The site is Gusev Crater, a 90-mile wide hole in the ground that probably formed three to four billion years ago when an asteroid crashed just south of Mars' equator. There's a channel system that drains into it, which probably carried liquid water, or water and ice, into the crater. "It's hard to imagine the landscape looking this way unless water was somehow involved," says Garvin.

Right now, inside the crater, researchers expect to find sediments, which may be nearly 3,000 feet thick. These sediments, which, researchers hope were deposited by water, may have been covered by dust and sand that's blown into the crater over the past two billion years.

But if there was once water in Gusev, its signature should still be there.

Water, of course, is important because it could signal an environment once-friendly to life. The landscape seen from orbit is very persuasive. But researchers can't be certain that Gusev contained liquid water until they examine the site up close.

"The Gusev landscape we see today could have been modified by lava, ice, and winds," notes Garvin. "Aspects of it could have been formed by standing water, or by intermittent floods." Spirit carries a suite of tools that will find out.

The rover will be able to grind away the surface cover on rocks and analyze minerals inside. It will be able to view its surroundings with unprecedented detail and precision. It can scoot over to the most interesting rocks it finds in order to examine them more closely.

With these tools and others, Spirit will work to find out what really happened.

One clear sign of past water will be in the rocks.

For example, if indeed Gusev once held a giant lake ( more than 10 times the size of the famous Crater Lake here in the US) certain key minerals are likely to be found in its rocks. Spirit might find evaporites -- minerals formed as water dries up. Salt or gysum are familiar ones here on Earth. Salt's component parts -- sodium and chloride -- are separated, dissolved in sea water, but as the water dries up, the sodium and chloride join together to form the mineral "halite," for example.

On Mars, Spirit might find evaporites like gypsum, or calcium magnesian sulfate. It might also find minerals involving carbonates (i.e., calcium carbonate). These are sometimes, although not always, produced by or from living organisms. But they are almost always a sign of water--"at least here on Earth," notes Garvin.

Another sign will be in the way the sediments are organized. For example, if the sediments were blown in by winds, the layers may be more erratic, to reflect the changing directions of airflow (as in fossil dunes here on Earth). If they were deposited by water, they are more likely to be layered evenly, one on top of the other in rhythmic stacks.

The most exciting result, says Garvin, would be proving that liquid water existed at the surface of this site for a long time. "Persistent standing bodies of water are possible habitats for life," he explains.

But whatever information Gusev yields will be important. Water or not. Life or not. Whatever it tells us will help determine the course of future explorations on Mars.

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