BioEd Online

A cracked rock called Escher is helping NASA scientists reconstruct the history of water on Mars. They say that the crater currently being explored by the Opportunity rover may have been shaped by a second watery episode long after the wet period when the rocks first formed.

The flat rock carries a network of fissures that looks like cracked mud at the bottom of a dried up riverbed. "When we saw these crack patterns, right away we thought of a secondary water event significantly later than the episode that created the rock," says John Grotzinger, rover team geologist from the Massachusetts Institute of Technology, Cambridge.

Long after the sedimentary rocks in the area had formed, water may have welled up from underground to form a small lake within the stadium-sized Endurance crater. As the surface dried out again, the rock slowly cracked apart under the weak martian Sun. Alternatively, climate changes on Mars could have melted and frozen water within the rock, which opened out the channels over thousands of years, says Grotzinger.

However, the scientists cannot yet rule out the possibility that the rock was cracked during the impact that formed the crater. They plan to send the rover to look for a crust of water-soluble minerals on a nearby rock called Wopmay, which would strengthen the case for wet conditions after the crater formed.

The history of this area would then become much clearer. Large sheets of sedimentary rocks formed underwater billions of years ago, before a meteorite impact dug an enormous hole in the dry surface. Later, the martian climate must have again been warm enough to see liquid water - perhaps fleetingly - in the crater.

Aches and pains

On the other side of the planet, Opportunity's twin rover Spirit has overcome steering problems to press further up the Columbia Hills.

"Just as we worked our way deeper into Endurance crater with Opportunity, we'll work our way higher and higher into the hills with Spirit, looking at layered rocks," says Steve Squyres, principal investigator for the mission from Cornell University, Ithaca, New York. "All the rocks in the hills have been altered significantly by water," he adds.

Earlier this week, NASA reported that two of Spirit's troublesome wheels had become locked into place by faulty steering motors. Engineers were worried that they might have to deliberately blow the brake fuses to get Spirit moving again. But the problem seemed to solve itself two days ago.

"We don't have a root cause for this event but as [the rovers] age we'll see more aches and pains," says Jim Erickson, rover project manager at NASA's Jet Propulsion Laboratory, Pasadena, California.

Now that the rovers have made it through the darkest days of the martian winter, their solar energy supply will increase through the coming year. Although the mission has officially been extended for another six months, mission scientists hope that the rugged robot geologists could last even longer. "These rovers are gonna go for a long time," says Squyres.

NASA mission managers are now planning Opportunity's escape route out of Endurance crater. The rover should leave within the next 2 to 4 weeks, and then trundle about 200 metres to the heat shield that was ejected from the descent module as it delivered Opportunity safely to the planet's surface.

This should provide the first direct evidence about the performance of the heat shield, says Squyres, which will be vital information for future Mars missions.