Are we truly alone in the universe? Or is Earth just one of many inhabited worlds? These are some of the most fascinating questions facing humanity, and soon, thanks to a 2016 federal budget allocation for a mission to Jupiter's moon Europa, NASA may be able to help answer them.
But first scientists must figure out how to actually detect life on the world, whose frozen outer shell conceals an immense ocean of salt water that many think is our best chance for finding other life in our solar system. That challenge motivated a workshop held Wednesday in California at NASA's Ames Research Center, where planetary scientists and those who study the potential for alien life, called astrobiologists, gathered to discuss strategies for finding life on Europa.
"Europa is clearly such a prime target for astrobiology that having a workshop like this to try and figure out all the ways in which we could possibly sample its ocean … [is] critically important," said Kevin Hand, an astrobiologist at California's Jet Propulsion Laboratory, who was at the meeting.
Roughly the size of Earth's moon, Europa is structurally like a liquor-filled chocolate, with a liquid ocean that could be 62 miles (100 kilometers) deep standing in for the cherry brandy. Scientists have long thought that that sea is one of the best potential alien incubators in our solar system. But it wasn't until recently that sending a spacecraft to study it became more than a theoretical possibility.
Now, in a move that has astrobiology researchers buzzing in excitement, the 2016 federal budget request includes funds for the first phase of a Europa mission. NASA administrators are optimistic that such a mission will happen—even though it might not launch until the 2020s.
"We are going to do a Europa mission, and I'm very excited about that," said John Grunsfeld, a former astronaut who is now NASA's associate administrator for the science mission directorate. "I think it's unlikely that Congress is going tell us, 'No, NASA shouldn't be doing a Europa mission.' Very unlikely."
The current mission concept, called the Europa Clipper, would cost an estimated $2.1 billion and could launch as early as 2022. As planned, the mission will send a spacecraft to orbit Jupiter and brush by Europa about four dozen times over three and a half years, snapping photos and taking a good, close look at the moon.
The mission has been under review for years. Now, though, with preliminary funds in hand and the mission optimistically a go, scientists are considering modifying it to include a cache of gadgets capable of searching for signs of life itself. Grunsfeld, for one, thinks it's time to take that quest seriously—and maybe even get a little crazy with the ideas. After all, he notes, the next mission to Europa probably won't occur in our lifetimes.
"I'm very impatient," he said. "Let's try and answer that question now."
Flying Through Life Itself?
One of the wilder ideas is to blow a hole in Europa's crust and catch the stuff that flies out (scientists refer to this as a "precision impactor"). But there are more sedate versions of alien life–hunting that might be just as informative. One includes gently depositing a probe on the moon's surface, then having it look in the ice for the signatures of life, called biomarkers. And then there's the ideal scenario: melting through the crust and looking for life in Europa's oceans using a submersible. That level of ambition is out of reach for this trip.
But—as discussed at the workshop—some scientists think the moon's ocean could be tested more simply, by using plumes of water vapor that erupt from the moon's surface into space.
In 2012, Hubble Space Telescope observations picked up a cloud of water vapor erupting from somewhere near the moon's south pole. Many scientists think that plume contains water from Europa's ocean. In theory, then, it should be possible to fly through the plume, collect samples, and figure out what's in there. Sending a spacecraft through plumes has been done before (notably at Saturn's moon Enceladus), and scientists argue that it's easier and cheaper than sending a lander to scratch around on the moon's surface.
But there's at least one major problem with the plume approach: The plumes are hard to spot, and they may erupt only sporadically. They haven't been seen since 2012, though Hubble continues to look.
"The plumes, if they exist, offer an incredible opportunity to go in and measure something and even search for life itself," said Scott Bolton of the Southwest Research Institute, in San Antonio. "We don't want to design an entire mission that's based on that because they're hard to see … but we do need to take advantage of this."
In a presentation, the JPL's Hand also noted that if there is cellular life in the plumes, the number of cells is likely to be minuscule, making them very tough to detect. Instead, he says, it might be worth sending a small lander down to the spot where a plume breaks through the crust. "We know that if we go to the surface, we will get a sample," he reasoned.
Looking for Life as We Don't Know It
Then there's the more fundamental problem of what to actually look for in a sample: Organisms? Cells? Proteins? Europan organisms could use different chemistry than we expect from Earthly creatures, and life's signature might be written in invisible ink or in a language Earthlings can't read. In other words, scientists speculate, even if we flew through a plume and grabbed handfuls of Europan microbes, it's possible we might not even know it.
NASA astrobiologist Chris McKay argues that we should be looking for amino acids, which are used by life on Earth to build proteins, or for ratios of amino acids that indicate biological activity. Others argue for different biomarkers.
"We need to go look for complexity," said Lee Cronin of the University of Glasgow. Complex molecules—those with branches and different types of atoms—are usually clues that some kind of living factory is churning away nearby, he said.
In the end, some argued that getting a variety of data might be better than any one approach alone. "The data we take from these spacecraft will last forever. And we're not going to get a lot of opportunities to do these kinds of things," says planetary scientist Sarah Hörst of Johns Hopkins University, in Baltimore. In the future, she says, when we might have a better idea of how to detect alien life, we will want to have enough data to mine for those extraterrestrial signatures.