Could life have travelled between Earth and Mars on meteorites?
It is possible in principle, because rocks have travelled from Mars to Earth, but there is no evidence that life ever made the journey.
We know rocks travel between planets: several hundred Martian meteorites have reached Earth. This raises a fascinating question: could tiny living things, such as microbes, have hitched a ride?
The idea. This idea is called lithopanspermia, meaning life spreading on rocks. If life existed on early Mars, an impact might have blasted rocks containing microbes into space. Some of those rocks might have landed on Earth, possibly bringing life with them. The reverse, Earth to Mars, is also possible, though harder because Earth's gravity is stronger.
Can life survive? There are three dangerous stages: - Ejection: the shock of an impact. Some Martian meteorites, such as ALH 84001, show that parts of rocks were not heated above about 40 °C during ejection, suggesting that microbes inside could survive. - Space travel: radiation, vacuum and cold for thousands to millions of years. Some hardy microbes and spores survive surprisingly well in experiments, especially if shielded inside rock. - Entry: only the outer millimetres of a meteorite get hot, so the interior stays cool.
Evidence? No living organisms or convincing fossils have been found in meteorites. The 1996 claim of possible fossils in ALH 84001 is not accepted by most scientists.
Implications. If life is found on Mars, scientists will want to know whether it is related to Earth life, which might suggest exchange, or completely separate.
This is why sample-return missions follow strict rules to avoid contamination in both directions.