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Opportunity to leave Martian crater

mars rover

NASA has decided to drive Opportunity out of the crater before a wheel failure prevents it from climbing out (Source: NASA/JPL)

Scientists from NASA have decided it's time for one of its Mars rover to get a move on and climb out of the big hole in the ground it has been exploring for nearly a year.

Opportunity will leave Victoria crater, having provided scientists back on Earth with a bevy of pictures and chemical scans of ancient rocks taken during the sojourn.

Understanding the role water played in Mars' history is believed to be key to learning if the planet ever supported life.

"It's been a fruitful year," says project scientist Dr Bruce Banerdt with NASA's Jet Propulsion Laboratory.

Quiet achiever

"We've been able to verify a lot of our hypotheses. There haven't been any marquee discoveries, no first-of-its-kind ... but science goes forward by lots of differently sized steps, not just the headline-grabbing discoveries."

That was the case when Opportunity almost immediately discovered that its landing spot in an equatorial region known as Meridiani Planum was once covered with a shallow, acidic ocean.

Opportunity touched down in January 2003. A twin robotic scout named Spirit settled on the other side of the planet.

In Victoria, Opportunity discovered the same types of sulphate-laced sandstones found during its last campaign in the nearby but smaller Endurance Crater, though Victoria's walls showed subtle chemical differences as the rover progressed deeper and deeper into the basin.

Investigating layers

Impact craters give scientists the opportunity to peel back layers of geologic history by examining how older, deeper rock compares with rock closer to the planet's surface.

"We could see just by looking at the images that some layers looked different," says Banerd. "Some were coarser grain, some finer grain. It gives us clues to the conditions of the environment."

The team determined that wind deposited Victoria's building materials, and then water cemented its walls into place, which is basically the same scenario uncovered at Endurance.

Of particular interest is how sediments, such as potassium, sodium and magnesium, settled into the sandstone. The deposits tell scientists about the water conditions on Mars.

With a floor about 60 metres below ground level and a basin about 800 metres in diameter, Victoria has been the most challenging terrain for the rover, which is about the size of small ride-on lawn mower.

Although there is still much to explore in the crater, managers of the program decided to get out while Opportunity still can.

Stiff shoulder

The rover is operating well, with only a stiffened shoulder joint for scientists to work around, but an unexplained and transitory electrical signal has recently caught the team's eye.

A similar reading from Spirit preceded the breakdown of one of its wheels.

"We don't know if this was a mechanical fault or a sudden strain of the motor because maybe the other wheels slipped on some loose soil, but when it occurred, everybody sat back and thought about the condition we were in," says Banerdt.

The team had been hoping to drive Opportunity to a rock at the base of a cliff, but decided the science gain would not be worth risk to the rover should a wheel failure prevent it from climbing out of the crater.

"If Opportunity were driving with only five wheels, like Spirit, it probably would never get out," says JPL mission manager Bill Nelson.

While outside scientists review the team's research prior to publication, Opportunity opens a new chapter in its exploration of Mars - studying small rocks called cobbles that are transplants to the region.

The stones are coloured differently than the surrounding terrain, and if they were made of sandstone like the rocks lining the craters' walls, they would have been blasted to bits by now by Mars' heady winds. Rocks that survive are usually harder and made of different materials.

"These things are scattered pretty much all over the plains, and we've been driving past them for years," says Banerdt. "Now finally, in the eyes of many scientists, we're going to get a chance to look."
