By Paul Rincon
Science reporter, BBC News, Houston
A pair of meteorites discovered in Antarctica are in a class all of their own, a major space conference has been told.
One of the pair, known as GRA 06128, whose origin is unknown
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.
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."
Chip Shearer, from the University of New Mexico in Albuquerque, US, raised the possibility of a Venusian origin in the title to his conference talk. But at the meeting, he acknowledged the reference was intended to be "provocative".
Dr Shearer agreed the 4.5-billion-year ages of the meteorites indicated the likely source was an asteroid.
"The history of this rock involves partial melting on a fairly primitive body," he explained.
Even if the rocks had been blown off Venus in an impact 4.5 billion years ago, they could not have drifted in space for such a vast length of time before landing in Antarctica recently, scientists said.
The identity of the object that spawned the two meteorites may be elusive, but researchers have been able to draw up a basic profile.
They know, for instance, that the parent body had "differentiated" - that is, had been reprocessed into a layered object, usually with a core, a mantle and a crust. Stony meteorites which have undergone this reprocessing are known as achondrites.
"There has to be a finite number of differentiated parent bodies," said Dr Zeigler, who also thinks an asteroid was the likely parent body.
"It's got to be 200km across or so to make an achondrite - to differentiate into basalts and a core. We think we know where they all are, and - greater than 200km - there are about 25.
"Now, some could have been destroyed and so the number might be higher than that, but it's not like there's an infinite number of parent bodies in our Solar System where these could come from."
So far, just one asteroid has been tied to a class of meteorites on Earth. Spectral observations of the object 4 Vesta suggest it has about the same composition as the so-called Howardite, Eucrite and Diogenite (HED) meteorites.
Dr Caroline Smith, meteorite curator at London's Natural History Museum, who was not involved in the studies, commented: "The meteorite family tree is getting bigger and bigger."
She told BBC News: "Like a family tree for humans, when the tree gets bigger, you begin to see the various relationships between them."
The Graves-Nunataks space rocks do share some similarities with another rare class of meteorite known as the brachinites. But there are important differences which would preclude their easy inclusion in this category.