Delving into the desert to determine if Mars is too dry for life
Is Mars too dry support life? To find out, NASA sent a team from the Ames Research Center to the Atacama Desert in Chile. Infamous for being the driest place on Earth, the purpose of the Atacama study was sent to identify the limit is that divides the point where microorganisms can live, and where the best they can hope for is very temporary survival, and where Mars lands on this scale.
Water is one of, if not the, most important factors for life to exist. All life as we know it is dependent on water to support even the most basic of biological functions, and if there is no water, there is no life. It's for this reason that astrobiologists look for water before considering any other factor in determining if some place is habitable. That's because you can have a world with the perfect temperature, an Earth-like atmosphere, low radiation, and all the other elements needed for life, but if there isn't any water, none of that matters.
Such is the case in the Atacama Desert on the west coast of South America. It has a beautifully Earth-like environment, since it's on Earth, but it lacks one thing – water. In fact, it's about as close to Mars as you can get on Earth, with rainfall measured in millimeters per decade. Some areas are so dry that not even bacteria can survive. The nitrates fall from the sky that bacteria would normally gobble up, but these remain uneaten. In addition, the 10 million-year-old desert is surprisingly cool with a Mediterranean climate, but that only means there's less energy available for growth and reproduction.
The question is, where's the dividing line? When does it become too dry for life? According to NASA, the Atacama is a very good place to find out because this 1,000 km-long (620-mi) strip of land isn't uniformly dry, but is less dry at the southern edge and drier in the northern region. The idea is that by looking at variations in the extreme environment, it should be possible to find out when the water level drops below the biological minimum.
The important point is to determine when a collection of microbes are living or just surviving. That is, are the organisms feeding, growing, and reproducing, or are they just carrying out very basic biological functions to stay alive. If the former, then the colony will continue and change with each succeeding generation. If the latter, then the microbes will fade out in one generation and even sporulation (when nearly dormant forms of bacteria are formed) will be unable to keep them from eventually dying.
One way of seeing the difference is to look for signs of stress. Microbes that are just hanging on show no signs of stress, but ones that are growing and reproducing in a very harsh environment will be stressed. This will show in changes in the structure of the cells, like in the lipid molecules that form the outer surface of the cell membrane, which become more rigid. If this rigidity is missing from a very dry sample, then the microbes are not stressed and are, therefore, just surviving.
Another thing the team looked at was protein racemization. This is a standard dating technique used in archaeology and other fields that looks at how amino acids change after an organism dies.
Proteins and the amino acids that make up cells are complex, asymmetric molecules. In plain English, they come in left-hand and right-hand versions like gloves or shoes. In all terrestrial life, these molecules are left-handed. However, after an organism dies, this slowly changes as amino acids reform to become right-handed. This happens at a predictable rate until the left-handed molecules turning right-handed and the right-handed turning left-handed even out to achieve a 50/50 ratio.
What the team found was that in the driest regions, the ratio indicates that these have been dead for at least 10,000 years with remnants of life extremely rare.
What this says for Mars isn't very encouraging. The driest parts of the Atacama are almost entirely lifeless, with what little is there probably imports blown on the wind, but it's a paradise compared to Mars. NASA says the surface of the Red Planet is 1,000 times drier than the worst that the Atacama has to offer. This means that the chances of finding life on the Martian surface are very close to nil. But it may be possible to find remnants of life that once existed during the wetter epochs on ancient Mars.
"Before we go to Mars, we can use the Atacama like a natural laboratory and, based on our results, adjust our expectations for what we might find when we get there," says Wilhelm. "Knowing the surface of Mars today might be too dry for life to grow, but that traces of microbes can last for thousands of years helps us design better instruments to not only search for life on and under the planet's surface, but to try and unlock the secrets of its distant past."
The findings were published in Astrobiology.