Other, more recent studies have also been carried out and their results are suggestive that a very small number of bacteria may be able to eke out a microbial living in the Martian regolith.
But all of this is OK, according to Fairén and Schulze-Makuch. If these microbes can survive the harsh Mars environment, Earth has probably already infected the planet.
We know that meteorites from Mars have landed on Earth and, logically, meteorites have carried material from Earth to Mars. Energetic asteroid impacts on the solar system's planets' surfaces share material throughout interplanetary space. If a chunk of Earth material were to be blasted into space by a big meteorite impact, any bacteria inside could survive inside their rocky spaceship and crash-land into the Martian crust. Should they survive the voyage, they could already be on Mars, having been deposited there millions of years ago.
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This planetary microbe sharing is known as panspermia, but it is pure conjecture for now as there is no evidence that anything living has been transferred between the planets - although some Earth-based life has shown a certain flair for surviving for long periods in space, so the mechanism at least seems possible.