An 8-hour stratospheric flight on the NASA E-MIST balloon killed off even the hardiest microbes.

David J. Smith/NASA

UV light could easily kill microbial stowaways to Mars

Astrobiologists who dream of finding life on Mars also share a common nightmare. What if the very spacecraft intended to find martian microbes brings along Earth bacteria—and what if they fool experimenters, or contaminate the Red Planet? Now, a new study on Earth allays those fears, at least partially, showing that 8 hours of harsh ultraviolet (UV) light can decimate even the hardiest of bacteria.

Interplanetary contamination is not an unreasonable fear. Some of the organisms with the best chance of surviving on Mars also have good chances of stowing away in the first place. A 2003 survey found a particularly stubborn strain of the rod-shaped bacteria Bacillus pumilus in even the cleanest of clean rooms at NASA’s Jet Propulsion Laboratory in Pasadena, California, where Mars-bound spacecraft are assembled.

That strain, SAFR-032, has been able to survive as a spore in dry conditions with almost no nutrients. It can outlast waves of high-powered chemical disinfectants. It was even found to persist for 18 months in outer space, just outside the International Space Station (ISS).

UV rays from direct sunlight are the easiest way to kill it, according to those ISS experiments and others. The latest attempt to test its limits has confirmed that weakness by exposing SAFR-032 to an earthly place reminiscent of the Red Planet: Earth’s stratosphere.

Like Mars, the stratosphere is thin, dry, cold, nutrient poor, and irradiated by harsh UV light. So in October 2015, microbiologist David Smith of NASA’s Ames Research Center in Mountain View, California, and his team launched SAFR-032 samples on a balloon 31 kilometers over New Mexico and Texas.

Dangling from the balloon was a payload that extended flat plates out into the thin air and exposed tens of millions of spores—far more than the estimated 56,000 that rode on the surface of the Curiosity rover to Mars. “We sort of had a nightmare scenario and an intentionally high concentration in order to better understand how the bacterial population would respond,” Smith says.

After 8 hours, less than one in 100,000 spores survived, Smith’s team reports this month in Astrobiology. “That’s reassuring,” says NASA’s Planetary Protection Officer Catharine Conley in Washington, D.C., who is responsible for managing contamination risks. It shows that a relaxation in standards since the Mars Viking landers in the 1970s was justified, she says.

Once assembled, the twin Viking landers were each baked at 112°C for more than a day in an effort to reduce populations of bacteria by four orders of magnitude. By the time of the Pathfinder rover mission in the 1990s, engineers had dropped that step, concluding that Mars was dry and dead.

From this study and previous ones, it seems that sunlight sterilizes about as well as baking. “We’re probably still at about a similar level of confidence, statistically speaking, that we’re protecting Mars,” Conley says.

“The data are really good to know,” says microbiologist Ralf Möller of the German Aerospace Center in Cologne, who ran one of the studies of SAFR-032 exposed to space outside of the ISS. “For my lab at least it will have a big impact.”

But outside experts also point to the literal flip side of Smith’s experiment. Spores on shaded, upside-down surfaces didn’t seem to take a hit. Of all the harsh conditions in the stratosphere, only UV radiation from sunlight really mattered. And real space probes often contain plenty of shady hiding spots like coatings, creases, interior compartments, and even layers of already-dead spores.

“They are not building these spacecraft for UV microbial death,” says John Rummel, an astrobiologist at the SETI Institute in Mountain View, California. To really determine whether stowaway bacteria would survive on Mars, experiments need to use more complex surfaces, “not the kind of configurations that microbial ecologists find easy to count,” he says.

In future experiments, Smith hopes to test other surfaces and other kinds of organisms. Another interesting question would be to see whether bacteria that can survive the stratosphere are evolving genetic advantages, Möller says.

Ultimately, though, sending a probe into the wettest and warmest parts of Mars—the obvious places to search for life—will require stricter controls and maybe a return to old practices. Today, rovers are exploring relatively inhospitable parts of Mars. But areas that may contain near-surface water or ice, such as the recurring slope lineae where watery brines may periodically run down steep hills, are designated as “special regions” that require more care and would be currently off limits to spacecraft without further sterilization.

Exploring those regions or landing near cracks on the moons Europa and Enceladus, which may harbor subsurface oceans, could require reinstituting the baking step, Conley says. Rummel thinks that may be overdue, even though it adds heat resistance to the list of things engineers have to worry about. “The biggest problem we have with sterilizing spacecraft is that we didn’t do it again after Viking,” he says.