What’s First? A Life-Detection or a Human Mission to Mars?
Planetary protection guidelines were established by NASA and other space agencies to safeguard Earth's biosphere, but also protect possible life in extraterrestrial locations from accidental harmful interactions with Earth's biosphere. A public discussion of the appropriateness of the planetary protection guidelines will be held at the McGill Space Research Conference next week, and I’m one of the invited speakers to discuss the pro and cons of the guidelines. The discussion is timely as there have been concerns raised that some of the guidelines are too strict and hinder the exploration of Mars and other planetary bodies in our Solar System.
When talking about planetary protection, we must distinguish between backward and forward contamination. Backward contamination is when samples from an extraterrestrial location are brought back to Earth. This happens naturally all the time via meteorites, but what we are referring to are robotic probes or human missions that collect samples from planetary bodies and bring them back to Earth. This happened with the Apollo missions to the Moon and is scheduled to happen with Mars in less than ten years. Part of the current Perseverance Rover mission is to cache a sample and bring that sample back to Earth to a specially designed Sample Return Facility. When bringing the cache back from Mars, the samples could contain lifeforms, which could potentially be invasive and lead, in the worst case, to the extinction of species in Earth's biosphere. The classic movie “The Andromeda Strain” nicely portrays the potential dangers of backward contamination. Needless to say, these planetary protection guidelines have to be very strict.
However, in regard to forward contamination, which is the potential of bringing Earth life to extraterrestrial locations, which could be harmful to indigenous life, I think the planetary protection guidelines should be relaxed. Especially in the case of Mars, where overly cautious forward contamination guidelines hinder the exploration of Mars. We should act more decisive to directly address the question of life on Mars, which is now a realistic prospect but is slowed down by several myths:
Myth 1: Any spacecraft going on a mission to Mars is sterile after sterilization measures are applied.
Not true: As much as we try, we can never fully succeed in sterilizing a spacecraft. The chemical cleaning agents and radiation sources kill most of the “hitchhiking microbes,” but these are the ones that would be killed anyway on their travel to Mars or during surface exposure on Mars. This is because the environmental stresses on Mars are similar to the sterilization procedures used (UV radiation, oxidizing chemicals, etc.). Thus, by applying the sterilization measures, NASA is conducting an artificial selection experiment. Only those resilient microorganisms that survive also have a chance to survive on Mars.
Myth 2: The spacecraft needs to be sterilized as much as possible because otherwise, we cannot distinguish between indigenous Mars life and contamination from Earth.
Not true (anymore): If Martian life is biochemically similar to life on Earth or even related, we will be able to add it to Earth's DNA tree of life, which details the ancestry of all lifeforms on our planet. It would probably be there on its lower branches. If the microbial life is biochemically different, we will be able to identify the different building blocks of Martian life.
Myth 3: There is a high danger that Earth life may replace indigenous Martian life.
Very unlikely: Martian environmental conditions are very harsh and extreme in many ways. There is no environment on Earth that exhibits all the environmental extremes and stresses as Mars. Indigenous life on Mars may still exist, having billions of years to adapt to specific habitable niches on the Red Planet. There is, in my view, a pinch of arrogance to believe that Earth life may be better dealing with the Martian environment than Martian life. I think the danger of Earth life replacing Martian life on Mars is as likely as monkeys replacing penguins in Earth's Arctic!
Planetary protection guidelines for forward contamination have to be re-evaluated because if they are too strict, they make life-detection missions too expensive. This may be one reason why we have not had a life-detection mission to Mars after the Viking mission. In a 2019 paper, a team of scientists, including myself, suggested that we need to designate, describe, and analyze a few selected places on Mars, called Astrobiology Priority Exploration regions (APEX), to find out whether there is presently near-surface life on Mars. The forward contamination rules will likely be relaxed after a recent NASA review of the guidelines, but our most important point should be addressed as well: a mission designated for life detection should be sent before a human mission is launched. This has to be done already for the astronauts' sake to protect them from putative microbial life when they are exposed to the Martian environment and protect our biosphere upon their return to Earth.