Our new understanding of the ever-changing topography of Mars came into full focus in the last week with the release of three scientific papers and dozens of images by the University of Arizona's HiRISE camera team.

The images, taken over three Martian years (nearly six Earth years), demonstrate the seasonal changes in Mars' polar regions, where the permanent ice caps expand each winter with layers of frozen carbon dioxide 3- to 6-feet deep.

Six years of repeat observations by HiRISE and other instruments aboard NASA's Mars Reconnaissance Orbiter have reinforced earlier theories.

In spring, frozen CO2, commonly known as "dry ice," heats up and sublimates, changing from a solid to a gas without going through a liquid phase.

That heating and sublimation occur beneath the ice sheet, where the dry ice rests against the warming terrestrial surface. The gas levitates the ice crust, cracks it and explodes through, carrying dust and dirt in telltale patterns seen by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter.

Candice Hansen, lead author of one of three papers published last week in the journal Icarus, said the process, known as the Kieffer effect, was proposed years ago to explain spiderlike features near Mars' south pole - with gullies radiating from a central point.

The same process happens at the north pole, Hansen said, though the features created there look quite different. The CO2 frost there forms over sand dunes, which continue to shift in spring and summer winds. Features can be erased from season to season.

The surface of the planet is changing, sometimes gradually and sometimes quickly.

"One of the big contributions from Mars Reconnaissance Orbiter is this realization that Mars is really quite a dynamic place. There is a lot going on in today's climate. It didn't all happen hundreds of millions of years ago," Hansen said.

The mechanisms were known before, Hansen said, but were never this well-documented. "This is where we're trying to put it all together, put all the bits and pieces into a comprehensive look at what spring looks like in the northern hemisphere."

Hansen, a member of the HiRise team since its inception, is now with the Tucson-based Planetary Science Institute. Her co-authors on the Icarus paper include Alfred McEwen, principal investigator for HiRISE; and co-investigator Shane Byrne, both faculty members at the UA's Lunar and Planetary Lab.

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The HiRISE camera, developed at the University of Arizona, has been circling Mars aboard the Mars Reconnaissance Orbiter since March 2006.

The camera - its full name is the High Resolution Imaging Science Experiment - has captured a series of historic and detailed pictures of Mars' surface and in 2008 captured an image of the Phoenix Mars Lander as it descended to the red planet. In August, it photographed the Mars rover Curiosity descending by parachute toward the surface of Mars.

HiRISE is run by the Lunar and Planetary Laboratory from a building on the UA campus.

Contact reporter Tom Beal at tbeal@azstarnet.com or 573-4158.