The Plan to Hunt for Alien Life on the Most Promising Moon in the Solar System
If Enceladus has life, we will find it.
Enceladus's plumes. Image: NASA/JPL/Space Science Institute
Nestled within Saturn's ring system, a tiny, pearly moon shines brighter than almost anything else in our solar system. If alien life is out there, that lonely snowball could be the first place we find it.
"Everything that we measure about Enceladus tilts in the direction of habitability," astronomer Jonathan Lunine said at the inaugural ceremony of the Carl Sagan Institute, the new, Cornell-based research center devoted to searching for life within and beyond our solar system. "What we want to know is whether there is life there. So how do we do that?"
Hopefully, they'll do so with Lunine's newly proposed NASA mission, the Enceladus Life Finder.
Saturn's Cassini probe has given us plenty of reason to be excited about the prospect of life on this tiny moon. ELF, a space probe with an on-board chemistry lab, is tasked with finding the hard evidence. With NASA's blessing, ELF will fly to Enceladus, sample the material spewing out of the moon's cryovolcanoes, and use sophisticated instruments to analyze the samples for signs of life. If approved, the mission would launch in 2020 and arrive at the ice moon's south pole in 2030.
Thereafter, we'd have an answer in as little as a few weeks.
"We have these measurements in our solar system around these diverse worlds, that are very different and could all be habitable," Carl Sagan Institute director Lisa Kaltenegger told me. "They are helping us fill in our knowledge gaps and understand what it means to have habitability in the first place."
And if you wanted a prime example of a totally alien environment that could harbor life, Enceladus hits the mark like no other. The moon is tiny—at 310 miles in diameter, Enceladus could fit snugly between San Fransisco and LA. It's got no atmosphere, and its surface averages a frosty -330 degrees Fahrenheit. But Enceladus also bears some surprisingly Earth-like traits, including a hot, rocky core that's probably rich in iron and silica, two of the dominant elements on our planet. What's more, through gravity measurements, astronomers have used Cassini data to infer the presence of a liquid water ocean beneath the moon's south pole, one that could wrap around the entire world.
A geothermally heated ocean is intriguing enough, but it gets even better. In 2005, Cassini discovered plumes of material shooting out of cracks in the moon's south pole ice sheet and raining into Saturn's E-ring system. Researchers observed that the eruptions consist mostly of water ice, but also contains salts, methane, and simple organic molecules. And very recently, Cassini traced silica dust floating around in Saturn's rings to Enceladus's plumes.
"In their crystalline form, these silica grains would be quartz," Lunine told me. "This makes a case that you have quartz dissolving in hot water and precipitating out before getting shot out of the moon's surface."
In other words, hard evidence that the stuff sputtering out of Enceladus's ice volcanoes was hot water in a subsurface ocean.
"Enceladus has all the formal requirements for habitability," Lunine said. "All of them. And the material in its interior is coming out. It's offering free samples."
But Cassini's antiquated instruments will not give us definitive proof of life on Enceladus. That's where ELF comes in.
"Cassini doesn't have the kinds of instruments needed to determine whether this ocean is inhabited by life," Lunine told me. "They're simply too old and too crude. But a next generation of instruments, sent to do exactly the same thing—flying through the plume of Enceladus, just like Cassini does today—could potentially detect life."
The ELF mission concept is a solar-powered Saturn orbiter that would conduct roughly 10 passes through Enceladus's plume. ELF will harbor two state-of-the-art mass spectrometers, instruments that'll allow scientists to detect a variety of molecules and isotopes present in Enceladian seawater. As Lunine explained to me, ELF is designed to conduct three distinct tests, which together could provide powerful evidence for life.
The first two tests will look at patterns in organic molecules—specifically, protein-building amino acids and cell membrane-building hydrocarbons. Both of these classes of molecules can form via inorganic reactions. But if they're being produced biologically, we can expect to see very distinct patterns.
"When you look at composition in meteorites, the amino acids that are easiest to form are the most abundant," Lunine said. "Life makes amino acids in completely different patterns, which run counter to the thermodynamic solution."
Likewise, if living organisms are building hydrocarbon chains for lipid membranes, we'll expect to see certain molecules prevail over others.
"In a lab, you can cook up the building blocks of such membranes, but they don't have the same regular pattern you see in life," Lunine said. "It's a bit like what would happen if you smashed a LEGO piece with a hammer. You might still be able to build something out of it, but the pieces wouldn't fit together very well."
The final test for life looks at the abundance of carbon and hydrogen isotopes present in Enceladian water. Here again, we expect to see certain patterns emerge in living systems—greater quantities of the lighter carbon-12 isotope compared with carbon-13, for instance.
The ELF, Lunine says, should be able to carry out each of these three tests within its first five passes through the moon's south pole plume. The probe will simultaneously collect other types of information that'll shed light on ocean chemistry and how the moon formed.
"Then we've got five extra flybys, based on initial the data we collect," he said. "If we see something promising, we'll use those flybys to drill into certain issues more deeply, to ask if we really have a definitive indication of life."
To Lunine, the mission's simple, straightforward nature is one of its most compelling attributes.
"Ten flybys, that's it," he told me. "We don't have to worry about contaminating Enceladus, since we're sampling stuff that's in space. The data we collect will be astounding."
The ELF mission is still in its early stages and has several hurdles to clear. If NASA approves the proposal in September, Lunine and his team will spend the next nine months hashing out details. The final selection process will take place later next year.
But Lunine is optimistic. In his view, Enceladus is the bleeding edge of the search for life in our solar system. And under the umbrella of the Carl Sagan Institute, ELF would offer invaluable insights to researchers searching for life beyond our solar system as well.
"[ELF] connects up with the search for life elsewhere in the galaxy," Lunine said in his closing remarks at the Carl Sagan Institute inauguration. "It's the ground truth part of it. It's the visceral exploration part of it. And so, in this new institute, we will bring together two things that Carl [Sagan] was passionate about: the human exploration of the solar system through a robotic spacecraft, and the exploration of the entire cosmos for life, and perhaps, intelligent life like us."