Pics show Mercury 'shrinking and ageing'

1 Feb 2008, 11:03 am   -   Source: AAP
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The first pictures from the unseen side of Mercury reveal the wrinkles of a shrinking, ageing planet and a series of troughs shaped like a spider.

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The first pictures from the unseen side of Mercury reveal the wrinkles of a shrinking, ageing planet with scars from volcanic eruptions and a series of troughs shaped like a spider.

Some of the 1,213 photos taken by NASA's Messenger probe, released 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 troughs radiating out from it.

Mercury, the closest planet to the sun, has often been compared to Earth's dull 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."

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

Among the features shown in the photos are ridges, which scientists say provide evidence that Mercury is contracting.

Scientists had theorised 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 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 the Messenger on its nearly 8 billion kilometre mission in 2004. It will fly by Mercury two more times, this October and September 2009, before settling into orbit around in 2011.

Messenger will take pictures, measure the planet's atmosphere, hills and valleys and unusual magnetic field - Mercury is the only solar system planet other than Earth to have a magnetosphere.

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