SAN FRANCISCO, CALIFORNIA--You've got to hand it to NASA's Mars Exploration Rovers. Every time Spirit and Opportunity appear to have run out of discoveries, they unearth yet another dramatic surprise. Yesterday, at a meeting here of the American Geophysical Union, mission controllers announced that Spirit, in an accidental encounter with a rock beneath its wheels, may have stumbled upon the best evidence yet that the Red Planet could have once supported microbial life.
Spirit's discovery comes as the rover enters hard times. Its solar cells are coated with dust, and its power is down to 42% of peak capacity--a level that is expected to drop even further during the oncoming winter. For the past 2 years, Spirit has also been hamstrung by a stuck right-front wheel, which it must drag while driving backward. But that wheel has turned out to be a scientific blessing.
Last April, Spirit was probing Gusev crater, a region near the equator deemed promising for evidence of ancient water. The rover's errant wheel dragged up a bright white layer under the characteristic red dust. When a preliminary spectrographic analysis indicated the presence of water-soluble minerals, mission controllers at NASA's Jet Propulsion Laboratory in Pasadena, California, decided to investigate further. They spotted a small rock they thought might yield more clues about the layer's chemistry, so they instructed the rover to drive over it in an attempt to crush it. The rock survived the assault, but a fist-sized one nearby crumbled into powder.
The rock turned out to be a type of stone known as siliceous sinter. Containing 98% silica, the principal ingredient of glass, it could have only been produced by one of two processes: Either its constituent minerals crystallized from hot springs, or it formed from the residue of fumaroles, steamy caustic volcanic vents. That's big news, according to principal rover mission scientist Steven Squyres of Cornell University. On Earth, hardy microbes survive in fumaroles and hot springs, such as those at Yellowstone National Park. These energy-rich environments are top candidates for the habitats in which life first evolved. "We're very excited about this," Squyres said at a news conference.
Geoscientist Richard Alley of Pennsylvania State University in State College shares the enthusiasm. It's like finding "Yellowstone on Mars," he says.
Meanwhile, Opportunity is pursuing some good luck of its own. After several months of study and planning, mission controllers have found a way to send the rover safely into kilometer-wide Victoria crater, halfway around the martian equator from Spirit. There, it has discovered three sedimentary layers that suggest the martian surface may have experienced a prolonged drought following its initial wet phase--something Opportunity's previous 4 years of explorations had not encountered.