Messenger's mission to Mercury finds surprises

Six months after NASA's Messenger spacecraft began orbiting the planet nearest the sun, scientists have spotted a vast lava field at Mercury's north pole, weird sinkholes around some craters, and reason enough to throw out most theories for how the planet formed.

"In-orbit is definitely the place to be," said James Head III, a Brown University geologist on the team.

Messenger cameras looking down from a polar orbi have revealed surface details that could not be seen during three previous flybys, or by the Mariner 10 mission in 1974-1975. For example, Head said, "A mere few months has revolutionized our thinking about volcanism on Mercury."

A vast lava field covers 4.7 million square miles of the polar region. That's 6 percent of the planet's surface. On Earth, it would cover 60 percent of the continental U.S., and if the entire 1 million-cubic-mile volume were dumped on Texas, it would be 4.3 miles deep, Head said.

Scientists found photo evidence of "huge vents, 15 miles in length," where the lava spilled out, forming lava valleys and "teardrop ridges," he said.

During previous flybys, Messenger had also spotted brightly colored patches on some crater floors and rims. Closer inspection from orbit revealed — "to the utter surprise of the science team," according to David Blewett, of the Johns Hopkins Applied Physics Lab — odd "hollows," where surface material appears to have collapsed in a series of sink holes.

"We're not sure what process is responsible," Blewett said. But an emerging theory is that surface bombardment by micrometeorites and solar ions is breaking down volatile or easily evaporated materials, causing the collapses.

X-ray and gamma-ray spectrometry has confirmed "a surprisingly high abundance of volatiles," Blewett said. And that has cast doubt on several theories for Mercury's formation that required huge impacts or high solar energy, both of which would have driven off such volatiles.

What's left is a theory that Mercury formed by the relatively cool accumulation of metal-rich "chondrites," similar to certain meteorites found today. But "it's still an open question why Mercury looks like chondritic material and Earth doesn't," said APL staff scientist Patrick Peplowski.

The new Messenger findings are reported in this week's edition of the journal Science.

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