The ages of pedestal craters on Mars: Evidence for a late-Amazonian extended period of episodic emplacement of decameters-thick mid-latitude ice deposits

There is significant geomorphologic evidence for the past presence of longitudinally widespread, latitudinally zoned deposits composed of ice-rich material at the northern and southern mid latitudes on Mars (lobate debris aprons, lineated valley fill, concentric crater fill, pedestal craters, etc.). Among these features, pedestal craters (Pd) are impact craters interpreted to have produced a protective layer on top of decameters-thick ice deposits now missing in intercrater regions. The time during which these various deposits were present is still highly debated. To address this question we have analyzed the distribution and characteristics of pedestal craters; here, we use a population of 2287 pedestal craters (Pd) to derive a crater retention age for the entire population, obtaining a minimum timescale of formation of similar to 90 Myr. Given that the ice-rich deposit has not been continuously present for this duration, the timescale of formation is necessarily longer than similar to 100 Myr. We then compiled impact crater size-frequency distribution dates for 50 individual pedestal craters in both hemispheres to further assess the frequency distribution of individual ages. We calculated pedestal crater ages that ranged from similar to 1 Myr to similar to 3.6 Gyr, with a median of similar to 140 Myr. In addition, 70% of the pedestal ages are less than 250 Myr. During the 150 Myr period between 25 Ma and 175 Ma, we found at least one pedestal age every 15 Myr. This suggests that the ice-rich paleodeposit accumulated frequently during that time period. We then applied these results to the relationship between obliquity and latitudinal ice stability to suggest some constraints on the obliquity history of Mars over the past 200 Myr. Atmospheric general circulation models indicate that ice stability over long periods in the mid latitudes is favored by moderate mean obliquities in the similar to 35 degrees range. Models of spin-axis/orbital parameter evolution predict that the average obliquity of Mars is similar to 38 degrees. Our data represent specific observational evidence that ice-rich deposits accumulated frequently during the past 200 Myr, supporting the prediction that Mars was characterized by this obliquity range during an extensive part of that time period. Using these results as a foundation, the dating of other non-polar ice deposits will permit the specific obliquity history to be derived and lead to an assessment of volatile transport paths in the climate history of Mars. (C) 2013 Elsevier Ltd. All rights reserved.