Finding microbes on Mars would be the discovery of the century, and the last thing any self-respecting space agency would want to do after such an announcement is to sheepishly backtrack and say "sorry, it was from Earth all along." To make sure that any organisms dug up on the Red Planet are natives and not stowaways, NASA has launched bacteria into Earth's stratosphere aboard a balloon, to test how well they might survive under Mars-like conditions.
The Exposing Microorganisms in the Stratosphere (E-MIST) experiment does exactly what it says on the box: a huge helium balloon carries a payload of bacteria up to the stratosphere, some 19 miles (30.5 km) above the Earth, where the extreme conditions make for a good analogy of Mars: it's very cold, very dry, the air pressure is low and UV light exposure is high.
The microbe in question is Bacillus pumilus SAFR-032, a burden of a bacteria that's sometimes hardy enough to withstand the intense sterilization processes of NASA's rooms and equipment. On October 10, 2015, the team loaded a research balloon with a batch of these bugs in a dormant state called an endospore that some organisms retreat into to protect themselves from environmental stress, and sent them up to the stratosphere.
After eight hours of direct UV exposure, the samples were dropped back to Earth for study, where it was found that the barrage of radiation had wiped out 99.999 percent of the bacteria. The team re-analyzed the DNA of the few survivors, and found three tiny differences when compared to a control population that had stayed safely on the ground.
The study concluded that the vast majority of microscopic Earthly hitchhikers would be "inactivated" within hours of landing on Mars (if the journey there doesn't kill them first), but those that do survive "may be pushed in evolutionarily consequential directions" – essentially, they could evolve to live in those extreme conditions. Future tests will look into how plausible that scenario is, as well as address other questions raised by this experiment.
"Another point for consideration is that we only tested a single bacterial strain with this flight," says David J. Smith, leader of the study. "Follow-on studies will be needed with more test species so we can find out if every bug dies as quickly. What about the ones under a pile of dead endospores, or covered in dust? We don't know. These will be topics for future scientific balloon flights."
The research was published in the journal Astrobiology.