NASA’s Mars Exploration Rover Spirit, currently scaling Husband Hill above its Gusev Crater landing site, has found evidence of an explosive period in the region’s history, in which volcanoes or a massive impact showered the land with debris and possibly unearthed magma. Whether they were volcanic or impact explosions, however, is not yet known.

“Earlier in its history, this part of Gusev Crater was a violent place,” said Steven Squyres, lead scientist from Cornell University, Ithaca, NY, for the Mars Exploration Rover (MER) mission. “There were explosions going and there was stuff raining from the sky, and some of it was altered to a significant degree by a fairly modest size of water.”

Squyres and his fellow rover team members announced the finding, which is based on a trio of rock outcrops observed by Spirit’s cameras, during a May 24 press conference at an American Geophysical Union meeting in New Orleans, La.

“Really, for the first time since the start of the Spirit mission, we’ve got the kind of geology we can sink our teeth into,” Squyres said. “The last six weeks, I’d say, have probably been the most productive of the whole Spirit mission.”

Spirit’s sister rover Opportunity also has made progress, though not altogether scientific, at its Meridiani Planum. Opportunity is slowly but surely inching its way out of a deep sand dune, though mission managers don’t expect to free the robot for another few weeks.

It took the Spirit rover months to clamber up Husband Hill’s steep, slippery side. During that time the robot found little to suggest the region differed from the volcanic rock remains scattered across the rest of Gusev Crater.

But now halfway up Husband, after studying three rock outcrops, researchers are telling a different story.

“All of a sudden, we have geologic structure…everything changed,” Squyres said. “It was nothing more than you had to look at it from a different angle.”

Analysis by Spirit of rock outcrops known to researchers as Larry’s Lookout, Methuselah and Jibsheet contained signs of the Gusev’s tumultuous past, researchers said.

“Their chemical composition is very distinct from what we found out on the plains,” said rover science team member Richard Morris, of NASA’s Johnson Space Center in Houston, adding that there are signs of the mineral ilmenite — which is often formed in magma. “This is the first appearance of this mineral we’ve seen.”

While the rocks around Spirit share some compositional traits, the amount of weathering due to water differs among the outcrops, as do their textures. At Methuselah, for example, astronomers found the finest rock layers seen by Spirit to date, while Jibsheet sported a bulbous, globular look.

“Gusev has certainly turned out to be different than we expected it to be,” Squyres said, adding that he still believes that the crater was once the watery lake suggested by orbital photographs.

The rocks of the Columbia Hill chain, which includes Husband Hill, may completely predate that Gusev lake, rising like an island above the plains, Squyres added.

Opportunity ekes forward

While Spirit continues to explore Husband Hill, its robotic twin Opportunity is slowly but surely crawling out of a sandy quagmire on the other side of Mars.

The rover has moved about 27 centimeters (10 inches), which mission controllers say is good progress.

“We’re only traveling about half a percent of what we’re commanding,” said Jim Erickson, rover project manager at NASA’s Jet Propulsion Laboratory. “So it’s a very low number, but very consistent.”

At the current rate, it may be two more weeks before Opportunity once again reaches safe ground, Erickson added.

Opportunity is currently stuck in the outskirts of a region known as the etched terrain, which contains — scientists hope — exposed bedrock that could shed more light on water’s role in the history of Meridiani Planum. Astronomers know that the region was once awash with the liquid stuff.

“We’re learning that’s it’s a tough place to do business,” Squyres said of the area.

Now well past the one-year mark, NASA’s Spirit and Opportunity rovers still are going strong.

“We’re still trying to decide exactly how long they’ll go by running them until they wear out,” Erickson said. “We just don’t know how long these things are going to last.”