Microbes similar to those on Earth would have a tough time surviving the harsh environment of Mars, but it is not inconceivable that they could persist there given a little protection, according to a new study. The finding supports similar, previous work and lends credence to the theory that if microbial life ever arose on Mars, it could exist below the planet's surface to this day.

Mars is in most respects a terrible habitat for life as we know it: winter temperatures can dip below –100 degrees Celsius, the atmosphere contains little oxygen, and without the benefit of a robust ozone layer the Martian surface is bombarded with ultraviolet (UV) solar radiation.

In a paper submitted to Planetary and Space Science that was posted to the online preprint repository arXiv.org on February 22, a group of Italian researchers presents the results of submitting a number of terrestrial bacteria to simulated Martian conditions. Giuseppe Galletta, an astronomer at the University of Padova in Italy, and his colleagues introduced bacterial species, including three strains from the Bacillus genus and a strain of Mycobacterium smegmatis, to their LISA and mini LISA experimental chambers.

The enclosures provide a rough facsimile of Mars's surface—they are cooled by liquid nitrogen to create extreme Martian cold, bathed in UV light, and flooded with a low-pressure carbon dioxide atmosphere. (Mini LISA, a series of small enclosures, was built to prolong the running time of the experiments on a single tank of coolant; the laboratory is housed in a medieval castle in Padua, a historic site that can only accept coolant deliveries once a week.) The researchers varied the environmental parameters to simulate the drastically different conditions found on Mars depending on the season and the time of day.

Galletta and his colleagues found that the bacteria handled the temperatures, low pressures and lack of oxygen relatively well but that the UV intensity all but wiped out the colonies in minutes. Even the extremophile Deinococcus radiodurans, which can endure mammoth blasts of gamma rays hundreds of times more powerful than would kill a human, could not last 10 minutes under UV exposure.

Similar experiments have been carried out in the past, including many that have sought to determine the threat of terrestrial microbes stowing away on Mars-bound spacecraft and contaminating the Red Planet. In those studies, UV irradiation was also shown to be a lethal force for bacteria, although the other atmospheric effects seemed to at least retard the growth of bacterial colonies.

The Italian group found that bacterial spores fared much better than active cells; the Bacillus spores survived for more than an hour, albeit in low numbers. (Perhaps counterintuitively, more spores actually endured during simulated Martian winter, at –80 degrees C, than during summer, at 23 degrees C.) The study's authors report that spores shielded from UV light, as might occur on Mars beneath the frozen surface or in a cave, stabilize in a dormant state before reactivating in the presence of liquid water. "This is important for Mars, because if underground water exists this may mean that life could survive in some ecologic niches," Galletta says.

Galletta notes a major caveat of such research—the presumption that Mars life, if it ever existed, bears any resemblance to Earth life. The theory of panspermia, or the early transport of life between the planets, would imply that to be the case if proved out. But if life arose independently on Mars it could take a form completely distinct from terrestrial organisms. "The limitation is that we only ever test on terrestrial life," Galletta says. "We don't know if our conclusions will be universal."