DEFINING GEOMORPHOLOGIC CRITERIA TO IDENTIFY SITES WITH HIGH BIOSIGNATURE PRESERVATION POTENTIAL ON MARS

Cosmogenic radiation can destroy biosignatures that could exist on Mars to depths of 2 m below the surface. Therefore, outcrops that have recently been exposed to the surface from beneath ~2 m of overburden will have an increased potential for preserving biosignatures on Mars, and such outcrops will be astrobiologically-important exploration targets for future missions. In this study, we aim to define methods that can be used with orbital imagery to identify bedrock targets that have been recently exposed due to active scarp retreat. These methods can ideally be utilized to locate outcrops with the highest potential for biosignature preservation at candidate landing sites for the Mars 2020 rover. We propose that active erosion and scarp retreat can be identified in orbital images with analyses of scarp orientation, boulder shedding, and visible color (to constrain relative amounts of dust deposition from red/blue color variations). To test this methodology, we have performed a pilot study at Gale crater with ground truth from the Mars Science Laboratory (MSL) in Gale crater, Mars. MSL’s SAM instrument measured a cosmogenic exposure age of 78 ± 30 Ma at Yellowknife bay (YKB) [1], where the Sheepbed mudstone surface was exposed by parallel scarp retreat beneath the Gillespie sandstone, perpendicular to the dominant SW wind direction at Gale crater. Boulder shedding from the active scarp, small-scale streamlined ridges in the primary wind direction, and freshly exposed surfaces were observed at YKB by MSL and from orbit. For this study, we mapped for scarp orientation, boulder shedding and visible color in orbital HiRISE images of a 5.76 km² region surrounding the MSL traverse. Our mapping indicates that the dominant scarp orientation across the study region is generally NE-SW (average N55E), parallel to the known dominant wind direction. This result suggests that regional trends in scarp orientation, as observed in orbital images, can be indicative of a dominant direction of erosion, and therefore could be used to identify retreating scarps and recently exposed bedrock. Applying this methodology to other regions on Mars may help prioritize landing site selection for the Mars 2020 mission and could aid future missions in detecting signs of past life.

[1] Farley et al. (2013) Science, doi: 10.1126/science.1247166.