Mercury northern plains

In the center of this image mosaic of Mercury's northern plains are examples of "ghost" craters, preexisting craters that were buried by the emplacement of the plains. (Courtesy of NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Only three months after NASA's Messenger spacecraft became the first to orbit the planet Mercury, scientists are already tossing out some long-held ideas about the place, and wondering at some surprising and unexpected discoveries.

"In many cases, a lot of our original ideas about Mercury were just plain wrong," said Larry Nittler, a Messenger scientist from the Carnegie Institution of Washington.

Among the surprises from the Maryland-run mission:

• Mercury has unexpectedly high abundances of potassium and thorium — elements that scientists thought would have evaporated as the planet formed so close to the young sun. Now they'll need a new theory of how (and where) Mercury formed.

• Laser altimeter measurements appear to confirm that Mercury has polar craters deep enough to be permanently shadowed, supporting the astonishing notion that the planet nearest the sun may host even more water ice than Earth's moon.

• New measurements have revealed that Mercury's magnetic field is asymmetrical. Its magnetic equator rings the planet 20 percent north of its geographical equator. Scientists now have to figure out why.

Somewhere in the puzzle of Mercury there are answers to questions about the origins of all the rocky, "terrestrial" planets from Mercury to Mars, but especially about Earth and the life that has evolved here.

"Mercury is that end-member of the terrestrial planets that tells us something about the inner part of the solar nebula, and about how the planets formed," said Ralph McNutt Jr., Messenger project scientist at a news conference Thursday in Washington for the release of the latest Messenger findings.

"Mercury really has been a surprise," he said. But that's what the mission is all about. "The reason we're doing it is because you learn something by going to places you haven't been before."

Messenger was built, and is being managed, by scientists and engineers at the Johns Hopkins University Applied Physics Lab near Laurel. It is the first spacecraft to reach Mercury since Mariner 10 in 1975, and in March, after fly-bys in 2008 and 2009, became the first to orbit the planet.

It has now completed a quarter of the yearlong orbital phase of its $446 million mission. It's working beautifully so far, said Sean Solomon of the Carnegie Institution of Washington, the mission's principal investigator.

Only this week, he said, Messenger survived Mercury's first "perihelion," or closest approach to the sun, since entering orbit. It also logged one Mercury year in orbit and came back on line after passing behind the sun and out of communication with Earth. And Messenger successfully made its first orbital correction maneuver and flew within 120 miles of the planet's surface.

Its cameras have sent 20,000 scientific images in higher resolution than ever before. "We are confirming a few theories," Solomon said. "Many are being cast into the dustbin of science."

APL scientist Brett Denevi said images of Mercury's north pole have revealed a smooth lava field half the size of the United States, more evidence of early volcanic activity on Mercury that raises questions about current theories of the planet's evolution.

"The smooth plains are compositionally distinct from their surroundings," Denevi said. "We don't yet know their composition."

Data from the spacecraft's X-ray and gamma-ray spectrometers show unexpected abundances of volatile elements that some theorists said should have been driven off if Mercury formed near the young sun.

That has ruled out theories that Mercury formed much like Earth, except for its proximity to the sun. Another theory had Mercury forming from a particular class of metal-rich asteroids. That one, too, is now "not in agreement with our observations," Solomon said.

There is also an idea that Mercury formed like Earth but was struck by another planet in a collision that blew away its crust and part of its mantle. "At this stage we cannot rule out this model," Solomon said.

But "there will probably be many more models devised before we have an answer on this," he said.

frank.roylance@baltsun.com

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Maryland weather blog: Frank Roylance on meteorology

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