A Martian icecap, now in 3-D

by Patrick Morgan
Thursday, January 5, 2012

Move over, James Cameron. Researchers have created the first 3-D subsurface pictures of Mars' northern icecap — and they’re using these images to solve a 40-year-old Martian puzzle.

The puzzle centered around Chasma Boreale, an ice canyon in the northern icecap that is comparable in size to the Grand Canyon, and the spiral troughs that extend in a pinwheel-fashion from the icecap’s center. How each of these features formed has long mystified researchers.

Based on photographs of the surface of Mars from NASA’s Mariner and Viking orbiter missions in the 1960s and 1970s, scientists invoked nearly half a dozen hypotheses to explain the development of Chasma Boreale, including catastrophic outburst floods and wind erosion. As for the spiral troughs, researchers posited that wind erosion parallel to the troughs or processes such as glacial surges were responsible for their formation.

But these hypotheses rested on the assumption that the canyon and troughs were simply cutting into a pre-existing icecap. To determine whether this assumption was correct, it would be necessary to see below Mars' surface.

Jack Holt and Isaac Smith, researchers at the University of Texas at Austin’s Institute for Geophysics and primary authors of two papers published in Nature this week, used a shallow subsurface radar called SHARAD, an instrument on NASA’s Mars Reconnaissance Orbiter. “You’ve probably seen those fine German cakes that have 25 layers in them,” says Alan Howard, a geologist at the University of Virginia in Charlottesville. “That’s basically the kind of signal we’re getting back from SHARAD. It allows us to really decode the changes that have gone on, and how these polar caps have been assembled through time.”

Holt and Smith conclude in their respective papers that the subsurface structures they see in the SHARAD images are not just a function of the features cutting into the ice, but are best explained by asymmetric accumulation: As wind erodes the northern cliff face of the canyon or trough, ice is deposited along the opposite cliff face. The accumulation patterns are driven by katabatic winds, which form as cold, dense air descends along the icecap’s central peak. These winds were especially strong at Chasma Boreale, which lies directly downwind from the icecap’s polar high, Holt says. As a result, what was initially a valley became a deep chasm as deposition occurred adjacent to the canyon, he adds. “The most reasonable explanation for this is that, rather than cutting down through the ice, the winds persistently blew down this canyon and prevented the deposition of ice.”

Over several million years, as more ice was deposited and the icecap grew higher, Smith says, the troughs deepened. The spiral pattern, he adds, is due to the Coriolis force: As Mars spins on its axis, the katabatic winds follow a curved path spreading out from the icecap.

Although solving the 40-year-old puzzle of the formation of Chasma Boreale and the spiral troughs is noteworthy in itself, Holt says the images provide clues to other Mars mysteries. “[They change] our view of how climate shapes and molds ice on Mars,” he says. “We now know that ice deposition and modification are almost completely a climate process.” The ultimate goal, he adds, is to create a 3-D map of these climatic processes on Mars — which would be a project of "Avatar" proportions.

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