Evidence for thermal-stress-induced rockfalls on Mars impact crater slopes

Abstract

Here we study rocks falling from exposed outcrops of bedrock, which have left tracks on the slope over which they have bounced and/or rolled, in fresh impact craters (1–10 km in diameter) on Mars. The presence of these tracks shows that these rocks have fallen relatively recently because aeolian processes are known to infill topographic lows over time. Mapping of rockfall tracks indicate trends in frequency with orientation, which in turn depend on the latitudinal position of the crater. Craters in the equatorial belt (between 15°N and 15°S) exhibit higher frequencies of rockfall on their north-south oriented slopes compared to their east-west ones. Craters >15° N/S have notably higher frequencies on their equator-facing slopes as opposed to the other orientations. We computed solar radiation on the surface of crater slopes to compare insolation patterns with the spatial distribution of rockfalls, and found statistically significant correlations between maximum diurnal insolation and rockfall frequency. Our results indicate that solar-induced thermal stress plays a more important role under relatively recent climate conditions in rock breakdown and preconditioning slopes for rockfalls than phase transitions of H2O or CO2, at mid- and equatorial-latitudes. Thermal stress should thus be considered as an important factor in promoting mass-wasting process on impact crater walls and other steep slopes on Mars.