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NASA's Mars rover Opportunity spied hints Friday of a mineral that typically forms in water — a finding that could mean the dry and dusty Red Planet was once wetter and more hospitable to life.

NASA 's Mars Exploration Rover Spirit returned this image on January 30, 2004. This is one of the first images sent back from the panoramic camera on the Mars Exploration Rover Spirit since the rover fell ill on the 18th sol, or Martian day, of its mission. They were acquired at Gusev Crater, Mars on Sol 26 January 29, 2004, showing that the camera's health remained excellent during Spirit's recovery. [Reuters]

That is the very question Opportunity and its twin, Spirit, were sent to answer.

The preliminary discovery came hours before Opportunity was to roll its six wheels onto the martian surface for the first time. Engineers planned to command the rover to roll the 10 feet off its lander and onto Mars at 3:12 a.m. EST Saturday. Confirmation was expected three hours later.

NASA said the US$820 million double-barreled mission should begin in earnest by Sunday, once Opportunity is on the ground and Spirit, on the other side of the planet, is cured of the software problems that have crippled it for more than a week.

"The fat lady has finally gotten onto the stage, but the time of her aria has not yet arrived," project manager Pete Theisinger said at NASA's Jet Propulsion Laboratory.

The rovers face continuing perils, including bitter cold and rough terrain.

NASA scientists want Opportunity to find on the pebbly ground a mineral called gray hematite. The iron-rich mineral typically — but not always — forms in liquid water.

Scientists said the preliminary evidence suggests Opportunity has already spied the mineral in the ruddy soil around it by using its mini-thermal emissions spectrometer, an instrument that measures infrared radiation.

Confirmation should take a few days, while scientists check and double-check their data, said Ray Arvidson of Washington University, the deputy main scientist for the mission.

A NASA satellite called Mars Global Surveyor previously spotted hematite at Opportunity's landing site. Scientists believe the mineral covers as much as 20 percent of the surface at Meridiani Planum, an area hundreds of miles across.

As for the ailing Spirit rover, NASA deleted 1,700 files from its flash memory Friday and then rebooted the rover.

"I am pleased to report it appears to be working just fine," said Glenn Reeves, chief engineer for the rover's flight software. He said NASA should be able to declare Spirit "fully recovered" by Sunday.

Cornell University astronomer Steve Squyres, the mission's main scientist, said very little science will have been lost because of the setback.

NASA previously warned that each rover would probably lose one of every three days of work to unforeseen circumstances.

While on the mend, Spirit already has resumed its science work, snapping the first-ever microscopic image taken on Mars of the surface of a rock. Spirit should begin drilling into the rock, dubbed Adirondack, sometime in the next four days.

Initial measurements reveal the rock is an olivine-rich basalt. The volcanic rock is the most common type on the surface of Earth and does not require water to form. That disappointed scientists.

"It is not the kind of smoking-gun evidence we're looking for," Arvidson said.

Scientists want Opportunity to strike out for an outcropping several yards to its left. High-resolution images have revealed the presence of fine layers in the bedrock. The layers could have been laid down by water, wind or the buildup of volcanic ash.

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