The Moon’s Rolling Stones

Post by Valentin Bickel, PhD student, Department of Earth Sciences, ETH Zurich, CH & Department Planets and Comets, Max Planck Institute for Solar System Research, GER.

One of the most intriguing objects on the surface of the Moon are the “rolling stones”, also known as lunar rockfalls or rolling boulders (Image 1). These boulders are abundant all over the Moon and have sizes that range from a couple of meters to several 10s of meters. Lunar boulders are believed to be displaced by moonquakes or impacts and can carve tracks with lengths that range from a couple of meters to several kilometers (Image 1; Xiao et al., 2013; Kumar et al., 2016). Besides their value for geomorphological analyses, these boulder tracks provide insights into the mechanical behavior and the trafficability of the lunar “soil”, the regolith (Bickel et al., 2019).


Image 1: A number of large and small boulders with tracks at the bottom of a lunar slope. The analysis of tracks provides insights about the mechanical properties of the regolith and is performed using high-resolution satellite imagery, taken by NASA’s Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC).  Detail of NAC Image M113934119LC.

For example, a rolling boulder of identical size would sink more in a soft material than in a stiff material. While two major regions of the Moon have been successfully traversed by astronauts and rovers in the past (mare and highland regions), new high-priority targets for exploration and resource extraction such as pyroclastic deposits and polar permanently shadowed regions have almost completely unknown mechanical surface properties – we simply have never been there before. This lack of knowledge poses a risk for any future human or robotic mission, e.g. if mobile assets encounter unfavorable wheel sinkage and are rendered immobile. Unfavorable soil conditions have caused complications during the Apollo 15 and Soviet Lunokhod 2 missions (Costes et al., 1972) and forced NASA to abandon the Spirit rover on Mars that became trapped in soft soil. Boulder tracks provide insights into the local surface properties and allow us to mitigate risk, directly supporting our efforts to design a new generation of rovers for the exploration of terra incognita.

One measure for the regolith’s trafficability is its bearing capacity.  This parameter describes the ability of a soil to bear a load, such as a rover. This capacity can be derived using the dimensions of boulder tracks in relation to the boulders that carved them. These observations are then combined with basic soil properties, such as the angle of internal friction, to determine the ultimate bearing capacity of the soil at the respective location. Besides this quantitative estimation, a qualitative comparison of boulder track appearance can indicate similarities or differences in the mechanical properties across different regions as well (Image 2).


Image 2: A qualitative comparison of more recent (top row, distinct tracks) and older boulder track appearances (bottom row, slightly faint tracks) between mare, highland, and LPD regions does not show any significant differences, suggesting similar geomechanical properties in all analyzed regions.

Our analysis of boulder tracks shows that the pyroclastic regions of the Moon appear to have bearing capacities equal to or higher than mare and highland regions. While bearing capacity describes only one aspect of trafficability, this result increases our confidence that ground missions in these highly interesting regions are generally feasible (Bickel et al., 2019), paving the way for future science as well as resource extraction operations.

Further reading

Bickel, V.T. et al. (2019), Analysis of Lunar Boulder Tracks: Implications for Trafficability of Pyroclastic Deposits. JGR: Planets, 124(5), 1296-1314.

Costes, N. C. et al. (1972). Mobility performance of the lunar roving vehicle: Terrestrial studies – Apollo 15 Results. NASA Technical Report (December), p.87.

Kumar, P.S. et al. (2016), Recent shallow moonquake and impact‐triggered boulder falls on the Moon: New insights from the Schrödinger basin. JGR: Planets, 121(2), 147-179

Xiao, Z. et al. (2013), Mass wasting features on the Moon – how active is the lunar surface? EPSL, 376, 1-11.

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