If we assume that life got started during the warmer, wetter conditions of Mars' past, could it still be hanging on somewhere under its frigid, sparse atmosphere? Without a careful examination of hundreds of potential habitats around the red planet, that question is probably impossible to definitively answer. But we can get a sense of whether that's possible by examining life in extreme conditions on Earth.
In the latest effort of this sort, an international team of researchers have taken samples from deep in the Siberian permafrost and put them under conditions similar to those on Mars: freezing, low pressure, and devoid of oxygen. Despite the unpleasant environment, several clones of bacteria grew out. A bit of study showed that they were all relatives of a strain first found growing on refrigerated, vacuum packed meat.
Although this doesn't tell us about what might be growing on Mars, it does highlight a bit of danger. We can contaminate the red planet if we don't carefully clean the hardware we send there.
The approach of the study was quite simple: obtain samples of bacteria from the bottom of a patch of permafrost in Siberia that's been there for thousands of years. Grow them under normal conditions, then shift them to ever harsher conditions. Next, drop them to 0°C. Draw off most of the air, until all that's left is less than a percent of normal atmospheric pressure. Then replace what's left with CO2, getting rid of oxygen in the process.
Dropping down to low temperature didn't bother most of the microbes, which kept growing. But swapping Earth's atmosphere for carbon dioxide was enough to stop most of them. At that point, lowering the atmospheric pressure didn't seem to make matters worse, since so little was growing anyway.
But so little didn't mean "nothing" in this case. In screening thousands of bacterial colonies derived from the permafrost, six different clones managed to keep growing through all the conditions. In fact, they grew better under the harsh, Mars-like circumstances than they did at room temperature and atmospheric pressure.
DNA sequencing revealed all of the growing samples were from the genus Carnobacterium, named for its ability to grow on refrigerated, vacuum-packed meat. But the genus turns out to be rather widespread, although it does have a tendency towards frigid circumstances. It's been isolated from both the Arctic and the Antarctic, and it has been pulled out of the waters of Lake Vanda (an iced-over, hypersaline lake in Antarctica).
Does this mean these organisms can survive on Mars? The authors are appropriately cautious, noting these conditions, while harsh, don't fully capture just how tough the conditions are there. "[These] conditions are only a subset of the total potentially biotoxic physical factors constraining the survival or growth of terrestrial microbes on Mars, such as solar UV, extreme desiccation, solar particle events, and galactic cosmic rays," they write. "In addition, the Martian regolith itself contains numerous potentially biotoxic factors, such as salinity, pH, and Eh of available liquid water; oxidizing soils created by UV-induced processes and soil chemical reactions; or the presence of heavy metals"
In other words, we have only begun to approximate Mars.
But the fact some bacteria on Earth could adapt to at least some of the conditions seems like a promising sign. In fact, the researchers note, there may even be more species on Earth that can grow well under a cold, sparse, anoxic atmosphere. As part of their procedure, they required everything to grow for a short time at room temperature and a normal atmosphere. This means they actually selected species that could survive a huge range of conditions, including Mars-like ones. There may be some that can only grow in the cold and sparse atmosphere, but were killed by conditions we view as hospitable.
All of this should serve as a caution to any space agency considering sending hardware to the planet. We'd hate to go to study life on Mars, only to find out we were examining the descendants of life that was, until recently, from Earth.
PNAS, 2012. DOI: 10.1073/pnas.1209793110 (About DOIs).
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