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

B. P. Kokelaar , R. S. Bahia , K. H. Joy , S. Viroulet (1) , J. M. N. T. Gray

1 IPGP - Institut de Physique du Globe de Paris

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.

Mots clés: Moon; mass wasting; debris flow; granular avalanche; granular segregation; lunar regolith

Domaines: Planète et Univers [physics]

B. P. Kokelaar, R. S. Bahia, K. H. Joy, S. Viroulet, J. M. N. T. Gray. Granular avalanches on the Moon: Mass-wasting conditions, processes, and features. Journal of Geophysical Research. Planets, 2017, 122, pp.1893-1925. ⟨10.1002/2017JE005320⟩. ⟨insu-03748840⟩