ADAPTED FROM FARLEY ET AL. (2014)

remained until 78 million years ago, when wind erosion quickly stripped away enough of the overburden for the accumulation of cosmogenic nuclides to begin in earnest. The calculated exposure age of 78 Ma is old by the standards of the terrestrial surface, but much younger than the estimated K–40Ar age of the rock itself, which is over 4 Ga. The MSL authors interpret the exposure age in the context of a Martian land-scape in which winds from the northeast prevailed (FIG. 1). Scouring by wind over time led to “scarp retreat ” at a rate of ~75 cm/My over the relatively soft regions that include the Sheepbed mudstone (FIG. 2). The duration of surface exposure has some implications for the search for organic materials on Mars. Cosmic ray protons and their nuclear by-products have more than enough energy to destroy chemical bonds and to convert organic compounds with higher molecular weights to compounds with lower ones. When prospecting for unaltered Martian organics, therefore, rocks with lower doses of cosmic rays are preferred. Farley and coworkers argue that samples taken closer to the retreating scarp may have lower radiation doses and would be good material to analyze