On Earth, tiny organisms like bacteria or lichen can live in the crevices and holes that permeate rocks. These forms of life, already adapted to the uncomfortable environment inside a rock, have proven to be resilient when subjected to the harsh conditions of space, often surviving radiation and frigid temperatures when exposed for short periods. Could such forms of life be carried in their rocky home to another world, and then, once landed, set up shop on the alien planet?

This theory of life traveling between worlds is known as Panspermia. Some scientists have suggested that life on Earth could be alien-born, having originated on Mars or even further afield and then brought to Earth by a meteorite.

Today astrobiologists are testing the possibility of Panspermia in various ways. The STONE experiments of European Space Agency scientists sent microbes inside rocks into outer space to see if they could survive the journey.

"To be transferred from one planet to another, you have to survive atmospheric exit, you have to survive the conditions in space, and you also have to survive atmospheric re-entry when you reach the destination planet," says Charles Cockell, a microbiologist at the Open University in the UK who was involved in this study.

The first STONE experiment in 1999 was conducted just to confirm that sedimentary rocks could cross the Earth's atmosphere without being destroyed. The Mars rovers proved there are sediments on Mars, and yet sedimentary martian meteorites have never been found - the martian meteorites collected so far on Earth are all igneous. One reason could be that sedimentary rock disintegrates during atmospheric entry or shortly after impact.

"Sediments are conglomerates of small pieces of basaltic or volcanic rocks cemented together by carbonates or sulfate," says STONE lead scientist Andre Brack of the Centre de Biophysique Moleculaire in Orleans, France. "When you heat them, the risk is that the carbonate and the sulfate will crack, and everything will be destroyed. But sediments - stones made by deposition in water - are the best places to look for fossils."

The first STONE flight fixed three different types of rock into the heat shield of a Foton re-entry capsule: an igneous basalt, a sedimentary dolomite, and a simulated martian regolith. The sedimentary dolomite was not totally destroyed by the atmospheric re-entry, which indicated that it is possible for similar rocks from Mars to enter Earth's atmosphere intact.

The dolomite did not acquire a fusion crust. Instead, the surface exposed to the heat of re-entry burned off. This could point to one reason why we have not yet found a sedimentary martian meteorite - it lacks that tell-tale black fusion crust that meteorite hunters look for.

"It will be difficult to recognize them," says Brack. "There is no obvious sign or feature that they are meteorites. The only way would be to have a mass spectrometer to measure the oxygen isotopes 16, 17, and 18, as well as nickel, manganese, and chromium, because Martian silicates are expected to be enriched in these elements."

The entry speed of the satellite was 7.5 kilometers per second (for a small meteorite, the entry speed averages about 12 km/s.). The basalt sample was included in the test because it would develop a fusion crust at the appropriate speed. Unfortunately, the basalt was lost during the experiment, but the simulated martian regolith provided the proof they needed.

"The artificial martian meteorite was made of small pieces of basalt cemented by carbonate and sulfate, and this small bit of basalt had developed a fusion crust," says Brack. "So we know that the temperature was high enough to be close to the real speed of meteorite entry."

After launch, the rocket orbited the Earth for 16 days. It then re-entered the Earth's atmosphere, landing in the Kazakhstan desert. The dolorite popped out of its casing and landed in the surrounding soil, but it was recovered and the scientists collected the surrounding soil so they could subtract any added contamination.

The rocket's landing was softened by a parachute, which is not a comfort a meteorite ever enjoys. Would organisms traveling within a meteorite survive the hard impact of landing on the Earth's surface? Brack says that studies have shown that they would.

"To test the theory of Panspermia, people are looking at the stress bacterial spores receive when they are ejected from a planet body," says Brack. "Bacterial spores survive when they're subjected to one million G in a centrifuge. Other people, looking at impact shocks, put bacterial spores into bullets, and then shoot into sand. The spores survive this impact shock. So I think we have good evidence that, even if there is no parachute for a meteorite, the shock will not kill spores."

Cockell confirms that some microorganisms can survive very high impact shock pressures. In a study with his colleagues in Germany, microorganisms were put between two plates and a small explosion shocked the plates together. The microbes survived. However, the scientists discovered that photosynthetic organisms, which have large vegetative cells, cannot survive more than 10 gigapascals of pressure.

"That's not sufficient to be launched from the Earth into the solar system," says Cockell. "But it is sufficient to be launched from Mars to Earth" because of the lower gravity of Mars.

While an organism might survive the launch into space and even the impact of landing, they still might not survive the journey to another world. In the 2004 STONE experiment, all the organisms that were launched into space were killed. There was no evidence for their DNA, and no organisms could be cultured from the recovered rocks. The scientists say this suggests all the organisms had completely burned up from the heat of atmospheric entry.

"You might think this is uninteresting because there's no survival," says Cockell, who was in charge of the gneiss sample. "But in fact it's very interesting. Because cyanobacteria are photosynthetic, they have to live near the surface of a rock to get enough light energy for their growth.

During entry, the rock heated up to below the minimum depth at which life would be able to photosynthesize. In other words, because photosynthetic organisms need to be near the surface of the rock to get light, they end up getting extinguished during entry. What this demonstrates is a very specific dispersal filter against photosynthetic microbes being transferred from one planet to another."

Cockell says that organisms might be able to get around this problem by living deeper inside a rock. But still, this experiment shows that atmospheric re-entry is a very strong barrier against most photosynthetic organisms and even bacterial and fungal spores being spread between planets.

But the story does not end there. Cockell added new microbes to the metamorphic gneiss to see if anything would grow inside. He discovered that organisms grew very quickly; the glassy fusion crust that formed during atmospheric entry acted as a tiny greenhouse, improving the temperature inside the rock and also trapping moisture.

"This demonstrates that something biologically damaging, such as atmospheric entry, improved the habitat for these organisms which later colonized the rock," says Cockell.

So meteorites, while they can be destructive to life when they hit the Earth, and may not act as an effective spaceship for life traveling between worlds, can also create new opportunities for life in the aftermath of an impact.

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