Earth exists in that sweet spot in distance from the sun, often referred to as the “Goldilocks Zone,” where water is a liquid on the surface. Much closer and you can get a runaway greenhouse effect, which makes Venus hellishly hot. Too far away, and you freeze into an ice ball.
But there’s another place in our solar system where liquid water exists in abundance. The icy moons of the outer planets have tidally heated oceans covered by thick sheets of solid ice. Minerals from deep-water hot spots may infuse those oceans with nutrients that life can exploit just beneath this frozen crust. Using analogs of sea ice here on Earth, researchers are now suggesting this “shallow biosphere” may be detectable in upcoming missions to Jupiter’s moon Europa or Saturn’s moon Enceladus.
Of course, we’ve been on the verge of detecting life elsewhere in the universe for the past 25 years or so. In the mid-1990s, during an introductory astronomy class, my professor barely mentioned the possibility of life in our own solar system, let alone on those as-yet-to-be-discovered worlds orbiting other stars. He grew up in the era of Ray Bradbury’s The Martian Chronicles and, until the 1960s, some scientists had interpreted seasonal blue-green spots on the surface as vegetation. I’m sure he felt a stab of disappointment when the Magellan orbiter revealed them as desiccated wastelands. The Viking landers, sent in the 1970s, deployed simple life detection experiments that fizzled, a let-down that kept us from visiting the surface of Mars for nearly 20 years.
My only hope at the time for discovering life in the universe hung on the unlikely event that E.T. beamed a message detectable by our radio telescopes. As a grad student, I dutifully downloaded the “SETI@Home” screensaver, what we’d now call an app, designed to help astronomers analyze radio telescope data for signs of artificial signals. I installed it on every PC in the University of Washington computer lab to help hasten the search. In the meantime, I read Carl Sagan’s Contact over and over.
It was about this time that David McKay and his colleagues found what they claimed were evidence for microbial fossils in the Martian meteorite ALH84001. This generated enough hype that President Bill Clinton himself mentioned the discovery during a press conference. The research was rightfully met with extreme skepticism. “Extraordinary claims,” Sagan reminds us, “require extraordinary proof.”
When we started discovering the first exoplanets in the mid-1990s, NASA stepped up its research in astrobiology. I recall an American Astronomical Society meeting in Washington, DC, when NASA’s then-administrator Dan Goldin showed a fuzzy picture of a blue-green planet and suggested we’d have images of another “earth” within the next two decades. I was so galvanized by this prospect that I abandoned my work on small robotic telescopes to study astrobiology. I threw myself into researching earth-like exoplanets in an effort to help engineers design telescopes capable of detecting evidence of life on other worlds.
It’s now been a few decades, and a telescope capable of imaging another earth is still beyond NASA’s budget horizon, but the prospects of finding life in our solar system have gotten that much better. In the ensuing decades, the Cassini mission to Saturn passed near several of the outer planet’s moons, detecting geysers spewing from the surface of icy Enceladus. Later, Hubble saw similar features on the surface of Europa. It seems these frozen oceans, once feared permanently hidden beneath dozens of miles of ice, are in contact with the surface. This, along with the study suggesting a biosphere could be interacting with these plumes, puts not only the detection of life, but the novel idea of a sample return, just one mission lifetime away from completion.
In February 2017 NASA advanced the Europa Clipper flyby concept into the next design phase. The primary mission includes onboard sampling of the plumes but NASA is contemplating an instrument to collect samples in aerogel for return, similar to the Stardust mission that brought pieces of comet Wild 2 back to Earth in 2006.
Imagine being the first scientist to look for the signatures of life in a sample returned from what may be an alien biosphere. Not limited to the simple experiments of Viking, or probing the ancient minerals in ALH84001 for signs of life, or squinting at the vague hints of green on a pixelated image of an exoplanet, but examining water droplets from Europa with the most sophisticated laboratory tools available.
I’ll tell you, if I had to do it all over again, I’d be studying to be an oceanographer with a minor celestial mechanics.
And that’s what makes this new research so exciting. If solar energy can liberate molecules to fuel energetic reactions just underneath the ice, as the researchers suggest, and these biological communities get expelled through geysers so that we can sample them without having to drill through miles of ice, we may get our first glimpse of an alien ecosystem in just a few decades.
This time we mean it.
This post was written by a graduate of the online course Share Your Science: Blogging for Magazines, Newspapers and More, offered by Scientific American and the Alan Alda Center for Communicating Science at Stony Brook University, with sponsorship from the Kavli Foundation.