Scientists tend to get cagey when you ask them to quantify the likelihood of finding life on other planets. Until there’s evidence, probablys don’t get you very far.

But, you know, there’s probably life on Mars.

That was the subtext from NASA on Monday as scientists announced, in a landmark finding, that the Red Planet has water flowing on it. Not just hunks of ice or evidence of ancient, dried-up oceans—but wet, trickling, salty droplets of water on Mars right now.

A planet that’s alive with water may well be a planet that sustains life, especially in this case.

“We know there’s life on Mars because we sent it there,” said John Grunsfeld, a science director at NASA, during a press conference with reporters on Monday.

This was Grunsfeld being funny, a little, but it’s also an allusion to one of the big problems that comes with looking for signs of living organisms in a place you’ve never seen them before. Which is a problem NASA will face when it actually goes looking for water-supported life on Mars. Namely, how do you make sure the life you think you found didn't get tracked in on the bottoms of your shoes?

This is a question that will affect where scientists eventually seek life on Mars. It has to be a place that seems worth investigating, but also not so conducive to life that it ends up hosting micro-organisms that came in on a contaminated rover from Earth.

Such contamination may be unlikely, though. Studies have found that most organisms are unlikely to survive a trip to Mars—and even those that do wouldn't survive the planet's brutal environment. A 2010 study simulating conditions on the Red Planet found that E. coli cells could survive for several days on Mars, but couldn't actually grow there. But there's still a common refrain among astrobiologists: Planetary protection isn't just about shielding Earth from alien species. It goes both ways.

“We're being very careful that we don’t send a spacecraft to Mars with the intention of detecting Martian life—and find out that we detected the Earth life that we took with us,” Grunsfeld said. “That’s tough to do.”