Engineers swab the Curiosity rover's heat shield to test its cleanliness. Credit: NASA

LOS ANGELES—When the Curiosity rover lifted off toward Mars, the spacecraft carried a few stowaways—278,000 bacterial spores, by NASA’s best estimate. That is sparkling clean, by spacecraft standards—the mission’s components had been sterilized, wiped, baked and coddled in clean rooms to drastically reduce the bacterial burden.

Mars missions such as Curiosity are subject to strict planetary protection policies intended to preserve habitats in the solar system that might harbor life of their own. After all, invasive species are a big enough problem on Earth, and one can only speculate about how terrestrial microorganisms would fare on Mars.

That speculation is getting a bit more grounded, however. At a conference held here this week on the present-day habitability of Mars, numerous researchers described experiments carried out in Mars simulation chambers that can replicate some of the environmental conditions of the Red Planet. Perhaps most intriguingly, a new set of experiments described by Andrew Schuerger of the University of Florida indicate that three of the most hostile elements of the Martian environment—low pressure, low temperature, and a carbon dioxide atmosphere largely devoid of oxygen gas—are not insurmountable blockades for Earth organisms. On the contrary, some microbes don’t just hunker down and hibernate but actually grow under such conditions.

Schuerger, along with University of Florida colleague Wayne Nicholson and their collaborators, collected 24 microbial strains that have been found on spacecraft surfaces, in clean rooms and around Kennedy Space Center in Florida, as well as two extremophile species tolerant of hostile environments. The bacteria included common species such as Bacillus subtilis and Escherichia coli, but “our winner in this set of experiments,” as Schuerger put it, was Serratia liquefaciens, a widespread generalist microbe.

Most of the selected microbial species shut down at temperatures of zero degrees Celsius (which falls in the upper range of Martian surface temperatures), even without being subjected to low pressure or anoxic conditions. But S. liquefaciens succeeded not only in the low temperatures but also under the simultaneous exposure to a carbon dioxide–dominated atmosphere and Mars-like pressures of only seven millibars. (Sea-level atmospheric pressure on Earth is roughly 1,000 millibars.) The researchers reported their findings in January in the journal Astrobiology.

Whereas S. liquefaciens actually grew under the trio of harsh conditions, the others did not perish—they simply lay dormant. “All of these bacteria were not killed by the conditions they were exposed to,” Schuerger said. When returned to ambient laboratory conditions, the inactive bacterial species all resumed growth.

In a separate study, bacteria pre-adapted to survive in frigid conditions fared even better. In a study published in December in the Proceedings of the National Academy of Sciences, Schuerger, Nicholson and their colleagues reported that bacteria isolated from the Siberian permafrost thrived in Mars-like conditions. Those species, from the genus Carnobacterium, actually seemed to favor the low-pressure conditions. “When they grew at zero [degrees C] under CO₂, seven millibar atmospheres, they seemed to grow better, at higher rates, than under CO₂ at 1,000 millibars or under oxygen at 1,000 millibars,” Schuerger said.

But bacteria need not hail from extreme habitats to flourish under Mars-like conditions. Schuerger shared preliminary, unpublished research during the conference that indicates that low-pressure, or hypobaric, environments actually stimulated the growth of microbes harvested from an unusual source: human saliva. In petri dishes incubated at low temperature under carbon dioxide atmospheres, the salivary flora failed to grow at Earth-like pressures. “Yet these hypobarophiles have popped out” under Mars-like pressures of seven millibars, he said. The specific organisms that thrived in hypobaric conditions have not yet been identified, Schuerger noted in an email, but “the human oral cavity is not a place that one would expect to find microbes that yield such a strange response.”

Proving that some bacteria fare well under Mars-like pressures, temperatures and atmospheric compositions is nonetheless a long way from proving terrestrial life can flourish on Mars. Schuerger and his colleagues count 17 environmental factors on Mars that could be hostile to life, of which pressure, temperature and anoxia are only three. Two important threats to life that went unaddressed in the two bacterial studies were ultraviolet irradiation from sunlight, which on Earth is thankfully attenuated by ozone in our planet’s atmosphere, and the extreme dryness of the Red Planet’s surface. Schuerger noted that accurately simulating Martian desiccation would rapidly degrade the growth medium for the bacteria. “We had to reduce evaporation to carry out these experiments,” he said.

Which brings us back to those hundreds of thousands of spores on the Curiosity rover and its flight hardware. Even in light of the new research, the rover’s landing site appears extremely unlikely to suffer contamination by terrestrial biology. Direct and reflected sunlight likely sterilized the outside of the rover within the first day or two of the mission, Schuerger said. And any survivors are unlikely to find purchase at the Curiosity landing site, Gale Crater. “Even if the UV radiation doesn’t sterilize or kill off microbes on the outside of the vehicle or on the wheels, even if the microbes are dispersed, the extreme desiccating conditions of Gale Crater argue strongly against” the proliferation of stowaways from Earth, he added.