DISSECTED MANTLE TERRAIN: CALCULATING EXTENT AND ICE VOLUME OF THE LAST GLACIATION ON MARS

It is accepted that Mars experiences ice ages, and by examining the areal extent of dissected mantle terrain (DMT), a relict glacial landform, we can place constraints on the volume of ice present during these periods. DMT appears as a hummocky surface that breaks up smoother mantled regions. It is believed that during periods of high obliquity debris covered ice sheets covered much of the mid-latitudes of Mars. At the end of the last glacial period the ice sublimated and the collapse of the overlying debris formed the dissected mantle terrain.

We investigated images from the HiRISE camera on the Mars Reconnaissance Orbiter by map-projecting the images into ArcMap, and locating and measuring the areal extent of DMT within the images. Assuming a conservative 1-meter thickness of past ice (based on depth of the dissection pits), we calculated total volumes in our subset and applied the data to a whole-planet extrapolation. Over 90 HiRISE images were investigated from the region of 150-180 degrees longitude and 20-60 degrees south latitude, with 63 images found containing DMT. This area included over 5 billion square meters of surface area, and over 1.5 billion square meters of dissection. At higher latitudes periglacial processes begin to rework the DMT and the classic hummocky morphology of the DMT transitions into a more pitted appearance. These regions were labeled ‘transition’ terrain. Values of ‘Percent Dissection’ for each image were calculated from area values of dissection and transition terrain using tools in ArcMap.

Dissection was found to be most prevalent between 25 and 45 degrees south, with clear DMT transitioning into re-worked pitting between 45 and 55 degrees south. At latitudes higher than 55 degrees, polygonal terrain begins to dominate indicating the presence of stable ground ice. Calculations lead to a total projected volume of ice needed to form global DMT of 1.9E4 km³ (assuming conservative 1m deep pitting). If this ice was concentrated on the polar caps, it would form a layer 8.5 meters thick. These results are scalable to the depth of the ice sheet. For example, if the ice sheet was 100 meters thick these values would increase by 2 orders of magnitude. This research provides valuable insight into the amount of ice that is mobilized from the north and south polar caps during periods of high obliquity.