Return To Mars

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    "It looks like mud, but it can't be mud," says Steve Squyres, the mission's principal science investigator. "It holds together well. I'm not even prepared to speculate."

    One possibility is that the soil, while now dry, was once wet. Water rising to the surface or sinking down from above could have caused the dirt to congeal, perhaps leaving salty deposits that help hold it together. The Viking probes investigated similar-looking callused patches back in 1976. Investigators on those missions dubbed the features duracrust. The science team working this trip is eager to dig around in the stuff.

    The duracrust gave new relevance to a study published in 2000 by microbiologist Russell Vreeland of Pennsylvania's West Chester University. Vreeland discovered some 250 million-year-old salt crystals in New Mexico that contained tiny quantities of ancient water. The water held preserved spores that sprang back to life once their salt and nutrient levels were adjusted. Whether this is possible in the punishing, radiation-soaked environment of Mars is hard to say, but it certainly makes anything that hints at salt worth a closer look.

    A scattering of small craters within the larger Gusev Crater are attractive to mission planners as well. Such secondary-impact pits do geologists' excavation work for them, gouging away upper layers of soil and rock and offering a free peek at what lies below. There appears to be an especially inviting population of small craters to the east and southeast of the rover, providing one more reason for Spirit to head that way when it dismounts. "We've got a capable machine, but we can't dig 20-ft. holes with it," says Squyres. "The way to do that is to look at one of these craters."

    Hills too have caught the scientists' eyes. The far smaller Sojourner rover that landed on Mars in 1997 never moved more than 39 ft. from its home base. But Spirit is built to roam, and its cameras have spotted some inviting, 300-ft.-high mounds nearly 1.25 miles (2 km) away. The robot was designed to travel only about half that distance, but there's nothing to say it can't exceed expectations. Matt Golembek, a J.P.L. geologist, thinks Spirit has a shot at capturing the view from the summit of a mound. "Given how smooth and flat the terrain is," he says, "everyone feels pretty comfortable that that might be someplace we can get to."

    One thing Spirit won't have to travel far to explore is rocks. There's a whole quarry's worth of loose samples littering the Gusev floor, possibly carried there by river water or even glaciers. The rover is equipped with a robotic arm that includes two spectrometers for analyzing chemical and mineral composition, a microscopic imager and, most usefully for rock work, a miniature drill. "This thing's like a big Swiss Army knife," says Squyres.

    Choosing the rocks most worthy of the attention of all this hardware won't be easy. J.P.L. scientists admit that Gusev Crater looks a little less pristine than they had hoped. Since the time its water vanished, the terrain may have been covered by lava, blasted by incoming meteorites, and then further eroded by millions of years of winds. It will take some doing to find the rocks that have been least affected by all that. Making a choice and then getting to a prize sample could take more than five days. On a mission that may last no more than 90 days, that's a lot of time. "You've got to think hard about where you're going," says Knoll.

    Something the scientists do know is that once Spirit has its wheels in the dirt, it's going to act fast. Almost immediately, it will extend its robotic arm and begin sampling the soil directly in front of it. This will allow it both to calibrate its instruments and get the data flow streaming back to Earth. The Apollo astronauts used to do something similar, spending their first moments on the moon collecting what they called a contingency sample — a clump of lunar soil and rock they would tuck into a spacesuit pocket so they would have something to show for the trip if a sudden emergency forced them to turn around and come straight home. Spirit, of course, is never coming home. It will spend its entire useful life on the Martian surface and die there sometime before the end of spring. NASA scientists plan to make sure it makes the most of every day it has.

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