When a robotic probe finally lands on a watery world like Jupiter's moon Europa, what do scientists have to see to definitively say whether the place has any life?
"We looked at him with blank faces," recalls Jim Green, head of NASA's planetary sciences division. "What do we need to build to really find life? What are the instruments, what are the techniques, what are the things that we should be looking for?"
To get some advice, the agency recently asked the prestigious National Academies of Sciences, Engineering, and Medicine to gather some of the top experts in astrobiology for a meeting that begins Monday.
There's a growing interest in so-called biosignatures — or substances that provide evidence of life — because NASA has upcoming missions that have real potential to search for them. Those include a visit to Europa in the 2020s and the 2018 launch of the James Webb Space Telescope, which could scan the atmospheres of planets around other stars.
The last thing NASA officials want is a repeat of the experience with the Viking missions back in the 1970s, when analysis of Martian soil chemistry produced what was initially interpreted as evidence of life — but then later deemed a false-positive.
"I remember the aftermath of that," says James Kasting, a professor of geosciences at Penn State University, who was tasked with planning this week's meeting. "NASA was criticized heavily for looking for life before they had investigated the planet and for not having thought that through carefully. They're hoping to avoid that same experience."
Finding life means first defining life, and NASA's Green says the key features are that it must metabolize, reproduce and evolve.
But having that definition doesn't mean there's a consensus on what, exactly, to look for. "We have big debates about it, actually," Kasting says, "and that's part of what this meeting is all about."
In our own solar system, scientists ponder what kind of extant or extinct life might be found on Mars, icy moons such as Europa or Enceladus, or the strange methane lakes of Titan. If scientists found DNA or RNA, obviously, that would be like finding a smoking gun, assuming it wasn't a contaminant.
But the alien life probably wouldn't have exactly the same kind of genetic material. In fact, its chemistry might be unrecognizable.
"If I start just doing the usual things to look for life that are successful for looking for life that we know on Earth, there's no reason to believe that it will be successful in identifying life that has even a mildly different biochemistry," says Steve Benner, with the Foundation for Applied Molecular Evolution.
So he thinks that searches for alien life have to be able to detect something more generic. "It's a rather esoteric thing, but we're going to be looking for long, stringy molecules that have repeated, regularly spaced, backbone charges," Benner says.
Searching for life beyond our solar system poses different challenges, because there's no interstellar travel that would allow a spacecraft to visit a planet around another star and scoop up dirt or suck up liquid. All scientists will be able to do is look through telescopes and tease apart the light, searching for clues.
With that limitation, Benner says, "maybe the best we can do is look for Earth-like life," though not all scientists agree on that.
One signal from a planet in a distant solar system that would be pretty unambiguous would be the simultaneous presence of abundant oxygen and gases such as methane or nitrous oxide.
"Both oxygen and methane and nitrous oxide are produced predominantly by biology, and so it's very difficult to build up high concentrations of those gases, two or three of them simultaneously, without having life present," Kasting says.
An issue that's likely to come up at the meeting is whether it's enough to see oxygen by itself, or if there also has to be other gases linked to life. That's because if you looked at the Earth from far away, it would be relatively easy to detect the oxygen because it's so abundant, but harder to see the methane or nitrous oxide.
Of course, NASA doesn't just send up probes or telescopes — it also can send out people.
"As a field geologist, I have this strong bias that it's going to take people like me, on the surface of Mars, cracking open a lot of rocks, looking for those fossil signatures of early Mars life," says Ellen Stofan, NASA's chief scientist, noting that NASA has a goal of getting humans to Mars in the 2030's. "Because it's not enough just to say, 'Ah-ha, we've got one molecule that we think is biological,' you need lots of molecules, you need lots of fossil samples, to really understand what are the implications of life beyond Earth."
She's optimistic that we'll find signs of life off our planet in one or more places in the next couple decades.
"To think that within the next 20 years we're going to start answering some of these questions really blows my mind," Stofan says.
STEVE INSKEEP, HOST:
Today, scientists grapple with a profound question - if we were ever to find alien life in the universe, how would we know? Whatever's out there might be really strange, unrecognizable. And the top people in astrobiology are talking about what to look for. Here's NPR's Nell Greenfieldboyce.
NELL GREENFIELDBOYCE, BYLINE: NASA is planning to send a probe to an icy moon of Jupiter called Europa. Beneath the ice lies a vast ocean of liquid water. So former astronaut John Grunsfeld recently asked some NASA colleagues, when we get to this water world, how exactly will we search for life?
JIM GREEN: We looked at him with blank faces.
GREENFIELDBOYCE: Jim Green is director of planetary science at NASA. Scientists like him know how to recognize life on Earth because everything here shares a common ancestry. But he says life began way out there might look very different.
GREEN: What do we need to build to really find life? What are the instruments? What are the techniques? What are the things that we should be looking for?
GREENFIELDBOYCE: These are the questions NASA officials have asked the prestigious National Academy of Sciences to help them answer, with a meeting that starts today in California. The academy's rounded up the small crew of people who contemplate the chemistry of hypothetical aliens, including Steven Benner from the Foundation for Applied Molecular Evolution.
STEVEN BENNER: It's a bit of a crackpot field, right? If you're interested as a young scientist is setting up a career, it makes much more sense to do cancer research.
GREENFIELDBOYCE: All joking aside, the stakes here are huge. NASA doesn't want a repeat of what happened in the 1970s with the Viking landers on Mars. Their analysis of the Martian soil showed ambiguous signs of life that never panned out. Jim Kasting is with Penn State University.
JIM KASTING: I remember the aftermath of that. NASA was criticized heavily for looking for life before they had investigated the planet and for not having thought that through carefully.
GREENFIELDBOYCE: Four decades later, he says, there's still no scientific consensus on the best way to find alien life.
KASTING: We have big debates about it, actually, and that's part of what this meeting is all about.
GREENFIELDBOYCE: And these days, the search for life faces a new challenge. Scientists have detected planets beyond our solar system around distant stars. They're so far away you can't send a probe to scoop up soil or slurp up liquid. All you can do is use telescopes to get hints about a planet's atmosphere. Scientists argue over what combination of gases might suggest life is there. William Bains of MIT says, hey, a plant-like lifeform might produce hydrogen instead of oxygen. Over Skype, he told me he's pessimistic about finding aliens in his lifetime. Still, he says...
WILLIAM BAINS: Even getting a hint that there's life elsewhere would just be really sort of philosophically and socially a fascinating thing to find out. I think it's a really important quest.
GREENFIELDBOYCE: That's why he'll be there at the meeting with the usual suspects. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.