Another rover to land - on another Mars

Saturday night, if the heat shield holds, if the retro-rockets fire at just the right time, and if the airbags survive an estimated 40-foot drop, NASA's rover Opportunity will emerge from its protective shell to look upon a Mars unlike any humans have ever seen.

For three weeks, Opportunity's twin, Spirit, has offered the world a familiar view of Mars, swaddled in its blanket of red dust and pierced by sawtoothed rocks.

If scientists are right, though, Opportunity is now descending toward a Martian Mordor barely imaginable, where much of the characteristic salmon soil has been swept away and dark fields of puzzling ridges ripple as far as the eye can see.

These are the two faces of the great Martian riddle. Each site promises clues into whether liquid water - the essential element for life - ever existed on Mars.

Yet, in many ways, the peculiarly furrowed plains of Opportunity's destination, Meridiani, offer a more compelling story to scientists, and perhaps the greater promise of discovery.

"Our expectation is that Meridiani is going to be very different," says Michael Carr, an astrogeologist with the United States Geological Survey in Menlo Park, Calif.

The difference is the key to its significance and hints at a paradox at the heart of the rovers' missions. In recent years, the century-old search for liquid water on Mars has increasingly focused on telltale surface features, from fanlike patterns of what seem to be dry riverbeds to winding gullies.

Sensors on the two American spacecraft orbiting Mars, however, have looked at these areas and found no minerals that would suggest the past presence of water.

By contrast, Meridiani shows no signs that water ever shaped its surface, but it contains gray hematite, a substance that usually forms with water on Earth. Accordingly, NASA sent Spirit to Gusev Crater, which appears to be the bed of an ancient lake, while it dispatched Opportunity to the dark rock of Meridiani on the other side of the planet.

"The great thing about the two sites is that they are so complimentary," says Steve Squyers, lead scientist for both rover missions at the Jet Propulsion Laboratory in Pasadena, Calif.

If Gusev offers the hope of seeing how water might have reshaped the Martian surface, Meridiani could let scientists peer into the long-dead processes of the planet itself.

A leading theory suggests that the hematite might have formed in hydrothermal pools heated by volcanic activity - like an extraterrestrial Yellowstone on Mars's equatorial plains. Others point to the action of small surface pools or subterranean aquifers, while skeptics argue that volcanoes could have formed the hematite alone, without water.

Regardless, Meridiani offers the most convincing evidence yet of water on Mars billions of years ago. Moreover, it is one of the safest landing corridors on the planet - as flat as a baking sheet and as broad as Ohio.

What Opportunity will actually encounter if and when it hits the ground, though, remains a mystery.

While data from the orbiters suggest the furrows of the Meridiani plain are not steep enough or tall enough to interfere with the lander or the rover, scientists acknowledge that the formations are utterly baffling.

The ridges could be small dunes several feet high and tens of feet across like earthen yardangs in the deserts of Africa or Asia. They could be waves of solid rock, though hematite makes up only 5 to 10 percent of the material. Or they could be something else entirely.

"We think that it's OK [for landing], but we're exploring, and we don't know," says Dr. Carr. "There's a little tension over what those ripples are."

More likely the greatest threat to Opportunity will come during the complicated entry procedure, when the rover must decelerate from 12,000 m.p.h. to a tissue-soft landing in six minutes.

If Opportunity hits the correct angle of entry within 0.2 degrees, it will rip through the Martian atmosphere with its heat shield rising to temperatures equal to the surface of the sun - more than 2,600 degrees F. If it gets the angle wrong, it will skip off into the void or be incinerated.

At 30,000 feet, the real fun begins as the lander starts a dangerous dance with the oncoming surface. The object is to slow the craft to 0 m.p.h. just above the surface, so when the final cord is cut, the airbag-encased lander falls no more than 40 feet and bounces to a stop.

In those two minutes, though, a parachute must deploy, the heat shield must be jettisoned, and retro-rockets must use speed and distance sensors to fire at precisely the right time. In 1999, the retro-rockets on the Mars Polar Lander apparently malfunctioned at 300 feet, causing a fatal fall.

Mission managers are desperate not to have that happen again. Science from Spirit has already whetted their appetite, but the rover appeared to be having problems sending data Wednesday. Still, Spirit's arm has already shown that the Martian soil in Gusev is bonded more strongly than many had expected, possibly by salts.

It has also shown traces of olivine, a volcanic substance that breaks down easily in water. The fact that it is there, scientists say, shows that water hasn't been around for a long time.

But Spirit has always hoped to get beneath the dust - which could have blown in from anywhere - and look at the older, deeper levels of Martian soil.

It probably won't get that chance until it arrives at the smaller Bonneville Crater 800 feet away. There, these layers have been exposed and ejected by the ancient impact.

"It will be disappointing if [Spirit] can't get beneath the surface," says Roger Phillips, a geologist at Washington University in St. Louis. "Hopefully, this crater will do the job."