Originally posted on sciy.org by Ron Anastasia on Wed 12 Dec 2007 10:55 AM PST
Mars Rover Finding Suggests Once Habitable Environment
Channels on Mars that are carved in the surface by carbon dioxide gas, are visible in this image taken by a camera on board the Mars Reconnaissance Orbiter.
SAN FRANCISCO — The lame wheel on the NASA Mars rover Spirit has proved an invaluable science tool, turning up evidence of a once habitable environment, scientists said Monday.
Meanwhile, images from the Mars Reconnaissance Orbiter have largely unraveled the mystery of geological patterns called “spiders†that appear each spring around the south pole.
The scientists reported their findings here at a meeting of the American Geophysical Union.
The right front wheel of Spirit stopped turning in March 2006. Since then, the rover has been driving backwards, dragging the lame wheel along. This May, scientists noticed a bright spot in the trail of overturned dirt.
They turned Spirit around for a closer look, finding high levels of silica, the main ingredient of window glass. They then aimed the rover at a nearby rock, wanting to break it apart to determine if the silica was just a surface coating, or if the rock was silica all the way through.
The target rock survived Spirit’s charge, but a neighboring rock cracked open. The interior of that rock, which the scientists informally named “Innocent Bystander,†turned out to be rich in silica.
On Earth, such high concentrations of silica can form in only two places: a hot spring, where the silica is dissolved away and deposited elsewhere, or a fumarole, an environment, often near a volcano, where acidic steam rises through cracks. The acids dissolve other minerals, leaving mostly silica. On Earth, both environments teem with life.
Spirit’s twin, Opportunity, which has been exploring a spot on the other side of Mars, has found evidence of an environment once steeped in acidic groundwater. The silica discovery is the first time that Spirit has seen signs of widespread water in its surroundings, a 90-mile-wide impact crater known as Gusev Crater.
Gusev was chosen as a landing site, because, at least from orbit, it looks as if it were once a lake with what appears to be river channels flowing away from it. However, until now, the rocks that Spirit has examined have largely been volcanic basalt with little hint of water.
“This shows us a side of Mars we haven’t seen before, and my guess is that it’s more common than we had thought,†said Steven W. Squyres, the project scientist for the rovers. “Whichever of those conditions produced it, this concentration of silica is probably the most significant discovery by Spirit for revealing a habitable niche that existed on Mars in the past.â€
From far above the surface, the Mars Reconnaissance Orbiter has been taking a closer look at radial patterns of “spider†gullies, as well as bright and dark fan-like features that appear in the Martian landscape each spring.
Scientists first spotted the gullies several years ago in images taken by the Mars Global Surveyor. With the much higher resolution of Reconnaissance Orbiter, scientists saw for the first time that the gullies were wider at the center of the pattern. Another instrument allowed them to map the images onto the Martian topography; the centers of the spiders were at the top of the small hills. Those two bits of information indicated that the gullies were carved by something flowing uphill — and that pointed to carbon dioxide.
At a news conference on Tuesday, Candice Hansen, deputy principal investigator for the orbiter’s high-resolution camera, said it now appeared that a layer of translucent carbon dioxide ice, perhaps half a yard thick, formed over the south polar terrain during the winter months.
In the spring, sunlight warms the ground, vaporizing carbon dioxide at the base of the ice layer. The gas flows uphill, carving channels in the underlying soil. At weak points in the ice, the gas erupts in small geysers. The release of pressure causes the carbon dioxide gas to freeze solid and fall as white snow — the white parts of the fan-like patterns. Dust blown out with the carbon dioxide falls on the ground to form the dark parts of the fans.
“It is unlike anything on Earth,†Dr. Hansen said, though similar patterns have been seen on the Neptunian moon of Triton.
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