Signs of ancient life could be littered across the moon, just waiting for an intrepid explorer to find them. That's according to physicists who tested what would happen if a chunk of rock containing microscopic fossils from Earth were to be launched into space and smash into the lunar surface. Finding one could give us a pristine glimpse into past life on Earth.

Meteorites found on Earth that were created by impacts on the moon and Mars suggest that cosmic bodies regularly chuck rocks at each other. A few researchers have claimed that some of these meteorites show signs of fossilised bacteria, the most famous being Mars rock ALH 84001. However, the evidence is shaky – and misses a more fundamental question, says Mark Burchell at the University of Kent, UK.

"No one ever seems to have asked, even if the fossils did exist in a rock, would they survive?" he says. To find out, Burchell and his colleagues tried to simulate the conditions that fossilised diatoms – microscopic algae with detailed shells – would face on a trip from here to the moon.

The team powdered rock containing these fossils then mixed it with water and froze it to replicate a meteoroid. They then fired it into a bag of water using a large gas-powered gun. The force of the gun mimics what happens when a nearby impact launches a rock into orbit, and the rapid deceleration and high pressures of hitting the water simulates smacking into the moon at high speeds.

Microbes on the moon

None of the fossils survived perfectly intact, and the team found fewer and fewer recognisable fragments as they ramped up the impact speed from around 500 metres a second to a likely meteorite impact speed, around 5 kilometres a second. But being able to recover anything at all is promising, says Burchell. Because Earth is so geologically active, some rocks on this planet containing evidence of past life have been destroyed, but any fossils found on the moon would be better preserved.

"There is a good chance even if you found fragments, there would be things you have not seen before," he says. Finding out how old they are could provide a wealth of information about Earth's past.

Robotic and human explorers have brought back hundreds of lunar samples, but so far no one has found an Earth meteorite on the moon. Christian Koeberl at the University of Vienna, Austria, points out that Earth's dense atmosphere and high gravity – compared with the moon and mars – makes it more difficult for rocks to leave, but it could happen. "Even if it happens rarely, it is not impossible."

It is a prize worth pursuing, says Kieren Torres Howard at the City University of New York. "The idea that fragments of Earth rocks littering the lunar surface could be preserving a fossil record spanning much of Earth's history is intriguing," he says. "Actually discovering them would be amazing – another reason we should hurry back to the Moon."

Quelle: NewScientist