# Lobate Forms Around Craters on the Moon and Mercury: Origin From Landslides, Ejecta Flows and Modification Stage Collapse

A J Blance, A R Lennox, D A Rothery, M. Balme, J. Wright, V. Galluzzi,

Susan J. Conway

► To cite this version:

A J Blance, A R Lennox, D A Rothery, M. Balme, J. Wright, et al., Lobate Forms Around Craters on the Moon and Mercury: Origin From Landslides, Ejecta Flows and Modification Stage Collapse. Journal of Geophysical Research. Planets, 2025, 130 (9), 10.1029/2025je008980. http://dx.doi.org/10.5199/jgr-p-2025-05374856

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

## RESEARCH ARTICLE

10.1029/2025JE008980

**Key Points:**

* We find ~100 craters on both the Moon and Mercury with exterior lobate forms indicating significant flow, more than previously recognized

* Most examples extend downslope from their source crater into an antecedent crater, and are directly linked to source crater formation

* We suggest origins by exterior modification stage collapse (~71%), impact-triggered landslides (~24%), and flow of ejecta (~5%)

**Supporting Information:**

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

**Correspondence to:**

A. J. Blance,
alistair.blance@open.ac.uk

**Citation:**

Blance, A. J., Lennox, A. R., Rothery, D., Balme, M., Wright, J., Galluzzi, V., & Conway, S. J. (2025). Lobate forms around craters on the Moon and Mercury: Origin from landslides, ejecta flows and modification stage collapse. *Journal of Geophysical Research: Planets*, 130, e2025JE008980. https://doi.org/10.1029/2025JE008980

Received 27 JAN 2025

Accepted 15 AUG 2025

**Author Contributions:**

**Conceptualization:** A. J. Blance,
D. A. Rothery

**Data curation:** A. J. Blance

**Formal analysis:** A. J. Blance

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

**Methodology:** A. J. Blance,
D. A. Rothery, M. Balme, J. Wright

**Project administration:** A. J. Blance

**Supervision:** D. A. Rothery, M. Balme, J. Wright, V. Galluzzi

**Validation:** A. R. Lennox

**Visualization:** A. J. Blance, A. R. Lennox, D. A. Rothery, M. Balme, J. Wright, V. Galluzzi, 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.

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# Lobate Forms Around Craters on the Moon and Mercury: Origin From Landslides, Ejecta Flows and Modification Stage Collapse

A. J. Blance¹ ID, A. R. Lennox¹, D. A. Rothery¹ ID, M. Balme¹ ID, J. Wright¹,² ID, V. Galluzzi³ ID, and S. J. Conway⁴ ID

¹School of Physical Sciences, The Open University, Milton Keynes, UK, ²School of Physics and Astronomy, University of Leicester, Space Park Leicester, Leicester, UK, ³Istituto Nazionale di Astrofisica (INAF), Istituto di Astrofisica e Planetologia Spaziali (IAPS), Roma, Italy, ⁴Nantes Université, University Angers, Université du Mans, CNRS UMR 6112 Laboratoire de Planétologie et Géosciences, Nantes, France

**Abstract** Lobate forms emanate from impact craters and have morphologies indicating ground-hugging emplacement via flow. We present global surveys of lobate forms on the Moon and Mercury. Very few examples have previously been reported on either body, with debate over their interpretation as either crater rim landslides or ejecta flows. We identify 92 lobate forms on the Moon, and 98 on Mercury, a significant increase on previously reported examples. 98% extend downslope into an adjacent or overlapped antecedent crater. Lobate forms are directly linked to impact cratering processes: evidence for syn-impact formation, including impact melt on top of lobate forms, suggests ~71% formed due to exterior collapse of source crater material into antecedent craters during source crater modification. 24% of examples instead appear to be landslides on antecedent crater rims, triggered by the newer impact. Understanding the formation of these features therefore provides new insight into impact cratering and mass movements processes. Equivalent lobe mobilities and occurrence rates on the volatile-depleted Moon and the somewhat volatile-enriched Mercury suggest volatiles are not necessary for formation. However, lobate form mobilities on the Moon and Mercury are lower than for comparable martian features, thought to require ground-ice to form, potentially supporting the idea that very high volatile content enables more mobile mass movements. Two lobate forms on flat ground (Hokusai and Piazzolla craters, Mercury) are morphologically similar to martian layered ejecta, possibly indicating a more significant influence of volatiles for these examples or a more enigmatic formation process.

**Plain Language Summary** The surfaces of planets, moons and asteroids across the Solar System are strewn with impact craters, which are circular depressions formed by collisions with smaller objects. Fragmented rock debris is ejected from craters as they form; propelled upwards by the energy of the collision it then lands in the area surrounding the crater. This “ejecta” can be identified around craters using images of planetary surfaces, collected remotely by spacecraft. We have identified 178 craters on the Moon and Mercury that have “lobate forms” around them, exterior flows of debris which contrast with typical ejecta on these bodies. These flows can be tens of kilometers long. Previously a few lobate forms had been identified on these bodies, and it was suggested that they may be later landslides, since most extend downslope into neighboring craters. However, we identify many more, and suggest novel origins, highlighting evidence for formation during the initial impact event. We believe lobate forms are an underexplored component of impact cratering, and that understanding their formation provides a fuller picture of cratering on planetary bodies. Additionally, we suggest that unlike similar features on Mars, where ice may enhance flow, lobate forms on the Moon and Mercury form dry.

## 1. Introduction

Impact craters are ubiquitous on planetary surfaces across the Solar System. On airless and relatively volatile-poor bodies, for example, the Moon or Mercury, craters are typically surrounded by continuous ejecta deposits, which thin gradually away from the crater rim. However in some cases, craters have exterior lobate forms, whose lobate shapes, steep margins, and deposit thicknesses that are maintained away from the crater rim differ from typical continuous ejecta (e.g., Barlow, 2005; Barnouin et al., 2015; Boyce et al., 2010; Carr et al., 1977; Guest & Murray, 1969; Maloof et al., 2010; Strom et al., 1981; Sturm et al., 2013; Xiao & Komatsu, 2013).
Writing – original draft: A. J. Blance

Writing – review & editing: A. J. Blance,
A. R. Lennox, D. A. Rothery, M. Balme,
J. Wright, V. Galluzzi, S. J. Conway

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Examples of lobate forms across the Solar System. Lobe boundaries indicated by yellow arrows. (a) Mars—single layered ejecta crater, 23.6 N, 101.7 E, CTX mosaic, crater 11 km across. (b) Ceres—landslide on a crater rim, 58.035°N, 85.247°E, Dawn LAMO mosaic, crater 10 km across. (c) The Moon—Tsiolkovskiy lobate form, 20.25°S, 128.94°E, Lunar Reconnaissance Orbiter Camera Wide Angle Camera mosaic, crater 185 km across. (d) Mercury—Hokusai crater lobate form, 58.3°N 342.3°W, Messenger MDS BDR mosaic, crater 95 km across.</figcaption></figure>

Material excavated during crater formation is initially ejected on ballistic trajectories, then lands in the area surrounding the site of impact. It retains radial momentum on landing and experiences ground flow, incorporating secondary material from the local substrate. This process emplaces continuous ejecta deposits (Hörz, 1982; Hörz et al., 1977, 1983; Maloof et al., 2010; Oberbeck, 1975; Osinski et al., 2011). In the case of the Moon, remote sensing provides evidence for radial flow within continuous ejecta from small-scale features dispersed throughout the deposit, including dunes, interfingered flow lobes and flow deceleration features (Oberbeck, 1975). In this study we consider a subset of craters which have extensive lobate forms surrounding a significant proportion of their circumference (up to 360°), in which large sections or entire layers of material around a crater have steep lobate margins. The morphology of these features may indicate a more significant flow event than typical continuous ejecta deposits (Barlow, 2005; Oberbeck, 2009; Osinski, 2006; Xiao & Komatsu, 2013). These lobate forms are distinguishable from lobes of impact melt, which tend to be smaller relative to the crater size, and have characteristic properties such as a relatively low albedo and visible surface fracturing (Bray et al., 2010; Howard & Wilshire, 1975; Neish et al., 2014).

On Mars, lobate forms around impact craters are common. Layered ejecta structures frequently surround fresh craters, consisting of one or several lobate layers of material, each layer terminating in a steep margin or raised rampart (Figure 1a; Barlow, 2005; Barlow et al., 2000; Carr et al., 1977; Schultz & Gault, 1979). Thought to be primary features of the impact crater, the layers are interpreted as flow deposits of fluidized ejecta. They occur on
flat ground around craters, in addition to extending into topographic lows (e.g., Weiss & Head, 2018). Various mechanisms have been proposed to explain fluidization of martian ejecta, but most implicate volatiles in near-surface materials, particularly water from ground ice (e.g., Carr et al., 1977; Costard, 1989; Li et al., 2015; Wohletz & Sheridan, 1983). An alternative proposal is that the martian atmosphere gets incorporated into ejecta movements (e.g., Schultz & Gault, 1979). Material around terrestrial craters has similarly been interpreted as deposits of fluidized ejecta, with volatile components or the terrestrial atmosphere suggested as fluidizing agents (Kenkmann & Schönian, 2006; Sturm et al., 2013).

Other lobate forms around craters are suggested to be landslides. Lobes identified on both Mars and Ceres (Figure 1b) appear to be landslides with distinct source failure scarps (Crosta et al., 2018; Duarte et al., 2019). These examples extend downslope from failure areas on newer crater rims, into older, adjacent craters.

The variety of potential origins for lobate forms has led to these features being referred to using genetic terminology, for example, ejecta flow or landslide. While such terms may be reasonable in certain cases, some features have less obvious origins, so a morphologically defined descriptor is more appropriate. For this reason, we refer to these features as lobate forms, or lobes, throughout this work.

On airless or relatively volatile-poor rocky bodies, few lobate forms have previously been documented. The lunar crater Tsiolkovskiy has a lobate form extending into an adjacent antecedent basin (Figure 1c; Boyce et al., 2020; Guest & Murray, 1969; Magnarini et al., 2021; Morse et al., 2021), and 13 examples on Mercury have been reported (Lennox et al., 2025; Xiao & Komatsu, 2013), the majority extending down-slope into antecedent craters. Hokusai crater on Mercury, in contrast, has a lobate form on flat ground surrounding the entire crater (Figure 1d; Barnouin et al., 2015). The origin of lobate forms on these bodies is contentious: some authors suggest they result from flow within ejecta material (e.g., Guest & Murray, 1969; Howard, 1973; Morse et al., 2018; Xiao & Komatsu, 2013), but in the absence of significant volatiles or an atmosphere to fluidize ejecta, others have argued that they are landslides, a conclusion consistent with most known examples extending downslope into antecedent craters (e.g., Boyce et al., 2020; El-Baz, 1972; Magnarini et al., 2021; Morse et al., 2021). The lobate form on flat ground surrounding Mercury's Hokusai crater seems unlikely to be a landslide, however, and an ejecta flow interpretation is preferred (Barnouin et al., 2015). Mercury has evidence for abundant moderately volatile elements in its near-surface materials (e.g., Blewett et al., 2011; McCubbin et al., 2012; Nittler et al., 2011; Peplowski et al., 2011; Weider et al., 2016), which could potentially fluidize ejecta. Landslides may also be caused or enhanced by volatile involvement. For example, on Earth, water saturation can trigger landslides (e.g., Iverson, 2000), and volatiles may extend the runout of landslides on Earth and Mars (Goguel, 1978; Harrison & Grimm, 2003; Legros, 2002; Lucchitta, 1978, 1979).

Until now, research into lobate forms on the Moon and Mercury has been limited by sample size, with few reported examples and a particular focus on Tsiolkovskiy crater on the Moon. Despite their enigmatic origins, there has been little investigation into the global population of lobate forms on either body. Here we present the first comprehensive survey of lobate forms on the exteriors of craters on the Moon and Mercury. These bodies provide good points of comparison with lobate forms found on volatile-rich bodies such as Mars or the Earth, as the Moon is volatile-poor, whereas Mercury is between these extremes with some volatile content. We assess the prevalence of lobate forms, characterize their morphology, investigate potential origins, and compare the lobate forms on the two bodies and similar features elsewhere in the Solar System. We aim to investigate how planetary characteristics might affect the development of lobate forms, and to determine what information lobate forms provide about the influence of impact cratering on mass movement processes.

## 2. Methods

### 2.1. Data

We analyzed data sets in a GIS (Geographical Information Systems) environment using ArcGIS Pro 2.9.5 software. Mercury data included: the MESSENGER Mercury Dual Imaging System (MDIS) (Hawkins et al., 2007) monochrome 166 m/pixel Basemap Reduced Data Record (BDR), the Low Incidence Angle Basemap (LOI), the High Incidence Angle Basemap Illuminated from the East (HIE) and the West (HIW), and the 655 m/pixel Basemap Enhanced Color Global Mosaic (Denevi et al., 2018). Additionally the 665 m/pixel United States Geological Survey (USGS) Global Digital Elevation Model (DEM) (Becker et al., 2016), and the 221.7 m/pixel 30 m vertical accuracy DLR DEM for available areas (Preusker et al., 2017). Lunar data used
included: the Lunar Reconnaissance Orbiter Camera (LROC) 100 m/pixel Wide Angle Camera (WAC) Global Morphology Mosaic (Speyerer et al., 2011), the 118 m/pixel Moon Clementine UVVIS Global Mosaic (Lee et al., 2009), the 100 m/pixel 10 m vertical accuracy LROC WAC DTM GLD100 (Scholten et al., 2012), and the 118 m/pixel LRO LOLA DEM (Neumann et al., 2011). For both bodies, we used USGS ISIS3 to process individual images when full resolution was required, including MESSENGER MDIS Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) frames, and LROC NAC frames. In addition, we analyzed NAC mosaics using LROC QuickMap online (https://quickmap.lroc.asu.edu).

## 2.2. Survey Methodology

The primary basemap used for surveying Mercury was the MDIS BDR Mosaic, whereas for the Moon we used the LROC WAC Morphology Mosaic, resampled to match the 166 m/pixel resolution of the Mercury mosaic. We used other data sets listed in Section 2.1 for investigation of identified lobate forms, or occasionally to survey an area obscured by shadows in the primary basemap. Although the resolutions of the basemaps were matched, we note that the overall quality of the lunar mosaic is superior due to higher original data resolution, and more consistent illumination conditions.

We completed the global surveys by manual searching and observation, using grid squares to search the whole globe by smaller areas. We identified lobate forms using the following selection criteria: (a) lobe shaped deposits extending out from a crater, with a proximal boundary immediately adjacent to the crater rim or on its flank, (b) deposits appear to maintain thickness over some distance from the source crater, terminating in a steep distal margin. We focused search efforts on craters larger than 10 km in diameter due to basemap resolution limits and survey scale, and consider this the lower limit of crater size which was comprehensively surveyed. However, we did note some lobate forms around craters smaller than this, down to 3.5 km in diameter, and include these in our analysis.

We classified lobate forms according to whether they extended into antecedent craters, and by the presence and location of any source scarps. Distinguishing features that extend into antecedent craters is useful for assessing the topographic context of lobate forms. This classification is typically clear-cut and is important for investigating the origin of these features. Classifying by the presence and location of source scarps likewise gives information on lobe emplacement and was also typically a clear-cut decision, barring a few gradational examples.

## 2.3. Analysis of Lobate Forms

### 2.3.1. Source Crater Asymmetries

To investigate the potential influence of impact angle and antecedent topography in the development of lobate forms, we assessed source craters for asymmetries. Both crater shape and ejecta distribution are indicators of impact angle (Bottke et al., 2000; Gault & Wedekind, 1978; Herrick & Forsberg-Taylor, 2003), but antecedent topography also influences crater shape and structure (e.g., Aschauer & Kenkmann, 2017; Gulick et al., 2008; Włodarski et al., 2017). Below ~45° incidence angle from horizontal, impacts begin to have asymmetric ejecta distributions (Gault & Wedekind, 1978; Herrick & Forsberg-Taylor, 2003; Pierazzo & Melosh, 2000). For lunar impacts <25°, ejecta deposits become minimal in the up-range impact direction, resulting in an ejecta “forbidden zone” (Herrick & Forsberg-Taylor, 2003). Below ~5–12° impacts start to produce elliptical craters, with the major axis aligned to the impact azimuth (Bottke et al., 2000; Gault & Wedekind, 1978). Equivalent asymmetries are observed for craters on Mercury (Gault & Wedekind, 1978). Thus, ejecta distributions and crater shape can be used to estimate the angle of impact and its azimuth. We mapped ballistic ejecta around source craters and assessed for ejecta forbidden zones or preferred directions where possible. Crater shapes were also assessed for asymmetries. These results were then compared to the orientation of lobate forms around the source crater.

### 2.3.2. Lobate Form Runout and Mobility

The geometry of lobate forms can be quantified as an aid to understanding formation mechanisms. The runouts of mass movements are used to derive mobility parameters, which indicate the efficiency of material movement. We measure maximum runout length as the longest horizontal distance from the crater rim to the lobate form edge, or (if applicable) from an identified source scarp to the lobe edge. We measure this radial to the source crater, which corresponds closely to the emplacement direction for lobate forms. This measurement provides a basis to
calculate what we term lobe mobility, the ratio of lobe runout length divided by the source crater radius. This is analogous to ejecta mobility used in layered ejecta studies (e.g., Boyce et al., 2010; Mouginis-Mark, 1979). Conversely, in landslide studies, mobility usually refers to the ratio of horizontal to vertical distance traveled by a landslide. In terrestrial landslides, this is typically the distance traveled by the landslide's center of mass, though using remote sensing data, the maximum horizontal and vertical distances ($H_{max}$ and $L_{max}$) from source to lobe edge are easier to measure. We have estimated this parameter where DEM resolution permits reliable $H_{max}$ measurements, and refer to it as $L/H$, a common term, to avoid confusion with lobe mobility based on source crater size. Limits on DEM resolution leads to an underrepresentation of smaller examples for this measure. Available Mercury DEMs also have poorer resolution overall than lunar DEMs, so Mercury examples are likewise underrepresented.

### 2.3.3. Lobate Form Volume

Runout lengths and mobilities of mass movements are known to correlate with their volume, with trends that depend on mass movement type (Legros, 2002) and the planetary body on which they occur (McEwen, 1989). To estimate the volume of lobate forms, we used DEMs and shadow measurements to estimate their thickness, then multiplied this by the area of the lobe. The area measurement is typically well constrained, so the credibility of the volume estimate depends on the accuracy of the margin thickness measurement, and the consistency of thickness across the lobe's area. For lunar lobes, the average margin thickness can be reliably determined using comparatively higher resolution DEMs. For Mercury, DEM resolution is poorer, with less vertical accuracy, so only larger lobe margins can be estimated in this way. Some smaller lobe margins in areas of Mercury covered by the DLR DEMs can also be estimated, as this DEM is at higher resolution, but it still has lower vertical accuracy than the lunar DEMs (30 vs. 10 m). To include more mercurian examples, we used shadow measurements for lobes where possible, using illumination angles and trigonometry to estimate lobe margin heights from their shadow length. Details are in the supporting information.

The consistency of a lobe's thickness is likely the greatest uncertainty in the volume estimate. Based on their morphology (Section 3.2), lobes frequently appear fairly consistent in thickness throughout much of their length, but can have significant raised sections near their source. Terrestrial landslides are often observed to have an increased thickness near their source (De Blasio, 2011), as do examples on Mars, where volume estimates based on frontal thickness may be up to 7 times smaller than the true volume (Legros, 2002; McEwen, 1989). The converse risk, of overestimating lobe volume, would apply if lobe margins were thicker than the rest of the deposit. Layered ejecta on Mars has significant ramparts at its margin, with low-lying "moats" behind. We identify this distinct morphology for only two lobes in our survey (Section 3.2.3), but in the remainder of examples the distal margin is not significantly raised compared to the lobe interior. When considering these morphological observations together, it seems probable that volume estimates in this survey represent minimum values.

## 3. Results

We identified a total of 190 lobate forms around 178 craters, 98 on Mercury and 92 on the Moon (Figures 2 and 3; Table 1; Table S1 in Supporting Information S1). Most craters with lobate forms have one distinguishable example, though 12 craters have two separable lobate forms. Previously, just 14 lobate forms have been reported on Mercury (Barnouin et al., 2015; Lennox et al., 2025; Xiao & Komatsu, 2013), so this study expands the catalog seven-fold. For the Moon, only sporadic reports have been made (e.g., Guest & Murray, 1969; Howard, 1973; Magnarini et al., 2021; Sruthi & Kumar, 2014), but we identify numerous features not found in the literature. Hence, our catalog reveals lobate forms are more common on both bodies than previously realized.

We find that almost all lobes (98%) enter adjacent or overlapped antecedent craters that constitute areas of low topography around the newer source crater. Four examples are instead found on relatively flat ground (<4°) around their source crater, not extending into an identified antecedent crater. Most lobate forms extend only partially around their source craters, generally around 10%–40% of the circumference, usually entirely restricted to within an antecedent crater. A few exceptions occur for lobate forms on flat ground (e.g., Figures 1d and 3f) or lobes extending into antecedent impact basins much larger than their source crater. Scarps are commonly identified at the source of the lobate form (83%). No apparent self-secondary impacts from the source crater are identified on top of lobate forms, whereas self-secondary craters and ballistic ejecta textures are commonly
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Overview of lobate form survey results, dots indicate location of identified lobate forms sourced at craters. Mercury—MDIS BDR mosaic basemap, Moon—LROC Morphology mosaic basemap. Blue dots: category 1—with source scarp and entering antecedent crater, Pink dots: category 1a—source scarp on antecedent crater rim, Green dots: category 2—no source scarp and entering antecedent crater, Yellow dots: category 2a—isolated lobe entering antecedent crater, Red dots: category 3—on flat ground, not entering antecedent crater.</figcaption></figure>

identified beyond the distal margins. For 15 lobate forms, we identify dark, smooth and flat patches of material in local topographic lows on the lobe (we interpret these as patches of impact melt, see Section 4.2).

### 3.1. Properties of Source Craters

We identify lobate forms around craters 3.5–206 km in diameter, though 85% are in the 15–80 km range. They are typically complex craters, with only 19% of lobes around simple craters. On both bodies the source craters are usually fresh in appearance. Referring to a 5-class degradation system in which c5 are the freshest examples and c1 the most degraded (Kinczyk et al., 2020), most Mercury lobes are found around c4–c5 craters with some c3. The degradation state of the lobate forms themselves are indistinguishable from their source craters, based on the distinctness of boundaries, “crispness” of topographic features, morphological texture preservation, and brightness.

For craters with lobate forms, we identify 16 with preserved asymmetric ejecta (Mercury, 8; Moon, 8; Table S1 in Supporting Information S1). Of these, 12 have ejecta extending around the whole crater, with an asymmetric distribution, while four have a forbidden zone. The lobate forms mostly align with the direction of greatest ejecta extent, or are orthogonal to a forbidden zone. However, for two examples, part of the lobate form lies within an ejecta forbidden zone, including for Tsiolkovskiy crater on the Moon, as previously noted by Boyce et al. (2020).

We identify 37 craters with noncircular shapes (Mercury, 21; Moon, 16; Table S1 in Supporting Information S1). Yet for 33 of these examples, instead of being an ellipse, the crater rim approximates a circle segment interrupted on one side by a straight section (e.g., Figure 3d). The straight edge is always adjacent to the lobate form, and to the antecedent crater into which the lobate form extends. Hence, this noncircularity appears related to pre-impact

topography or the formation process of the lobate form, rather than relating to an oblique impact (see Sections 4.3 and 4.6). Supporting this, 31 of the identified craters with this shape have approximately symmetrical ejecta, which ought not to be the case if their shape were a result of oblique impact.

### 3.2. Classification of Lobate Forms

We classify lobate forms into three main categories with two additional subcategories, as listed in Table 1. Figure 3 illustrates representative examples. Next, we give an overview of each category of lobate form, describing their common characteristics, noting relevant features, and discussing key examples.

#### 3.2.1. Lobate Forms With Source Scarps, Extending Into Antecedent Craters—1 and 1a

We identify scarps at the source of 158 lobate forms extending into adjacent antecedent craters (Mercury, 73; Moon, 85). Including categories 1 and 1a, this is the most common type we identify. The scarps are steep slopes (>~20°) on the exterior of source craters, either directly on the exterior side of the rim crest, or located further down the rim flank, in both cases facing away from the source crater and bounding the upper margin of the lobate form. The distinction between category 1 and sub-category 1a is the location of these source scarps with respect to crater materials. For category 1, scarps appear to reside within material from the newer source crater, whereas for sub-category 1a the scarps cut into antecedent crater rim material.

The lobate forms and their source craters span the size range of surveyed features, from 3.5 to 180 km crater diameters. There is considerable variation in lobe morphology, with some common patterns identified between lobe size, form, and texture. Smaller lobes often appear relatively thick in comparison to their extent, forming “blocks” of material bound by failure scarps (e.g., Figure 3b). Larger lobes can have thicker proximal zones crossed by multiple minor scarps, which transition into thinner, more uniform frontal lobes bounded by the front-most minor scarp (Figures 1c, 3a, and 4c). Source scarp geometries are similarly varied, including crescent-
<figure><img src="image_5.png" /><figcaption>Figure 3.</figcaption></figure>
shaped “spoon-like” scarps resembling those found bounding rotational landslides on Earth (e.g., Figure 3b). Other scarps are linear, or follow the curved contours of the source crater, producing an outwardly convex trending scarp surface (e.g., Figures 1c, 4a, and 4c). These scarps mirror the interior walls of the craters they are found around, forming a ridge at the crater rim, with steep slopes on both sides.

For 26 category 1 examples (see Table S1 in Supporting Information S1), the lobate forms have similar textures to the surrounding source crater continuous ejecta, retaining clear distal lobe margins, but with gradational or unclear lateral contacts with adjacent ejecta (e.g., Motonobu crater Figure 4a). Source scarps for these examples often follow the contours of the source crater rim, or of the antecedent crater topography. In contrast, 37 category 1 lobes have consistently clear contacts, with a texture that differs from that of the source crater's continuous ejecta (e.g., Figures 4b and 4c). The remaining 50 category 1 lobes have unclear contacts and texture relationships with the ballistic ejecta, due to limited image resolution or lobe degradation.

Some category 1 lobe source scarps appear connected to internal terracing of the source crater. For Shirakatsi crater on the Moon, at the intersection between the source and antecedent craters, there is no clear distinction or boundary between the internal terracing of the source crater, the scarp bounding the lobate form, and minor scarps within the lobate form (Figures 5c-5e). The source crater has no clear rim here. Instead, a zone of scarps extend from the interior of the source crater into the antecedent crater. We identified similar situations in two other craters, and an additional tentative example at Klute W crater (Figure 4b). For one category 1 lobe, we identify a dark, smooth and flat patch of material at the base of the lobes source scarp, collected in a topographic low on top of the lobe (Figures 5a and 5b).

Category 1a lobes are prevalent within our survey (45 examples) and have some distinct morphological properties to category 1 (morphometric properties are also distinct, see Section 3.4), so we classify these lobes as a subgroup within lobate forms with source scarps. For these examples, the scarp cuts into rim material of the antecedent crater, forming a direct contact between the source crater rim and the antecedent crater basin (Figures 3b and 4d).

### 3.2.2. Lobate Forms Lacking Source Scarps, Extending Into Antecedent Craters—2 and 2a

Category 2 lobes appear to emanate directly from the source crater rim, forming a continuous unit from the rim until their steep distal margin (Figures 3c-3d and 6a-6c). The lobes lack a steep scarp at their source, and instead extend out at approximately the height of the crater rim, with a gentle downwards slope (<10°) in the upper section of the lobe body for some examples. We identified more examples on Mercury (19) than the Moon (5). Category 2a lobes similarly lack source scarps, but are less clearly delineated from their source crater's continuous ejecta, and have distinct properties we describe at the end of this section.

Occurring around some of the largest source craters identified in this survey (average diameter 58 km), category 2 includes a high proportion of the largest, most obvious, and morphologically striking lobate forms. We classify five out of seven of the features initially recognized on Mercury by Xiao and Komatsu (2013) in this category. These lobes have clear margins, and an approximately constant thickness along their length. Lobes often have a rough, hummocky texture throughout; they lack the outer radially ridged texture characteristic of ballistic deposits, but 11 examples have transverse ridges (e.g., Figure 3d).

Category 2 contains the clearest examples of the noncircular, segment shaped source craters discussed in Section 3.1. Nasir (Figure 3d) and Masuka craters on Mercury have obvious straight sections of rim adjoining the antecedent crater, with lobate forms extending from the straight part of the rim into the antecedent crater.

**Figure 3.** Examples from each category of lobate form. Yellow arrows indicate lobate form distal margin, green triangles indicate source scarps, and red dashed line indicates antecedent crater rims where not obvious. (a) Category 1, with Source Scarp and Entering Antecedent Crater. Moon—Von Neumann crater, 40.32°N 153.28°E, Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) mosaic, crater 75 km across. (b) Category 1a, Source Scarp on Antecedent Crater Rim. Moon—extending into Harkhebi H crater, 39.9°N 99.26°E, LROC WAC mosaic, crater 10 km. (c) Category 2, No Source Scarp and Entering Antecedent Crater. Mercury—Truffaut crater, 13.5°N 63.19°W, Messenger MDS BDR mosaic, crater 79 km across. (d) Category 2, No Source Scarp and Entering Antecedent Crater (example with non-circular crater shape). Mercury—Nasir crater, 20.35°S 26.69°E, Messenger MDS BDR mosaic, crater 42 km across. (e) Category 2a, Isolated Lobe Entering Antecedent Crater. Mercury - Castiglione crater, 40.88°S 87.95°E, Messenger Mercury Dual Imaging System LOI mosaic + EN1013806686 M, crater 78 km across. (f) Category 3, On Flat Ground, Not Entering Antecedent Crater. Mercury—Piazzolla crater, 34.36°N 0.44°W, Messenger MDS BDR mosaic, crater 37 km across.
<table><caption>**Table 1**
Summary of the Population of Lobate Forms on the Moon and Mercury, and Their Key Parameters</caption><thead><tr><th></th><th>No. of<br>lobate<br>forms</th><th>No. of<br>source<br>craters</th><th>Source crater diameter<br>average and<br>range (km)</th><th>Antecedent crater<br>diameter average and<br>range (km)</th><th>Max. runout of lobate<br>form average and<br>range (km)</th><th>Mobility of lobate form average<br>and range (runout/source crater<br>radius)</th></tr></thead><tbody><tr><td><strong>Moon</strong></td><td><strong>92</strong></td><td><strong>85</strong></td><td><strong>45.9 (6–206.4)</strong></td><td><strong>79.1 (7–236.9)</strong></td><td><strong>16.4 (2.2–76.5)</strong></td><td><strong>0.89 (0.3–2.6)</strong></td></tr><tr><td>1—with source scarp<br>and entering<br>antecedent crater</td><td>65</td><td>60</td><td>50.3 (12.4–183.6)</td><td>68.8 (7–236.9)</td><td>16.4 (2.2–71)</td><td>0.72 (0.3–1.3)</td></tr><tr><td>1a—source scarp on<br>antecedent<br>crater rim</td><td>20</td><td>19</td><td>16.9 (6–34.8)</td><td>105.1 (11.7–141.9)</td><td>11.1 (4–20.6)</td><td>1.45 (0.8–2.6)</td></tr><tr><td>2—no source scarp and<br>entering antecedent<br>crater</td><td>5</td><td>5</td><td>72.5 (21.6–163.1)</td><td>95.4 (35.1–153.2)</td><td>33.5 (12.6–76.5)</td><td>0.99 (0.8–1.2)</td></tr><tr><td>2a—isolated lobe<br>entering antecedent<br>crater</td><td>1</td><td>1</td><td>206.4</td><td>136.1</td><td>26.5</td><td>0.25</td></tr><tr><td>3—on flat ground, not<br>entering antecedent<br>crater</td><td>1</td><td>1</td><td>76.3</td><td>N/A</td><td>24.2</td><td>0.63</td></tr><tr><td><strong>Mercury</strong></td><td><strong>98</strong></td><td><strong>93</strong></td><td><strong>38.8 (3.5–153)</strong></td><td><strong>58.2 (4.4–205.8)</strong></td><td><strong>15.1 (2.6–44)</strong></td><td><strong>0.94 (0.4–2.9)</strong></td></tr><tr><td>1—with source scarp<br>and entering<br>antecedent crater</td><td>48</td><td>45</td><td>36.9 (5.6–115.1)</td><td>47.7 (8.3–167.1)</td><td>12.7 (3.2–38.4)</td><td>0.75 (0.4–1.5)</td></tr><tr><td>1a—source scarp on<br>antecedent<br>crater rim</td><td>25</td><td>25</td><td>13.7 (3.5–31)</td><td>48.7 (4.4–205.8)</td><td>8.1 (2.6–14.6)</td><td>1.37 (0.5–2.9)</td></tr><tr><td>2—no source scarp and<br>entering antecedent<br>crater</td><td>19</td><td>17</td><td>53.8 (26–105.4)</td><td>91.9 (31.4–188.9)</td><td>24.6 (10.2–40.4)</td><td>0.94 (0.6–1.2)</td></tr><tr><td>2a—isolated lobe<br>entering antecedent<br>crater</td><td>3</td><td>3</td><td>125.7 (76.5–150.5)</td><td>88.3 (71.9–104.6)</td><td>29.5 (16.7–44)</td><td>0.46 (0.4–0.6)</td></tr><tr><td>3—on flat ground, not<br>entering antecedent<br>crater</td><td>3</td><td>3</td><td>94.1 (36.5–153)</td><td>N/A</td><td>34.8 (20.6–42)</td><td>0.86 (0.5–1.1)</td></tr><tr><td><strong>All examples</strong></td><td><strong>190</strong></td><td><strong>178</strong></td><td><strong>42.2</strong></td><td><strong>68.70</strong></td><td><strong>15.7</strong></td><td><strong>0.92</strong></td></tr></tbody></table>

Note. Data for individual examples are found in Table S1 in Supporting Information S1.

Category 2a, or “isolated lobes” (Mercury, 3; Moon, 1), instead of having clear proximal margins adjacent to a source crater rim, appear to be continuous with their source crater's ejecta, with only distal lobate margins apparent (Figures 3e and 6d). This occurs where the ejecta overlies the rim of an antecedent crater, which for three of these examples does not quite overlap with the source crater. These lobes are proportionally smaller compared to their source crater size than for other lobes lacking source scarps. We designate these lobes as a sub-category due to their contrasting appearance, and potentially disparate origins to category 2 lobes.

### 3.2.3. Lobate Forms on Flat Ground—3

We found only four examples of lobate forms on flat ground, three on Mercury, and one on the Moon. These lobes extend across regional slopes <4°, and do not enter antecedent craters. For two examples, at Magritte crater on Mercury, and at King crater on the Moon, the lobes are limited in extent around the crater, extending directly from their source crater rims, forming individual lobes with steep edges. Lobe texture appears continuous with surrounding proximal ejecta. The two other examples, Hokusai and Piazzolla craters, are both in Mercury's northern smooth plains region. Piazzolla has a lobate form around approximately half of the crater, with ballistic ejecta
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Examples of lobate forms with source scarps (Category 1 and 1a). Yellow arrows indicate lobe margin, green triangles indicate source scarps. (a) Mercury—Category 1, Motonobu crater, 0.78°N 65.33°W, Messenger MDS BDR mosaic, crater 48 km across. (b) Moon—Category 1, Klute W crater, 37.98°N 143.28°W, Lunar Reconnaissance Orbiter Camera Wide Angle Camera mosaic, crater 30 km across. (c) Mercury—Category 1, Lobe extending into Akutagawa crater, 47.74°N 139.18°W, Mercury Dual Imaging System BDR mosaic, crater 36 km across. (d) Mercury—Category 1a, Lobe extending into Velázquez crater, 37.81°N 53.57°W, EN1033429525 M, crater 8 km across.</figcaption></figure>

textures around the remaining half (Figure 3f), but at Hokusai the lobate form occurs around the whole circumference (Figure 1d). The morphology of these examples is clearly distinct from typical ballistic ejecta and from other lobate forms in this survey, with a raised rampart ridge at their distal end, preceded locally by a low moat. These two category 3 lobes have a generally ropey texture and are remarkably reminiscent of the layered ejecta craters of Mars.

### 3.3. Mercury Versus Moon Comparison

The spatial density of lobate forms is $2.43 \times 10^{-6}$ per square km on the Moon, versus $1.31 \times 10^{-6}$ for Mercury. However, the Moon has a higher average crater density overall, with total crater numbers above 10 km diameter (which source the vast majority of lobate forms in our survey) being similar for both bodies (~16,000 lunar; Robbins, 2019; Rodionova et al., 1987; and ~17,000 for Mercury; Herrick et al., 2018). Allowing for this, each body has comparable occurrence rates for lobate forms around craters above this size, with Mercury having 0.0051 lobate forms per crater, or one lobate form for every 196 craters above 10 km diameter, whilst the Moon has 0.0055 lobate forms per crater, or one lobate form for every 182 craters above 10 km diameter.
<figure><img src="image_7.png" /></figure>

**Figure 5.** Yellow arrows indicate lobe margin, green triangles source scarps. Red dashed lines indicate outline of zoomed views. (a and b) Ponded, flat, smooth patch of material located in front of a lobate form source scarp, indicated with red arrow on (b). Van Gent X crater on the Moon, 16.31°N 159.67°E, crater 37 km across. (a) Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) mosaic. (b) LROC Narrow Angle Camera (NAC) Mosaic from LROC QuickMap. Zoomed in view of the proximal area of the lobe, adjacent to the source scarp. (c,d&e) Lobate form source scarp connected to internal terracing. Shirakatsi crater on the Moon, 12.2°S 128.57°E, crater 49 km across. (c) LROC WAC mosaic (d) LROC NAC Mosiac from LROC QuickMap. Zoomed in view of the intersection of Shirakatsi crater and an antecedent crater, at the source of the lobate form. Appears as a zone of failure connecting interior terracing to transverse scarps on the lobate form. (e) LROC DTM (GLD100) via LROC QuickMap. DTM shows no clear rim crest for Shirakatsi crater across the intersection with the antecedent crater.

Lobate forms are globally dispersed on both bodies (Figure 2). There is no obvious clustering, and they are found within multiple terrain types. However, a few areas, namely the lunar maria, and Mercury's Borealis Planitia, Caloris Planitia and the circum-Caloris plains, have a relative lack of lobate forms. The lower crater density in those areas not only reduces the number of craters for lobate forms to be found around, but also reduces the likelihood of a crater overlapping with an antecedent crater, the scenario in which most lobate forms are found (98%).

For every category of lobate form we define, we have found examples on both Mercury and the Moon. However, some notable differences between the two bodies exist. Mercury has more lobes lacking source failure scarps (19 vs. 5) and more lobate forms on flat ground that don't extend into antecedent craters (3 vs. 1). Hokusai and Piazzolla craters, the only examples of lobate forms on flat ground reminiscent of martian layered ejecta craters, are both on Mercury.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Examples of lobate forms without source scarps (Category 2 and 2a). Yellow arrows indicate lobe margin, red dashed line indicates antecedent crater rims where not obvious. (a) Moon—Drygalski crater, 79.77°S 87.65°W, Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) mosaic, crater 165 km across. (b) Mercury—Carvalho crater, 26.81°N 51.59°W, Mercury Dual Imaging System BDR Mosiac, crater 82 km across. (c) Moon—Kovalevskaya crater, 30.78°N 130.54°W, LROC WAC mosaic, crater 110 km across. (d) Mercury—Giotto crater, 12.45°N 56.59°W, Messenger MDS BDR mosaic, crater 140 km across.</figcaption></figure>

**3.4. Lobate Form Morphometry**

Figure 7a shows a trend of increasing maximum runout length with larger source craters, consistent across lobe categories. However, the lobe mobility parameter reveals some variation in trends within the population (Figure 7b). For most categories of lobate form, lobe mobility appears to slightly decrease with increasing crater size, representing a minor deviation from self-similarity (extent increasing proportionally with crater radius), which is the theoretical trend for ballistic ejecta extents (Housen et al., 1983). In contrast, category 1a lobate forms do not appear to have any significant trend in lobe mobility with source crater size. They also tend to have higher mobilities than other categories, with an average of 1.41 compared to the overall average of 0.92 (Table 1).

The average mobilities for all categories of lobes are shown in Table 1. Overall, there is more variation in mobility between lobe categories than between the two planetary bodies, the averages for each category being approximately the same for each body. However, average lunar and mercurian lobe mobilities (Moon, 0.89; Mercury, 0.95) are generally lower than reported ejecta mobilities for lobes elsewhere in the Solar System, including layered ejecta craters on Mars (single layered ejecta, 1.53; multiple layered ejecta, 2.17; Barlow, 2005) and
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Runout and mobility measures for lobate forms. Measurements of source crater size taken as diameter but have been changed to radius for plots for ease of comparison with lobe mobilities, which are calculated with respect to radius, as is convention for ejecta mobilities. (a) Source crater radius versus maximum lobe runout length. (b) Source crater radius versus lobe mobility. (c) Source crater radius versus $L_{max}/H_{max}$. (d) Lobe mobility versus $L_{max}/H_{max}$.</figcaption></figure>

Ganymede (double layered ejecta, 1.86; Boyce et al., 2010), and for ejecta flows around terrestrial craters (e.g., Ries crater, 1.12; Sturm et al., 2013). Of the individual lobe categories, category 1a lobes (mean mobility 1.41) approach the mobility of martian single layered ejecta craters.

Figure 7c shows $L_{max}/H_{max}$ plotted by source crater radius. $L/H$ is generally positively correlated with source crater size. However, mercurian lobes lacking source scarps are notable outliers above the general trend with exceptionally high $L/H$ ratios.

If we assess the lobes identified in this survey as landslides, then most examples, not just the outlier mercurian lobes, could be considered long runout landslides. The defining characteristic of long runout landslides is a runout longer than expected for a given height drop, as assessed using simple frictional models (Legros, 2002). We are not aware of particular models for Mercury or the Moon, but landslides with $L/H$ ratios significantly above 1.7, which most of our examples are, may qualify. This is conventionally given by the reciprocal, $H/L$, or apparent coefficient of friction, where examples significantly below 0.6 are not predicted by “normal” frictional spreading, and an additional process may enhance runout by reducing friction (Campbell, 1989; Goren & Aharonov, 2007; Johnson et al., 2016; Legros, 2002; Magnarini et al., 2021; Melosh, 1979). What this process might be is contentious, and
volatiles (Legros, 2002), basal heating (Goren & Aharonov, 2007), acoustic fluidization (Johnson et al., 2016; Melosh, 1979), and many more mechanisms have been suggested. If the surveyed lobes on Mercury and the Moon are landslides, then a similar process may be needed to explain their large ratio of runout to height drop.

The graph of L/H versus lobe mobility (Figure 7d) summarizes our findings on mobility. Most examples have lobe mobilities between 0.6 and 1.2, and Lmax/Hmax ratios of around 5–10. Mercurian and lunar lobes have mostly indistinguishable mobilities for equivalent categories and source crater sizes. Two groups of outliers exist: lobes with source scarps on antecedent crater rims have lobe mobilities as high as 2.9, and lobes on Mercury lacking source scarps have Lmax/Hmax ratios as high as 44.

3.4.1. Lobate Form Volume Estimates

Lobe volume increases around larger craters, appearing to follow a power law (Figure 8a). Some category 1a lobes are slightly offset from this trend, having relatively larger volumes for the diameter of their source crater. This offset agrees with morphological observations, suggesting additional material for these lobes is derived from the rim of an antecedent crater (Section 3.2.1).

Lobe volume is positively correlated with lobe runout, matching trends observed for landslides on the Earth and Mars, and for martian layered ejecta (Figure 8b). A consistent trend can be seen across the different features, with most types of mass movement having a power law best-fit with approximately the same slope. Offsets are apparent, however, with martian landslides having runouts typically lower than terrestrial landslides of equivalent volume. The placement of the lunar and mercurian lobate forms overlaps considerably, with no apparent offset between them. They follow the trend of the martian landslides and layered ejecta closely, but plot on the lower end of the range for runout length for a given volume. Given that our volume estimates are minima, the true offset from the martian features may be greater, with even lower runouts in comparison to volume.

3.4.2. Overlapping Crater Geometry

We find most lobate forms at adjacent or overlapped crater pairs, with lobes extending from newer source craters into antecedent craters. We therefore assess how the geometries of paired craters may influence lobe development, noting variety in the relative sizes of paired craters, and in the degrees of overlap between them.

To assess overlap, we digitized crater rims and central points, then constructed a radial line connecting both craters' central points. Along this line, we measured the length of the newer source crater's radius sitting within the antecedent crater, or alternatively measured the gap between the rims as a negative value if they were not in contact. This measurement was then put into terms of crater radii by dividing it by the newer crater radius. Thus, an idealized pair of circular craters with an overlap of one would have the newer crater placed exactly one radius within the antecedent crater, with its central point on the antecedent crater rim. An overlap of zero corresponds to craters with rims touching at a point, and any negative overlap would mean there is some distance between the two crater rims. A figure illustrating this method is given in the supporting information (Figure S1 in Supporting Information S1).

Most crater pairs with lobate forms overlap, but no lobate forms are identified around craters overlapping by >1, so for all crater pairs the central point of the new crater lies outside the rim of the antecedent crater. Some crater pairs do not overlap at all, with 12 lobate forms identified extending into adjacent craters some distance from their source crater. There is a positive correlation between degree of crater overlap and lobe mobility, consistent across both bodies for lobate forms with and without identified source scarps (Figure 9a). There is separation along this trend by category: lobate forms without source scarps have the highest degrees of overlap, and higher mobilities. The exceptions are the lobes with scarps on antecedent crater rims, which as previously noted have high mobilities, but these do not seem correlated with high degrees of crater overlap.

Another important aspect of crater pair geometry is the relative sizes of the two craters. This relationship can be assessed using the ratio of antecedent crater to source crater diameter. We plot this ratio against lobe mobility (Figure 9b), and find that there is a positive correlation between the two. This correlation holds across lobe types and for both Mercury and the Moon. Lobes with scarps on antecedent crater rims have the highest lobe mobilities and the proportionally largest antecedent craters, though there is greater variance in this population too. Figure 9 excludes examples where lobes are confined by the boundaries of their antecedent crater, demonstrating this trend cannot solely be explained by accommodation space.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Plots using volume estimates for lobate forms, which we were able to calculate for 77 examples. (a) Log-log plot of source crater radius versus lobe volume estimate (b) Log-log plot of volume versus max runout length, with lobate forms compared to terrestrial and martian features. Terrestrial data from: Hayashi and Self (1992), Siebert (1984), Hazlett et al. (1991), Siebe et al. (1992), Legros (2002), Hampton et al. (1996), Lipman et al. (1988), Iverson et al. (1998), Mothes et al. (1998), Pierson et al. (1990), King (1999), Sosio et al. (2008), Crosta et al. (2007), Kelfoun and Druitt (2005), Shreve (1987), Voight et al. (1983), Young et al. (1998), Moretti et al. (2012). Martian data from: Lucas et al. (2011, 2014), McEwen (1989), Legros (2002), Quantin et al. (2004), Barnouin-Jha et al. (2005).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Plots assessing relationship between source and antecedent crater geometries, and the mobility of lobate forms. We were able to assess overlap for 145 examples, with some lobate form hosting crater pairs excluded due to degradation of their antecedent craters, which precluded accurate overlap measurements. (a) Degree of crater overlap versus lobe mobility. (b) The ratio of antecedent crater to source crater radius versus lobe mobility.</figcaption></figure>

# 4. Discussion

## 4.1. Lobate Form Emplacement

Agreeing with previous interpretations of lobate forms, we suggest a ground-hugging style of emplacement. There is a strong tendency for lobate features to extend into antecedent craters (98% of surveyed lobes), which are usually the lowest elevation area surrounding the source crater, and likely also the lowest point in the pre-impact
topography for the source crater impact. This indicates a topographic control on lobate form development, as would be expected for gravity-driven slides and flows. For two examples, topographic rises also divert lobate forms (Figure S4 in Supporting Information S1), consistent with a ground-hugging emplacement style. The morphological characteristics of the features, including lobate margins, maintained thicknesses and steep distal edges, are also consistent with emplacement from a flow and/or slide (Barnouin-Jha et al., 2005; Carr et al., 1977; Xiao & Komatsu, 2013).

Category 1 lobes, defined by the presence of a scarp at their source, give further evidence for a gravitationally controlled lobe origin. Given the location of the scarps on topographic boundaries between craters, and that they form the upper boundary of lobes which exhibit evidence for downslope ground-movement, the scarps are most likely the exposed parts of failure surfaces (Boyce et al., 2020; Crosta et al., 2018; De Blasio, 2011; Duarte et al., 2019; Lucchitta, 1978), resulting from gravitational collapse of material into the antecedent crater.

## 4.2. Timing of Lobate Form Production

Several lines of evidence indicate lobate forms are produced within the time span of the formation of their source crater. We identify dark, smooth, and flat patches of material in local topographic lows on top of 15 lobate forms (e.g., Figures 5a and 5b; Table S1 in Supporting Information S1). The morphology of these patches indicates pooling of once molten material, so we interpret these patches as impact melt, agreeing with previous studies of four lobate form examples (Barnouin et al., 2015; Lennox et al., 2025).

We suggest that identified impact melt is sourced from the lobate form's source crater, due to its proximity, the lack of alternative fresh craters nearby to source melt, and the patches isolated nature making a volcanic origin unlikely (see Howard & Wilshire, 1975). The stratigraphic position of the melt above the lobate form suggests the lobes must form during the impact cratering process, while impact melt is still molten and able to pool in low-lying areas of the lobe. This restricts timing to the excavation and modification stages of impact cratering (Osinski et al., 2011). Though we identify impact melt patches only on a minority of surveyed lobes (~8%), their occurrence may be unrecognized in other cases, because impact melt deposits are mostly identified around fresh craters (Howard & Wilshire, 1975; Neish et al., 2014; Plescia & Cintala, 2012), and evidence may have been lost at more degraded examples. We identify more examples on the Moon (9) than Mercury (6), where image resolution is poorer, so it is possible more melt patches exist on Mercury below the current resolution limit. Melt is identified across lobate form categories 1, 2 and 3, though not in subcategory 2a.

Four lobate forms have source scarps that appear continuous with terracing within the source crater (e.g., Figures 5c-5e). The lobes therefore probably formed simultaneously with the terracing, which occurs during the modification of the transient crater (Melosh, 1989). This suggests simultaneous inwards and outwards directed failure of material surrounding the transient crater during this modification stage. Twenty-six category 1 and four category 2a lobate forms appear contiguous with the proximal ejecta of a crater, with shared textural characteristics (e.g., Figure 4a). This material is likely ejecta from the source crater that has flowed down-slope during the impact process, forming a lobe. Category 3 lobes likewise appear to consist of ejecta material, though occur on flat ground. The interpreted origin of several other groups of lobate forms, which we discuss in Sections 4.4 and 4.5, also indicates syn-impact formation.

The degradation state of a crater is mostly tied to its age, and is often a useful indicator of the relative ages of features (e.g., Galluzzi et al., 2016; Kinczyk et al., 2020). We find that lobate forms and their source craters are consistently indistinguishable in degradation state. Given the continuity and adjacency of the craters and their lobes, it seems probable they have degraded at similar rates, and therefore have geologically similar ages. Lobate forms are therefore unlikely to result from crater rim landslides that happened long after the crater-forming impact.

Though the overall body of evidence suggests early formation of lobate forms, 118 examples lack specific evidence to suggest syn-impact formation. We in fact identify one example on each body where there is evidence for lobe formation after the impact cratering process, where a source scarp appears to crosscut the source crater's rim (Figure S3 in Supporting Information S1). However, these examples are greatly outnumbered by examples with evidence for syn-impact formation (72), so it seems unlikely that many lobes are later landslides. There is also a theoretical argument against large numbers of landslides occurring long after source crater formation. The topographic expression of impact craters diminishes over time, tending towards a flat plain through mass
movements and diffusive processes (Fassett & Thomson, 2014; Fassett et al., 2017). If a large landslide is to occur around a crater, it therefore would be more likely to occur while the crater is still young. The largest interior mass movements within craters are terraces, which form immediately after the excavation of the crater, due to the gravitational instability of the transient crater. The same scenario may also occur on the exterior of craters, in scenarios where they intersect topographic lows from antecedent craters.

### 4.3. Modification Stage Collapse

Most lobate forms appear to result from collapse and/or transport of material under gravity, as evidenced by topographic control on their formation, and examples with failure scarps at their source. Combined with observations suggesting early formation, including syn-impact formation, we propose that many lobate forms result from outward collapse of source crater material into antecedent craters during crater modification. This includes ejecta, but may also include structurally uplifted target material, which in combination with ejecta constitutes the raised rims and flanks of transient craters (Melosh, 1989). Observations of category 1 and 2 lobate forms are consistent with this collapse process, in which they extend into antecedent craters, plot along similar trends morphometrically (e.g., Figures 7a, 7b, 8 and 9), and both display evidence for early formation. These categories of lobe are also almost all found around complex craters (94%), which form due to substantial internal collapse of transient craters (e.g., Melosh & Ivanov, 1999). This highlights modification stage collapse as a potential mechanism for formation of these lobes. Other lobe categories can be more commonly found around simple craters (e.g., category 1a—59% simple source craters), which result from more minor transient crater collapse. We note that lobe material may be emplaced as part of a continuous process, in which material flows downslope immediately following initial ballistic emplacement and/or the cessation of target uplift, depending on the source of lobe material. This is comparable to internal collapse of transient crater rims in complex craters, which never fully settle after uplift, and collapse immediately on formation (Kenkmann et al., 2013; Melosh & Ivanov, 1999).

Variations in the degree of crater overlap between source and antecedent craters may explain differences in lobate form morphology within categories 1 and 2 (Figure 10). In crater pairs with minimal overlap, only sections of the source crater's continuous ejecta overlap with the antecedent crater (Figure 10a). The flow of this material (including both primary ejecta and incorporated substrate from ballistic sedimentation) into the topographic low during crater formation results in a lobate form that may retain an ejecta-like texture and appear laterally continuous with the surrounding ejecta (e.g., Motonobu; Figure 4a). With increased crater overlap, structurally uplifted target rock that surrounds the source transient crater may also intersect the topographic low (Figure 10b). This material is uplifted during crater excavation (Sharpton, 2014; Sturm et al., 2016), and although it may not extend radially as far as ejecta (e.g., Collins, 2015), close to the transient crater it accounts for significant elevation rises compared to the pre-impact topography (e.g., 80% of the rise of final crater rims on the Moon; Sharpton, 2014). This material may collapse outwardly in addition to typical internal transient crater collapse, with a lack of support on either side of the low-strength transient rim. Collapse of the target material in addition to ejecta may lead to the loss of an ejecta-like texture for the lobate form (e.g., Figures 4b and 4c). Examples can retain failure scarps, however, in which they mirror the internal scarps bounding crater terraces (e.g., Figure 4c), or even appear connected to internal terracing (e.g., Figures 5c–5e).

The mobility of category 1 and 2 lobate forms correlates with the degree of crater overlap (Figure 9a). Greater degrees of overlap increase the proportion of the source crater that may be involved in collapse, increasing lobe volume independently of source crater size, and therefore increasing lobe mobility (in which the runout of mass movements depend on volume; Barnouin-Jha et al., 2005; Davies, 1982; Legros, 2002; McEwen, 1989). Greater degrees of overlap also represent a larger topographic differential across the pre-impact topography, with source craters intersecting deeper parts of the antecedent crater. This may further increase flow of material into the antecedent crater. The correlation between lobe mobility and the relative size of the antecedent crater (Figure 9b) may be explained similarly, where new craters intersecting with relatively larger antecedent craters collapse more substantially.

In the most extreme cases of overlap, the whole source crater may be involved in asymmetric collapse (Figure 10c). The asymmetric form of some high-overlap craters supports this, in which the overlapped side has a topographically minimal rim and crater fill material extending up to it, whereas the non-overlapped side has a prominent rim and terracing (e.g., Figures 3c and 3d). This may explain the lack of failure scarp in examples with high overlap (Figure 9a): instead of coherent sections of marginal source crater material collapsing into the
<figure><img src="image_12.png" /><figcaption>Figure 10.</figcaption></figure>
adjacent topography, the topographic gradient across the developing crater causes material flow. This is reflected in category 2 mobility measures, as in addition to high mobilities relative to crater size, there is especially high measured L/H (Figure 7c), in which measuring Hmax from the final crater rim to the edge of the flow does not reflect the full topographic gradient that formed the lobe. The timing of collapse may also be relevant. Topographic steps closer to the point of impact may initiate collapse earlier, at a time when excavated and uplifted materials had low-strength, outward momentum and were potentially fluidized (Kenkmann et al., 2013; Melosh & Ivanov, 1999). These examples may therefore be more akin to an ejecta-like flow, whereas slightly later collapse in less overlapped examples occur in more cohesive material, hence their forming of failure scarps (De Blasio, 2011). However, we suggest that even for less overlapped examples a transient weakening mechanism could be involved to some extent (Sections 4.3.1 and 4.7), also proposed to explain the formation of crater terraces, which are likewise bounded by failure scarps (Melosh & Ivanov, 1999).

It is notable that no lobate forms are identified in crater pairs with a degree of overlap greater than 1 (Figure 9a). In this scenario the newer source crater rim overlaps the antecedent crater by more than 1 radius, placing its center within the antecedent crater rather than on its rim. The point of impact for the new crater, approximated by its center, would already be within the lower topography, removing the opportunity for outwards collapse, and therefore no lobate form is produced.

The formation mechanisms proposed here are consistent with the observations and interpretations of Hood et al. (2024), who investigated overlapping complex craters on the Moon, but were not concerned with the presence of lobate forms. Based on asymmetries in crater rim height for overlapping craters, they identified evidence for a net flow of source crater material into antecedent craters during crater modification, likely due to asymmetric internal collapse of the transient crater. We propose that lobate form production could be viewed as an extension of this process. In addition to asymmetric internal collapse within the source crater, external collapse may also occur, resulting in a lobate form extending into the antecedent crater.

Modification stage collapse into antecedent craters may explain instances of source craters with non-circular shapes (Section 3.1), with recessed sections of crater rim adjacent to the lobate form. At the intersection with an antecedent crater, uplifted material around the transient crater may collapse outwardly in addition to typical internal collapse, so interior terracing, and hence rim crest location, stops closer to the topographic step at the antecedent crater edge. In the most extreme examples (e.g., Figure 3d), the recessed edge of the crater approximates the boundary of the estimated transient crater size (see supporting information), with no identifiable terraces internal to this edge, suggesting dominance of external collapse in these locations. A later mass-movement, after final crater formation, is unlikely to form the recessed straight rim in these examples, as projecting an originally circular rim into the overlapped section shows it would already intersect the antecedent crater floor. There would therefore be no obvious instability to trigger a later collapse. The straight sections of rim maintain an approximately constant height within the antecedent crater, and appear continuous with the rest of their rims, suggesting they are original features of the final crater rim as it formed. We note that many lobe's source craters are approximately symmetrical, even for examples with large mobilities and overlaps, so this process has not occurred for all lobate form related collapse.

### 4.3.1. Modification Stage Collapse—Ejecta Flow or Landslide?

Lobate forms resulting from modification stage collapse are produced by mass movements under the action of gravity, so in this sense they are landslides. However, this also involves flow of material recently ejected from a developing crater, so they could also be described as ejecta flows. Examples with and without source scarps may indicate slightly different timings, movement mechanisms, or origin from uplifted target and/or ejecta material, but overall, lobes are produced with features characteristic of both ejecta and landslides, leading to difficulties in interpretation. Some previous studies of the Tsiolkovskiy crater lobe identified ambiguous morphological indicators, and hypothesized mass-movement origins that could be closely linked to crater formation (Guest &

**Figure 10.** Schematic model for formation of lobate forms from external modification stage collapse, with varying degrees of crater overlap. Black arrows indicate material movement. Not to scale, features are vertically exaggerated. (a) Minimal overlap between source and antecedent crater, resulting in a category 1 lobate form with a source scarp and a texture consistent with ejecta. (b) Moderate crater overlap, resulting in a category 1 lobate form with a source scarp but a texture distinct from that of the ballistic continuous ejecta. (c) High crater overlap, resulting in a category 2 lobate form lacking a source scarp, with a distinct texture from typical ballistic continuous ejecta.
Murray, 1969; Magnarini et al., 2021a), but discussion about lobate forms on the Moon and Mercury in general has often been framed as an ejecta flow versus landslide debate. From our interpretations, this framing may be misleading and does not lead to a satisfactory explanation of all the characteristics of these features. A helpful distinction may be made between syn-impact processes like modification stage collapse, and much later landslides that could occur around a final crater rim. From the observations made in this work, we suggest that few lobate forms are later landslides, and the majority are mass movements that occurred during the modification stage of crater formation.

It has previously been proposed that particular mechanical effects associated with impact cratering are needed to explain the formation of terraces and central peaks in complex craters (e.g., acoustic fluidization; Melosh, 1979), on the grounds that simple debris sliding models are not sufficient (Melosh, 1989). This could also be true for lobate forms, where the mechanics of crater collapse and long runout landslides have been hypothesized to share common characteristics (Melosh & Ivanov, 1999). Regardless of whether the term landslide or ejecta flow best describes these features, this work demonstrates that lobate form production is directly related to the impact cratering process, and may require transient conditions or energy released in an impact event to occur.

In addition to the distinction between lobate forms and landslides occurring long after crater formation, it is also worth highlighting their difference to layered ejecta craters, a well-studied fluidized ejecta morphology. Apart from the Hokusai and Piazzolla lobate forms on flat ground, surveyed lobes have morphologies distinct from layered ejecta craters, and occur mostly in different contexts. They therefore probably originate through different mechanisms.

## 4.4. Landslides

Some lobate forms appear less directly related to modification of the source crater during impact cratering, and although intimately connected to impact cratering processes, could more simply be described as landslides. This includes two category 1 lobate forms that appear to post-date final crater formation, with source scarps that appear to cut into the final crater rim (Figure S4 in Supporting Information S1). These two exceptions appear to be later landslides around a source crater. Additionally, we identify many more examples of category 1a lobes, with source scarps appearing to cut into antecedent crater rims. Supporting this, some of these examples have scarps that are too deep, and lobes that are too large to involve only material excavated and deposited by the newer impact (e.g., Figures 3b and 4d). Agreeing with interpretations of similar features elsewhere in the Solar System (Crosta et al., 2018; Duarte et al., 2019), we interpret these lobes as landslides on the older crater rim, triggered by the newer adjacent impact. The newer craters are often located on the rims of much larger basins (Figure 9b), so that the topographic low into which the lobe extends dominates over the topography of the newer crater. In combination with large lobe volumes compared to the newer crater size, this leads to high lobe mobilities in this category (Figure 4b). We calculate lobe mobilities based on the newer “source” craters, yet the volume and the $H_{max}$ of the landslide, which are both correlated with runout, are largely controlled in this case by the geometry of the antecedent crater, leading to much longer runouts than the size of the newer crater would suggest. By examining mobility indicators that are not normalized to the newer “source” crater size, such as volume versus runout or $L/H$ (Figure 7c), we find that these lobes plot similarly to the bulk of surveyed lobate forms.

Some category 1 lobes also approach the high lobe mobilities of category 1a (Figures 7b and 7d). This suggests gradation between the two categories, in which lobe formation involves the simultaneous collapse of newer source crater material and antecedent crater rim material. This may be expected, given that we suggest category 1 lobes can result from collapse of uplifted flank material around the source transient crater, which will include antecedent crater material as structurally uplifted target rock. What constitutes source or antecedent crater material in this context is therefore gradational. Some category 1a examples have source scarps that appear to cut through large sections of source and antecedent crater materials, leaving the source crater with a segmented non-circular shape (Section 3.1) in which parts of it appear “missing” (Figure 4d). The collapse of the antecedent rim removes material that would otherwise make up the uplifted flank and rim of the newer source crater.

## 4.5. Ejecta Flows

The morphologies of some lobate forms indicate that they were emplaced as ground-hugging flows of ejecta. Category 2a lobes are contiguous with their source crater's ballistic ejecta deposits, extending into antecedent craters which intersect the ejecta. Continuous ejecta has been shown to deposit preferentially into topographic
lows (e.g., Morse et al., 2018), so initial ballistic emplacement on the slopes of antecedent crater walls may enhance ballistic sedimentation downslope into the antecedent crater, forming a lobe immediately during ejecta emplacement. The immediate radial flow of ejecta on landing accounts for the lack of failure scarp in these examples. Some category 1 lobes also appear to have ejecta textures and may have similar origins (Section 4.3), but these lobes have source scarps, which could indicate wholesale collapse of more coherent sections of ejecta.

Two lobate forms on Mercury are reminiscent of layered ejecta on Mars (Hokusai and Piazzolla; Figures 1d and 3f). They have no source scarps, and they do not extend into antecedent craters nor down any regional slopes, so they are not landslides. By analogy with martian layered ejecta, their morphology instead indicates highly fluidized ejecta, with ramparts and ropey texture resulting from flow dynamics (Baloga et al., 2005; Boyce & Mouginis-Mark, 2025; Carr et al., 1977; Komatsu et al., 2007; Mouginis-Mark, 1979). However, there are some notable differences in the Mercury examples, with less continuous terminal ramparts than on Mars, and rampart sections stacked one behind the other. There is no obvious separation of stacked ramparts into different layers, so this arrangement is not equivalent to multiple layered ejecta on Mars, and it is unclear how these examples would be classified using the martian system (Barlow et al., 2000). The mobility of the mercurian fluidized ejecta is also less than the martian counterparts (ejecta mobility of ~1 vs. 1.5 for martian single layered ejecta, or 2.17 for multiple layered). Finally, the fluidized ejecta at Piazzolla crater occurs only on one side of the crater, with typical ballistic ejecta textures on the opposing side, which is atypical of martian examples (Barlow, 2005; Li et al., 2015). Despite these differences, their morphology is still strikingly like martian layered ejecta, which combined with their formation on flat ground indicates two clear examples of highly fluidized ejecta on Mercury.

The occurrence of layered ejecta-like features on Mercury has interesting implications. It suggests an atmosphere is not a prerequisite for ejecta fluidization, but leaves open the possibility of volatiles within the target material having a role. Moderately volatile elements (potentially in the form of sulfides; e.g. Nittler et al., 2011) in Mercury's surface materials may aid in ejecta fluidization, whereas the Moon's lack of equivalent materials may explain why it lacks these features. However, the ability of Mercury's moderately volatile elements to fluidize ejecta is debatable, whereas the martian near-surface contains high proportions of easily volatilized water-ice that is mostly lacking on Mercury (except from at the poles (e.g., Chabot et al., 2014), far away from the two layered ejecta-like examples). The occurrence of fluidized ejecta features potentially reflects the relative abundance and type of volatiles on the two bodies: Mercury hosts only two such features, whereas on Mars they are commonplace. The ejecta around Hokusai crater may alternatively been fluidized by an excess of impact melt (Barnouin et al., 2015), with impact melt also proposed as an important factor in martian layered ejecta (Osinski, 2006). However at Piazzolla crater we find no exterior impact melt morphologies, and it is also a considerably smaller crater than Hokusai (37 vs. 95 km diameter), so proportionally less melt was formed in its impact event (Cintala & Grieve, 1998). A minority view, that layered ejecta in general could form dry (without requiring volatiles or an atmosphere) has some support from modeling work (Wada & Barnouin-Jha, 2006). The limited number of examples on Mercury should engender caution in interpretation. It is possible that special and unrecognized properties of the local terrain or even of the impactors could have led to these two fluidized ejecta examples on Mercury.

## 4.6. Occurrence Rate of Lobate Forms

Though a pre-impact topographic low in the form of an antecedent crater is implicated in the development of most lobate forms, it is unlikely to be the only controlling factor. If many lobate forms result from modification stage collapse of source crater material into antecedent craters, why do we not identify more on Mercury and the Moon? The occurrence rate around craters >10 km (Mercury, 1 in 196; Moon, 1 in 182) is clearly less than the number of intersecting craters on such densely cratered bodies. Here, we explore possible additional factors:

1. *Relative depth of pre-impact topographic low.* In Sections 3.4.2 and 4.3 we demonstrated that relatively larger steps in pre-impact topography correlate with increased lobe runout. Thus, where craters overlap without a significant topographic step (e.g., for new impacts adjacent to very degraded or comparatively smaller craters) a lobe may not be produced, or be too small to be apparent at survey resolution. Lobate forms in this survey tend to extend into antecedent craters that retain topographic expression, and are at least half the size of, though typically larger than, the source crater (Figure 9b).

2. *Oblique impact.* Ejecta distribution is known to be influenced by impact angle and direction. The formation of lobate forms could potentially be facilitated in the preferred ejection directions for oblique impacts (Lennox
et al., 2025). In estimating the impact obliquity for source craters in our survey, we focus on ejecta distribution. Other characteristics, such as crater shape are also linked to oblique impact (Gault & Wedekind, 1978), but as previously described, asymmetries in shape are also likely linked to uneven pre-impact topography (Aschauer & Kenkmann, 2017) and lobate form production (Section 3.1). We find 16 craters with asymmetric ejecta, of which 14 have their lobate forms orientated in the preferred ejecta direction (Table S1 in Supporting Information S1). This suggests oblique impact could be an additional factor in developing lobate forms, given the tendency for alignment of lobes and ejecta distribution. However, few source craters show evidence for recognizably oblique impacts, and we identify two examples where a lobate form extends into the ejecta forbidden zone of a source crater. We therefore conclude oblique impact is not a requirement for lobate form development.

3. *Volatiles*. Lobate forms occur at similar rates on the volatile-depleted Moon and on Mercury, known to have moderately volatile elements in its surface materials. The runout of lobate forms is also equivalent on the Moon and Mercury, and lobes on Mercury are not correlated with the location of other volatile-suggestive features (e.g., hollows or faculae; Blewett et al., 2011; Thomas et al., 2014). We therefore suggest volatiles are not a driving factor in the development of lobate forms on these bodies. The occurrence of the two layered ejecta-like features in Mercury's northern plains may be an exception.

4. *Impact velocity*. Faster impact velocities increase the mass of melt and vapor produced in an impact, and additionally may fracture target materials to a greater degree (Melosh, 1989). These factors may aid in lobate form production. However, Mercury has a greater average impact velocity than on the Moon (42.5 vs. 19.4 km/s; Le Feuvre & Wieczorek, 2008), but lobe occurrence rates are comparable for both bodies, so this is unlikely a controlling factor.

5. *Impact melt*. Barnouin et al. (2015) suggested impact melt as a potential fluidizing agent for the lobate form on flat ground at Hokusai. Impact melt is also preferentially distributed into pre-impact topographic lows around impact craters (Neish et al., 2014; Stopar et al., 2014), so melt concentration into antecedent craters could aid in lobate form production in these settings. However, lobate forms are distinct from common impact melt suggestive morphologies, and where melt is identified it forms separated ponds or flows on top of the lobe, potentially indicating a separate slightly later emplacement (Osinski et al., 2011). It is possible a minor melt component within the body of a lobe may aid in fluidization; melt ponds on lobes could result from accumulation of melt from other lobe materials post-lobe emplacement. Regardless, we suggest a limited role for melt in lobe fluidization, as although proportionally more impact melt is produced in larger impacts (Cintala & Grieve, 1998), beyond >~15 km diameter craters, there is no obvious preferential increase in lobate form occurrence with crater size. Additionally, due to higher average impact speed, Mercury impacts should produce more impact melt on average than on the Moon, yet this is not reflected in lobe occurrence rates.

Without a clear factor in addition to topographic-related processes, it remains unclear why lobate forms are only identified in limited scenarios. One possibility is that material collapse into antecedent craters is occurring often, but generally does not result in lobate forms. Diffuse movement of material into an antecedent crater may occur, or limited slumps may be produced that do not fit our lobe identification criteria, or which may be hard to identify using remote sensing data. Additionally, for many overlapping craters, net flow of source crater material into the antecedent crater may occur through asymmetric internal collapse (Hood et al., 2024).

## 4.7. Planetary Comparisons

We have shown that the overall populations of lobate forms on Mercury and the Moon have similar occurrence rates and mobilities. Most lobate form categories are also represented in similar numbers on each body (Table 1). No influence of Mercury's more abundant volatiles is apparent and, given our proposed origins, is likely not a necessary component. Lobate forms on flat ground are potential exceptions, given their absence from the Moon, and only two examples on Mercury. Layered ejecta craters on Mars, a volatile rich body, are much more common, suggesting the Mercury examples are special cases.

Lobate forms have mobilities equivalent to long runout landslides, but gravity alone (as would be the case for landslides on a crater rim long after impact) may not be the only factor involved in mobilizing material. We have demonstrated that lobate form production is directly linked to the impact cratering process, which may explain the high mobility characteristics of these features (Crosta et al., 2018). Previous studies of long-runout landslides on Mars and the Earth, apparently not associated with impact events, conclude that such landslides require volatiles or fluid to generate the measured runout to height drop ratios (Legros, 2002). Terrestrial landslides appear to have
generally greater runout compared to martian landslides of equivalent volume, which may be related to fluid
availability (e.g., Johnson, 1978; Voight et al., 1983), or the effects of gravity on yield strength (McEwen, 1989).
However, some authors suggest that many large landslides, regardless of planetary context or volatile presence,
plot along the same volume to mobility trend (Lucas et al., 2014). Overall, the role of fluids or volatiles in
mobilizing long runout landslides remains contentious, as it does for fluidized ejecta. What is not contentious
though is that we have identified lobate forms with mobilities equivalent to long runout landslides, on both an
unambiguously volatile-poor body (the Moon) and on an essentially water-ice free world (Mercury, poles
excepted). Our results indicate that volatiles are not required to achieve these mobilities. Instead, the conditions or
energy released in an impact event may mobilize material to a degree not achieved in dry, non-impact related
conditions. However, lobate forms on Mercury and the Moon do have shorter runouts than martian and terrestrial
landslides of equivalent volume (Section 3.4.1), and lower mobilities than martian layered ejecta craters (Sec-
tion 3.4). This indicates volatiles could be increasing mobility of mass movements on Mars and the Earth,
especially given that Mars and Mercury have roughly equivalent gravity.

**5. Conclusions**

We have identified 98 lobate forms on Mercury and 92 on the Moon, a many-fold increase on previously reported numbers. We identify evidence for ground-hugging emplacement for these features, and find that 98% of examples extend into adjacent or overlapped antecedent craters. The formation of ~71% of lobate forms on these bodies appears to result from modification stage collapse into antecedent topography, whereas ~24% of examples are landslides on antecedent crater rims triggered by newer impacts. These origin ideas are supported by evidence for syn-impact formation, including impact melt identified on top of 15 lobate forms. Previous interpretations of these features were generally focused on either an ejecta flow or landslide interpretation, however we show that collapse during the modification stage of impact cratering can produce features with the observed mixed characteristics seen in lobate forms. Morphological variation (e.g., presence of source scarps) may indicate differences in the timing and extent of collapse, largely dependent on the degree of source and antecedent crater overlap, but lobe formation overall appears directly linked to the impact cratering process in almost all cases.

Lobate forms are therefore a result of the fundamental process of impact cratering onto bodies with uneven topography, and may be expected to form elsewhere in the Solar System. Future work could assess their occurrence on other bodies. Of particular interest may be Mars or Ceres, where mass flow features identified at the intersection between craters have already been recorded in landslide databases (Crosta et al., 2018; Duarte et al., 2019). We find no evidence for volatile involvement in the development of the mercurian and lunar features, and occurrence rates and mobilities are equivalent for both bodies. These mobilities are less than for similar martian features, so though this work demonstrates that volatiles are not a required component, they may still aid in fluidization of material elsewhere. We also identify two examples of fluidized, layered ejecta-like craters on flat ground on Mercury, but these have unclear origins.

New data from the upcoming orbital operations of the joint ESA-JAXA *BepiColombo* mission are likely to enhance understanding of mercurian lobate forms via higher resolution images and compositional information, bringing the data available in line with that of lunar examples. Increasing understanding of lobate forms on different bodies would aid in investigating more general planetary scale processes, and how they may alter with differing planetary conditions. Of particular relevance to lobate forms are the modification of transient craters, and the mechanisms involved in long runout mass movements.

**Conflict of Interest**

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

**Data Availability Statement**

Data used in this study is available from the United States Geological Survey (USGS) data portal "Astropedia: Lunar and Planetary cartographic catalog" (https://astrogeology.usgs.gov/search?target=&system=&p=1& MCCScope=&searchBar=). We also accessed additional data for Mercury from the NASA PDS Mercury Orbital Data Explorer (https://ode.rsl.wustl.edu/mercury/index.aspx). Images were also obtained from the USGS Planetary Image Locator Tool v4 (https://pilot wr.usgs.gov/). USGS Integrated Software for Images and Spectrometers was
Acknowledgments

A.J.B and A.R.L. are funded by the UK Research and Innovation Doctoral Training Partnership and formerly the Space Strategic Research Area at the Open University. J.W. acknowledges support from the European Space Agency (ESA) as an ESA Research Fellow and from the Horizon Europe program as a Marie Skłodowska-Curie Actions Postdoctoral Fellow, Grant 101208070, received during the write-up of this work. J.W. 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. We thank Giulia Magnarini and Gordon Osinski for their detailed and helpful reviews which greatly helped us to improve this manuscript.

used for processing image products (https://isis.astrogeology.usgs.gov/8.1.0/). Data analysis was primarily conducted in ArcGIS Pro version 2.9.5. With analysis of some lunar data using LROC QuickMap (https://quickmap.lroc.asu.edu).

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