</td><td>19.2 ± 0.6</td><td>—</td><td>0.88 ± 0.03</td></tr><tr><td>Lorde</td><td>44.0 ± 0.3</td><td>14.2 ± 0.9</td><td>—</td><td>0.65 ± 0.04</td></tr><tr><td></td><td>Unnamed (Figure 1f)</td><td>12.2 ± 0.1</td><td>5.3 ± 0.5</td><td>—</td><td>0.87 ± 0.03</td></tr></tbody></table>

Note. Values for Nairne and Magritte, the main case studies in this report, are in bold and we have calculated their lobe heights. The approximate distal rampart height of Hokusai established from MLA data is from Barnouin et al. (2015) and other morphometric parameters are based on our measurements.

no clear evidence of volcanic infilling, and the isolated patch-like nature of the deposits is not suggestive of a volcanic origin.

Nairne's distal ejecta covers the largest surface area of all its mapped units, with a gradational boundary between it and the surrounding terrain. The axis of symmetry for the ejected material is roughly oriented north-northeast to south-southwest. Distal ejecta extend slightly further northwards, depositing material 2.1–2.3 crater radii beyond the crater rim in this direction. Substantially less ejected material (proximal or distal) is deposited on the east and west of the crater. The highest concentration of secondary crater chains produced by Nairne occurs to the south/southwest of the crater and these follow a northeast-southwest trajectory. There is a discernible scarcity of crater chains at the northern and to a lesser extent southeast azimuth. Nairne seems to crosscut an apparently deep crater chain oriented roughly east-west, that, to the east of Nairne, aligns with a crater-hosted tectonic scarp (Figure 4d) in a crater that contains probable explosive volcanic vents. Superposition suggests that this chain predates Nairne's impact.

### 3.3.2. Case Study 2: Magritte

We also interpret Magritte as a c4 crater (Kinczyk et al., 2020) of Mansurian age. At ~150 km in diameter, it is nearly three times larger than Nairne. Magritte exhibits a discontinuous peak-ring preserved primarily but not exclusively in the southern half of the crater. The diameter of the peak-ring is approximately 55 km, indicative of a ring circumference of around 170 km. Given that the currently visible cumulative total length measured along the inner edge of the peak-ring is just 95 km, we infer that around 45% of the peak-ring is absent. It may have formed incompletely or portions may subsequently have been destroyed (Rodriguez et al., 2023). The remaining peak-ring material has been embayed by large volumes of melt, either impact-generated or the product of post-impact volcanic infilling on the crater floor, on which a graben system has developed. Unlike Nairne with its uniformity in spectral signature between the smooth crater floor material and probable pools of impact melt, Magritte's smooth crater floor is characterized by a high reflectance signature, redder than its external impact melt deposits and hummocky crater floor, and contrasts with the bright pale blue signature that borders the central peak materials. Consequently, Magritte's smooth floor is likely to represent post-impact effusive volcanism (Figure 5). Graben formation by the cooling of large thicknesses of infilling lavas is noted in other peak-ring basins such as Rachmaninoff, Raditladi and Mozart (Blair et al., 2013) where it is postulated that thin late-stage lavas have buried grabens within the centermost region of the peak-ring (Blair et al., 2013). Magritte's grabens occur as curved troughs of concentric grabens demarcating the buried northwest portion of the peak-ring and, unlike
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Nairne crater. (a) Nairne crater and ejecta in enhanced color. (b) The same extent as (a) showing the colorized DEM overlay. In frames (a, b), the rims of underlying craters associated with Nairne are denoted by a white dashed line and are labeled "a–d." Referred to in text as Nairne<sub>a, b, c, d</sub>, these include a degraded c1 crater ("a") and a cluster of three craters ("b," "c," "d") towards which Nairne's lobate ejecta has flowed. (c) NAC frame EN1065303226M (104 m/pixel) showing Nairne's lobes. The yellow arrow points to the edge of the perched very smooth deposit. (d) Monochrome basemap of Nairne providing local context. The white (a, b) and red (c) dashed boxes indicate the fields of view of previous frames. The cyan arrows in (c, d) point to linear features: (1) a frontal lobe graben; (2) lineations texturing the lobe; and (3) a tectonic scarp. The white arrows in (d) point to flow fronts and the green arrows to deep crater chains.</figcaption></figure>

Magritte's larger counterparts, crosscut the peak-ring center, oriented roughly north-south. We observed a small number of concentric grabens outwith the peak-ring, concentrated in the smooth infilled material in the northeast region of the crater floor. Additionally, there is a smattering of superposing impact craters on Magritte's floor, six of which are >1 km in diameter and one, located directly south of the peak-ring, is both large (23.5 km in diameter) and fresh enough to have its crater materials mapped (Figure 3b).

In contrast to Nairne, Magritte's proximal ejecta is continuous around the crater rim (though not consistently thick) and its lobes are spread over a wider range of azimuths. The ejecta units do not contrast in color with the background terrain. Well-developed lobate forms emanate from a wider portion of Magritte's crater rim in an arc that extends from the south-southwest to the west. Lobate ejecta here extends ~0.5 crater radii from the rim and terminates in steep margins (Figure 5c). Lobe front heights vary substantially, with height measurements at six points differing by nearly 900 m (Table 2). A narrower, less distinct form still exhibiting distal escarpments occurs at the opposite eastern extent.

Magritte's distal ejecta is also continuous, extending 1.0–1.7 crater radii from the rim in the directions of preferential deposition (south and west), and notably far less in a zone in the southwest. The area deficient in the distal ejecta consists of a V-shaped notch with the apex occurring at 0.2 crater radii from the crater rim (Figure 3b) centered around a bearing of 336°. This is accompanied by a lack of crater chains observable in this region. The
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Magritte crater. (a) Magritte crater and ejecta in enhanced color. (b) The same extent as (a) showing the colorized DEM. In frames (a, b), the rim of an underlying crater associated with Magritte (Magritte<sub>a</sub>) is outlined by a white dashed line labelled “a.” (c) Monochrome basemap of Magritte providing local context. The white and black arrows point to steep, well-defined lobate ejecta fronts and a secondary, less convincing lobate ejecta form respectively (the latter could arguably be mapped as normal proximal ejecta). White (d) and red (e) dashed boxes indicate the fields of view for subsequent frames. (d) NAC frame EN023161430M (160 m/pixel) showing the NE rim of Magritte crater and Magritte<sub>a</sub>. The yellow arrows point to the edge of perched very smooth material. Field of view ~110 km. (e) Close up view showing the interior of the peak-ring. Field of view ~55 km. The cyan arrows in (d, e) point to linear features: (1) a scarp-like, hilly feature; (2) grabens crosscutting the peak-ring center; and (3) sinuous grabens located in the position of the missing peak-ring portion.</figcaption></figure>
crater rim elevation at this azimuth is relatively low. This notch is consistent with an upr ange forbidden zone
resulting from a moderately oblique impact.

Magritte has a smaller (65 km diameter) underlying crater located beneath its lowest crater rim elevation in the northeast (designated Magritte<sub>a</sub>; Figure 5). Perched very smooth material we interpret as impact melt is located both in the interior and partially surrounds the exterior of Magritte<sub>a</sub>, situated contiguous to Magritte's rim with some apparently pooled atop the eastern edge of the well-developed lobe (Figure 5d). More perched impact melt can be found between Magritte's crater wall terrace structures. The impact melt patches differ from the smooth material on Magritte's floor as they are spatially confined and cover areas from 25 km<sup>2</sup> to 4 × 10<sup>5</sup> km<sup>2</sup> in size, whereas the smooth floor material covers an approximate area of 6 × 10<sup>6</sup> km<sup>2</sup>.

Magritte's crater wall in the southeast appears to have a mass wasting deposit in the form of an inward landslip, causing wall material to have slumped into the smooth crater floor. Additionally, directly opposite this in the northwest near the crater rim associated with Magritte<sub>a</sub>, there is a hilly linear formation of what we suspect to be crater wall material crosscutting the smooth crater floor. This is potentially an earlier inward landslip at the septum between Magritte and Magritte<sub>a</sub>, separated from the crater rim by lava from volcanic infilling.

Magritte differs significantly from all other H15 and Xiao and Komatsu (2013) candidates in that the ejecta lobes appear not to have flowed into a topographic low that in other cases is accommodated by a pre-existing crater, nor does it seem strongly influenced by regional slope. The location of Magritte<sub>a</sub> is not associated with Magritte's lobate forms and is situated 90° around the rim from Magritte's dominant lobe. That is not to say that the Magritte-forming impact occurred onto completely even topography: the underlying terrain comprises tracts of smooth, intermediate and intercrater plains, the latter being undulating to uneven in nature. These terrains practically exhibit no regional slope: the maximum slope is <3.5° in the direction of lobate ejecta deposition.

**4. Discussion**

**4.1. Timing of Formation**

Previously, there has been no definitive evidence to demonstrate whether lobate ejecta on Mercury was produced
during or after the impact event that formed the host crater (Xiao & Komatsu, 2013). Our interpretation that
perched very smooth patches, confidently identified on Nairne's lobes and tentatively on Magritte's, are impact
melt is based on their setting, texture, and color. That interpretation is consistent with impact melt deposition
predominantly late in the cratering process, toward the end of the crater modification stage (Osinski et al., 2011).
Thus, formation of the lobes must have occurred prior to this associated with either the crater excavation and
ballistic emplacement or early modification stage. The impact melt on Nairne's lobe shares a similar color
signature with its smooth crater floor, suggesting a similar composition and is consistent with synchronous
formation. The presence of an impact melt perched on Nairne and Magritte's lobate ejecta provides the first
evidence that the lobate ejecta formation must have occurred during the impact process and cannot be the result of
a mass wasting event that occurred after the impact.

4.2. The Influence of Topography on the Formation of Lobate Ejecta

Considering all lobate ejecta deposits in H15 (Figure 1) and previously documented occurrences (Barnouin et al., 2015; Xiao & Komatsu, 2013), all bar Magritte and Hokusai have formed by impact onto a pre-existing crater with deposition of the lobe into topographic lows accommodated by the pre-existing crater floors. This strongly implicates topography as a control on the formation process, with the specific topographical influence being an underlying crater.

The frequent but not exclusive association between the presence of a pre-existing crater and development of lobate ejecta means topography may allow for the distinction of lobate ejecta classes. Xiao and Komatsu (2013) categorized their small sample of lobate ejecta deposits into two morphological groups: those with sinuous margins and those with semi-circular, tapered, or tongue-like morphologies. Having now almost doubled the database of examples, we hypothesize that such morphologies are likely to represent a continuum where morphology is perhaps dependent on the local topography amongst other factors. A more suitable classification might consider the nature of the topography onto which the crater impacts, and the specifics regarding the emplacement scenario may differ accordingly. Thus, most candidates fall into a more typical “uneven topography” category, where the uneven topography is defined by a pre-existing crater creating a substantial slope to direct the course of material flow and
<table><caption>**Table 3**
Craters With Lobate Ejecta at Mercury and the Occurrence of Asymmetrical Morphologies and/or Characteristics Indicative of Formation by an Oblique Impact</caption><thead><tr><th></th><th>Asymmetrical ejecta distribution</th><th>Asymmetrical crater rim shape</th><th>Asymmetrical terracing</th><th>Topographic control</th></tr></thead><tbody><tr><td>Nairne (Figure 1a)</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Magritte (Figure 1b)</td><td>✓</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>Unnamed (Figure 1c)</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>MacNicol (Figure 1d)</td><td>✓</td><td>x</td><td>✓</td><td>✓</td></tr><tr><td>Lorde (Figure 1e)</td><td>✓</td><td>✓</td><td>x</td><td>✓</td></tr><tr><td>Unnamed (Figure 1f)</td><td>(x)</td><td>✓</td><td>x</td><td>✓</td></tr><tr><td>Hokusai</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>2A</td><td>✓</td><td>x</td><td>x</td><td>✓</td></tr><tr><td>2B</td><td>✓</td><td>✓</td><td>x</td><td>✓</td></tr><tr><td>2C</td><td>x</td><td>✓*</td><td>✓</td><td>✓</td></tr><tr><td>3A</td><td>x</td><td>✓*</td><td>x</td><td>✓</td></tr><tr><td>3B</td><td>x</td><td>✓*</td><td>✓</td><td>✓</td></tr><tr><td>3C</td><td>x</td><td>✓*</td><td>x</td><td>✓</td></tr><tr><td>3D</td><td>✓</td><td>x</td><td>✓</td><td>✓</td></tr></tbody></table>

Note. Ticks with asterisks indicate that, while the crater does exhibit asymmetry in shape, evidence points to this being a function of slope rather than obliquity. Bracketed results indicate that, due to size constraints, there is an absence of evidence for that type of asymmetry, and not evidence of absence. Lobate ejecta candidates of this study are highlighted.

basin-bound topographic lows to accommodate lobate ejecta deposition. Of this data set, only Magritte and Hokusai belong to a different category in which lobate ejecta develop by impact on less topographically variable ground. We contextually define this as an area with no pre-existing basins into which the lobate forms are directed, and little to no discernible slope. While the impactor forming Hokusai occurred onto remarkably smooth terrain (Hokusai is located within the Borealis Planitia region of the northern smooth plains, forming on <1° regional slope), Magritte demonstrates that this category may tolerate a moderate level of local terrain complexity. The fact that Hokusai and Magritte form on differing terrains likely reflects that the degree of slope has a greater influence on lobate ejecta formation than the amplitude or wavelength of the uneven terrain.

Not all morphological characteristics of craters with lobate ejecta are explained by pre-existing topography. For example, if Nairne's crater rim eccentricity was caused by the underlying crater, it could be expected that the diameter would be greatest at orientations parallel to the slope direction (Aschauer & Kenkmann, 2017; Plescia, 2012). Other examples of lobate ejecta (3B in Xiao & Komatsu, 2013) being the best example) exhibit the antithesis, with the smallest diameters in the downslope direction. In this instance, the craters do not approximate ellipses like that of Nairne. If they were elliptical, the ellipse's foci would align with its midpoint. However, what we observe are crater rims that are largely circular around much of the circumference with consistent radii, with an altered morphology only at the boundary overlapping the antecedent crater. The radius is shortened between the crater center and the overlapping crater rim and shortest when measured following the direction of slope and lobate ejecta flow. As a result, the midpoint of the crater's diameter in this orientation is offset from the center of the circle. The degree of radius shortening seems dependent on the degree of overlap between the two craters.

That topography cannot be the sole control on lobate ejecta formation is further demonstrated by the occurrence of such features throughout H15. Across the quadrangle, we counted 89 craters ranging in degradation state from c2 to c5 that impact on to the rim of a pre-existing crater but did not produce lobate ejecta. We excluded c1 craters because, being heavily degraded, the underlying topography is often difficult to ascertain and these craters have rarely preserved ejecta. There is a high likelihood that even if lobate ejecta were an original feature, the ejecta would by now be overprinted. Just 6.3% of eligible craters that impact onto a pre-existing crater rim produce a lobate ejecta form in H15. Thus, while we confirm a strong correlation between formation and pre-existing topography, topography alone cannot be the controlling factor. To that end, we explore another option that may control the formation of lobate ejecta, the influence of an oblique impact.
4.3. Obliquity and Direction of Impact

Some morphological characteristics of lobate ejecta at H15 indicate formation by an oblique impact in which the
angle between the impact vector and the surface is less than the most probable angle of 45° (Gilbert, 1893;
Shoemaker et al., 1962). Listed in Table 3, we explore asymmetrical ejecta distribution, crater rim morphology
and terracing as potential criteria for oblique impact identification. Additional observable asymmetries found at
H15 include changes in wall slope and rim relief (Gault & Wedekind, 1978; Schultz, 1992), the size and symmetry
axis of central peak material (Schultz, 1992), and the apparent volume and placement of impact melt (Collins
et al., 2011; Gault & Wedekind, 1978; Hawke & Head, 1977; Howard & Wilshire, 1975). Though impossible to
decipher for craters exhibiting circularity and radial symmetry, these elements of asymmetry may help direct
identification of azimuthal impact for our case studies.

Nairne's morphology alludes to an impact angle in the range of 20–40° from the surface. Crater ellipticity de-
velops below impact angles of 45°, as does variations in the axial symmetry of ejecta distribution (Gault &
Wedekind, 1978). Nairne exhibits a dearth of ejecta at eastern and western azimuths. Preferential deposition of the
proximal ejecta (lobate ejecta) in south/southwest is likely to be controlled by topographical slope. Distal ejecta
deposition preferentially in the north/northeast accompanied by a lack of crater chains in primarily northern
azimuths may be symptomatic of a roughly south-north projectile path.

Due to infilling at Magritte, the size and symmetry axis of the peak-ring material and any asymmetry in terracing
are impossible to discern: Ekholm and Melosh (2001) demonstrate that significant volcanic infilling can displace
the apparent crater center, overprinting original wall-slope or central peak asymmetries. However, the asymmetric
distribution of Magritte's ejecta may indicate an impact angle less than 45°. We suggest an impact angle of 30–
40°, sufficiently oblique to exaggerate the azimuthal zones of limited ejecta distribution while maintaining a
radially symmetrical rim. Depleted ejecta in the southeast with a line of bilateral symmetry running southwest to
northeast suggests a southwest to northeast trajectory. This locates the proto-forbidden zone in the proposed
uprange direction and offsets the ejecta blanket downrange.

Aside from the case studies, the majority of the other examples in H15 exhibit subtly eccentric crater rims (c, e, and f in Figure 1), though none are as exaggerated or as obviously elliptical as Nairne. All have impacted onto or share a boundary with a pre-existing crater, and often their shortest radius is in the direction of flow of the lobate ejecta. Those candidates where the change in crater rim morphology is differently oriented than that of the change is slope and so not associated with the overlap between superposing and antecedent craters are more convincing indicators of having formed by an oblique impact. For H15 candidates that exhibit asymmetrical ejecta distribution, this tends to manifest in ejecta extension at azimuths associated with the lobate ejecta and the back azimuth, but is notably lacking at perpendicular orientations. While topography can influence the ejecta deposition pattern, this is commonly evidenced by a decreased volume or absence of ejecta deposited at higher elevations (Aschauer & Kenkmann, 2017), which is the back azimuth in the case of lobate ejecta. Most of the smaller candidates do not differentiate between proximal and distal ejecta or the only ejecta preserved is the lobate deposit, as is the case for the particularly small unnamed crater in Figure 1f. Finally, c, d, and f all show subtle asymmetries in terracing, whereby the back azimuth to the crater rim associated with the lobate ejecta has wider terraces.

We found similar asymmetries when we reanalyzed the Xiao and Komatsu (2013) examples. These tend to be
noncircular, often with the shortest axis being in the direction of extension of the lobate ejecta (2C, 3A, 3B, 3C).
The same examples exhibit asymmetry in terracing with a poorly developed terrace and narrow crater wall at
azimuths associated with lobate ejecta. It is possible that this reflects increased inward wall slumping at azimuths
associated with higher elevations, opposite azimuths to those associated with the lobate ejecta. As such, of the
Xiao and Komatsu (2013) candidates, only the asymmetries exhibited by 2A and 3D may be linked to obliquity.
Table 3 compares the asymmetrical morphologies of all H15, Xiao and Komatsu (2013) and Hokusai craters.

At Nairne, we suggest that a combination of sloping topography and an oblique impact might together have
caused lobate ejecta to form. At Magritte, the influence of topography appears negligible, but there is consistent
evidence of formation by an oblique impact. Ostensibly, Magritte's closest morphological analogue is Hokusai.
Aside from previously discussed parallels associated with topography, Hokusai and Magritte share a number of
similarities in asymmetry: both have radially extensive and asymmetric continuous ejecta, lobate ejecta that
spreads over a wide range of azimuths and terminates in steep margins, a partially complete peak-ring and
asymmetric terracing. Being young (late-Kuiperian in age (Ernst et al., 2018) (<230 Ma (Banks et al. (2017))),
Hokusai has preserved its impact ray system, the asymmetrical nature of which has provided evidence for an oblique impact angle of around 30–40° following a northeast to southwest trajectory (Ernst et al., 2018; Xiao et al., 2016). Obliquity is also attributed to the shallow nature of Hokusai, which achieves smaller rim crest to floor depths than similarly sized or even smaller craters (Barnouin et al., 2015).

Due to the pervasive evidence for oblique impacts, we hypothesize that low impact angles may act as a stimulus to lobate ejecta at Mercury. This is not a control on formation explored by Xiao and Komatsu (2013), and we recognize that the asymmetries in their examples are not unequivocally caused by oblique impact or topography. Morphological features at Nairne, Magritte and Hokusai, however, convincingly substantiate an oblique impact and have allowed for estimations of impact trajectories.

## 4.4. Formation Mechanisms for Lobate Ejecta: Clues From Other Planetary Bodies

Due to the prevalence of analogous features across planetary bodies of the inner solar system, drawing comparisons may provide opportunities for investigation into alternative influences on formation at Mercury, other than that of topography and impact obliquity.

Mars was previously considered quite distinct from both the Moon and Mercury because of its variety in ejecta morphologies and propensity for ejecta with lobate or layered morphologies (single-/layered ejecta (S/LE); Li et al., 2015; Barlow & Perez, 2003). Particularly, SLE are comparable to our six examples in that both exhibit a lobate ejecta blanket formed by additional processes other than solely ballistic emplacement. That said, SLE is typically associated only with craters smaller than 20 km that form on flat ground (Barlow & Perez, 2003); the LE tends to radially surround the crater; and ejecta margin heights are lower but exhibit a raised terminal rampart. With a variety of runout distances, SLE produces a range of EM values, with different studies finding an average EMs of 1.18 (Weiss & Head, 2014), 1.26 (Li et al., 2015) and 1.53 (Barlow, 2005) compared to mercurian averages of ~0.95 in this study and ~1.10 in Xiao and Komatsu (2013). These martian LEs are thought to have been in a fluidized state during emplacement (Carr et al., 1977; Gault & Greeley, 1978; Mouginis-Mark, 1979). Their final morphology is explained by either impact into and vaporization of sub- or near-surface volatile reservoirs (Carr et al., 1977; Mouginis-Mark, 1987; Stewart et al., 2001) or by entrainment of Mars' atmosphere into the ejecta (Barnouin-Jha & Schultz, 1998; Schultz & Gault, 1979) or by some combination of the two.

At Mars, the volatile acting as fluidization agent is understood to be water, as ice or vapor. At Mercury, radar-bright features that are confined to large, permanently shadowed north and south polar craters provide evidence for surface water ice (Chabot et al., 2012; Slade et al., 1992), and additional small-scale deposits are postulated to exist inside microcold traps associated with rougher terrains mapped only in northern polar latitudes due to MESSENGER's periherm only ever being in high northern latitudes (Mcadams, 2003). However, we do not suggest water as a fluidizing agent for our Mercury examples. The H15 examples show no correlation with permanently shadowed craters. Moreover, neither Hokusai, the examples from Xiao and Komatsu (2013), nor additional examples in the global survey by Blance et al. (2024) are restricted to polar regions. If volatiles are implicated for Mercury, they must instead originate as moderately volatile minerals present within the wider crust. We have found no strong association between lobate ejecta and other indications of volatile content. This does not rule out volatiles playing a role in the formation of lobate ejecta on Mercury, but there is no observational evidence yet that indicates that they do.

An alternative fluidizing agent could be impact melt; Venusian flow-like ejecta termed “outflows” are considered to have formed from large volumes of melt produced by the crater-forming event (Basilevsky & Head, 2003). At Mercury, a layer of impact melt some 200–400 m thick is postulated to have pooled at Hokusai's smooth crater floor (Barnouin et al., 2015), providing convincing evidence for excess impact melt. Despite differing hypotheses for melt generation mechanisms (Barnouin et al., 2015; Kinczyk et al., 2018), this increased volume of impact melt has been proposed as a possible fluidizing agent for its ejecta flow, involving intermixing of the melt and solid ejecta (Barnouin et al., 2015). Barnouin et al. (2015) described Hokusai's ejecta as morphologically identical to SLE on Mars, even exhibiting the characteristic moat separating its flow margin from the continuous ejecta, which is not shown by the H15 or Xiao and Komatsu (2013) examples.

Finally, Xiao and Komatsu (2013) draw comparisons to the lunar far side of Tsiolkovskiy crater and its associated lobate ejecta-like feature (termed the Tsiolkovskiy landslide e.g. Boyce et al., 2020), reporting similar EM values of 1.22. The Tsiolkovskiy crater was formed by an oblique impact (Guest & Murray, 1969) on the rim of the
underlying Fermi crater, with material partially coinciding with the Fermi crater floor despite its situation pre-
dominantly within Tsiolkovsky's forbidden zone (Morse et al., 2018; Wilhelms et al., 1987). Though debated, the
current consensus is that Tsiolkovskiy's lobate ejecta-like feature formed by dry granular flows, which required no
volatile involvement (Barnouin-Jha et al., 2005; Boyce et al., 2020), making Tsiolkovskiy one of just two major
lunar long runout landslides (the other being the Light Mantle avalanche on the nearside (El-Baz, 1972; Mag-
narini et al., 2021)). Tsiolkovskiy's landslide was probably caused by the over-steepening and subsequent collapse
of the northwest rim of the crater, the flows themselves stimulated by low friction between the excavated material
and the surficial geology (Aschuaer & Kenkmann, 2017; Boyce et al., 2020). The timing of formation of the
landslide relative to the crater origin has not been conclusively determined, but in their initial geomorphological
mapping of the Tsiolkovskiy crater, Guest and Murray (1969) suggested that the landslide occurred syn-impact,
and crater counting by Boyce et al. (2020) revealed that there could have been only a short time between the
impact and landslide emplacement, setting an upper limit of a few seconds to thousands of years.

Formation mechanisms responsible for producing features analogous to lobate ejecta on Mars, Mercury and the Moon offer two options for emplacement scenarios: one that requires some means of fluidization of the ejecta flow, and one that does not. While both possible scenarios remain feasible for the formation of lobate ejecta within H15, the lack of evidence for volatile- or impact melt-induced flow makes fluidization via these mechanisms unlikely. Additionally, the correlation between lobate ejecta and oblique impacts, and the presence of perched impact melt pooled on top of ejecta lobes, suggest that flows are directly impact-generated. As such, we suggest a dry granular flow that occurred syn-impact (rather than thousands of years post-impact) to be the most reasonable option.

4.5. Expanding the Data Set of Lobate Ejecta at Mercury

If the previously reported occurrences of lobate ejecta were representative of their global distribution, our six
additions confined to H15 alone would seem anomalously high. Blance et al. (2024) have shown that there are
many examples of lobate ejecta found globally absent from the Xiao and Komatsu (2013) database. However,
should oblique impacts into sloping topography provide the most favorable conditions for lobate ejecta deposits to
develop, concentration of such features at Mercury's polar regions may well be expected given that the poles of
planetary bodies tend to experience fewer steep or near vertical impacts (Gault & Wedekind, 1978).

**5. Conclusions**

We report six previously undocumented examples of lobate ejecta in Mercury's south polar quadrangle (H15). Our observation of perched impact melt pooled on top of lobate ejecta, best observed on the larger of the two lobes at Nairne, provides the first evidence for syn-impact formation of ejecta lobes, so clearly not all lobate ejecta features were formed post-deposition as mass wasting or landslide deposits, and it is possible that all or most other examples formed equally soon. This will be testable when higher resolution imaging and better spectroscopic and other compositional observations can be made using BepiColombo (e.g., Rothery et al., 2020).

Our results corroborate the influence of pre-existing topography on the formation of many lobate ejecta deposits, but we also show that this cannot be the only controlling factor: not all impacts onto antedating craters produce lobate ejecta. We also show that oblique impact geometries are commonly associated with lobate ejecta. Although we cannot rule out a potential role for volatiles, we have found no evidence to suggest that they play a key role. Comparisons to analogs constrain the emplacement scenario to either a fluidized ejecta flow or a dry granular flow in which there was practically no time between the impact event and mass movement. We conclude that lobate ejecta on Mercury is far more prevalent than previously thought. Future studies could help clarify exactly how common they are at Mercury and better constrain the mechanisms and timing of formation.

**Conflict of Interest**

The authors declare no conflicts of interest relevant to this study.

**Data Availability Statement**

The data used for mapping in the study are available for download from the United States Geological Survey (USGS) data portal "Astropedia: Lunar and Planetary cartographic catalog" and accessed on https://astrogeology.usgs.gov/search?target=&system=&p=1&accscope=&searchBar=. Additional images were obtained from the
Planetary Image Locator Tool (PILOT) v4 using Projection on the Web using the free, open-source software Integrated Software for Images and Spectrometers (ISIS) for processing (Hare et al., 2016). PILOT can be accessed at https://pilot.wr.usgs.gov/ and information on ISIS available at https://isis.astrogeology.usgs.gov/8.2.0/. All mapping and figure making was conducted on ArcGIS Pro version 3.1.3.

## Acknowledgments

A.R.L. and A.J.B. are funded by a UK Research and Innovation Doctoral Training Partnership and formerly the Space Strategic Research Area at the Open University. JW acknowledges support from the European Space Agency (ESA) as an ESA Research Fellow. JW also acknowledges additional support from the Open University as a Visiting Fellow. S.J. C. is grateful for the support of the French Space Agency, CNES, for her BepiColombo-related work.

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