Bacterial fry-up: Living and fossil samples of bacteria were attached to the heat shield of the ESA's Foton M3 spacecraft.
Credit: ESA
PARIS: A novel experiment has dealt a setback to a theory that life on Earth was kick-started by bacteria which hitched a ride on space rocks.
The 'panspermia' hypothesis is that cells were transported to the infant Earth on rocks that were bumped off other planets or came from another star system.
The theory gained notoriety in 1996 when a group of U.S. scientists proposed that a famous meteorite found in Antarctica had traces of fossilised bacteria that once lived on Mars.
Artificial meteorites
Seeking to find out more, a European Space Agency (ESA) team devised artificial meteorites to see what happens when rocks bearing fossil traces and living bacteria are exposed to the fiery heat of entering Earth's atmosophere.
In research unveiled today, at the European Planetary Science Congress in Muenster, Germany, the team report that they attached small rocks, two centimetres thick, to the ESA's unmanned Foton M3 capsule. This was launched in September 2007, and returned to Earth 12 days later.
The samples were imbedded on the capsule's heat shield, which reached a peak velocity of 27,200 km/h during the controlled descent. One sample comprised a 3.5-billion-year-old piece of sedimentary rock from Pilbara, Australia, that contained carbonaceous microfossils.
The other was a piece of lake sedimentary rock from the Orkney Islands, Scotland, containing chemical traces of past organisms.
Hardy germ
The back of both rocks was smeared with a living bacterium called Chroococcidiopsis – a hardy, primitive species that lives on the underside of stones in the desert, surviving on tiny droplets of moisture. Some scientists have considered it, or a relative of it, to be a good candidate for a Martian germ.
Recovered and analysed after the return, the Pilbara sample was found to be covered with a creamy-white 'fusion crust' about half a millimetre thick but, underneath, its microfossils were intact.
The Orkney sample lost nearly a third of its mass, but otherwise survived, as did its biomolecules.
But there was bad news for the Chroococcidiopsis. The bugs were burnt to a crisp, although their carbonised outline remained intact.
"The STONE-6 experiment suggests that, if Martian sedimentary meteorites carry traces of past life, these traces could be safely transported to Earth," said investigator Frances Westall, of the Centre of Molecular Biophysics in Orleans, France.
Panspermia
This experiment is hardly the end of panspermia - how about using a rock the size of a car, with lots of cracks and fissures, where bacteria has been found living at great depths of Earth's mantle.
All the French have proven is that bacteria has a rough ride on a re-entering nose cone!
Panspermia
I agree with the previous comment. Assuming that the cooling Earth continued to be quite heavily bombarded for perhaps hundreds of millions of years by rocks of all sizes, the solitary experiment in question can hardly be of great significance.
Of course, were not even talking of the abundant complex prebiotic precursor molecules observed in relatively local regions of interstellar space, and it may be even more likely that those were our ancestors.