Galileo’s tantalizing finds
Almost all knowledge of Europa comes from a limited data set gathered by NASA’s Galileo mission to the Jupiter system. Launched in 1989 after years of delay due to the space shuttle Challenger disaster, Galileo rewrote planetary science textbooks despite being plagued by one severe problem. The spacecraft’s 16-foot-wide umbrella-shaped high-gain antenna was supposed to unfurl after traveling far enough from the Sun. This would allow large images to be sent back every minute for years. Instead, the motor got stuck, and the antenna never raised.
Galileo’s inadequate connection turned a data deluge into a trickle. Observations were forced through a secondary dish using a signal 10,000 times fainter. With only a dozen close flybys of Europa (the spacecraft went into safe mode on two of those), JPL scientists had to make the most of their opportunities. The single highest-resolution Europa image ever taken has just 6 meters per pixel resolution and isn’t even in color.
Still, Europa’s first photo album was startling. Galileo imagery seemed to confirm what astronomers suspected: Europa was best explained as a spinning shell of ice atop a large liquid water ocean. The surface also gave clues to Europa’s history. Its fractured and icy terrain crawls around, breaking up and pushing together in a process akin to plate tectonics — the only known world other than Earth with such geology (See “Ice tectonics,” p. 25).
Galileo’s magnetometer also detected an induced magnetic field between Jupiter and Europa. The easiest way to interpret that is with a salty, global subsurface ocean. Ice simply isn’t conductive enough.
The team only managed to gather enough data for a frustratingly low-resolution map of the moon, but the few high-resolution images were enough to whet scientists’ appetites. Many of the current small crop of Europa scientists worked as graduate students using Galileo data. To them, a follow-up mission seemed obvious. But in the years since Galileo’s 1995 arrival at Jupiter, scientists watched NASA launch 11 missions to Mars without a single craft to explore Europa.
The mother of innovation
Europa mission Project Scientist Robert Pappalardo is among those scientists in waiting. He figured his work on Galileo would lead to a job on an eventual Europa trip. And, around the turn of the millennium, that dream looked likely to come true when a group of astronomers proposed the Europa Ocean Discovery mission.
Ultimately, the small craft was seen as unrealistic and NASA moved in favor of New Horizons’ trip to Pluto instead. Still, the decadal survey, a document summarizing the planetary science community’s priorities, ranked Europa near the top. Astronomers wanted a Europa orbiter.
Eventually the space agency asked Pappalardo and a team of JPL scientists for an intermediate-class mission. The team combined efforts with the European Space Agency’s mission to explore Jupiter’s other icy moon, Ganymede. They called it the Jupiter Icy Moon Explorer — JUICE. This time, the astronomical community said the concept wasn’t worth the cost. NASA backed out, leaving the Europeans to build JUICE themselves. (That mission is planned to launch in 2022.) “I said I’ll go to JPL and if after three years you don’t get a mission, I’ll go back to academics,” Pappalardo says. He didn’t give up. “After nine years, I’m still here. It was a little like Lucy and the football.”
The complicating factor in all these missions was that Europa’s orbit sits some 400,000 miles (644,000km) from our solar system’s biggest planet. That’s about twice as far away as the Moon is from Earth. Any spacecraft needs heavy radiation shielding to withstand the deadly downpour of high-energy electrons streaming off Jupiter at nearly light speed.
After the latest orbiter was rejected, NASA asked its scientists to look at alternatives, and they found several. One plan seized on something genius NASA had already mastered: studying the moons of Saturn. JPL’s Brent Buffington worked miracles calculating the extended Cassini mission trajectory. Buffington’s colleagues compare the crackshot astrodynamicist to Rich Purnell, the “steely eyed missile man” who saves the day in Andy Weir’s The Martian. Thanks to Buffington’s calculations, Cassini’s last seven years use the remaining 20 percent of fuel for 155 orbits that swoop by Saturn’s moons in daredevil flybys, finding signs of hydrothermal vents on Enceladus, rainfall on Titan, and other new science. During its closest flybys, Cassini scrapes within 16 miles (25km) of the surface.
Buffington was asked for the same magic as mission designer for a Europa flyby spacecraft. He divided the moon into 14 overlapping regions that allow a global map in high resolution. The best images will match those streaming from orbiters at Mars or the Moon. And, because it doesn’t stay close to Jupiter like a Europa orbiter would, the flyby design doesn’t require intensive shielding.
“We have an architecture that is much better tuned for a mission of discovery,” says Europa mission Project Manager Barry Goldstein. “By having a mission where we loop in and out of the regime, we’re able to observe Europa from afar, and if we see a plume while we’re far out, we can adjust our flyby.”
If the moon has active plumes like the ones Hubble tentatively detected, the spacecraft could taste Europa’s ocean and determine its composition thanks to targeted flybys. The final proposal includes at least 45 flybys — radiation eventually will kill the spacecraft — almost all of which come within 60 miles (100km) of the surface. And it pulls it off for a low $2 billion. That’s half the expected cost of prior designs.
The combination of imagery and topographical data will revolutionize Europa science like the first 3-D data from the Moon and Mars did, says Europa Imaging System Principal Investigator Elizabeth Turtle of Johns Hopkins University’s Applied Physics Lab. But getting the photos isn’t easy. Inner solar system images benefit from the Sun’s brightness, as well as steady orbits around their less hostile targets. The Europa flyby mission’s closest passes will skim the surface at a height equal to Earth’s best spy planes. The ground below will move fast under extremely low-light conditions.
So, instead of borrowing technology from Cassini, whose CCD cameras have been challenged by similar conditions at Saturn, Turtle’s team turned to New Horizons’ LORRI camera. Its high-resolution flyby images of far-off Pluto have stunned the public over the past year. Instead of a CCD camera, the spacecraft uses CMOS, a detector that works better in low-light conditions.
“It’s a kind of detector that’s in a lot of digital cameras these days,” Turtle says. “It’s good for Europa because it’s more radiation tolerant than CCDs and it can do a much more rapid readout.”
The briny deep
Like his colleagues, Kevin Hand also learned Europa while working on images from Galileo. But the data-starved years since then have driven NASA’s deputy chief scientist for solar system exploration to extremes in search of new discoveries.
He trekked Alaska’s North Slope and explored Antarctica’s dry valleys. He also dove to the Mid-Atlantic Ridge and East Pacific Rise with Hollywood filmmaker James Cameron. There, he explored the Lost City Hydrothermal Field, which spews methane and hydrogen into the saltwater in a process much different from the black smokers found by Ballard and other explorers in the 1970s.
There are no tube worms or clams. Instead, vastly different life-forms persist like snails, mollusks, and crustaceans. Many scientists believe similar sites served as the starting point for all life on Earth.
“Obviously, NASA’s mantra has long been ‘follow the water,’” Hand says of the space agency’s search for life. “We think [Europa] is where the water is.” But life requires more than that. It also needs energy. And it needs the elements to form life. Those things can come from volcanism recycling Europa’s rocky seafloor. However, for life to be widespread on Europa, vents aren’t enough. The life-forms Hand saw at Lost City are actually dependent on oxygen dissolved in the water. On Earth, that oxygen is derived from living things that use the Sun. Hand wanted to know if there was another way to get that oxygen on Europa.
He and his colleagues at JPL created what they call “Europa in a can.” The laboratory experiment allows the astronomers to replicate Europa’s temperature, pressure, and radiation conditions on tiny ice samples. The Galileo spacecraft saw hydrogen peroxide (H2O2) in just one place on Europa’s surface. So the scientists showed in the lab how the chemical could form on the icy moon and eventually decay to oxygen (O2). Finally, they used the twin W.M. Keck Observatory telescopes in Hawaii to map the moon’s hydrogen peroxide deposits. Hand’s group also examined the strange colors that stand out on Europa’s cracked surface. They showed the browning yellow streaks are actually a byproduct of what happens when salt is hit by Jupiter’s radiation. “I think part of the discoloration we see on Europa’s surface is damaged sea salt,” Hand says, implying ocean water reaches the surface.