Certain hardy bacterial spores, such as the Bacillus subtilis in the picture were exposed to space aboard the ISS, but shielded from solar UV-radiation, and demonstrated a high survival rate. The space vacuum and temperature extremes alone were not enough to kill them off. These remarkable bugs could be capable of surviving an interplanetary space flight to Mars and live there, under a thin layer of soil, were they to be accidentally deposited by a spacecraft.
This finding has huge implications; if microorganisms, or their DNA, can survive interplanetary spaceflight, albeit by natural means, it leaves open the possibility that life on Earth may originally have arrived from Mars, or elsewhere.