Mercury is shrinking, Nasa space probe reveals

The smallest planet in the solar system is shrinking, a space mission to Mercury has revealed.

The Messenger space probe, which began its 5 billion mile journey to Mercury four years ago, has provided new evidence that the closest planet to the sun is contracting in on itself.

Nasa scientists revealed images of cliffs and faults crossing the planet's surface, which they say are result of the crust buckling as Mercury's molten core cools down and solidifies.

The planet may have lost around 3 miles from its 3,000 mile diameter through out its history.

They said Messenger had revealed the shrinking caused by this cooling is far greater than had previously been thought.

In a special issue of the journal Science, researchers reveal that volcanoes have covered large swathes of the landscape with smooth plains of lava flow during the early part of Mercury's history – around three to four billion years ago.

Messenger, which managed to take photographs of around a fifth of the planet's surface in a fly-by in January this year, has also solved the mystery surrounding the source of Mercury's magnetic field.

Scientists had been unsure whether the magnetic activity was actively produced by the stirring of the planet's molten core, as it is on Earth, or was simply the result of large deposits of iron that act like a giant bar magnet, as is the case on the Moon.

The mission revealed that Mercury is similar to the Earth's, raising hopes that the planet can be used to study how Earth would have behaved and looked in its early history.

"After five months of analysis we know have some fantastic results," said Sean Solomon, principal investigator on the Messenger Mission and a researcher at the Carnegie Institution of Washington, Washington DC.

"The dominant landforms on Mercury are features called lobate scarps. These are huge cliffs that mark the tops of faults that formed during the contraction of the surrounding area.

"The data we have from the fly-by tells us that the total contraction is at least one-third greater than we had appreciated."

Mercury is unusual compared to other planets in the solar system because it has such a large core.

Scientists estimate that the mainly iron-based core accounts for 60 per cent of the planet's mass and 75 per cent of its diameter.

As solid iron is more dense than liquid iron, as the planet's core cools, its innards contract, causing the crust to buckle inwards and create the faults seen on the surface.

Scientists estimate that Mercury's diameter has decreased by about one tenth of one percent - a big shrinkage in geological terms.

During its first fly-by of Mercury in January this year, Messenger flew within 125 miles of the planet's surface to take pictures and carry out other tests.

The images sent back by Messenger have solved a 30 years controversy about patches of smooth rock first seen on the surface by the Mariner 10 mission in 1975.

Researchers debated whether these were hardened lava or material thrown out by asteroid impacts.

The images showed a large volcanic vent in the centre of one of these smooth patches, which have now been confirmed as the remains of lava flows that spewed out of the vents like a volcanic fountain.

Another vent also showed signs of molten rock that had oozed outwards over the surface.

Colour images of an area known as the Caloris basin, showed that it was completely filled with these smooth volcanic plains.

Scientists also said they were unable to rule out that there was still volcanic activity taking place on the surface of the planet.

Messenger is due to return to Mercury for another fly-by in October this year and again in September next year before setting into an orbit around Mercury in 2011.

It is hoped that far more of the planet's surface can be studied in greater detail and evidence for ongoing volcanic eruptions may be found.

James Head, a planetary geologist at Brown University, Providence, Rhode Island, warned, however, that eruptions will have become more infrequent as the planet's surface contracted.

He said: "The more shrinkage there is the less lava will get out and Mercury has experienced a huge amount of shrinkage.

"The volcanic activity we have seen seems to date from the first half of the solar system's history, but because the interior of the planet is still active we will be looking for evidence that there are still eruptions."

The probe has also shown how violent volcanic eruptions have helped to shape the surface of the planet.

By Richard Gray [Via telegraph.co.uk]