OPPONENTS of the idea that life originated on Mars, and came to Earth on meteorites, have always been able to point to the huge impacts needed to eject rocks from the Martian surface. Surely, they argue, this would have killed any life they carried. Not so, says a study of the forces involved.

Rocks can be ejected when a giant meteorite impact sends an intense pressure wave across the surface of a planet. To see if microbes could withstand these pressures, Dieter Stoffler at the Fraunhofer Institute for High-Speed Dynamics in Freiburg, Germany, and his team put thin layers of microbes between two sheets of gabbro, a rock similar to the coarse-grained basalts found on Mars. They then detonated a mix of explosives on a steel plate on top of the rock, generating shock waves that subjected the microbes to pressures of up to 50 gigapascals, the equivalent of 500,000 Earth atmospheres.

Two of the three microbes tested, a bacterial spore and a type of lichen, survived pressures up to 40 gigapascals. "If Mars had early life there is no doubt that it was able to survive the pressures of impact ejection," Stoffler says.

While none of the Martian meteorites so far found on Earth shows clear evidence of having carried microbes, the new study tells us that a Martian origin for life is not ruled out. "Did Mars have a period where it was teeming with bacteria? That is a quite different question," says Brett Gladman of the University of British Columbia, Vancouver, Canada, who studies how impacts move objects within the solar system.

From issue 2588 of New Scientist magazine, 27 January 2007, page 17