Calef pointed out that over 1000 m of sediments fill the southern portion of Gale Crater. He then posed the question of what’s beneath them. Most certainly there are impact melt breccias—rocks melted from the shock/heat of the impact event—but how deeply are they buried? And how deep was Gale before it was filled, buried, and now exhumed? These are all big open questions when it comes to Gale.

Nicholas Warner of the State University of New York at Genesco spoke next on regolith thickness estimates at within the proposed InSight landing ellipse(s) in Elysium Planitia. His estimates for the regolith thickness were 3 to 5 meters. This is within the depth range to which InSight’s heat probe can penetrate—good news, because InSight’s engineers don’t expect the probe to be able to penetrate bedrock. So, thicker regolith means a greater penetration depth for the probe. He observed that some craters in the area have “rocky” ejecta—littered with boulders—while others don’t. So, he looked to see if there was a size cut-off at which “rocky ejecta craters” did vs. didn’t form. After analyzing over 3000 craters, he found that only craters larger than 200 meters in diameter had rocky ejecta. This suggests that these craters are excavating an additional layer in the subsurface relative to smaller craters: One that erodes into boulders, and one that does not. This tells us about the material’s resistance to erosion. Warner found that craters less than 800 million years old within the landing ellipses still preserved boulders on their ejecta blankets. This leaves the question of what happened to smaller craters with rocky ejecta. The calculated degradation rates in this region are too low to have caused the complete destruction of small craters and boulders, so they haven’t simply eroded away. It could be matter of image resolution limitations and/or lighting conditions, or the smaller craters might not excavate to the depth of the boulder-producing unit in the subsurface.

The rest of the session focused on gullies and recurring slope lineae. Matthew Sylvest of the University of Arkansas showed experimental results for gully slope constraints on Mars based on carbon-dioxide-sublimation-induced granular flows. In a small chamber under martian conditions, carbon dioxide frost was sublimated and then condensed onto regolith slopes consisting of a Mars soil simulant and two different grain sizes of sand. Videos of these experiments showed that carbon dioxide frost sublimation off the slopes triggered mass wasting—that is, landslides (but on a VERY small scale in the lab compared to nature). Interestingly, the greatest levels of mass wasting activity occurred on areas of the slope devoid of frost. In some rare cases, “large” carbon dioxide ice blocks dislodged on the slope. Across the three regolith types, the Mars soil simulant experienced the most mass wasting compared to the fine and coarse-grained sand. Sublimation triggered mass wasting on slopes as low as ~13°. This is significant as that’s way below what’s called the “angle of repose” for dry material. The angle of repose is the maximum angle at which entirely dry material can sit before sliding downhill under the force of gravity.