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In this photo sand and ice are cascading down the slipface of a dune,
covering seasonal ice. The arrow points to a small cloud of dust kicked
up by this cascading.
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Extraterrestrial sand dunes exist on Mars and Titan. There has been some question about how active the dunes that cover a large region of the northern latitudes of Mars are. Although there have been some indications of small changes, most data suggested that the dunes are currently stable, perhaps held together by ice. However recent images from the HiRISE experiment show substantial changes that are triggered by the sublimation of CO2 as the seasons change. This work is reported by
Hansen et al. in Science this week (vol. 331, no. 6017, pp. 575-578, 2011).
Sand dunes have a windward side, where the sand is pushed up onto the dune, and a slip face on the lee side. Sand moves on the dunes by saltation (bouncing) and avalanching (called "grainfall" in this Science article). On earth, they have a variety of shapes and, in spite of the differences in climate, have similar morphologies on Mars.
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Kelso dunes in the Mojave desert, California,
Photo by Mark A. Wilson, Wiki |
Every year, in Mars winter, the polar regions are covered by seasonal caps of CO2 frost. In the spring, when the CO2 sublimates sand grains can move forming the avalanches and dust cloud shown in the figure above. Hansen et al. found that the dunes showed new alcoves, gullies and extension of their aprons. These changes were also remobilized by the wind, forming ripples and erasing the gullies. Even if the dunes contain a large amount of ice internally, their surface layers, perhaps to a depth of 25 cm, is dessicated and able to move.
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