Antarctica's unique space rocks

A pair of meteorites discovered in Antarctica are in a class all of their own, a major space conference has been told.

Studies of the extra-terrestrial rocks have revealed qualities that set them apart from any meteorites previously known to science.

Researchers are pondering where in our Solar System the meteorites could have originated.

An origin on Planet Venus has been discussed, but now looks unlikely.

The notion of a meteorite hailing from this hothouse world is highly contentious. As yet, nobody has found one, probably because it is very difficult for rocks to escape Venus' thick atmosphere and strong gravity.

Several scientists propose that the Antarctic meteorites broke away from a previously unrecognised reservoir of asteroids before falling to Earth.

The space rocks are much older than the majority of Venus' surface - appearing to rule the planet out as the source.

The results have been discussed here at the 39th Lunar and Planetary Science Conference.

Origins unknown

The paired meteorites, known as GRA 06128 and GRA 06129, were discovered in the Graves-Nunataks region of Antarctica in 2006.

The rusty, slab-shaped rocks have defied classification, not fitting into any of the existing groupings drawn up for meteorites.

The pair's distinctiveness has been revealed by analyses of their mineral make-up and of the ratios of different forms - or isotopes - of oxygen present in them.

Dr Ryan Zeigler, from Washington University in St Louis, US, has been studying samples from GRA 06128. He told BBC News: "It's unique - it's the only meteorite that has this much plagioclase (a form of feldspar) of this composition.

"There are other meteorites that have minerals of the same composition but not in anything approaching the same proportions." ...

•	Antarctica's unique space rocks BBC News (UK), Thursday, March 13, 2008 Byline: Paul Rincon, Science reporter, BBC News, Houston