Cynthia Phillips was mesmerized when she saw the latest pictures of her favorite moon. Here at last was a fresh look at Europa, an icy satellite of Jupiter. The moon resembles a truffle drizzled haphazardly with strips of melted white chocolate, as if the universe had rushed to finish a baking-show challenge. The images gave us a new sense of Europa’s topography, its collection of ridges and troughs appearing more intricate than ever. The lighting was different this time, Phillips told me, and the shadows brought out dramatic shapes in the terrain.
The images were taken by a Jupiter-observing spacecraft as it swept past Europa last week, coming within just 222 miles (358 kilometers) of the frozen surface. Phillips, a planetary geologist at NASA’s Jet Propulsion Laboratory, immediately reached for the Europa globe on her desk and began comparing the real-world views with the topography stretched over the plastic orb. The last probe to come this close to Europa was disposed of in 2003, when NASA deliberately plunged it into Jupiter’s atmosphere after the mission started running low on fuel. “We haven’t seen Europa's surface in this level of detail for 20 years,” Phillips said.
Europa might be the best place to search for life elsewhere in the solar system. Scientists are almost certain that beneath the Jovian moon’s frozen surface is a salty ocean with more water than all of Earth’s oceans combined. And such a watery subsurface environment could provide a comfortable home for small Europan life forms.
[Read: Jupiter's frozen moon is studded with 50-foot blades of ice]
Two years from now, another NASA spacecraft will depart for the Jupiter system, specifically designed to study Europa. The probe will swing past the moon dozens of times, sometimes coasting just 16 miles (25 kilometers) above the surface. Each pass will bring scientists closer to understanding the workings of this celestial truffle and its mysterious interior. Beneath that frosty coating could be the answer to one of our biggest questions: Is there life anywhere else in the universe?
Europa is enveloped in a thick coat of water ice. (Some other moons in our solar system have ice made of methane and nitrogen—the cosmos is a weird place.) The criss-crossing lines visible in the new pictures are actually cracks and fissures in that frozen exterior. Scientists suspect that they’re caused by the stretching and squashing that Europa experiences as it orbits giant Jupiter. The moon’s terrain is sprinkled with chemical compounds such as sodium chloride and magnesium sulfate—more commonly known to Earthlings as table salt and Epsom salt—and they could indicate briny waters below.
Scientists got their best evidence that a Europan ocean might exist two decades ago, when that earlier NASA spacecraft detected a magnetic connection between Europa and Jupiter that could easily be explained by the presence of a salty, global sea. This deep into the solar system, Europa’s underground ocean wouldn’t feel the warmth of the sun; it would stay liquid because of Jupiter’s gravitational tugging. In recent years, telescopes have detected signs of plumes of water vapor spewing out of the cracks and into space. Scientists believe that Europa’s ocean could be as old as the moon itself, about 4 billion years or so, which would give life plenty of time and a stable environment in which to evolve, Phillips said.
[Read: Europa's mysterious glow]
The data suggest that Europa has a rocky mantle—the layer between the moon’s crust and core—and when rock and water come together, magical things can happen: Chemical interactions between them are known to produce hydrogen-rich materials for tiny creatures to metabolize. “On our own planet, hydrothermal systems at the seafloor provide energy for communities of microorganisms,” Samantha Trumbo, a planetary scientist at Cornell who studies icy ocean worlds like Europa, told me.
The upcoming NASA mission, named Clipper—a nod to the speedy, lightweight vessels favored by 19th-century merchants—will study nearly every bit of the Europan surface. If it gets lucky, the spacecraft could fly through some plume particles, take a sip, and analyze the contents. Alyssa Rhoden, a planetary geophysicist at the Southwest Research Institute who studies Europa, is most excited about a Clipper instrument designed to detect warmer-than-usual spots on the moon’s surface. “When you look at Europa’s surface, you can see a lot of pits where the surface seems to have dropped down a little bit, places where the surface has been disrupted,” Rhoden told me. “We think that that’s happening from heating coming from below.” That signature could simply indicate the presence of melted bits of ice near the frigid crust—or it could mean a roiling sea has floated toward the surface, perhaps bringing any tiny inhabitants with it.
[Read: Exploring the oceans of the outer solar system]
The Clipper mission is not meant to find definitive proof that life exists on Europa, only explore whether the moon has the right conditions and chemistry to make life possible. Evidence of life will require more missions, guided by Clipper’s data, that could land on the Europan surface and drill into the ice. NASA is also searching for life elsewhere in the solar system, notably on Mars, where a rover is collecting samples from a dried-up river delta. But Europa is a more tantalizing target, and so are the other ocean moons sprinkled across the solar system, such as Enceladus and Titan, which orbit Saturn, and Triton, around Neptune. The Mars mission is designed to search for signs of fossilized life that existed several billion years ago, when water once flowed on the planet. “It’s quite possible that Mars could have had life in the past, in a warmer-weather era, and it’s possible that there are subsurface pockets on Mars that could have remnants of this living biosphere,” Phillips said. “But on somewhere like Europa, life could exist there now.”
And what might humanity, by way of carefully engineered machines, find on Europa, once we’ve figured out which melty bits to inspect? “I would love for there to be Europan whales swimming around in that ocean,” Phillips said with a laugh. But alien life, if it exists, is likely to be small and simple. Energy sources are limited in the Europan depths, and scientists don’t think the environment can support the development of more complex organisms, Phillips said. Still, even the discovery of a single microbe would mark an explosive event in human history. It would mean that life had managed to spring up in two different places around the same star—in a universe absolutely brimming with stars. If it happened more than once here, in our own solar system, it’s likely happened elsewhere in the cosmos, around someone else’s sun. This is why scientists are so eager to catch a glimpse of Europa, and prepare as much as they can for the exploration to come. “We all want it to be water,” Rhoden said. “We all want it to be a cool plumbing system in the shell with lots of activity, and someday we’ll get down there and find little Europan sea urchins clinging to the bottom of the ice.”
