������ý

Mercury, arguably our solar system’s weirdest planet, just got a little less puzzling. Researchers have figured out the origin of a huge but until now unexplained feature on its surface.

Unusually rich in magnesium, the feature covers 15 per cent of the surface, making it the size of Canada. It was identified by NASA’s Messenger spacecraft before its mission ended when it crashed into the planet last year. But no one knew how it formed and why it differs from the rest of the surface.

The inner-most planet is unusual because its iron core takes up 85 per cent of its radius, compared with just 50 per cent for planets like Earth and Venus. Not only is Mercury’s mantle shallow, it is made up of rocks that aren’t oxidised to form rust-like minerals as they are on Earth and Mars. And despite the relatively huge iron core, the mantle is lacking in the stuff.

“We have a hard job explaining where in the solar system Mercury formed, and how it happened,” says of the UK’s Open University, who wasn’t involved in the latest research.

To solve the mystery of the feature, a team at the NASA Johnson Space Center in Houston, Texas, created a powdered version of rare meteorites that most closely mimic the primitive mantle rock found on Mercury, from which all its other rock types originate. They then heated and crushed the mix over the range of temperatures and pressures found from top to bottom of the planet’s mantle.

Under pressure

The results showed that the surface feature probably originated from intense crushing and heating of rock 400 kilometres down, where the planet’s mantle meets the iron core. The rock was then spat up by volcanic activity, accounting for the oldest rocks found on the surface. By contrast, the younger rocks found elsewhere originated from milder conditions found higher up in the mantle.

“The pressures were high, up to 50,000 times the Earth’s atmospheric pressure, and the sort of pressure where you can form diamonds,” said team-member , who revealed the results at the Goldschmidt geosciences conference in Yokohama, Japan. “This is the pressure of Mercury’s core-mantle boundary.”

The meteorites she and her colleagues mimicked – called enstatite chondrites – are extremely rare, accounting for just 2 per cent of all space rocks recovered so far on Earth. But being rich in magnesium, their mineral and elemental content closely resembles that identified in the surface feature by X-ray spectrometers aboard the Messenger probe.

Boujibar’s conclusions corroborate those of another , which also cooked up their own fakes.  “The work we published reached the same conclusion,” says at the Massachusetts Institute of Technology. “We cite several others who’ve said that enstatite chondrites are a good parent composition for Mercury.”

“Essentially, both teams are saying that the composition of the magma you’d get by melting Mercury’s mantle at different pressures – and therefore at different depths – is different, and that this can account for the composition variations across Mercury’s surface,” says Rothery.