Modification of martian impact craters by ice-related processes as a function of age and latitude

The role of ice in sculpting craters on Mars remains uncertain, particularly regarding erosion of equatorial craters during early martian history. We review the implications of an evolving view of ice-related crater erosion processes during the Hesperian and Amazonian periods as clues to early equatorial Mars. Ice accumulation on the rims of scattered equatorial craters led to sculpting of deep valleys on interior walls and alluvial fan sedimentation on crater floors. Whether the ice occurred as repeated short-term shallow accumulation and melting or as persistent valley glaciers is uncertain, because of post-ice crater wall degradation. Consideration of the effects of gravity on erosion related to glacial ice, however, suggests that basal slip and scour by martian glaciers is substantially reduced, whereas subglacial runoff and erosion is enhanced due to reduced closing of cavities by ice inflow, increasing the likelihood that episodic valley glaciation runoff sourced the Hesperian alluvial fans, such as in Saheki crater. A set of craters on the northeastern rim of the Hellas basin at about 30S, 83E, centered at Batson crater, provide evidence of widespread valley and plateau glaciation manifested primarily in fluvial and lacustrine landforms. In addition to several remnant valley glaciers and a few small moraines, more extensive glacial activity prior to ~600 Ma is primarily recorded in alluvial fans and deltas on the crater floors, deep crater rim valleys like those hosting equatorial alluvial fans, as well as probable tunnel valleys, eskers, and widespread hummocky terrain likely sculpted by subglacial runoff. In the central mid-latitudes (~40), many craters presently host thick ice-rich deposits on crater floors, often exhibiting multiple episodes of glacial flow originating from crater walls and central peaks. Many craters larger than 20 km in diameter also provide evidence of episodic fluvial activity in channelized valleys and fan-like deposits, although the degree to which fluvial flows have eroded the crater walls and floors is unclear because of the modern ice cover. Landforms including valleys that are relatively shallow and morphologically fresh in appearance, and craters with exit breach channels on their exterior crater walls, may have been sculpted beneath a thick ice cover. At the higher mid-latitudes (>50) crater interiors, rims, and inter-crater plains are largely encased in ice-rich deposits, so that any past fluvial activity is obscured. Most degraded craters in the equatorial region lack obvious deep crater wall valley incision, fans, deltas or other signatures of fluvio-glacial activity, indicating the general lack of glacial sculpting, with a few exceptions such as Dawes crater. Post-Noachian degradation may partially explain the general lack of glacier-related features, but the characteristic morphology of most equatorial craters suggests long-term, weathering-limited degradation under arid conditions with a short period of enhanced runoff during the Noachian-Hesperian transition without deep accumulation of ice. The strong crater degradation and associated sedimentation across the cratered highlands is likewise inconsistent with a Noachian deep ice cover.