National Geographic : 2014 Jul
life beyond earth 43 any life that might exist there. Up at the surface, comets periodically crash into Europa, deposit- ing organic chemicals that might also serve as the building blocks of life. Particles from Jupiter’s radiation belts split apart the hydrogen and oxy- gen that makes up the ice, forming a whole suite of molecules that living organisms could use to metabolize chemical nutrients from the vents. The big unknown is how those chemicals could make it all the way down through the ice, which is probably 10 to 15 miles thick. The Voy- ager and Galileo missions made it clear, however, that the ice is riddled with cracks. Early in 2013 Hand and Caltech astronomer Mike Brown used the Keck II telescope to show that salts from Eu- ropa’s ocean were likely making their way to the surface, possibly through some of those cracks. And late in 2013 another team of observers, using the Hubble Space Telescope, reported plumes of liquid water spraying from Europa’s south pole. Europa’s ice is evidently not impenetrable. This makes the idea of sending a probe to orbit Europa all the more compelling. Unfortunately the orbiter mission the National Research Coun- cil evaluated in its 2011 report was deemed scien- tifically sound but, at $4.7 billion, too expensive. A JPL team led by Robert Pappalardo went back to the drawing board and reimagined the mission. Their Europa Clipper probe would orbit Jupiter, not Europa, which would require less propellant and save money, but it would make something like 45 flybys of the moon in an attempt to under- stand its surface and atmospheric chemistry, and indirectly the chemistry of the ocean. All told, Pappalardo says, the redesigned mission should come in at under two billion dollars over its whole life span. If the mission concept goes forward, he says, “we envision a launch sometime in the early to mid 2020s.” If that launch takes place aboard an Atlas V rocket, the trip to Europa will take about six years. “But it’s also possible,” he says, “that we could launch on the new SLS, the Space Launch System, that NASA is currently developing. It’s a big rocket, and with that we could get there in 2.7 years.” The Clipper likely wouldn’t be able to find life on Europa, but it could help make the case for a follow-up lander that could dig into the surface, studying its chemistry the way rovers have stud- ied Mars’s. The Clipper could also scout out the best places for such a lander to set down. The next logical step after a lander—sending a probe down to explore Europa’s ocean—could be a lot tougher, depending on how thick the ice is. As an alternative, mission scientists might try to reach a lake that may be entirely contained within the ice near the surface. “When that undersea explorer eventually does come to fruition,” says Hand, “in evolutionary terms, it’ll be like Homo sapiens to the Australopithecus we’ve been testing in Alaska.” The relatively crude rover Hand and his crew are testing at Sukok Lake crawls along under a foot of ice, its built-in buoyancy keeping it firmly pressed against the frozen subsurface, sensors measuring the temperature, salinity, pH, and other characteristics of the water. It doesn’t look for organisms directly, however; that’s currently the job of the scientists working on another as- pect of Hand’s project across the lake, including John Priscu of Montana State University, who last year extracted living bacteria from Lake Whillans, half a mile under the West Antarctic ice sheet. Along with geobiologist Alison Murray, of the Desert Research Institute in Reno, Nevada, and her graduate student Paula Matheus-Carnevali, Priscu is investigating what characteristics frigid environments need to make them friendly to life and what sorts of organisms actually live there. Useful as the study of extremophiles is to contemplating the nature of life beyond our planet, it can only provide terrestrial clues to an extraterrestrial mystery. Soon, however, we Frank Drake is still looking for extraterrestrial signals—a discovery that would trump everything else.