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