WAS ICE PUSHING AN IMPORTANT GEOMORPHIC PROCESS IN ANCIENT MARTIAN LACUSTRINE AND MARINE SETTINGS?

The Mars rovers have found evidence of ancient liquid water as near-surface groundwater or fluvial features (Perseverance Valley) or as fluvial and lacustrine sediments.

Even with 25cm/pixel images, it isn’t obvious from orbit that surface water played a role in the development of these landing sites. At Gale crater, for example, though fluvial and lacustrine sediments have been positively identified, the morphology along the traverse to date suggests eolian deflation of these ancient materials.

Opportunity discovered sedimentary bedforms and stratigraphy, and aqueous geochemistry indicating at least areally- or temporally-limited lacustrine environments. We compare three features at Endeavour crater with ice scour- and push-related morphologies in terrestrial cold climate settings:

1) Subtle ridges and troughs on the west flanks of Endeavour crater. The ridges are sometimes visible in HiRISE images, but appear similar to wind streaks elsewhere on Mars. From the rover, they consist of rows of rocks ranging from pebbles to boulders in size. The ridges don’t appear to be structurally-controlled in orientation. The troughs are shallow features, typically a few centimeters deep and a few tens of centimeters wide. With the exception of longitudinal troughs in Marathon Valley, most of these aren’t identifiable from orbit. Ridges and troughs are typically oriented east-west near the crater rim, though more subtle troughs in Marathon Valley show no preferred orientation.

2) The Endeavour crater rim appears weakly stratified, with apparent dips downward from the rim crest toward the east into the crater and to the west outward from the crater rim.

3) Many bedrock surfaces appear scoured at scales of centimeters to a meter in width. The scour marks indicate a direction up to the rim crest from the west outside and from the east inside the crater. Scoured surfaces may appear “stacked” with the stratrigaphy, with successive offlaps being “out of plane” with one another. Where scour marks meet at the crater rim crest, the change in indicated flow direction is almost instantaneous. The scoured surface appears to cross the tops of disconnected patches of outcrop, where sides of blocks don’t appear scoured or streamlined – suggesting that they are either not wind-related or are of great age (surrounding material has been deflated).