Though MSL was not designed to be a life-detection mission per se, let’s suppose for a moment that Curiosity finds traces of life on Mars. How could we make sure that the microbes we find are not coming from Earth, brought to Mars by our own spacecraft?
It’s known that certain microbes can survive the hostile conditions of space including ultraviolet light, cosmic rays and dramatic shifts in temperature. In an 2008 experiment, microbes were placed on the outside of the ISS to see how they would cope with this environment. 553 days later, many were still alive. [ref]Beer microbes live 553 days outside ISS:
http://www.bbc.co.uk/news/science-environment-11039206[/ref] That’s quite impressive and shows that a contamination of Mars could be possible.
I talked about this with Dr. Catharine A. Conley, NASA’s planetary protection officer. Yes, she definitely has the coolest job title on Earth! Based on Article IX of the outer space treaty, her office is responsible for minimizing the biological cross-contamination resulting from the exploration of the solar system.
What has been done to protect Mars from contamination
Protecting Mars already begins with choosing the right landing site. Gale Crater, in addition to being the site preferred by science, was also preferred by planetary protection. Says Dr. Catharine A. Conley:
Gale Crater is very near the equator, and observations suggest there is a minimal chance that the Curiosity rover could contact water ice near the surface. Without water, in the conditions at Gale Crater, there are no known Earth organisms that could grow.
It’s not possible to eliminate all microbes from a spacecraft, but the number can be greatly reduced. The entire MSL spacecraft was cleaned to ensure only a minimum of Earth organisms was transported to Mars. And some parts, like sample-acquisition hardware, were heated to sterilize their surfaces. At temperatures ranging from 110 to 146 degrees Celsius, some components have been baked for up to 144 hours.[ref]Mars Science Laboratory Mission and Science Investigation:
Dr. Catharine A. Conley:
We measure heat-resistant organisms that grow on a particular kind of petri plate as a proxy for cleanliness, because this allows spores of Bacillus bacteria to grow and these organisms are known to be among the most resistant to the space environment. The requirement, using this proxy, is that there be less than 300 ‘spores’ per meter of spacecraft surface — in practice, quite a lot of the time the MSL planetary protection team measured zero spores. This means that the assembly personnel did a really good job of keeping the spacecraft clean, by wearing proper gowning, wiping things down with isopropanol as they were working, and covering the hardware when they were done.
The MSL mission has been classified as a Category IVa mission,[ref]Category IV description at the Office of Planetary Protection:
http://planetaryprotection.nasa.gov/about-categories#4[/ref] allowing no more than 300,000 spores in total on the landed system (including the rover, parachute and back shell). So although the NASA team has done a great job cleaning the spacecraft, there still could be some microbes left to contaminate Mars.
What if we find traces of life on Mars?
Let’s make it clear: Curiosity is not designed to directly identify organisms on Mars – there’s no microscope to look at Mars soil and search for bacteria. But instruments like ChemCam can identify chemical compounds which could indicate potential biosignatures or past habitable environments.
Dr. Catharine A. Conley:
The challenge, with any mission, in identifying possible Mars life and telling the difference between that and possible Earth life, is to be able to do follow-on experiments — because any set of measurements we make at first is much more likely to provide hints and raise more questions, than it is to give a definitive answer.
Take the Viking mission for example, two probes sent to Mars by NASA in 1976. They carried three different experiments designed to determine whether life exists on Mars. One of them came out positive for life, the other two came out negative. So in 1976 there was no definite answer and until today the question is still open.[ref]Life on Mars Found by NASA’s Viking Mission?
Curiosity’s SAM instrument can look for organics in Martian soil by heating up samples to even higher temperatures than Viking did. So we might get some good answers from Curiosity – but we’ll most likely also get a lot of new questions.