Crater morphology and evolution at the Phoenix landing site: Insights into timing of modern geological processes and subsurface ice

Abstract

We report results of an in-depth study of the morphology of small (km-scale and smaller) craters over a 4000 km2 region on Mars that includes the Phoenix landing ellipse, focused on the nature and timing of geological processes that have been active in the region. The study seeks to constrain regolith depositional rates, thickness of ground ice layer, abundance of ice in the subsurface, and the rate of crater degradation. Measurements of the size of ejecta blocks around craters show that the region has not been a net-depositional site over the last billion years. Most craters smaller than 200–350 m lack ejecta blocks regardless of age, indicating that there is a layer approximately 20–35 m deep that either does not form or does not retain ejecta blocks. This layer is expressed both within and outside the extended ejecta blanket of Heimdal crater, implying that it is not associated with the ejecta blanket. We propose that this unconsolidated layer consists dominantly of ice-cemented soil, which raises questions about the emplacement of ice at depth. We also note clear progression in crater degradation morphology, which allows us to place constraints on the rate and style of crater degradation. Small craters (20 to 200 m in diameter) appear to lose their relief due to slow infilling of the bowl over time scales of 5–100 kyr. The infilling of small craters, perhaps by a combination of ice and dust, presents a mechanism by which ice is buried down to a few tens of meters. On the other hand, larger craters (~200 m to 1 km) do not exhibit evidence for infilling, but do exhibit concentric sets of fractures within and outside the rim. We propose that loss of relief of the larger craters is due to viscous creep over time scales of a few to tens of Myr. This latter interpretation would require the presence of water ice in abundances greater than pore-filling down to depths of at least tens of meters.