Spirit, one of two rovers with the project, has been scouring the surface of Mars for the past two years looking for clues to its watery past.
"A week ago, we got to 'Home Plate' (in the Gusev Crater), and 12 hours ago we found this fine-grained stuff," Steven Squyres, the project's lead scientific investigator, said Thursday during a packed keynote speech at the RSA Conference 2006 here. "In a few weeks, we hope to learn what it's made of."
Scientists have learned plenty in the past two years as the Spirit and Opportunity rovers have roamed the Mars terrain.
When Spirit first landed in the Gusev Crater in January 2004, scientists were worried that the rather flat landscape, filled with lava rocks such as the Adirondack, would offer few clues to Mars' watery past, compared with finding information-rich sedimentary rock.
Spirit at first offered scientists just a teasing view of a range of hills in the far distance, later dubbed the "Columbia Hills." The rover was expected to remain among the lava rocks because of the estimated time it would take to travel to the Columbia Hills, Squyres said. The rovers, after all, were expected to last only 90 days on the Red Planet.
But as the 100th day approached, a decision was made to head for the hills. And on the 156th day after landing on Mars, Spirit reached the base of Columbia.
When Spirit began to explore a section of the hills, in an area called "Husband Hill," scientists were excited to find sedimentary rock. "We started to see different stuff," Squyres said. "There was a fine layering of rock...which would only be formed if water was present."
Over the next 100 days, Spirit explored the area and climbed Husband Hill, noting a white semicircular area in the distance, dubbed Home Plate.
When Opportunity landed on Mars two years ago, its descent was better than a hole-in-one shot by Tiger Woods, Squyres said. The Mars rover, with the aid of blustery winds, landed inside a 20-meter crater, later named Eagle Crater.
"When we opened our eyes, we saw bedrock exposed in the walls of the crater," Squyres said, noting he couldn't have asked for a better situation.
As Opportunity explored the crater's terrain, it discovered the surface was littered with small balls of rock 4 millimeters to 6 millimeters in size that resembled blueberries embedded in a muffin. The concretions, or blueberry rocks, were a significant discovery.
"It was evidence that water at one time had saturated the ground here," Squyres said, noting that as the ground began to erode, concretions were left.
Opportunity also discovered patterns in the terrain that indicated water not only had soaked the ground but had risen to the top of the surface at one point, he added.
After spending 60 days in Eagle Crater, Opportunity headed onto Endurance Crater, a much more perilous trip for the rover, with its steep cliffs. "We didn't know if the rover could climb up or down the hills of the crater," Squyres said.
Opportunity has successfully mastered the crater's hills during its 10-month stretch inside the crater, but it wasn't as lucky trying to master a sand dune. The vehicle became stuck, requiring the team on Earth to develop a mock setup to devise a rescue plan.
After spending more than two weeks trying to develop a plan, the team discovered the optimal technique to use, Squyres said. "It was 'put it in reverse and gun it.'"
Opportunity made its way out of the sand dune and is currently at the Erebus Crater, which has a virtually flat terrain. But if scientists have their way, Opportunity will next head for the Victoria Crater, which is an estimated 900 meters in diameter and 60 meters deep.
"It'll be spectacular if we can get there," Squyres told the crowd, to a standing ovation.