New Images Show Filled-In Craters On Mercury

An image of Mercury taken by NASA's Messenger i i

An image of Mercury taken by NASA's Messenger probe in early October. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington hide caption

itoggle caption NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
An image of Mercury taken by NASA's Messenger

An image of Mercury taken by NASA's Messenger probe in early October.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Craters on Mercury i i

Some of Mercury's craters as shown in images taken by Messenger. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington hide caption

itoggle caption NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Craters on Mercury

Some of Mercury's craters as shown in images taken by Messenger.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

New images of filled-in craters and large, smooth plains on the planet Mercury are helping scientists understand how volcanic eruptions helped shape this alien world.

A NASA spacecraft called Messenger zoomed just 125 miles above Mercury's surface earlier this month, and scientists are now analyzing hundreds of photos that show previously unseen regions of the planet, which is just a little larger than Earth's moon.

Some of the photos add to evidence suggesting that volcanoes have been very active on the planet.

"The bottom line is vulcanism was very important in the history of Mercury," says Mark Robinson, co-investigator and professor at Arizona State University School of Earth and Space Exploration.

Before this year, Mercury had only been visited by one spacecraft, a probe called Mariner 10 that flew past the planet three times back in the mid-1970s. But it glimpsed less than half of Mercury's surface.

When scientists looked at those Mariner 10 images, "there was actually a fair amount of ambiguity about whether or not vulcanism was even an important process on the planet," says Maria Zuber, co-investigator and head of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology. "We had a lot of materials on the surface, and we didn't know their origin."

That's changing as new images come in from the Messenger probe. The latest images were taken early on Oct. 6, in the second of three planned flybys of the planet.

Messenger launched back in August 2004 and is currently on a 4.9 billion mile trip that will send it around the sun more than a dozen times before it finally gets in position to start orbiting the planet in March 2011.

At a press conference, Zuber showed off one new image of two adjacent craters. Both are about 60 miles in diameter. But one of them is four times deeper, because the other crater is filled in with solidified lava — a huge amount of lava.

To get an idea of how much, Zuber explains, you could imagine the entire Baltimore-Washington region covered with a layer of solidified lava about 12 times the height of the Washington monument. "So it's a great, great deal of vulcanism," she says. "That's an awful lot of volcanic material in one place for such a little planet."

Other instruments on board Messenger took advantage of the latest flyby to measure calcium, sodium, and magnesium in the planet's tenuous atmosphere.

And Zuber says the spacecraft's laser altimeter saw hints that one hemisphere of Mercury appears to be about 30 percent "smoother" than the other. "The Western Hemisphere is smoother than the Eastern Hemisphere," says Zuber. "So there's some process that acted to smooth one hemisphere more than the other hemisphere."

Mercury is one of the more mysterious planets in the solar system, despite the fact that it was known to ancient astronomers and is one of Earth's rocky neighbors.

Messenger's final quick flyby will come next September. After that, when it finally begins to orbit Mercury, it will circle the planet for a year.

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