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Before any nation launches a spacecraft, the U.N.’s Outer Space Treaty requires the craft to undergo extensive cleaning, to “avoid harmful contamination of space and celestial bodies.” Basically, we don’t want our gross Earthly microbes to invade new regions of space and kill off other life that might potentially be living there. That’s especially true for Mars, because scientists think there’s a chance that single-celled organisms may have evolved there in the past, and might even still live on the Red Planet today.

NASA spacecraft get disinfected about 10 to 30 times before they launch, says Stephanie Smith-Rohde from the University of Idaho. But those decontamination sessions can’t catch everything. Smith-Rohde and her colleagues analyzed swabs that were taken from the surface of the Curiosity rover after cleaning and prior to launch. Their preliminary results, which were presented at a meeting of the American Society for Microbiology, turned up 377 organisms from 65 bacterial species.

Smith-Rohde’s team attacked these microbes with a battery of tests meant to simulate the harsh conditions of space and Mars, and they found that many of the microbes survived just fine—even, surprisingly, the bacteria that don’t form protective spores.

Scientists need to do longer-term studies, but so far the evidence suggests that some of these bacteria may have been capable of surviving a journey to Mars.

“Have we contaminated Mars already, or is there no way those microbes could survive the journey?” asks Smith-Rhode. “We don’t have answers to those questions yet. These studies allowed us to narrow down the organisms that we want to focus on.”

If Earth microbes did make it to Mars, here are the ones that are most likely to make themselves at home.

1. Staphylococcus

These bacteria (top), typically found in soil and on human skin, persevered in petri dishes that contained 20 percent salt. That’s really salty—by comparison, the ocean is only about 3 percent salt. It may be that Staphylococcus could also thrive in Mars’ salty sands and waters.

2. Enhydrobacter

During laboratory tests, Enhydrobacter colonies withstood a 2000-joule zap of radiation, “which is a pretty decent dose of UVC radiation,” says Smith-Rohde. They also endured a two-week desiccation experiment, wherein they had absolutely no access to water, with no major problems.

3. Moraxella

Nearly 50 percent of Moraxella bacteria outlived a one-hour dunk in a 5 percent hydrogen peroxide solution—a common cleaning agent meant to kill microbes on spacecraft.

4. Streptomyces

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Normally noted for their role in decaying organic matter, Streptomyces microbes are surprisingly hardy. In experiments, they were able to grow in the 20 percent salt solution as well as the two-week desiccation period, withstood low temperatures, and tolerated a pH of 9—similar in acidity to the soils of Mars.

5. Gracilibacillus


Gracilibacillus one of a handful of types of bacteria that can eat the perchlorates found in Martian soil. “Gracilibacillus would definitely be a top contender to survive on Mars,” says Smith-Rohde.

Up next, Smith-Rohde and her colleagues plan to learn more about each of these microbes. They will expose the hardiest species to multiple extreme conditions at once, which is more like what they’d really experience in space, and also may determine whether they could endure the nine-month journey to Mars.