Section of a debris-flow deposit on the NE wall of Kepler Crater. Part of LROC-NAC image M173165404RE courtesy of NASA/GSFC/ASU.
Context image depicting the location of all granular avalanche sites investigated. Lunar LRO LROC-WAC Mosaic global 100 m image courtesy of NASA/GSFC/ASU.
Lunar granular avalanche deposits and experimental analogs. a. Deposit fingers near the foot of Bessel crater wall. b. Experimental deposit from a single release of an 80/20 fine ballotini/coarse carborundum mixture onto a roughened 27° slope. Granular segregation formed a coarse resistive flow front that developed fingering instabilities and deposits similar to Bessel examples (a), although orders smaller in scale. c. Deposits in Virtanen F crater comprising numerous ribbon-like fingers with no distinct levees or coarse terminations. d. Experimental granular wave traveling down an erodible layer of the same material at its repose angle (carborundum, 35.2°). Moving grains in the wave are blurred and behind it slight lateral levees form with an intervening trough that is just below the level of the original surface. Such waves, single or in series, are inferred to form the ribbon-like trails in Virtanen F crater (c). e. Experimental granular wave formed by dumping yellow sand onto a layer of red sand at its angle of repose (34°). The wave steadily erodes substrate at its front while re-depositing material in its wake, so that the triggering (yellow) mass is gradually left behind. Such waves can involve minor triggers and yet travel far on erodible layers at repose angles, such as are common in lunar craters. f. Kepler crater wall with avalanche deposits extending across darker, relatively coarse talus that accumulated on former ledges and in places was remobilized to form downwards-tapering dark slope streaks. g. Experiment (overhead view) devised to simulate remobilization of lunar crater-wall talus by avalanches of finer debris. Coarse black sand rests at repose at 31.7° on a narrow ledge surrounded by steeper slopes. An avalanche of fine white ballotini causes the sand to mobilize and form dark streaks while a mixture continues to lesser slopes. Images a, c and f courtesy of NASA/GSFC/ASU; b and d also in Baker et al. 2016 and Edwards et al. 2017