Further analysis suggested that such a rock likely originated from a volcanic eruption rather than from space.
"We find deposits that are compositionally basalt, which is kind of a volcanic rock, and very similar to other rocks in the vicinity that are clearly volcanic in origin," Squyres explained.
So far, however, scientists have not found a volcanic vent.
"What we have found is evidence suggesting that the material at Home Plate may actually be much more extensive than the Home Plate [itself] is," Squyres said.
That suggests the volcanic vent may be somewhere other than Home Plate.
"We suspect that much of the volcanic explosives have been eroded away and that Home Plate is a small erosional remnant of a formally more extensive deposit," he added.
"Basalt Goes Boom"
But what would have caused the volcano to explode?
The presence of basalt may offer a clue.
"One way to get basalt to go boom is to have it come into contact with water," Squyres said.
"If there is water below the surface or near the surface and it suddenly comes into contact with very hot lava, it will flash into steam and you can get a steam-driven explosion.
"We speculate that the explosion here may have come about as a consequence of lava coming into contact with Martian groundwater," said Squyres.
Scientists believe that as much as half of Mars's surface may contain frozen water. (Read related story: "Mars's Ice Patchy, Water Cycle Quite Active, Study Reveals" [May 2, 2007].)
Larry Crumpler is a curator of vulcanology and space sciences at the New Mexico Museum of Natural History and Science in Albuquerque and a member of the Mars rovers-science team.
Finding a volcanic vent, he said, may help scientists understand "the way volcanoes erupt, what the gases were when they erupted, and how the gases have been contributing to the atmosphere early in the history of Mars."
"That would be very exciting," he added.
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