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On October 6, NASA’s Messenger space probe swooped down to within 125 miles of the surface of Mercury, and the just-released images from that flyby are shaking up astronomers’ ideas about the planet’s geologic history. The remarkable pictures reveal a vast patch of lava, indicating that the planet was shaped by a long age of volcanic eruptions. Astronomers used to dismiss Mercury, the planet closest to the sun, as mere “dead rock,” little more than a target for cosmic collisions that shaped it, said MIT planetary scientist Maria Zuber. “Now, it’s looking a lot more interesting,” said Zuber [AP].

Messenger’s cameras spotted a crater of about 60 miles in diameter that was not as deep as other nearby craters, and determined that it had been filled in with a huge amount of solidified 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” [NPR News]. Researchers think the eruption happened between 3.8 and 4 billion years ago.

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Yesterday, the Messenger space probe swooped down and flew 124 miles from the surface of the innermost planet, Mercury, furiously snapping more than 1,200 pictures of a side of the planet that has never before been seen by a spacecraft. Today, after the NASA probe turned its antenna back towards Earth, it began sending home remarkable photos of Mercury’s pockmarked surface.

The second Mercury flyby of Oct. 6 comes after a first flyby on Jan. 14, which looked at a different side of the planet. “When these data have been digested and compared, we will have a global perspective of Mercury for the first time,” said [astronomer] Sean Solomon…. Launched in August 2004, MESSENGER - short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging - is the first spacecraft in 33 years to greet Mercury up close since NASA’s earlier Mariner 10 mission of the 1970s [SPACE.com].

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Last January, NASA’s Messenger probe swooped past Mercury, dipping down to within 125 miles of its surface to take pictures and scientific measurements of the solar system’s smallest and hottest planet. Now the results have been analyzed, and researchers say the little planet is full of surprises.

Among other revelations, researchers realized that Mercury has a molten iron core that accounts for its strong magnetic field, but that its core is cooling, which explains the fact that the planet has shrunk more than a mile in diameter. Mission scientist Sean Solomon explains: Because solid iron is denser than the liquid form, “as you grow the inner core, the whole volume of the core shrinks,” he says, causing the planet itself to contract [Scientific American]. Messenger’s pictures reveal steep cliffs called “scarps” that researchers believe were pushed upward by the planet’s contraction.

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