Are day-night heating cycles a trigger for launching the “stones” on tour?

Boulders are a major surface feature on solid planets and small bodies, including asteroids and comets. Interest in these clasts range from applications relevant for landing site selection to geomechanical parameter characterization of the soil on which they rest [1], to measurements of their size frequency distributions [2] which is relevant for an understanding of their formation and erosion processes. On the Moon boulders are generally found in association with craters, hilltops, rilles, and other steep relief forms. Two main mechanisms of boulder formation are bedrock fragmentation and excavation by impacts, and progressive exposure of pre-existing blocks and fractured bedrock by removal of regolith from steep reliefs by diffusive creep.

An important issue are transport processes which can move the stones on the surface of their parent bodies. On the Moon, one group of boulders, frequently called “rolling stones”, have left tracks on the surface which can cover large distances. Mainly two mechanisms, meteoritic impact and moonquakes [3], have been cited in the literature as drivers of boulder displacements. Much less attention has been given to the hypothesis that other processes like thermal solar-induced rock breakdown [4] could deliver the initial momenta that could initiate the movement of meta stabile rocks.

From an AI -based mapping of the distribution of boulders with tracks on the lunar surface [5] we know that the majority of these boulders are found – not surprisingly - within craters. However, as the AI-based procedure strongly underestimated the number of boulders with tracks, we have conducted a new investigation to map these boulders. However, such a mapping it is only one prerequisite in understanding whether a thermally-induced breakdown could be responsible for an initial triggering of boulder movements. Boulders moving down the slopes disturb the mature regolith and move fresh lunar soil to the surface. This process should remotely be detectable through the stronger spectral features of the fresher optically immature regolith. The number of non-decayed boulders along crater walls should therefore be correlated with the strength of the absorption bands in spectra taken from those crater walls. Spectral characteristics of the refreshed crater walls are measurable through various quantities in the VIS-NIR (e.g. color ratios, etc.)

To start addressing the question to what extent a solar-induced breakdown can trigger rock movements, we have chosen lunar craters for which we have generated new boulder maps. For these craters we determine spectral characteristics and mineralogical composition based on a nonlinear spectral mixing model using M3 hyperspectral imager data from Chandrayaan-1. We are reporting the first results of spectral feature mapping for these craters and discuss the mineralogical interpretation, as well as the existence of a correlation between the number of observable boulders inside craters and identified spectral features of the regolith.

References:

[1] Filice, A., 1967, Science, 1967-06-16 156(3781): 1486-1487. [2] Ruesch, O. et al., 2022 Icarus, 387, 115200. [3] Kumar, S. et al., 2016, J. Geophys. Res. Planets, 121, 147– 179. [4] Molaro, J.L. et al., 2017, Icarus, 294, 247-261. [5] Bickel, V.T. et al., 2020, Nat Commun 11, 2862.