PEDESTAL CRATER HEIGHTS ON MARS: CONSTRAINING THE THICKNESS OF AN ICE-RICH, MID-LATITUDE PALEODEPOSIT

We have recently presented evidence for an armoring and sublimation-driven formation mechanism for pedestal craters on Mars. In this model, pedestal craters form via impacts into ice-rich surface layers deposited at mid-latitudes during periods of higher obliquity. The ice in these deposits later sublimates as obliquity changes, lowering the elevation of the surrounding terrain and its exposed snow/ice. Beneath the armored cover of the pedestals, however, the ice-rich deposit is preserved. The heights of the pedestals thus offer direct evidence for the elevation of the icy paleosurface at the time of impact; they can be used to derive the minimum thickness of the paleodeposit. We measured the heights of 2300 pedestals between 60°N and 65°S using MOLA data. These can be used in conjunction with other pedestal crater attributes to place constraints on the distribution, volume, and timing of ice accumulation at mid-latitudes during obliquity excursions. From this analysis, we conclude that: (1) 82% (1885 of 2300) of pedestal craters have a height of <60 m; pedestal heights can reach up to ~260 m. (2) The mean mid-latitude pedestal height in the northern hemisphere is 48.0 m. In the southern hemisphere, the mean is 40.4 m. (3) Neither hemisphere shows any significant variation in height as a function of latitude, but longitudinal variations are present. In the northern hemisphere, the tallest pedestals are located in Utopia and Acidalia Planitia. In the southern hemisphere, the tallest pedestals are in Malea Planum. (4) Pedestal heights have no statistically significant correlation with pedestal crater diameters, nor with any other measured attribute such as the sinuosity of the pedestal perimeter or the extent of the pedestal. This supports the notion that the resulting height of the pedestal is determined by the thickness of the ice-rich target deposit and not the impact process. (5) Pedestal craters with significantly different heights are found in close proximity to each other, and in some cases pedestals are partially draped or completely superposed on other pedestals. This implies multiple distinct episodes of the deposition of a mid-latitude, ice-rich layer.