A novel experiment has dealt a setback to a theory that life on Earth was kickstarted by bacteria that hitched a ride on space rocks.

The "pan-spermia" hypothesis is that cells were transported to the infant Earth on rocks that were bumped off other planets or even came from another star system.

The theory gained a boost in 1996 when a group of US scientists proposed that a famous meteorite found in Antarctica may have held traces of fossilised bacteria that once lived on Mars.

Seeking to find out more, European scientists have 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 to be unveiled on Thursday, they attached small rocks two centimetres (0.8 of an inch) thick to a Russian unmanned Foton M3 capsule that 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 7.6 kilometres (4.75 miles) per second (17,000 miles, or 27,200 kms, per hour) during the controlled descent.

One sample comprised a 3.5-billion-year 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.

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 (0.02 of an inch) thick but, underneath, its microfossils were intact.

The Orkney samples 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.

"However, the results are more problematic when applied to pan-spermia," she said a press release.

"STONE-6 showed at least two centimetres (0.8 of an inch) of rock is not sufficient to protect the organisms during [atmospheric] entry."

The study was scheduled to be presented on Thursday at the European Planetary Science Congress in Muenster, western Germany.

So far 39 meteorites have been found on Earth that have been attributed, through their chemical signature, to a Martian origin.

The notion is that they were knocked off the planet in the distant past by an asteroid impact. They then wandered in space before landing here.

But all of these meteorites are of basalt, or volcanic origin.

None is sedimentary, a term for rocks that are laid down in beds or strata as a result of wind, water or gravity. This has perplexed scientists, as there is abundant evidence for sediments on the Red Planet.

The outcome of the STONE-6 experiment, though, shows that Martian sedimentary rocks could survive entry through Earth's atmosphere.

The Foton capsule generated temperatures of around 1,700 degrees Celsius (927 degrees Fahrenheit), although its speed was somewhat slower than that of a meteorite.

Meteorites normally attain a velocity of 12-15 kms per second (26,800-33,500 mph, 42,800-53,600 kph) depending on their angle of descent.

A third piece of rock, a control sample of basalt, was lost during the descent.