There's no way to be absolutely certain, but we're pretty sure that no germs have ever survived the grueling journey from Earth to Mars. But the latest rover to explore the Red Planet might just take along some microscopic colonists.
NASA takes a number of steps to ensure that the probes that reach the Martian surface don't bring any stowaways along with them. All equipment is sterilized in clean rooms, and then the actual journey does a ton of heavy lifting — the journey to Mars is one long bath in cosmic rays, and spending time on the surface of Mars adds some additional deadly ultraviolet radiation to make sure that no lifeforms survived the journey.
But some slight changes have been made to the landing procedure for the upcoming Mars Science Laboratory (MSL) mission. Unlike previous missions, the MSL uses a parachute and downward-firing thruster that will allow the rover Curiosity to be lowered directly down onto the planet surface. That's a departure from previous rovers, which first spent two days on a landing platform before making direct contact with the surface. That allowed time for any microbes that survived the journey through space to die on the platform before the rover proceeded with its mission.
NASA microbiologist Andrew C. Schuerger ran simulations on the microbe Bacillus subtilis to see how it dealt with Mars-level exposure to ultraviolet light. First, he and his team simulated UV exposure when the rover was on the platform. They found that this exposure killed off 96.6% of all bacteria after just six hours of exposure, and Schuerger is confident that the microbes would have been killed off completely if they had continued the exposure for another 7 to 12 days.
He then simulated what would happen if the rover landed directly on Mars and then started moving around immediately. The researchers found that 31.7% of the soil that the rover came in contact with showed signs of bacterial growth. That said, this contamination level dropped to only about 15% after just a day's worth of exposure to ultraviolet light, so this isn't exactly an indication that the Curiosity rover could seed Mars with new, Earthly life.
Indeed, it's also worth stressing that Schuerger was intentionally using as much as 100,000 times the amount of microbes one would actually expect to end up on the Martian surface, once you consider the multiple sterilizations that all rovers endure and the harsh cosmic rays of space. Schuerger calculates that the journey to Mars should kill about 75% of the microbes that survive the sterilization - and there's no real reason to think that number will be particularly high.
"Although this paper suggests we could be transferring bacteria to martian surface, we don't know for certain yet. We could very well be losing most due to the exposure to vacuum in space, cosmic rays and hard radiation. Even if cells are present on a rover wheel at launch, they might be dead by the time they get to Mars."
Still, as minute as the risk is, there is something vaguely alluring about the idea of some microbes actually manage to survive and colonize Mars, even if they don't last all that long. Schuerger did point out that the experiment was unable to account for the effects of multiple rover wheels passing over the same soil, and that added force could serve to push the microbes down far enough that they could escape the wrath of the UV radiation.
Perhaps it's just a reflection of my generally low expectations about human colonization of Mars in even the medium-term future, but I'll admit there's something weirdly cool about the thought of sending any Earthly colonists to Mars, even if they're just microscopic. That may be pushing the phrase "One small step" to its absolute limits, but I'll take what I can get.
Via Astrobiology. Image via NASA.