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Author Topic: Craters Reveal Ice in Martian Dust  (Read 5910 times)
Andy
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« on: March 30, 2009, 07:41:36 AM »

When it comes to planets, there's a fundamental reality: Cratering happens.

Things slam into other things and make holes all the time. Big or small, old or new, simple or complex, impacts are the most ubiquitous geologic features in our solar system. Roughly 1,600 named craters (and countless lesser pits) scar the Moon's ancient surfaces. On Earth, where wind and water continually wear down the land, the census of confirmed impact craters stands at just 176.

Mars, a mixed bag of ancient and modern terrains, lies somewhere in between. Over the years spacecraft have glimpsed ever-finer features in the Martian landscape. These days, the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter (MRO) can pick out objects only 1 foot (0.3 m) in size; the High Resolution Stereo Camera on the European Space Agency's Mars Express is no slouch either, with a ground resolution of 7 feet (2 m).

So HiRISE researchers were elated, but not particularly surprised, to discover some small, freshly gouged craters in images taken last year. Seen at five sites over a latitude range of 43 to 56 north, the excavations are typically 10 to 20 feet across and a foot or two deep. How fresh is fresh? One cluster must have appeared sometime between June and August, and a somewhat larger pit showed up between January and September.

What did astound the team were splashes of white seen in and around a handful of these craterlets. Could it be water ice? Colleagues operating the spacecraft's CRISM instrument soon confirmed, for the one case large enough to yield a spectrum, that it was! Apparently fist-size impactors had punched into a layer of ice hidden by a foot-deep topping of dust.

In the months that followed, these snowy splashes gradually faded from view. Water ice isn't stable at the craters' latitudes, so most likely it gradually sublimated (vaporized) into the atmosphere, leaving behind veneer of any dust that had been mixed with it. The disappearing act might also be due in part to a coating of dust blown in from the atmosphere. Either way, notes HiRISE investigator Shane Byrne (University of Arizona), the icy deposits had to be at least a couple of inches (several centimeters) thick, and they couldn't have been unearthed from more than a foot or two down.

Byrne announced these findings today at a meeting of solar-system specialists in Houston, Texas. He points out that prior surveys, particularly one done by the neutron spectrometer aboard NASA's Mars Odyssey orbiter, show that vast reservoirs of ice lay barely buried across most of the planet's polar and mid-latitude regions.

But scientists are only now realizing just how near the surface the ice lies and how easily it can be reached. When NASA's Phoenix lander dropped onto a northern polar plain last May, its braking engine blew off a few inches of loose dirt and revealed slabs of nearly pure ice.

The irony in all this is that the Viking 2 lander, which arrived in September 1976, sits just 500 miles (800 km) southeast of the ice-splashed craterlet shown above, and scientists now realize that a layer of water ice almost certainly lies not far beneath its footpads. "It's probably just tens of centimeters down," says HiRISE team leader Alfred McEwen. Had Viking's sampling scoop been able to dig a little deeper, he adds, "we might have sampled ice on Mars 30 years ago."
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Andy
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« Reply #1 on: March 30, 2009, 07:46:33 AM »

The "icy" patches can't be seen by the main cameras, which are perched atop a spindly mast about 7 feet off the ground. Instead, they came to light when spotted by a monochromatic camera mounted near the 8-foot-long robotic arm's scoop. When the team used this robotic-arm camera, or RAC, to peek underneath, they were stunned to see that the descent engines' exhaust had cleared away the topmost few inches of dirt to reveal three flat areas, each roughly a foot across.

Peter Smith, the mission's chief scientist, said someone exclaimed "Holy cow!" when this remarkable image first appeared on monitors at the science-operations center in Tucson, Arizona and that's what they've been calling this area ever since. Uwe Keller, who heads the RAC team, notes that the extended flat areas suggest that the ice exists as an extended "table" just a few inches down.

Phoenix's scientists had suspected that the landing area would be especially ice rich. A global "prospecting map" compiled by the orbiting Mars Odyssey implied that this location, at 68.2N, 125.7W, would yield abundant ice within inches of the surface. (Smith had even dubbed this region the "Sweet Spot" when he proposed the Phoenix mission to NASA.)

Moreover, simulations conducted two months ago at NASA's Ames Research Center predicted that Phoenix's powerful pulsed engines would strip away the topsoil as it landed to reveal the barely hidden ice layer. "It's a gleeful day," smiled Smith during a telephone hookup with reporters over the weekend. "We'd worried that the ice might lie 40 or 50 cm [16 to 20 inches] down," which would be near the limit of the arm's reach.

The ice has to be frozen water, not frozen carbon dioxide (dry ice), because temperatures at the landing site range from -30 to -80C (-22 to -112F) incredibly cold, yet too warm to sustain CO2 ice. (You can check out the lander's weather reports here.)

Unfortunately, even the agile, four-jointed arm won't be able to get much closer to the underbelly area. Instead, its camera has concentrated on an icy patch near the footpad dubbed "Snow Queen." Light-emitting diodes (LEDs) mounted next to the camera can bathe the ground near it in red, green, and blue light, which should allow the RAC team to reconstruct color images.

Meanwhile, on Sunday, Phoenix's seventh Martian day (or "sol") on the ground, the robotic arm successfully practiced scooping up some loose soil. That's paved the way for gathering the first shovelful for testing by the two miniaturized laboratories on board. The first of these to get a "taste" of Mars will be the Thermal and Evolved Gas Analyzer (TEGA), despite problems with a heating filament that's forced a switch to its backup.

Also awaiting a sample is the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA), actually a cluster of four tools that will subject the ruddy soil to a series of tests.

Image (you must be logged in to view): This fresh crater has dredged up barely buried water ice and splashed it onto the Martian surface. The HiRISE camera aboard NASA's Mars Reconnaissance Orbiter recorded this color close-up on November 1.
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