POSSIBLE ICE PUSH STRUCTURES AND GEOMORPHOLOGY AT ENDEAVOUR CRATER, MARS

Over the last four years, the Opportunity rover has been exploring the west rim of the 20-kilometer diameter crater Endeavour. The rim of Endeavour consists of a broken ring of inselbergs a few meters to a few hundred meters in height above the surrounding and onlapping Burns and Grasberg Formation sulfates. The rim inselbergs appear rugged and blocky, similar to the highland terrain a few tens of kilometers south of Endeavour. Along the Opportunity traverse, the Endeavour rim materials, more than several meters above the Grasberg “bench” in elevation, are striated by ridges and grooves oriented in a generally east-west direction. This striation occurs at all scales in Opportunity imagery, from the panorama and navigation cameras to the microscopic imager. The largest striations consist of subtle ridges and grooves that can extend up to a hundred meters or more in length. The ridges consist of linear piles of pebble- and cobble-size lithic fragments. The smallest striations are millimeter to centimeter-scale structures resembling tool marks on rock surfaces imaged by the rover’s Microscopic Imager. At the crest of the Endeavour rim, blocks up to boulder-size appear imbricated toward the rim crest, with an inferred flow direction from the west outside the crater and from the east inside the crater, and often meet at the crest. The Ridges and grooves, most rock fluting, and apparent dip slope stratigraphy are interpreted to be the result of floating lake or sea ice being shoved back and forth across the crater rim as winds shift direction, or as the ice expands and contracts laterally with changes in temperature. If this interpretation is correct, it has very specific implications about climate at the time the process was active.