Andrei Ivanov, who is based at the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow, Russia, spent two decades puzzling over the fist-sized Kaidun meteorite before he decided that it must be a chip off Phobos, the larger of the two Martian moons (Solar System Research, vol 38, p 97).

"I can't find a better candidate," Ivanov told New Scientist. The Kaidun meteorite is like no other in the world- and 23,000 of them have been catalogued. It is made of many small chunks of material, including minerals never seen before.

Working with Michael Zolensky of the NASA Johnson Space Center in Houston, Texas, Ivanov used an electron microscope to look at the space rock's crystal structure, peered through its minerals using X-rays and vaporised fragments to catalogue the elements inside.

And every sample turned out to be something "new and weird", says Zolensky. Among the odd materials in the meteorite were two fragments of volcanic rock- which only forms in massive, planet-like bodies with a core, mantle and crust.

But much of the meteorite is a kind of carbon-rich material that only occurs in asteroids. Zolensky thinks this paradox could be resolved if the meteorite comes from a Martian moon. Both Phobos and Deimos are thought to be asteroids captured by Mars as they wandered through space.

That would explain the carbonaceous material. And the pieces of volcanic rock could be bits of Mars, thrown into orbit when other asteroids crashed into the planet. Phobos is the more likely candidate: it orbits only 6000 kilometres from the planet's surface, much closer than Deimos, and so has probably mopped up a lot more fragments of Mars rock.

The idea is plausible, if somewhat speculative, says Sara Russell, a meteorite expert at the Natural History Museum in London. "There have been no landers sent to Phobos and so almost nothing is known about the composition and geology of this body."

Zolensky thinks that an unusual asteroid could have been the source. Hope of resolving the mystery rests with the European Space Agency, which has been asked by UK scientists to consider sending a mission to Phobos as part of its Mars exploration programme.

This article will appear in full in the April 24 issue of New Scientist

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