"Mars ain't the kind of place to raise your kids..." sings Elton John in the song Rocket Man and he is right. Temperatures are mostly well below zero, the atmosphere is thin and devoid of oxygen, the existing water is mostly frozen at the poles, and the planet is blasted with so much radiation that it is doubtful that the even hardiest microbe could survive exposed on the surface. Nonetheless, NASA has sent its Curiosity rover over 350 million miles to test the "habitability" of the red planet for life, or for evidence that it was inhabited by life forms in the distant past. I would like to help Curiosity by suggesting some life-forms it might be on the look-out for.
Here on Earth, of course, life is ubiquitous and inhabits some of the harshest microenvironments imaginable. Martian life may not have a direct evolutionary link to Earth life forms, but some of our odder flora and fauna suggest what is possible.
If liquid water exists on Mars, it is likely in subsurface pools that would be very cold and extremely salty. Halophiles are Archean bacteria that can survive in salt concentrations at the limit of solubility. They have been found in the Great Salt Lake in Utah and as well as in the subglacial lakes in Antarctica. Ancient halophiles have been isolated from fluid inclusions in mined salt crystals, including a previously unknown species, Halococcus salifodinae.
D. Radiodurans is a bacteria isolated first from canned meat after it was supposedly sterilized with gamma irradiation. It is extremely resistant to radiation, surviving a dose of 5000 Gy with no loss of viability. Its resistance to radiation may actually be a side-effect of its extreme resistance to desiccation, which would also be very handy on the surface of Mars. Though radiodurans is the original and best studied, it turns out that there are a variety of bacteria on Earth with similar radiation resistance, including species of cyanobacteria, actinobacteria and the Archean Thermococcus gammatolerans.
Whatever they are, the term "nanobacteria" is certainly a misnomer, as these structures are not bacteria, and most likely are not living in the sense that we usually use the term. They are calcified structures of about 200 nanometers in diameter. Their fossilized remains were found in one of the oldest rocks ever examined, the Allan Hills 84001 meteorite thought to have originated on Mars. Nanobacteria have also been suspected of causing kidney stones in orbiting astronauts. NASA scientists caused something of a sensation when they found that nanobacteria propagated five times faster under low gravity conditions.
Lichens are symbiotic combinations of a fungus with a photosynthetic partner. They are ubiquitous on Earth, thriving in polar regions, on high mountains, and in the driest deserts. On a flight of a Soyuz rocket, the European Space Agency exposed two species of lichen—Rhizocarpon geographicum and Xanthoria elegans—to the vacuum and radiation of space for 15 days. The lichens were brought back to Earth and were found to be undamaged and still viable. More recently, Agency scientist showed that certain lichens could survive simulated Martian conditions for thirty four days. Should the decision ever be made to terraform Mars, the deliberate introduction of lichens may be a good place to start. These are frequently pioneer species on Earth, one of the first life forms to occupy areas after a volcanic eruption or other violent disturbances. Plus, some are edible, if you are hungry enough.
The wonderful pictures sent back from Curiosity and earlier Mars rovers show no bug-eyed monsters or obvious evidence of macroscopic life. But in the end, what we do find may surprise us, stretching our definition of what it means to be alive.