Earth needs to ensure Mars rocks don't have alien germs NASA and the European Space Agency are gearing up to bring home a pristine sample of Martian rock. But given the small chance of life on the red planet, they have to grapple with safety questions.

NASA is bringing rocks back from Mars, but what if those samples contain alien life?

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LEILA FADEL, HOST:

NASA is planning its first-ever mission to pick up dirt and rocks from Mars and bring them back to Earth. But before that consequential event even happens, the space agency needs to hash out some details. A big one is how to protect Earth from any alien life that might hitch a ride back. Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: On Mars, a NASA rover called Perseverance is exploring a crater that seems to have once been flooded with water. Every once in a while, the rover drills down into the rocky surface.

JIM BELL: And then it pulls up a plug of that rock, a core, that's about the size of a dry-erase marker.

GREENFIELDBOYCE: Jim Bell of Arizona State University is a member of the rover team. He says a little robotic arm processes each rock sample.

BELL: Takes pictures of it, measures its volume and then hermetically seals it in one of our 42 empty sample tubes that we brought with us.

GREENFIELDBOYCE: The rover brought those metal sample tubes to Mars because NASA intends to pay the red planet another robotic visit to pick up those carefully selected rocks and send them back to Earth. The question is, what else might come back with them? Peter Doran is a geologist at Louisiana State University who studies life in extreme environments.

PETER DORAN: It would be a big deal if some pathogen got back to Earth and caused problems. You know, it could be any level of problems. But we definitely have to take the stance of protecting Earth, at least in the early missions.

GREENFIELDBOYCE: He says Mars is a harsh place - cold, dry, blasted with UV radiation. But on the other hand, the chances of life aren't zero.

DORAN: Even though it's highly unlikely, there is a possibility it could eke out a living there. And there's examples of microbes on Earth that could do it.

GREENFIELDBOYCE: So as NASA and the European Space Agency are working towards a sample return mission that could launch around 2028, NASA is trying to assess the potential environmental impact on Earth and inviting the public to weigh in.

DORAN: Maybe this is the most important environmental assessment that humans have ever done.

GREENFIELDBOYCE: The agency is having public meetings today and tomorrow to lay out their plans and get feedback. One of the speakers will be Brian Clement, a planetary protection expert at NASA's Jet Propulsion Laboratory. He says, over the years, panels of scientific experts have considered whether bringing home Martian rocks could endanger any life here.

BRIAN CLEMENT: Those panels have all agreed that the potential hazard is very, very low.

GREENFIELDBOYCE: He says, nonetheless, NASA is being conservative and will either sterilize or securely contain anything that has touched Mars before it comes home. Mars researcher Jim Bell says what worries him is not the sci-fi scenario of Martian germs getting out, but Earth stuff getting in - that would contaminate the pristine samples NASA will have gone to so much trouble to collect. These samples will have to be opened in a special lab, and Bell can't wait.

BELL: I just want to see the stuff with my own eyes. I mean, we've been looking at this world through robotic eyes for so long, and I want to see that famous red dust, and I want to see the insides of some of these rocks and little grains that may have formed in a watery environment 3, 4 billion years ago.

GREENFIELDBOYCE: If all goes as planned, samples could land in Utah at an Air Force facility in about 10 years.

Nell Greenfieldboyce, NPR News.

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