Mixed fates for Mars rovers after 5-plus years SCIENCE

A worker at NASA's Joint Propulsion Laboratory digs sand to try to replicate Spirit's situation on Mars. A worker at NASA's Joint Propulsion Laboratory digs sand to try to replicate Spirit's situation on Mars. Photo: NASA/JPL Photo: NASA/JPL Image 1 of / 1 Caption Close Mixed fates for Mars rovers after 5-plus years 1 / 1 Back to Gallery

The latest news from NASA's robot explorers on Mars today is both good and not so good.

Opportunity, one of the pair that landed there more than five years ago, has discovered ancient wind-blown sand dunes and signs of water amid the sedimentary rocks inside the rim of an ancient Martian crater, and it's sending back pictures showing hundreds of tiny purple pebbles formed long ago from rising water levels on the sloping ground outside the crater.

But Spirit, Opportunity's twin on the opposite side of the planet, is in trouble.

It's stuck hub-deep and up to its belly in a patch of soft sand. Its five working wheels were spinning fruitlessly in the sand until Mission Control engineers back home stopped them, and now the teams are puzzling how to get the stranded robot moving again. It could take them several weeks to figure it out, or - if worse comes to worst - let it remain where it is while its instruments continue analyzing the soil around it and the weather above.

Good news or bad, it does seem incredible that these two veteran adventurers can still function at all. When Spirit and Opportunity landed on Mars in January 2004 three weeks apart, their mission was to last only three months.

But both their lives have been charmed - until now, at least - as they've reported new findings from their instruments every day, sometimes directly to Earth through their own transmitters and more often through either of two orbiting spacecraft that relay their signals home.

Spirit has been showing signs of age lately: bouts of amnesia and erratic behavior, and its right front wheel has long been immobilized so it can't turn at all and has had to rely on the rover's five other independently powered wheels to push it forward or pull it backward.

The vehicle got stuck as it traveled around a small plateau named Home Plate. It had tried to climb over the plateau, but failed because the obstacle was all of 20 inches high - just too steep for Spirit to handle, given its dead wheel.

Testing rescue strategies

The scientists and engineers at NASA are working day and night to solve Spirit's problem. A full-scale model at Mission Control of the sand patch on Mars can duplicate the ground where Spirit is stuck, and the engineers will use a full-scale replica of the robot itself to test various strategies for rescuing it.

"We're heading into the season of increasing dust storms when dust devils can cover Spirit's solar panels and limit their ability to generate electricity," said John Callas, the rovers' project manager at NASA's Jet Propulsion Laboratory in Pasadena. That electricity runs Spirit's driving engine and powers all its instruments and radio systems.

"Fortunately, winds have cleared the panels four times in the past month, no storms are forecast, and the panels are completely clear," Callas said. "So there's plenty of electricity for Spirit's science instruments to tell us more about the nature of the ground around it even while it's stuck.

"Spirit is in a very difficult situation, but the winds have bought us time so we can move very cautiously, and we're pretty optimistic we can solve the problem."

In the five-plus years they've been on Mars, Spirit has traveled 7.7 kilometers (4.8 miles) on its voyage of exploration and Opportunity has logged 15.9 kilometers (9.9 miles), Callas said.

Trek to Victoria crater

Most of Opportunity's miles have piled up traveling to Victoria crater, exploring its rock-strewn rim and looking for a place where it could safely climb down to sample inside the rim.

"It's been a fantastic bit of driving," said Steven Squyres of Cornell, the mission's chief scientist. "Getting the rover right up to the very edge of the cliffs above the crater, and then stopping right at the tip, took real control by the drivers. But we've gotten fantastic images of the crater's walls inside the rim."

He said he was thrilled with the performances of the rovers, adding that it's not surprising that Spirit is stuck.

"Mars is just full of treacherous, perilous rover traps. It's horrible, slippery, salty stuff," Squyres said.

In a paper published Friday in Science, Squyres and 33 of his colleagues report that Opportunity has found thick layers of windblown sedimentary rocks that once were sand dunes millions of years ago. The rock layers are rich in water-formed sulfate salts that the rover sampled inside the rim of Victoria crater, and they indicate there must have been a body of water there in the distant past, Squyres said.

Indication of past water

As it traveled across rising ground for about 3.7 miles toward the crater, Opportunity came upon tiny purple-gray pebbles known as concretions barely a quarter-inch in size. The scientists have called the pebbles "blueberries." They are strewn across the red sands and are rich in the iron mineral hematite. Blueberries just like them exist on Earth and are known to be formed in water.

As the ground rose higher, Squyres said, Opportunity encountered fewer and fewer blueberries - another indication that water once lay deepest beneath the Martian surface a billion or more years ago. Because Opportunity found similar blueberries inside the crater where it landed in 2004, and also inside another crater, scientists reason that the water must have played a major role in shaping the surface throughout planet's Meridiani region where Opportunity has been exploring, Squyres said.

"That water must have been a kind of acid brine back then," Squyres said.

Although ancient Mars was cold and wet, the brine could have remained liquid well above the normal freezing temperature of water, he said.