Scientists believe they have found an underground lake of liquid water on Mars, which has space explorers all the more eager to visit.

“Seems like a good time to start working on landing BFR,” tweeted the SpaceX engineer in charge of making sure Elon Musk’s next big rocket, the Big Falcon Rocket, can safely reach Martian soil. NASA is already planning a mission to return samples from Mars in 2020. The agency is also plotting an $8 billion mission to Jupiter’s moon Europa that could fly through plumes of water ejected by the icy planetoid.

Water on Mars, or anywhere in space, is exciting for two reasons: First, with the right technology, humans could purify it to drink, and use it to produce oxygen and even rocket fuel, making long-term exploration far more feasible. Second, water offers the tantalizing possibility of extraterrestrial life: Scientists think life on other planets could evolve much like life on earth—out of the ocean.

But that’s part of the problem: Since microbes on our planet love water, human expeditions to another planet could contaminate it, potentially leading to a solar system-scale invasive species. That would in turn screw-up scientific research—and, if actual extra-terrestrial life exists, potentially disrupt its ecosystem.

Advocates of space exploration often reach for comparisons to Europe’s first encounters with the American continents as a metaphor for their ambitions. But the European explorers carried with them bacteria that wiped out indigenous peoples, even as they encountered new illnesses that decimated their own delegations. Scientists hope to avoid such unintended consequences on the high frontier.

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