Monday, 13 October 2014

Interpreting landslide deposits in the Mariner Valley, Mars.

Landslides on Mars typically have much greater runout distances than those on Earth, due to the planets lower gravity and thinner atmosphere. This can lead to areas of layered deposits from different landslides quite distant from the source, particularly within the larger canyons on Mars. Since it is possible to produce approximate ages for such deposits based upon the number of impact craters on the surface of the deposit (meteors are thought to fall onto planets at a steady rate, therefore there should be a direct relationship between the number of impact craters on a surface and its range), it is therefore possible to try to understand the long-term evolution of these deposits once in situ, and therefore potentially the nature of the material in such deposits.

In a paper published in the journal Geology on 29 July 2014, PeterGrindrod of the Department of Earth and Planetary Sciences and Centre forPlanetary Sciences at Birkbeck College, University of London and Nicholas Warner of the Jet Propulsion Laboratory at the California Institute of Technology and the Department of Geological Sciences at the State University of New York at Geneseo, attempt to understand the sequence of events that led to the formation of a layered landslide deposit in the Ophir Chasma area of the Mariner Valley visible in images from the Mars Reconnaissance Orbiter.

By counting craters over 100 m in diameter on the various deposits in the area, Gridrod and Warner conclude that the oldest deposits in the sequence were around 423 million years old and the youngest are about 216 million years old. However the relationship between craters of 30-50 m in diameter was not consistent with predictions, suggesting that craters are being eroded over time.

Landslide ages from crater count studies. Mars Reconnaissance Orbiter Context Camera base map showing location of the six landslides dated using crater counting. Numbers represent relative stratigraphic position of the landslide as inferred from superposition relationships. ILDs—interior layered deposits. Inset shows relative stratigraphic age determined from landslide boundaries in direct contact. Grindrod & Warner (2014).

Based upon the apparent rate of erosion of craters, and the apparent diversion of later landslides by layered deposits that have subsequently been eroded, Grindrod and Warner conclude that the deposits are being eroded at a very high rate for Mars, with about 500 m of material having been removed since the deposits were laid down.

Grindrod and Warner suggest two possible ways in which this could occur. Firstly aeolian abrasion, in which deposits are abraded away by friction with wind-blown particles (saltation), which could potentially erode away 500 m of sediment in the Mariner Valley in 18 to 101 million years. Secondly ice sublimation, in which ice particles (carbon dioxide rather than water) in the deposits are sublimated away (turn directly from solids to gasses) as they warm. This could potentially lead to the loss of 500 m of deposits in as little as 57 million years. However the OphirChasma deposits are quite close to the Martian Equator, where ice deposits are not currently predicted; if substantial amounts of ice were present in deposits laid down between 200  and 400  million years ago, then significantly cooler temperatures must have existed in the region at these times than can be accounted for by our understanding of Martian climate cycles.

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