Hints of Water Intrigue Scientists

If you plopped the region around Gusev Crater -- the proposed landing site for NASA’s first Mars rover, Spirit -- down on Earth, its features would be recognizable to even amateur geologists:

* The long, Grand Canyon-like Ma’adim Vallis, stretching the same distance as from San Diego to San Francisco, that seems carved out of the soil by a raging river;

* The effluvial delta where the canyon empties into the crater itself, appearing to dump soil and carving myriad channels like those at the mouth of the Mississippi River;

* The floor of the crater, smooth and flat, like the bottom of a massive lake created as sediment was deposited over eons by standing water.

Geologists can provide complicated explanations involving winds, earthquakes and lava flows for how these features were formed on Mars, but scientists are intrigued by the possibilities offered by the simplest explanation -- that massive amounts of water once coursed over the surface of Mars before eventually evaporating into space or freezing into subsurface deposits.

With water, there is the possibility that life once existed on Mars. Finding traces of that life is the ultimate goal of the ongoing missions to the planet, and Gusev is a promising place to begin.

Selecting landing sites requires a delicate balance between the competing interests of geologists, who are looking for exciting rock formations to explore, and engineers, who want a location where the rover will not be destroyed on landing.

The first criterion is that a potential site must lie in a narrow band near Mars’ equator so the craft will receive the maximum possible amount of sunlight on its solar panels. It must be relatively flat, so the lander can come to rest with a minimum number of bounces. It must be free of large or sharp boulders that could puncture or destroy the air bags that cushion the landing.

The site should be relatively free of strong winds that could damage the parachute or capture it and blow it off course. And then, of course, there must be something interesting for the rover to look at, once it touches down.

Gusev is not the most geologically desirable landing site, but the apparent presence of sedimentary rock promises a wealth of information, and its location should not unnecessarily endanger the lander.

Named for the 19th century Russian astronomer Matvei Gusev, the Connecticut-sized crater was produced by a massive asteroid impact almost 4 billion years ago. The crater floor has since been partially filled in, perhaps by sediment from a lake, by lava or by wind-blown sand. It is pocked with many other impact craters that researchers hope will expose the lower layers for study.

Ma’adim Vallis stretches northward like the Nile River, from the highlands in the south to the southern rim of the crater. In places, Ma’adim Vallis gapes more than 16 miles wide and 1.2 miles deep, dwarfing comparable features on the Earth’s surface.

Geologists speculate that it was carved more than 2 billion years ago.

Some believe water could have filled Gusev Crater to a depth of about 3,000 feet, leaving behind an estimated 2,100 feet of sediment. Eventually, they believe, the water could have spilled out over a break in the western rim of the crater.

Gusev Crater is now “cold, dry, barren and desolate,” said Steven Squyres, a Cornell University planetary geologist and the chief scientist of the Mars mission. “But we see clues that long ago -- I don’t know how long ago -- it may have been very different.”

The Mars rover’s mission, he concluded, will be “to read the story in the rocks.”