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In the news this month: seasonal changes in the northern dunes of Mars

Polar sand dunes from HiRISE
Polar dunes made of basalt and gypsum grains in the northan polar region of Mars. CREDIT: NASA/JPL/University of Arizona
While the planet Mars is home to the largest volcano in the solar system, and the enormous Mariner Valley which cuts through a huge region of the planet's surface, the crust of the planet is today fairly inactive. Unlike the Earth, where plate tectonics cause the continents to move and ever-so-slowly collide with each other, Mars today has no such large-scale geological activity. But that doesn't mean the surface never changes with time.

Several spacecraft have been imaging the surface of the red planet over many years, some at very high resolution, and the images sent back are showing an amazing amount of surface change on short timescales. In 2010, images released from the High-Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter showed evidence of avalanches on sand dunes first imaged by Mariner 9 in the 1970s located in the northern polar region. Most older images of the region suggested that it was mostly stable with very little variation over time, but last year's images from the MRO show clear evidence of sediment transport with one image even catching a dust cloud kicked up by an avalanche. Now, a team led by Candice Hansen at the Planetary Science Institute in Arizona in the USA, have analysed the images and determined the cause of the avalanches.

The team found that numerous dunes in the northern polar region showed evidence of morphological changes over the course of a Martian year. Seasonal variations have been seen in images from previous Martian orbiters, but this is the first time they have been seen in such detail that the underlying processes can be studied. Some of the physical processes causing the changes are the same as those seen on sand dunes here on the Earth, but on Mars there is an additional process not found on the Earth.

The Earth's polar caps are made of ice, but the atmosphere of Mars has a very high percentage of carbon dioxide, a gas which freezes in the cold temperatures of the Martian winter and settles on the surface. In the Martian spring, the carbon dioxide ice sublimates, turning back into a gas and returning to the atmosphere. This sublimation process can destabilise the dunes, especially at the top where more sunlight is received and the side of the dune is steepest, causing loosened sand to cascade down the side of the dunes, creating gullies and aprons of material.

Over the area studied, a dune field 6.4- by 19.2-km in size, roughly twenty per cent of the dunes showed measurable changes over one Martian year and another twenty per cent showed slight changes. The widespread nature of these variations and the pristine appearance of the dunes suggests that sand transport on the surface is an active and ongoing process. Similar image comparisons of dunes at lower latitudes further from the polar regions show no changes in dune shape over the same period, adding to the evidence that the observed changes at high latitude require carbon dioxide ice.



This blog post is a news story from the Jodcast, aired in the March 2011 edition.

Hansen CJ, Bourke M, Bridges NT, Byrne S, Colon C, Diniega S, Dundas C, Herkenhoff K, McEwen A, Mellon M, Portyankina G, & Thomas N (2011). Seasonal erosion and restoration of Mars' northern polar dunes. Science (New York, N.Y.), 331 (6017), 575-8 PMID: 21292976

Posted by Megan on Wednesday 02nd Mar 2011 (06:25 UTC) | Add a comment | Permalink

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