While studying Pluto craters, I was faced with a somewhat confounding question: how would a crater form if you have a thick layer of volatile (methane or nitrogen) ice overlaying the much stronger water ice bedrock. I’ve been studying crater degradation, and it’s important to know the difference between a crater that has degraded vs a crater that just started degraded. That is what brought me to this, not so recent paper, Senft and Stewart (2008) that studied what water ice layers, at the surface and in the subsurface, do to the morphology of an impact crater that forms.
They studied a range of scenarios, but I want to focus primarly on the water ice surface and subsurface morphologies, at impact and right after impact. They assume the bedrock is basalt and the ice is water ice; they list the all the variables within the model (CTH, a Eulerian shock physics code) in their paper. Naturally, the strength of the material plays a major role in what the final morphology will look like, but the melting temperature (or pressure?) is also an important factor. That seems to be a major reason for the variations in morphology.
Ice appears to limit the lateral propagation of the shock wave/pressure, yet it enables larger shock waves to extend deeper down, on the hole. The highest pressures are not present in the water ice, but the smaller, but still large, pressures extend further. Of course, the net results is simply less extreme pressures at the bedrock surface. The addition of a subsurface layer interferes with the pressure wave but not as significantly. The ice seems to experience heating over a larger surface area, but the extremes are not as large as in the bedrock. Perhaps it relates to phase change (I don’t quite remember what they say in the paper). There should clearly be vaporization and melting of the water ice (almost all of it in fact), and that likely feeds into the morphologies they observed.
A change in morphology is almost inevitable when the pressures are reduced. The first thing that occurs is the formation of a less distinct rim, even with their thinnest ice layer (100 m). However, the rim that does form is sufficient to prevent the surface ice from (at least immediately) flowing back into the crater. However, there seems to be a critical thickness (maybe ice thickness > 0.5*crater depth) where the rim cannot prevent the surrounding ice from flowing back into the crater. It is almost fluid in nature, and perhaps it is, given the temperatures they predicted. I’m curious whether that fluidity would have happened if the ice was not melted. Naturally, I can’t help but wonder if this is applicable to a Pluto scenario with bedrock water ice and a surface volatile ice layer. See my paper for more discussion of that.
The team also considered a subsurface ice layer of varying depths and thicknesses. This scenario is interesting because I had suspected these morphologies for Pluto when I first started considering what craters would look like in volatile ice over stronger bedrock water ice. I thought the depression would leave volatile ice residue in the basin and on the walls, but that seems to only happen for subsurface layers. I suspect it relates to the bedrock basalt acting as a blockade to the surrounding ice, until a critical thickness is reached for surface ice. While in subsurface ice, the hinging of bedrock basalt does less to block the ice than it does to open the way for it like a door swinging open. The craters begin degraded with water ice in the base. As the layer thickens, the crater becomes less and less observable, but there is a characteristic sloping of the bedrock layer. This is an interesting observation that would be cool to observe directly.
Big picture, it is clear that volatile ices/layers can affect crater morphology. This isn’t exactly news as marine impacts produce similar results (Bray et al., 2022). I think it would still be advantageous to pursue this problem for Pluto specifically, but even the Mars work may be in need of a refresh given it has been nearly a decade an a half.