Messenger probe shows off side of Mercury never seen before

WASHINGTON (AP) -- The first pictures from the unseen side of Mercury reveal the wrinkles of a shrinking, aging planet with scars from volcanic eruptions and a birthmark shaped like a spider.

Some of the 1,213 photos taken by NASA's Messenger probe and unveiled Wednesday help support the case that ancient volcanoes dot Mercury and that it is shrinking as it gets older, forming wrinkle-like ridges. But other images are surprising and puzzling.

The spidery shape captured in a photo is "unlike anything we've seen anywhere in the solar system," said mission chief scientist Sean Solomon of the Carnegie Institution of Washington. The image shows what looks like a large crater with faint lines radiating out from it.

Mercury, the closest planet to the sun, has often been compared to Earth's dull black-and-white moon. But the new photos, which reveal parts of Mercury never seen, show the tiny planet is more colorful and once had volcanic activity.

With the help of NASA high-tech enhancement, Messenger photos showed baby blues and dark reds.

"It has very subtle red and blue areas," said instrument scientist Louise Prockter of Johns Hopkins University, which runs the Messenger mission for NASA. "Mercury doesn't look like the moon."

Mariner 10 was the last NASA spacecraft to visit Mercury. In 1975, it took pictures of just 45 percent of the planet.

Messenger, which will do a couple more flybys of the planet before going into a long-term orbit, already has taken pictures of another 30 percent of Mercury, Prockter said. The rest will be seen eventually.

Planetary scientist Robert Strom, who was part of both the Mariner 10 and Messenger teams, said, "This is a whole new planet we're looking at."

And Prockter noted "there are some features we haven't been able to explain yet."

Example No. 1 is what scientists are calling "the spider." It is in the middle of a basin formed billions of years ago when space junk bombarded an infant Mercury.

Mariner had seen only part of the crater. When Messenger took a look with sharper cameras and a better angle, it photographed the odd central plateau, jutting about half a mile high with dozens of tiny ridges radiating out.

It is as if "something is pushed up," said Maria Zuber, a Massachusetts Institute of Technology planetary scientist who is part of the science team.

Prockter guessed that it could be remnants of a volcano. Other scientists think the leg-like features could be the same ridges seen all over Mercury.

The planet's ridges, first seen in the 1970s, now seen to more widely provide evidence that Mercury is contracting, the scientists said.

Scientists had theorized that as the core of Mercury cools, it contracts, and the whole planet shrinks. That was even a 19th century theory for why Earth had mountains, but one that was later proved wrong, Solomon said. But with Mercury, that seems to be the case. As the planet shrinks, a bit of crust is pushed over another, forming what Prockter calls "wrinkle ridges."

Besides having what looks like the leftovers from volcanoes, Mercury has at least one crater that seems to be filled with what would be that planet's version of lava, Prockter said.

NASA launched the $446 million Messenger on its nearly 5 billion-mile mission in 2004. It will fly by Mercury two more times, this October and September 2009, before settling into orbit in 2011. Messenger will take pictures, and it will measure the planet's tenuous atmosphere, hills, valleys and unusual magnetic field. Mercury is the only planet in the solar system other than Earth to have a magnetosphere.

Quirky Mercury is one of the bigger question marks in the solar system, probed not nearly as much as Mars, Jupiter, Venus or Saturn.

Strom, a retired University of Arizona scientist who worked on Mariner 10, said that as he awaited Messenger's flyby earlier this month, "I couldn't sleep at all. I was like a kid on Christmas Eve."

Only he had to wait 30 years for his presents. It was worth it, he said: "What I saw was astounding to me." E-mail to a friend

Copyright 2008 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.

All About Unmanned Space Exploration • Planetary Science • NASA