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Mars' dried up lakes and carbon dioxide ice
KEITH COOPER
ASTRONOMY NOW
Posted: September 16, 2009


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Cracks intersecting into a network of polygonal shapes on the floors of large craters and impact basins on Mars are the result of dessication caused by the evaporation of lakes, according to a new analysis presented today at the European Planetary Science Congress (EPSC) in Potsdam, Germany.

A comparison between dessication patterns in dried up lakes on Earth (left) and on Mars (right). Image: Google/NASA/JPL.

The findings are the result of work done by a PhD student, M Ramy El Maary, of the Max Planck Institute for Solar System Research. “I got excited when I saw that the crater floor polygons seemed to be too large to be caused by thermal processes,” he says. El Maary had set up an analytical computer model to test the assumption that the cracks had formed by thermal contractions in the Martian permafrost, and he found this assumption to be false. Thermal contractions should create smaller cracks and polygons no larger than about 65 metres across. The polygons El Maary was seeing in images produced by the Mars Orbiter Camera that had been on NASA’s Mars Global Surveyor craft, and the HiRISE camera onboard the Mars Reconnaissance Orbiter, were on average between 70 and 140 metres wide, with the largest up to 250 metres wide. The cracks themselves are between one and ten metres wide.

El Maary saw similarities in the polygonal pattern with patterns seen in dried-up lakes on Earth. “These are the same type of patterns that you see when mud dries out in your back garden, but the stresses that build up when liquids evaporate can cause deep cracks and polygons on the scale I was seeing in the craters.”

A closer look at the polygons found in Mars' craters. Image: NASA/JPL.

El Maary believes that some of the polygons are relatively young on a geological timescale. The current model of Mars’ climate in the past describes how it was warm and wet until about 3.8 billion years ago, although there is evidence of liquid water existing on the surface far more recently than that (for example, read our story here). El Maary also points out that meteorite impacts on the surface of Mars and the resulting heat could have melted underground ice to fill the crater with water. “Even under current climatic conditions, these [lakes] may take many thousands of years to disappear, finally resulting in dessication patterns,” he says.

Meanwhile, in another discovery about climatic conditions on Mars, results from the neutron spectrometer on NASA’s Mars Odyssey spacecraft have mapped the varying concentration of carbon dioxide ice (more commonly known as dry ice) at the red planet’s poles. The research, led by Thomas Prettyman of the Planetary Science Institute in Tucson, Arizona, has been published in the ‘Journal of Geophysical Research’ and has found that about 25 percent of Mars’ atmosphere is recycled through the polar dry ice. Near the north pole, the dry ice is concentrated in a region known as Acidalia, and may be deposited there by cold winds blowing in from a nearby large canyon called Chasma Boreale. Meanwhile, in the southern hemisphere the dry ice is offset from the south pole, indicating variations in surface composition, says Prettyman.

Changing carbon dioxide ice at Mars' north pole, both imaged by Hubble (left) and Mars Odyssey's neutron spectrometer (right). They show the how the ice evaporated as spring approached. Image: NASA/JPL/STScI/Thomas Prettyman.

“The regions outside the residual cap consist of water-ice mixed with rocks and soil that are warmed during summer. This delays the onset of carbon dioxide ice accumulation in the autumn. In addition, heat stored in water-rich regions is gradually released during autumn and winter, further limiting ice accumulation,” he says.

Using the neutron spectrometer, Prettyman and his team were also able to measure the amount of gases such as nitrogen and argon that don’t freeze out in winter, but remain in the atmosphere. “The observed time variation of the concentration of nitrogen and argon provides information on local circulation patterns,” says Prettyman. “This includes the timing and strength of the polar winter vortex – a large scale, cyclonic flow that inhibits the mixing of nitrogen and argon with air from lower latitudes.”

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