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

This blog was written by Prof. Peter Kokelaar (formally Liverpool University) and Ricci Bahia (University of Manchester SEES PhD student) about a new paper published in JGR-Planets. 

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The Moon is believed to have formed from the same giant impact event that formed the Earth, and also records a history of asteroid bombardment that affected the Earth throughout the past 4.5 billion years. Furthermore, it is a large-scale natural laboratory where surface processes occur without the presence of atmosphere or liquid, making it a relatively accessible analogue of other airless rocky planets and asteroids in our Solar System.

Recently very high resolution images (~0.5 meters per pixel) of the Moon have been taken from the LROC mission narrow angle camera, allowing for our new study of lunar avalanches to better understand planetary mass wasting processes.

The lunar surface primarily consists of two geological groups: the lunar highlands of primordial crust and the lunar maria of lavas formed from ancient volcanic eruptions. Would there be differences in the avalanches that occurred within these different terrains? The Moon also has much smaller gravity than Earth (0.16 g), so would this effect the style of avalanches and form of their deposits?

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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.

Our work focuses on seven case study lunar avalanche sites (see image above): four located on impact crater walls within the lunar maria, and three on crater walls within the lunar highlands. The avalanche record on the north-east wall of Kepler Crater (see image below right) is examined in detail, as it shows evidence of a variety and complexity of debris avalanche processes. We compared our findings with avalanche deposits found on Earth, and with the results from laboratory experiments, to detect differences and understand the lunar processes.

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Section of a debris-flow deposit on the north east wall of Kepler Crater. Part of LROC-NAC image M173165404RE courtesy of NASA/GSFC/ASU.

We found that many of the lunar avalanche deposits were similar to those found ‘in the field’ on Earth and in laboratory experiments, and we showed theoretically why this is to be expected. The reduced gravity on the Moon does not affect the morphology of the avalanche deposits, although the timescale of motion is longer on the Moon. We have identified a new type of avalanche deposit that has never been seen naturally ‘in the field’ on Earth, and the associated flow has only recently been discovered and understood from small-scale laboratory experiments (see image below). This new type of flow produces long and narrow ribbons of material, along tracks reflecting reworking of substrate, with minor levees and no coarse terminations. We deduce that these ribbon-like deposits result from granular erosion-deposition waves that propagate down repose-angled slopes dominated by fine-grained particles (i.e., lunar regolith). The extremely long runout of flows on the Moon simply results from the existence there of very long granular slopes close to the material angle of stable repose, a situation that does not occur on Earth. Our work establishes the variety of flow deposit types that can arise where there is no influence of atmosphere or liquid, and thus it benchmarks the nature of dry granular flow types and their deposits. This has important bearing on inferences of liquid involvement in debris flows (e.g., gullies) on other planets, such as Mars.

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Lunar granular avalanche deposits and experimental analogs. a. Deposit fingers near the foot of Bessel impact 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

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Paper reference: Kokelaar B. P., Bahia R. S., Joy K. H., Viroulet S. and Gray J. M. N. T. (2017) Granular avalanches on the Moon: Mass-wasting conditions, processes and features. Journal of Geophysical Research: Planets. DOI: 10.1002/2017JE005320

The paper is now online in JGR-Planets

 


The paper started through Pete inviting Katie to give a Herdman seminar at the University of Liverpool about the Moon, and a discussion afterwards about lunar debris flows. The topic then became the focus of Ricci’s research for his Geology with Planetary Science 4th year MSci project and further developed into a wider collaboration with researchers from the University of Liverpool and SEES and the School of Maths at the University of Manchester. 

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Additional  resources

LROC posts about lunar debris flow observations: http://lroc.sese.asu.edu/posts/583  ; http://lroc.sese.asu.edu/posts/329  ;  http://lroc.sese.asu.edu/posts/334

Access LROC NAC images via quickmap: http://target.lroc.asu.edu/q3/

Lunar surface resources http://www.lpi.usra.edu/lunar/surface/ and a 101 lecture about lunar surfaces can be accessed via http://www.lpi.usra.edu/lunar/moon101/

Prof Peter Kokelaar research ( https://www.liverpool.ac.uk/environmental-sciences/staff/brian-kokelaar/

Prof Nico Gray’s reseach http://www.maths.manchester.ac.uk/~ngray/

Dr Katherine Joy’s research https://www.research.manchester.ac.uk/portal/katherine.joy.html

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About Katherine Joy

Hello! I am Katherine Joy. I am part of the University of Manchester Isotope Geochemistry and Cosmochemistry group. More details about my research interests can be found at http://www.seaes.manchester.ac.uk/people/staff/profile/?ea=katherine.joy
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