A plateau on Mars known as Home Plate shows evidence of long-past explosive volcanic activity, say scientists on NASA's Mars Exploration Rover mission. And data collected during the rover Spirit's initial pass across the 90-meter (295 feet) wide plateau also supports earlier findings indicating that water once existed at or beneath the planet's surface.

Home Plate's finely layered appearance made it one of the most tantalizing targets within Spirit's reach in Gusev Crater, said Steve Squyres, the mission's principal investigator and the Goldwin Smith Professor of Astronomy at Cornell. The rover captured its first panoramic image of Home Plate in August 2005 from the summit of Husband Hill and reached the plateau in the Columbia Hills' inner basin in February 2006.

It quickly sent back an image Squyres called "one of the neatest pictures we've taken with the rovers." The image shows a small (4 centimeter) rock fragment nestled within a downward deflection in otherwise ruler-straight lines of layering -- a feature likely to be what geologists call a bomb sag. These usually form when a rock fragment (the bomb) is thrown upward in an explosion; then lands in deformable material, causing the material to sag beneath it.

Chemical analysis shows the rock is made of the same material (basalt) as volcanic rocks around it, indicating the explosion was not the result of an impact by an exotic source (such as a meteorite). The rock also shows tiny spherical particles that look like accretionary lapilli -- coagulated bits of ash that typically rain down after a volcanic explosion.

Any volcanic activity at Home Plate probably happened billions of years ago -- but part of what makes it intriguing, said Squyres, is its similarity to regions on other parts of the planet.

"There are lots and lots of places on Mars where, from orbit, you see layered deposits locally that kind of look like this," said Squyres, "and so it really raises the possibility that a lot of these things all over the planet could be explosive volcanic deposits."

That the rocks at Home Plate are basalt -- not a material normally associated with explosions -- also hints that water was involved. "When basalt erupts, it often does so as very fluid lava, rather than erupting explosively," Squyres said. But a notable exception comes when hot basalt meets water to cause a steam-driven explosion.

The bomb sag -- now dry, but shaped as if the rock sitting in it landed with a splat instead of a thud -- is a second hint that the surface was once wet. A third is the material's high chlorine content, which may point to past exposure to a briny fluid.

Home Plate may be the site of an early impact crater that was later filled in by volcanic debris, according to stereo images that show the layered rocks around its edge all sloping in toward the plateau's center. Billions of years of erosion could have stripped away the surrounding material but left the debris protected by the crater's rim -- resulting in the current plateau.

The Science paper is based on data collected during a frenetic few months in 2006, as Spirit was chugging down the Columbia Hills toward a safe place to ride out the Martian winter.

The route to safety included a path across Home Plate -- leaving Spirit's drivers on Earth with a dilemma.

"There was all this fabulous science around us," Squyres said. But with winter approaching, the team had the harrying task of getting Spirit to its destination in time, while gathering as much data as possible along the way. "We got an amazing amount of science done, all things considered," he said. "But there's more work to be done here." Spirit is now back at Home Plate, continuing exploration there.

Another sol, another discovery ... (and no one's yawning)

A year after Spirit first reached Home Plate plateau, the rover and its twin Opportunity are still healthy and plugging away.

Spirit, having made it through the winter, is back for a second pass at Home Plate (now driving ably on five wheels after its right front wheel died last year). And the baseball theme for names at Home Plate continues: Last year, rocks in the area were named to honor players from the Negro Leagues of the early 20th century; discoveries this year celebrate women from the All-American Girls Professional Baseball League of the 1940s and '50s.

Meanwhile, Opportunity, on Meridiani Planum on the opposite side of the planet, has been exploring the rim of Victoria Crater and is now heading back to an alcove called Duck Bay. From there, it will look for a place to start the tricky descent into the crater.

Both rovers are well past sol 1,100 of their 90-sol warranty (a sol is a Martian day). Mission members in Ithaca and at NASA's Jet Propulsion Lab in Pasadena have settled into a routine Squyres calls "a sort of Earth time-Mars time hybrid." And there's always more to do.

"We now have an operations concept that is sustainable in the long run," said Squyres. "We can keep doing this as long as we need to."

Source: Cornell.