Here is a very nice and through paper on the subject: "Natural Transfer of Viable Microbes in Space" https://doi.org/10.1006/icar.1999.6317 https://doi.org/10.1

Here is a very nice and through paper on the subject: "Natural Transfer of Viable Microbes in Space" https://doi.org/10.1006/icar.1999.6317 https://doi.org/10.1006/icar.1999.6317 In the abstract: "The conclusion is that if microbes existed or exist on Mars, viable transfer to Earth is not only possible but also highly probable, due to microbes’ impressive resistance to the dangers of space transfer and to the dense traffic of billions of martian meteorites which have fallen on Earth since the dawn of our planetary system. Earth-to-Mars transfer is also possible but at a much lower frequency."