2007-05-10 | SCIENCE
STONEs in Space

Can life travel from planet to planet? When a rocky world is hit by a meteorite, the impact can send pieces of the planetary surface out into space, and eventually these ejected rocks can travel to other planets in the solar system. Here on Earth we have collected many meteorites that originated from the moon and from Mars, and there are also likely rocks from Earth sitting on the surfaces of our planetary neighbors. 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 André 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.

The preceding news links are provided as a public service for interested users. The views and claims expressed in external internet sites are not necessarily those of NASA.