Mercury meteorite puzzle

By Dr David Whitehouse BBC News Online science editor

The first meteorite that may have come from Mercury has been identified.

NWA 011 was found in the Moroccan Sahara in December 1999 and was immediately regarded as something unusual.

It clearly had a molten past and was formed from lighter materials than most meteorites. This implied it had once been part of a much larger body.

It was originally classified as a eucrite, a group of meteorites thought to be from the asteroid Vesta. But a detailed analysis of NWA 011 showed it to be different.

Now, researchers speculate that it is the first known meteorite from our Solar System's innermost planet, Mercury.

Rocks blasted off Mercury by a large impactor would have a difficult journey to reach the Earth, say the researchers - but not impossible. Nevertheless, the calculations show such rocks would be an extremely rare find on Earth.

NWA 011 has an oxygen isotope ratio that indicates it came from a body larger than a big asteroid.

Japanese researchers say the basalt in NWA 011 suggests the body from which it did originate had a core of molten iron with an outer covering of silicon and aluminium that formed a basaltic crust.

And that means a planet-sized body. Could it really be Mercury?