From the hottest deserts to the freezing polar regions, microorganisms keep turning up in Earth's most extreme environments. And if that's the case, why wouldn't it hold that life can do the same on other planets? To test whether certain hardy microbes can survive the harsh conditions of space or Mars, colonies were placed on the outside of the International Space Station (ISS) for almost 18 months – and many managed to survive.

For the project, known as the Biology and Mars Experiment (BIOMEX), several hundred samples of Earthly microbes, including bacteria, archaea, mosses, lichens, fungi and algae, were placed in containers attached to the outside of the Russian Zvezda module on the ISS. Some samples were kept in soil and air that mimicked that of Mars, to see whether they could survive on the Red Planet.

For 533 days between October 2014 and February 2016, the microorganisms were exposed to the extreme conditions of space, enduring a vacuum, wild temperature swings and intense ultraviolet radiation. The bugs were returned to Earth in June 2016, and have been extensively studied ever since.

At least some members of every species in the experiment managed to survive, with some thriving and others only just scraping by. Several archaea and bacteria species fared the best, while more complex, multicellular organisms like fungi and lichens struggled a bit more.

"Some of the organisms and biomolecules showed tremendous resistance to radiation in outer space and actually returned to Earth as 'survivors' from space," says Jean-Pierre Paul de Vera, scientific manager of the BIOMEX project.

The key outcome of the experiment was to show that life is hardier than we give it credit for, and as a result Mars may not be as inhospitable as once thought. Microscopic organisms could be thriving underground, or in salty liquid lakes.

"Of course, this does not mean that life actually exists on Mars," says de Vera. "But the search for life is more than ever the strongest driving force for the next generation of missions to Mars."

BIOMEX also helped define what kinds of biosignatures we should be looking for on Mars and other planets and moons. And with both NASA and the ESA launching new Red Planet rovers in 2020, an answer to one of the most profound questions humans have ever asked could be on the horizon.

The research has been the subject of dozens of published papers in recent years, and it's all summed up in a special issue of the journal Astrobiology. The final results of BIOMEX will be presented at a scientific conference this week.

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