...via meteorites. This has been suggested before, but now there is more suggestive evidence, thanks to an "artificial meteorite."
The STONE-6 experiment tested whether sedimentary rock samples could withstand the extreme conditions during a descent though the Earth’s atmosphere where temperatures reached at least 1700 degrees Celsius... sedimentary martian meteorites could reach Earth. The fact that we haven’t found any to date could mean that we need to change the way we hunt for meteorites. Most meteorites have been found in Antarctica, where their black fusion crust shows up clearly against the white snow. In this experiment we found that the sedimentary rocks developed a white crust or none at all. That means that we need to expand our search to white or light-coloured rocks.
The rocks also transported living organisms, a type of bacteria called Chroococcidiopsis, on the back of the rocks, away from the exposed edge. Unfortunately the heat of reentry was so high, even with a protective two centimetre-thick rock coating, that the organisms were carbonised. They died but their cells still remain as “pompeified” forms.