Many bedrock plains are clastic rocks, not lavas

figure-1The Martian surface is dominated by loose dust, sands, and rocks, but high spatial resolution imaging has permitted the detection of numerous flat-lying exposures of ancient, intact bedrock. These “bedrock plains” have previously been interpreted as lava sequences, perhaps similar to lava plains found in the dark parts of the lunar nearside. Here we [Rogers, et al.] show evidence [in Geophysical Research Letters] that bedrock plains may instead be composed of sedimentary rocks, airfall volcanic ash, or impact-generated airfall materials.

First, the bedrock plains should have developed a thick regolith over time, due to repeated pummeling by impactors over billions of years. But they lack a regolith, suggesting that they break up easily into fine particles that are then easily moved away from the bedrock by wind. Second, the bedrock plains show morphologies that are similar to wind-eroded soft rocks on Earth. Third, the bedrock plains have fewer small craters than adjacent surfaces, likely due to the relative ease in which craters can be erased through erosion.

Bedrock plains are found at all of the proposed landing sites for the upcoming Mars2020 rover; nonlava origins of these rocks should be considered. Direct analysis of these rocks will provide insight into the origin(s) of these globally important materials. [More at link]

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