Granular avalanches on the Moon: Mass‐wasting conditions, processes, and features

Abstract

Seven lunar crater sites of granular avalanches are studied utilizing high‐resolution images (0.42–1.3 m/pixel) from the Lunar Reconnaissance Orbiter Camera; one, in Kepler crater, is examined in detail. All the sites are slopes of debris extensively aggraded by frictional freezing at their dynamic angle of repose, four in craters formed in basaltic mare and three in the anorthositic highlands. Diverse styles of mass wasting occur, and three types of dry‐debris flow deposit are recognized: (1) multiple channel‐and‐lobe type, with coarse‐grained levees and lobate terminations that impound finer debris, (2) single‐surge polylobate type, with subparallel arrays of lobes and fingers with segregated coarse‐grained margins, and (3) multiple‐ribbon type, with tracks reflecting reworked substrate, minor levees, and no coarse terminations. The latter type results from propagation of granular erosion‐deposition waves down slopes dominantly of fine regolith, and it is the first recognized natural example. Dimensions, architectures, and granular segregation styles of the two coarse‐grained deposit types are like those formed in natural and experimental avalanches on Earth, although the timescale of motion differs due to the reduced gravity. Influences of reduced gravity and fine‐grained regolith on dynamics of granular flow and deposition appear slight, but we distinguish, for the first time, extensive remobilization of coarse talus by inundation with finer debris. The (few) sites show no clear difference attributable to the contrasting mare basalt and highland megaregolith host rocks and their fragmentation. This lunar study offers a benchmarking of deposit types that can be attributed to formation without influence of liquid or gas.

Martian gullies: a comprehensive review of observations, mechanisms and insights from Earth analogues

“…Conway and Balme (2016) compared the morphometries of the catchments of martian gullies to dry mass wasting features on Earth (talus slopes), on the Moon and ungullied crater walls on Mars and found that martian gullies were statistically dissimilar from these nominally "dry" landforms. The runout of dry granular flows should not extend very far beyond slopes greater than the dynamic angle of repose (~20°; Kleinhans et al, 2011;Pouliquen, 1999) which has been confirmed to be the case for dry avalanches on the Moon (Kokelaar et al, 2017) yet the majority of martian gully-fans are shallower than this. Conway et al, 2015a, measured a median slope of 14° for 67 gully-fans and Kolb et al (2010) concluded 72% of the 76 fans they studied were likely emplaced by fluidised flows.…”

“…In the most generic sense martian gullies are a form of gravity-driven mass wasting system, where material is removed from the top and transported towards the base of the hillslope. However, they are distinguishable from simple fall-deposits (scree, talus or colluvium) by the presence of the transport channel and/or chute as pointed out by Malin and Edgett (2000) and the presence of a depositional fan below the dynamic angle of repose (Kokelaar et al, 2017). Hence, it has generally been acknowledged that a channel is the essential attribute for identifying a martian gully (e.g., Balme et al, 2006).…”

Landslides on Ceres: Inferences Into Ice Content and Layering in the Upper Crust

“…A reduction in friction during transport can occur via various proposed mechanisms (De Blasio, ; Legros, ; Voight et al, ). These include fluidization from air (Fahnestock, ; Kent, ; Shreve, ; Shreve, , ), suspension by particle distributions or vibrations (Hsü, ; Kokelaar et al, ; Melosh, ), and an increased mobility by warmed ice or liquid water (De Blasio, ; Erismann & Abele, ; Legros, ; Singer et al, ). The latter is commonly invoked for landslides on other planets (De Blasio, ; Lucchitta, ; Singer et al, ).…”

Spectral Trends of the Surface Regolith in Lunar Craters

“…After a crater has been formed, its steep walls collapse gravitationally over time by mass wasting and topographic diffusion by continuous impact meteoroids on its surface; thermal contraction/expansion and seismic shaking due to internal geodynamic processes or ongoing bombardments nearby may also contribute to crater degradation (e.g., Fassett & Thomson, ; Kokelaar et al, ; Melosh, ). Fassett and Thomson () evaluated the degradation state of small mare craters with diameters ranging from 800 to 5 km by solving diffusion equations that fit topographic profiles of craters.…”