Signs of Spring on Mars
Solid carbon dioxide, commonly known as dry ice, thaws directly to gas and forms starburst patterns under the seasonal carbon dioxide ice caps when spring comes to Mars' polar regions. Image taken by the HiRISE camera on the Mars Reconnaissance Orbiter.
Credit: NASA/JPL-Caltech/University of Arizona

A Martian orbiter has spotted seasonal footprints of spring creeping up on the red planet.

Seasonal polar caps formed from carbon dioxide have begun vaporizing or changing directly from solid ice to gas, and have kicked off a chain of events detected by NASA's Mars Reconnaissance Orbiter (MRO).

"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 Candice Hansen-Koharcheck, an MRO scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif.

Ice caps form each Martian winter as carbon dioxide changes directly to frost and builds dry ice layers more than three feet thick. The arrival of warmer spring temperatures thaws the solid carbon dioxide and thins the ice cap from both top and bottom.

The carbon dioxide gas beneath the ice cap often flows in the same places each year, eventually creating channels or troughs in the planet's surface. MRO has spotted many such spidery networks of cracks that remain even after the ice caps have vanished.

Pressure from the newly thawed gas also builds up beneath the thinning ice cap, which can lead to puffs of escaping gas and dust where the ice cap has cracked.

"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 dust ends up swirling about in the wind before settling in fan-like or starburst patterns, pointing in the direction of the prevailing wind at the time. Each pattern represents a jet of gas that was active at one time.

The Martian process differs from springtime thaws on Earth, where frozen water melts from solid to liquid and becomes runoff. Scientists have yet to spot flowing water on the Martian surface, though some suspect liquids may lie beneath the surface or even inside a volcano.

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