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.