Meanwhile the European Space Agency's Mars Express spacecraft has probed beneath Mars's northern hemisphere, revealing 11 buried impact craters beneath the hemisphere's relatively smooth surface. The buried craters range from 80 miles (130 kilometers) to 290 miles (470 kilometers) in diameter.
The findings, reported in this week's issue of the journal Nature, help explain why Mars's sparsely cratered northern hemisphere differs so drastically in appearance from the southern hemisphere's craggy highlands.
The scientists speculate that the northern craters were covered up fairly recently, geologically speaking, by the smother, younger crust visible at the surface. Exactly what caused this burial remains a mystery.
NASA's twin Mars rovers, Spirit and Opportunity, are also making new finds—nearly 900 days after their original, 90-day mission was originally slated to end.
Opportunity recently completed a 21-month, four-mile (seven-kilometer) uphill trek to arrive at the edge of a large crater known as Victoria.
(See photo: "Mars Orbiter Spies Victoria Crater" [October 6, 2006].)
The rover has snapped high-resolution images of the crater's interior, revealing layers of ancient rock characteristic of windblown deposits.
It "looks just like a place on Earth," said Steve Squyres of Cornell University in Ithaca, New York, principal investigator for the rovers' mission.
Opportunity also reencountered "blueberries," tiny spherical deposits filled with minerals that typically form in the presence of water.
The blueberries were probably created in the area when subsurface water rose, reaching the surface at lower elevations, such as Opportunity's landing site, Squyers said.
But at the higher elevations Opportunity traveled on in recent months, the blueberries most likely remained below the surface. The newfound blueberries were probably flung free during the impact that created Victoria Crater.
The blueberry-forming water seems to have evaporated pretty quickly, even on the rare occasions when it reached the surface, Squyers added.
"Mostly it was an arid environment," he said.
Opportunity's twin, Spirit, is also back on the move after its second Martian winter—though it has had to travel mostly backward ever since one of its front wheels was damaged.
The rover has found water-altered minerals and basalt rocks in the Columbia Hills, an area typical of much of Mars's surface.
Spirit has also spotted water-ice clouds.
"We didn't see that last winter," said Ray Arvidson, NASA's deputy principal investigator for the rovers.
"This shows that Mars has significant interyear variability."
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