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article i nfo Article history: Accepted 25 August 2009 Available online 25 September 2009 Amazonian non-polar ice deposits on Mars record periods and events when the climate differed substantially from that of today. Particularly evident are examples of ice-rich deposits in the martian mid-latitudes (lobate debris aprons, lineated valley fill, and concentric crater fill). Uncertain, however, is the amount of ice remaining in these deposits today, and the thickness of ice that might have existed when they formed. Here, we use HRSC, CTX and HiRISE imagery and MOLA topographic data to document an occurrence of concentric crater fill within which the past minimum volume of ice can be constrained. An ~8 km impact crater is superposed on the rim of a ~32 km impact crater near the contact between the Phlegra Montes and the Vastitas Borealis Formation in the northern mid-latitudes of Mars. We find evidence for flow from the larger crater into the perched smaller crater that indicates an earlier period of significant ice accumulation and glaciation within this double crater. Lobate ridges observed outside of the perched younger crater suggest that ice filled and overtopped the crater rim, providing minimum estimates of ice thickness and volume within the system. Glacial ice must have been at least ~1000 m thick to overtop the rims of both craters and induce gravitational flow onto the surrounding plains, with a minimum volume of ice of ~750 km 3 . This is the first volumetric measurement of this kind on Mars for concentric crater fill craters, and the thickness is comparable to that measured in a lineated valley fill glacial system along the dichotomy boundary at a similar latitude. We also document late-stage episodes of more localized glacial flow that include ridges on valley walls that we interpret as late-stage glacial high-stands, and concentric crater fill (CCF) that characterizes most of the present-day crater floor. Similar deposits in a crater ~60 km to the northeast suggest that such episodes were at least regional in nature. This sequence provides evidence for significant spin-axis/orbital parameter-driven shifts in the Late Amazonian climate of Mars and suggests that regional ice sheets may have existed in the mid-latitudes of Mars within the last several hundred million years. |
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