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The Messenger probe reveals a feature - dubbed 'the Spider' - found inside the Caloris basin on Mercury. A set of troughs radiates outward in a geometry unlike anything seen by Mariner 10. ((NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington) )

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 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 colourful 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."

'A whole new planet'

The last time a NASA spacecraft went to Mercury was Mariner 10 in 1975. It took pictures of just 45 per cent 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 per cent of Mercury, Prockter said. The rest will be seen by the time the probe enters orbit around Mercury in 2011.

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

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

Wrinkles show Mercury's age

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

Now seen more widely than when first noted in the 1970s, the ridges 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 proven 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 Messenger on its nearly eight-billion-kilometre mission in 2004. It will fly by Mercury two more times, this October and in September 2009, before settling into orbit in 2011. Messenger will take pictures, measure the planet's tenuous atmosphere, hills and valleys and unusual magnetic field — Mercury is the only inner solar system planet other than Earth to have a magnetosphere.