HiRISE sees signs of an unearthly spring

"Spring on Mars is quite different from spring on Earth because Mars has not just permanent ice caps, but also seasonal polar caps of carbon dioxide, familiar to us on Earth as dry ice," said HiRISE deputy principal investigator Candice J. Hansen-Koharcheck of the NASA Jet Propulsion Laboratory in Pasadena, Calif.

The seasonal polar caps form each winter because carbon dioxide in the atmosphere changes directly to frost, which builds into a dry ice layer over Mars' surface. Dry ice caps can be as much as a meter thick, Hansen-Koharcheck said.

Then, as winter wanes and spring arrives, warmer temperatures thaw solid carbon dioxide directly from ice into gas.

The seasonal ice cap thins both from the top and the bottom.

The gas beneath the ice cap can flow in the same places year after year after year, eroding troughs in the surface of the planet. Scientists know that the gas carves troughs into the surface of the planet itself because the troughs are still visible after the caps are gone entirely.

"What happens on Mars, we think, is that as the seasonal ice cap thins from the bottom, gas underneath the cap builds up pressure," Hansen-Koharcheck said. "And where gas under the ice finds a weak spot or a crack, it will flow out of the opening, often carrying a little dust from the surface below."

That pressure buildup beneath the carbon-dioxide ice is a key difference between this process and springtime thawings of frozen water on Earth. When warmed, water ice and snowpacks on Earth can melt to liquid and either run off beneath the frozen layer or soak into the ground.

Fans at the top of the picture are oriented in one direction, while fans at the bottom are oriented in a different direction. This suggests that as the ice layer thins, a set of gas jets becomes active and dies down, then another set, farther away, becomes active at a later time when there is a different prevailing wind direction, Hansen-Koharcheck said.

She estimated that the ice cap seen in the images is about a half-meter thick.

Credit: NASA/JPL-Caltech/University of Arizona
High resolution image

This image, covering an area about 1 kilometer (0.6 mile) across, is a portion of the HiRISE observation ESP_011842_0980, taken on Feb. 4, 2009. The observation is centered at 81.8° south latitude, 76.2° east longitude. The image was taken at a local Mars time of 4:56 PM and the scene is illuminated from the west with a solar incidence angle of 78°, thus the sun was about 12° above the horizon. At a solar longitude of 203.6°, the season on Mars is northern autumn.


Credit:
High resolution image (0.0 MB)

This is a reduced-resolution image from the HiRISE Observation observation ESP_011934_0945, taken on Feb. 11, 2009. The observation is centered at 85.4° south latitude, 104.0° east longitude. The image was taken at a local Mars time of 6:12 p.m.and the scene is illuminated from the west with a solar incidence angle of 79°, thus the sun was about 11° above the horizon. At a solar longitude of 207.9°, the season on Mars is northern autumn.

Source: University of Arizona
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