Jupiter’s moon Europa likely has water plumes—great geysers of saline ocean blasting from its icy shell. That's what scientists announced earlier today, reaffirming previous observations of plume activity on the Galilean moon. The findings raise the stakes for the agency's next flagship planetary mission, slated to launch for Europa around 2022.
The plumes, which are estimated to rise 125 miles above Europa's surface, were captured using the Hubble Space Telescope, NASA's 26-year-old space-based orbiting observatory. "We are working at the limits of Hubble's unique capabilities," said William Sparks, an astronomer with the Space Telescope Science Institute, during a press teleconference. To find the plumes, scientists used what’s known as transit imaging operations, observing the silhouette of Europa against the bright surface of Jupiter. It took 50 million observation events to generate the plume images, which were carefully processed through specially designed software.
NASA teased the results last week, promising "surprising activity" on the moon. Plumes fit the bill, and have implications beyond celestial wonder and geologic thrill-seeking. Europa is an ocean world, and is thought to have all of the ingredients necessary for life. Though it is only about the size of our own moon, it hosts a saltwater ocean with twice the water of Earth's oceans. That water is sandwiched between an ice shell and a rocky mantle. When water touches rock, interesting chemical processes result—especially if there are hydrothermal vents from the planet's interior blasting hot water into the ocean. The conditions for life on Europa are at least as hospitable as what can be found in the deepest parts of Earth's ocean. The plumes help kick open the door for alien, Europan creatures, though what we might find there remains a mystery.
The problem has always been getting to Europa’s ocean to take a sample. The ice shell is likely many miles thick—beyond the drilling capacity even here on Earth. Plumes solve that problem. We don't need to go to the ocean; we can have the ocean come to us. The Europa Multiple Flyby Mission in development by NASA will see a spacecraft enter orbit around Jupiter, circling the gas giant hundreds of times. Each time it comes around to Europa, it will scan and image the icy world. This technique, rather than a direct Europa orbiter, allows the spacecraft to avoid the worst of the punishing radiation belt in the Jovian environment. Scientists can now plan trajectories that allow the spacecraft to fly through the plumes in order to analyze their composition.
However, nobody is promising life detection yet. The multiple flyby mission is designed to study habitability. Follow-up missions—likely Europa landers—would actually determine the life question. (The plumes help a lander as much as they do a flyby, since water that blasts from Europa's shell would rain back down to the surface. A lander could make extraordinary observations by drilling only a few inches down.) Lander concepts are currently under development at Jet Propulsion Laboratory in southern California. The multiple flyby spacecraft is being developed jointly by JPL and the Johns Hopkins University Applied Physics Laboratory in Maryland.
"For a long time, humanity's been wondering whether there is life beyond Earth," said Paul Hertz, director of the astrophysics division at NASA headquarters. "We're lucky to live in an era when we can address such evidence scientifically."
For now, the discovery could only have been made with the sensitivity of the Hubble. "Hubble is the only telescope that we have right now capable of observing Jupiter and Europa at this detail in ultraviolet light," said Jennifer Wiseman, senior Hubble project scientist at NASA's Goddard Space Flight Center. The space telescope's science mission was recently extended by five years, until 2021, at which time NASA will have to decide whether to keep the space telescope operational. The powerful James Webb Space Telescope, Hubble's successor, is slated to launch in 2018. "We're particularly excited about using the James Webb Space Telescope ... to look for additional evidence of Europa water,” Wiseman says.