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Mars Saucer Mystery Baffles the Experts

Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1899a
DOI: 10.1126/science.310.5756.1899a

SAN FRANCISCO, CALIFORNIA—

Planetary geologist Michael Malin brought a long-standing, almost personal, problem to a joint planetary sciences/hydrology meeting session. Despite years of contemplating images returned by his camera now orbiting on Mars Global Surveyor, Malin can't for the life of him figure out how hundreds of impact craters on Mars were filled with some sort of sediment, some to overflowing, and then partially emptied of kilometers' worth of fill. Perhaps the terrestrial geologists in the audience could help?

Towering remains.

This 1-kilometer-tall pile of sediment (digitally stretched vertically) is taller than the surrounding crater rim, suggesting that sediment once covered the entire crater.

CREDIT: M. MALIN

Malin's prime example was 160-kilometer Henry Crater near the equator in the ancient highlands of Arabia Terra. A broad mound now covers much of the impact crater's floor and rises nearly as high as the crater's rim. The mound shows flat-lying layers, often monotonously uniform in thickness, which match layers in material still adhering to the crater wall.

To a geologist, it looks as if Henry was once filled to the top with sediment—40,000 cubic kilometers of it—and then was largely emptied. Some other craters were buried well above their rims, to judge by the height of lingering sediment piles. “We're pretty confident it happened,” said Malin, of Malin Space Science Systems Inc. in San Diego, California. “We don't know how it happened.” There is no obvious high ground that could have eroded to produce the sediment and no apparent gaps or channels through which running water might have carried sediment into or out of the crater. In fact, there's no sign of what erosive agent was at work. And there is no clue to where the kilometers of missing sediment have gone.

Malin has some ideas, of course. Rhythmic climatic variations—perhaps paced by the nodding of Mars's rotational axis over the millennia—probably turned sedimentation on and off to produce the layering. With no clear signs of wind-deposited layers, the sediments might initially have been laid down in seas, says Malin, although no one else has suggested seas up to several kilometers deep in the highlands. And wind is the leading candidate to whisk dust-size sediment particles from craters, but today's wispy atmosphere hardly seems up to the task. “It's hard to wrap your imagination around it,” says planetary scientist Robert Sullivan of Cornell University, “whether it was all water, all wind, or a combination. I remain as puzzled as Mike.”

The audience could offer no immediate solutions either, forcing Malin to fall back on the next probe to Mars. The Mars Reconnaissance Orbiter arrives in March, bringing far sharper eyes to bear on the cryptic half-full saucers of Mars.

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