# Mass Wasting on Mercury - Regolith Properties and Volatile Content

Susan J. Conway, Camille Aubry, Chris Malliband, Valentina Galluzzi, Lorenza Giacomini, Jack Wright, David A Rothery

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Susan J. Conway, Camille Aubry, Chris Malliband, Valentina Galluzzi, Lorenza Giacomini, et al. "Mass Wasting on Mercury - Regolith Properties and Volatile Content." 56th Lunar and Planetary Science Conference, LPI, Mar 2025, The Woodlands (Texas), United States. [hal-05395821](https://hal.science/hal-05395821v1)

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Submitted on 3 Dec 2025

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Distributed under a Creative Commons Attribution 4.0 International License
**MASS WASTING ON MERCURY – REGOLITH PROPERTIES AND VOLATILE CONTENT** S. J. Conway¹,
C. Aubry¹,², C. Malliband³, V. Galluzzi⁴, L. Giacomini⁴, J. Wright³ and D. A. Rothery³ ¹Nantes Université, l'Université d'Angers, Le Mans Université, CNRS UMR6112 Laboratoire de Planétologie et Géodynamique, France (susan.conway@univ-nantes.fr), ²Géosciences Le Mans, Le Mans Université, France. ³School of Physical Sciences, Open University, UK ⁴Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy

**Introduction:** Data from NASA's MERCURY Surface, Space ENVironment, GEochemistry, and Ranging - MESSENGER - spacecraft revealed small-scale (~100s metres to 10s kilometres) surface features such as faculae [1] and hollows [2] showing the influence of volatiles in shaping the hermean landscape. On other bodies, such as Mars and Vesta [3,4], downslope mass wasting can be driven by volatiles, hence the objective of this study is to investigate the possible relation between volatiles and mass wasting on Mercury.

**Approach:** We performed a survey of MESSENGER Mercury Dual Imaging System (DIS) Narrow Angle Camera (NAC) images with a spatial resolution better than 100 m/pix between 20°N and 60°N latitudes. These images cover 53% of the surface of Mercury at these latitudes. We did not extend the survey further north to avoid deep shadows, nor further south where images at better than 100 m/pix are rare, because of MESSENGER's elliptical orbit. Steep slopes were searched within impact craters and volcanic vents. We systematically catalogued the presence of: i) spur-and-gully morphology - this manifests as regularly spaced alternating high relief spurs and depressions extending downwards from the slope crest (Fig. 1a), and ii) downslope-oriented high albedo lineae - these are characterized by relatively bright streaks extending downslope from at or near the slope crest, ending mid-slope or at the foot of the slope with or without associated relief (Fig. 1b,c).

**Results:** 23 sites were documented with spur-and-gully morphology, 141 with lineae and 10 sites with both. Spur-and-gully morphology is only found within

volcanic plains and on steep slopes with visible talus aprons. It is only clearly visible in images with medium to high incidence angles, and at spatial resolutions better than ~40 m/pix. The Nathair Facula vent hosts a particular morphology of spur-and-gully morphology, where channels extend to mid-slope and end in distinctive debris cones [5] – hence are not included in this survey. Lineae are found at all latitudes and longitudes with two notable clusters, one in Rustaveli a ~200 km diameter peak-ring basin and another at Budh Planitia directly north of Phidias Crater. Lineae are most easily visible in low incidence images, but can be seen in images with medium and low incidence angles. They can be found on any sloping terrain and are visible in images up to and sometimes even beyond 100 m/pix.

**Discussion:** Spur-and-gully morphology is formed by the natural degradation of competent bedrock or ice exposures [e.g. 6]. On the Moon this morphology is rare [7] probably because of the thick regolith. On the Moon spur-and-gully morphology is only visible in 50 cm/pix images and is located partway downslope in steep craters within the mare materials (i.e. volcanic plains). On Mercury the regolith is thought to be 3x thicker than on the Moon [8], so the finding of widely distributed spur-and-gully morphology is surprising and suggests either the regolith is locally thinner than predicted, or there is another local factor that consolidates the regolith (e.g. sintering [9], fluid circulation, magma intrusion, or something else). Higher resolution imaging by BepiColombo should reveal more occurrences of spur-and-gully morphology, as well as further details on their characteristics (e.g., presence of layering).

<figure><img src="image_2.png" /><figcaption>Figure 1: MESSENGER NAC images of: (a) spur-and-gully morphology on the steep wall of Waters Crater, (b) bright lineae on a crater wall, and (c) bright lineae on a rounded, cratered peak-ring element in Rustaveli Basin.</figcaption></figure>
The lineae on Mercury can be most simply explained by differential erosion of materials with different albedos down steep slopes without any involvement of volatiles. Although lineae have a similar spectral signature to hollows [2], they are only rarely found emanating from hollows. Many hollows at the tops of steep hillslopes do not produce any lineae, despite this configuration being commonplace. On the other hand, some lineae are found on shallow degraded hillslopes (Fig. 1c), where factors other than gravity would be required to initiate motion, suggesting an involvement of volatiles. Higher resolution imaging by BepiColombo will help to determine if lineae show other signs of volatile involvement.

The two clusters of lineae are found in some of the smoothest plains materials on Mercury. Rustaveli is one of the youngest basins on Mercury [10] and is associated with plains that are smooth at the km-scale from laser altimeter data [11]. Budh Planitia is notably radar bright [12] and so also likely smooth at the km-scale, which can be verified by BepiColombo's laser altimeter. Lineae are visible in craters that punch through these young plains materials. Hence, it is seems likely these plains are the origin for the bright materials. However, it is unclear if these plains have a distinctive composition or whether the bright materials are better preserved because of their young age.

**Conclusion:** We report the discovery of widely distributed spur-and-gully morphology within fresh

craters and vents in volcanic plains on Mercury, likely revealing thinner or more consolidated regolith in these areas. Bright lineae are common on sloping terrains across the whole of Mercury and sometimes show evidence for volatile involvement. Two clusters of lineae are found in Rustaveli Basin and Budh Planitia – areas with young smooth plains – hinting at an enrichment in bright materials in the subsurface in these two areas. Both mass wasting morphologies warrant further investigation by the upcoming BepiColombo mission.

**Acknowledgments:** Thanks to image analysis tools: J-Mars, USGS Planetary Image Locator Tool (PILOT) and Projection on the Web (PoW). SJC is supported for her BepiColombo related work by the French Space Agency, CNES.

**References:** [1] L. Kerber et al. (2011) *PSS, 59*, 1895–1909. [2] D.T. Blewett et al. (2011) *Science*, **333**, 1856–1859. [3] J.E.C. Scully et al. (2015) *EPSL, 411*, 151–163. [4] L. Roelofs et al. (2024) *Commun. Earth Environ.*, **5**, 125. [5] D.T. Blewett et al. (2018) *Chapt. 12 in “Mercury” CUP*, pp.324–345. [6] N. Le Becq et al. (2025) *Icarus*, **428**, 116353. [7] J.N. Levin et al. (2022) *Icarus*, **373**, 114774. [8] M.A. Kreslavsky et al. (2014) *GRL*, **41**, 8245–8251. [9] A.Yu. Zharkova et al. (2020) *Icarus*, **351**, 113945. [10] V. Galluzzi et al. (2021) *GRL*, **48**, e2020GL091767. [11] W. Fa et al. (2016) *GRL*, **43**, 3078–3087. [12] J.K. Harmon et al. (2007) *Icarus*, **187**, 374–405.

<figure><img src="image_3.png" /><figcaption>Figure 2: MESSENGER MDIS Wide Angle Camera (WAC) enhanced colour mosaic and mapped lineae and spur-and-gully morphology. Lighter tones indicate the footprint of MDIS NAC frames at better than 100 m/pix.</figcaption></figure>