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BY KEITH COOPER ASTRONOMY NOW Posted: 10 February, 2009 The erosion of craters on Mars is providing researchers at the Planetary Science Institute (PSI) in Tucson, Arizona, with fresh new evidence about the role that water-ice plays in the geology of the red planet.
An image from the THEMIS instrument on the Mars Odyssey spacecraft showing erosional and depositional features. The crater towards the bottom is 16 kilometres wide, and is actually at the centre of a larger 60-kilometre wide crater, with part of its northern wall visible at the top displaying north-facing lobate flows. Image: NASA/JPL/MOLA. The erosion of craters on Mars is providing researchers at the Planetary Science Institute (PSI) in Tucson, Arizona, with fresh new evidence about the role that water-ice plays in the geology of the red planet. PSI's Daniel Berman, David Crown and Leslie Bleamaster studied several hundred craters in Mars' mid-latitudes, half in the northern hemisphere in Arabia Terra, and the other half in the southern hemisphere east of Hellas Basin. Each crater is larger than 20 kilometres across and has been thoroughly mapped by orbiting spacecraft. The PSI team catalogued a variety of features in the craters, such as lobate flows (lobe-shaped depositional features on the walls of craters), channels running through the craters, valleys, gullies or alcoves in the crater walls, arcuate ridges (curving, arc-shaped ridges surrounding depressions inside craters where glacier like features have evaporated), and debris aprons that are deposits on the crater floor. The study found that, in craters between latitudes of 30 and 45 degrees, the lobate flows, arcuate ridges and gullies were all facing the pole. Between latitudes of 45 and 60 degrees they were generally pointing towards the equator. Furthermore, in the southern hemisphere, large valleys in crater walls pointed towards the equator, whereas narrower channels faced the south pole. So what does all this mean? It has been shown that throughout history, without the presence of a large moon to stabilise it, Mars has wobbled quite dramatically on its axis. Currently it is tilted at 25.2 degrees, but in the past has been tipped over by as much as 60 degrees, and this is bound to play mayhem with the way different parts of the surface are heated by the Sun. As one pole is tilted towards the Sun, the ice there will melt and then evaporate, before falling as snow at the opposite pole. Uneven heating on crater walls will see the ice on craters pointing towards the Sun melting quicker, widening gullies, enhancing the size of lobate flows and increasing the number of arcuate ridges. As melt-water ran down crater walls, it would have flowed to the other side of the craters, creating visible slopes of up to three degrees in small craters, where material has been deposited. "Studying crater degradation in potentially ice-rich environments is vital to understanding the geology of craters and their surroundings, as well as for determining whether the ice comes from the atmosphere or from below the ground," says Berman. It also illustrates the dramatic effect that the relationship between the axial tilt of the red planet and the presence of water-ice has had in shaping surface features. Berman and his team will publish the results of their study in the journal 'Icarus' which can be found online at www.sciencedirect.com |
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