by Jim Bell
“We’re talking about a moon whose geology changes the same way the weather changes on our planet,” points out Rich Terrile. “It’s something right out of science fiction. And yet, it’s right here, orbiting Jupiter.” The little moon is turning itself inside out trying to get rid of all that internal heat.
My JPL planetary science colleague Rosaly Lopes recalls, “I was a student at the time Voyager discovered Io’s volcanoes, and I thought that an incredible discovery. Voyager detected a dozen active volcanoes, and that blew our minds at the time.” Rosaly later earned a spot in the 2006 edition of The Guinness Book of World Records as the discoverer of the most active volcanoes anywhere—a total of seventy-one on Io.
Voyager’s images at Jupiter’s second-closest big moon, Europa, were also full of surprises, although what was found to lie under the surface is what gained the most attention, then and since. Europa was only rather poorly photographed by the Pioneer missions earlier in the 1970s, and Voyager 1 was able to see its bright icy surface only from a long distance away since its trajectory was optimized for close passes by Io, Ganymede, and Callisto. Four months later, Voyager 2 passed through the Jupiter system and got close-up views of Europa. Oh my goodness, was it worth the wait! The details seen in Voyager 2’s photos were truly new and exciting. Indeed, perhaps the most common first impression among the imaging team when seeing those first close-up photos was “Wow—that’s flat!” And it sure is. Europa is about the same size as our moon, but unlike the 3 to 5 miles of rugged elevation difference found among the mountains and valleys of our moon, the largest “mountains” and deepest “canyons” on Europa are only around 30 to 50 feet tall or deep. That is to say, if Europa were the size of a bowling ball, the tallest bump on its surface would be less than the thickness of a piece of thread! Another surprise was (again) the relative lack of impact craters—the scars left on ancient planetary surfaces by asteroid and comet impacts over the eons. This implied that some process must be resurfacing Europa, erasing the craters that must surely have built up over time. To everyone’s amazement, Voyager had discovered one of the flattest and youngest surfaces (though not as young as nearby Io) in the solar system.
But why is it so flat? A clue came from the crazy-quilt series of lines that crisscross Europa’s surface—dark cracks that separate the icy crust into a jumble of curvy or triangular sections, almost like the pieces of a big jigsaw puzzle. In some places, the puzzle pieces seem to have rotated relative to one another, and in other places to have spread apart, letting reddish-brown material ooze up in the intervening spaces, maybe similar to the way fresh, new volcanic lava oozes up between tectonic plates that make up the Earth’s mid-ocean ridges. Indeed, one of the most common and obvious reactions to Europa’s cracked and platelike surface was to compare it to melting sea ice—a thin layer of frozen water floating on top of liquid water, sloshing around. On Earth, waves and the warmth of summer break up polar sea ice into millions of little icy “plates.” If that is essentially what is happening on Europa as well, the implications would be enormous. Life on Earth may have begun in the ocean. Does Europa have one?
Frustratingly, the Voyager view of Europa was only fleeting—two quick flybys, and one at a pretty far distance away. The known presence of strong heating from tidal forces (heavily in evidence at Io), the super-flat surface, the sea ice–like plates with distinctively colored material appearing to ooze up from the depths below—all these pieces of evidence pointed toward the possibility of a subsurface ocean. I like to imagine what would have happened if Voyager had been outfitted with a high-speed submarine probe that could have penetrated Europa’s thin ice shell and plunged into the watery depths below: Turning on its headlights, the sub relays real-time video as it dives deeper and deeper. Finally approaching the seafloor, the water becomes murky, and the probe’s thermal sensor detects a hot spot up ahead. Jupiter’s tidal energy is flexing Europa, heating its icy and rocky interior, and the chemical sensor identifies hot, sulfur-rich water and gases leaking out of the crust here just like at many mid-ocean ridges on Earth. With the outside pressure rising and beginning to stress the sub’s hull, the onboard mass spectrometer starts an analysis for organic compounds in the hot waters. Switching to wide-angle mode, the probe begins to scan for any signs of motion. The pressure is getting critically high and the sub’s signal is beginning to weaken as we’re now more than 60 miles below the surface. The video is getting ratty. There’s a flash of some kind—then static—then another flash, and then a strange, curved silhouette of—something? But then the signal is lost and we all just stare, dumbstruck, at the static. What had we just seen?
“Launch a class-five probe, Number One!” Star Trek’s Captain Picard would have ordered, had the USS Enterprise encountered this ice-covered water world. “And fit it for higher-pressure submarine operations!”
But as Europa receded from view, all the Voyager team could do was make the best of the limited data in hand and dream about the day when they’d be able to go back, at least virtually, and take a longer, deeper look at this enigmatic world. US Geological Survey planetary geologist and Voyager imaging team satellites subteam lead Larry Soderblom remembers the sort of deer-in-the-headlights feeling that he and many others on the imaging team had after taking only those two brief passes by Europa. Although he is an expert geologist, his Earthbound experience left him stumped time and again when trying to interpret the strange new landscapes that were revealed in the Voyager images.
“Although we only had a glimpse of Europa, the fact that there were so few impact craters on its surface left no doubt that its surface was geologically young—maybe only 100 million years old,” he explains, being sure to note that 100 million years truly is “young” to geologists. “Europa formed about four and a half billion years ago along with the rest of the solar system, so 100 million years is only about 2 percent of its lifetime. Surely, we all thought, its surface must still be changing today. But what causes those changes? We all wished we’d had a chance to take a closer look.”
Imagine for a moment that you had been dreaming all your life of someday seeing the Grand Canyon in person. Then imagine you decided to walk there. You set foot to the pavement day after day, and after half a year, give or take, you made it! You would hike to the bottom, set down your tent, raft the Colorado River, and explore. But wait: what if your only option was to keep walking right by it, peering over the rim, and dreaming wistfully of seeing those spectacular layers of colored rocks and feeling that ice-cold water running through your toes? That’s what Larry and others felt like after Voyager’s too-quick glimpses of Europa . . . we were so close and yet so far.
The chance for a closer look would not come for more than sixteen years, when NASA’s Galileo Jupiter orbiter mission began making close flybys of Europa and the other Galilean satellites in 1995. Galileo had the advantage of spending lots of time in the Jupiter system, orbiting the giant planet thirty-five times, on trajectories that took the spacecraft close to Europa eleven times. High-resolution color images of Europa’s cracks and other features, measurements of the variety of ices and minerals on the surface, and the discovery that the subsurface is electrically conductive all provided additional evidence for Voyager’s initial hypothesis of a deep, salty ocean under Europa’s icy crust. The conductivity measurement in particular is especially intriguing, because just a few percent of dissolved salts (much like NaCl, or table salt) in a deep liquid water ocean could explain the measurement. That kind of salinity would make Europa’s ocean very Earthlike. In fact, Galileo data are consistent with Europa having the largest ocean in the solar system—with maybe two or three times the volume of water in all the Earth’s oceans combined.
Still, though, the evidence is indirect, and many of us yearn for proof and details about Europa’s putative liquid-water ocean. Is it really there, or is there just more slushy ice under Europa’s frozen crust? If it’s really there, how deep is it? How warm is it at the
bottom, where the tidally heated rocky part of Europa’s crust is in contact with the water? Are there organic molecules in that ocean? Heat sources, organic molecules, liquid water: these are all the hallmarks of a habitable environment for life as we know it. Exciting, for sure, but evidence that a place is habitable is not necessarily evidence that it is inhabited. Is there life in Europa’s ocean?
I feel the same way the Voyager team did in 1979: we have to go back! And we have to go back for a longer visit, a dedicated visit, to find out. We can send missions to do more flybys and eventually to orbit Europa and map its surface in detail, to map the thickness of the icy shell of frozen water and find the places where it’s thinnest. We can land a mission there, perhaps robotic, perhaps human-crewed, and drill into that thin ice and find proof of the ocean below. If it’s there, and if we can get through the overlying ice, we can send a real version of my imaginary submarine down there and take pictures and make chemical and biologic measurements, maybe even collect samples to bring back to Earth. Oh, for one of Captain Picard’s class-five probes! In the decades ahead, we are in for a grand adventure exploring the number-one nearby locale in the search for living organisms beyond Earth. I predict that these missions will give us the answer about life in Europa’s ocean. I am trying to eat well and exercise regularly so I can live to see that exploration pay off.
After the bizarre and unexpected revelations at Io and Europa, many on the Voyager team could easily have figured, That was it. How could it get any better? They had discovered amazing secrets of the Jovian system. But still, Voyager marched on. Every precious moment of each spacecraft’s three-day plunge past the giant planet and its moons had been filled with the maximum number of photos and other measurements that the power supply and tape recorder could handle. It would have been an incredible set of sights to behold if we could have magically traveled aboard Voyager and looked over the shoulder of those cameras as they snapped their timeless photos, revealing strange and lovely new vistas at every turn of the scan platform.
Passing by Ganymede, the largest moon in the solar system (larger than the planet Mercury!), both Voyagers revealed evidence for past movement of a grooved, icy, platelike crust similar in some ways to Europa’s but apparently much more ancient because the many impact craters on its surface had been preserved in the ice. Ganymede was apparently not subject to the constant oozing of material from below like Europa, leaving much of its cratered surface intact. The team speculated about the possibility of a subsurface ocean on Ganymede because it was also tidally heated by the orbital resonance along with Europa and Io, but no convincing evidence was seen in the flyby data. Instead, as it had for Europa, it took the more detailed and frequent flybys by the Galileo mission in the 1990s to discover that Ganymede has a magnetic field (the only moon in the solar system with its own) as well as a conductive subsurface layer under its icy surface—perhaps another salty ocean waiting to be confirmed. We’ll have to wait a while to find out, however, as the next robotic mission to Ganymede, the European Space Agency’s Jupiter Icy Moons Explorer or JUICE spacecraft, won’t launch until 2022 and won’t orbit Ganymede until 2030.
The one large Galilean moon that is not part of the resonant dance around Jupiter with the others is Callisto; while its surface is not as exciting as that of its large siblings, Voyager data nonetheless revealed that Callisto has mysteries of its own. Callisto’s surface is heavily cratered, covered from stem to stern with impact scars from billions of years of pummeling by asteroids and comets. That observation alone helps us appreciate the significance of the nearly craterless surfaces of Io and Europa, and the mild cratering of the icy surface of Ganymede. Callisto, living as it does roughly in the same vicinity as those three other moons, tells us that its siblings had to have been smashed countless times as well. But their surfaces are so much younger and more dynamic that much or all of the evidence for those past impacts has been covered over or wiped away. Callisto’s relative lack of internal heating has made it a more passive world, taking its blows in stride. One of those strikes photographed by Voyager is an enormous, more than 2,300-mile-wide multiringed basin called Valhalla that preserves evidence for a whopper of a giant impact early in Callisto’s history. Despite its apparent lack of interesting surface geology, a more detailed study of Callisto by the later Galileo mission revealed evidence that there might even be a thin liquid water layer—an ocean of sorts—beneath that moon’s thick icy crust.
Voyager and subsequent missions have shown us that Io, Europa, Ganymede, and Callisto are a sort of mini solar system revolving around their “sun,” the giant planet Jupiter. Tidal forces from Jupiter and from one another, and perhaps some radioactivity from the moons’ deep rocky and metallic cores, heat the insides of these worlds and lead to massive high-temperature eruptions of sulfur-bearing volcanic rock on Io and probably to liquid-water layers—subsurface oceans—on Europa and Ganymede and perhaps even Callisto. Voyager’s discoveries at Jupiter included other moons as well, including the first close-up views of the small potato-shaped moon Amalthea, the fifth known moon of Jupiter (discovered in 1892), and also the discovery of three new moons (Metis, Adrastea, and Thebe), all of which are too small and faint to be seen from Earth, and all of which also orbit close-in to the planet like Amalthea.
Maybe the most amazing “small moon” discovery, however, took advantage of Voyager’s unique perspective of being able to look back, sunward, toward Jupiter after having flown past on its closest approach. Anyone who’s ever driven westward around sunset knows that driving into the sun’s glare causes all the dust and grime and bugs on your windshield to light up, making it hard to see. This effect is known as forward scattering, and it was a trick that was exploited by the Voyager imaging team to try to search for small particles from dust while pointing the camera back toward the general direction of the sun. Lo and behold, the strategy worked, and a newly discovered set of thin, dark rings around Jupiter was spotted in Voyager’s images. They turned out to be very small “moons” indeed; most are just motes, and the largest ring particles are only about the width of the thinnest human hair.
Voyager’s moon and ring discoveries were exciting and historic to be sure, but the clear highlight of the flybys in terms of sheer photographic beauty was the imaging of the planet itself. My former Caltech professor and research mentor Andy Ingersoll was one of the visionaries who helped plan a time-lapse movie of Jupiter’s swirling storm clouds as the Voyagers approached the planet. From far away, Voyager’s pictures weren’t much better than the best pictures that could be taken from Earth telescopes at the time. But as the cameras got closer, richer and subtler details began to emerge. The Great Red Spot, first seen more than three hundred years earlier, wasn’t just a single storm but was instead revealed to consist of lots of smaller storms swirling within and around the edges of a giant, multilayered, multihued, high-pressure vortex more than three Earths across. Watching Andy’s Voyager approach movies makes it feel as if you are riding along with the spacecraft, watching in wonder and perhaps a little fear as the imposing cyclones loom larger and larger. . . .
Voyager’s closest-approach imaging of Jupiter’s clouds provided more visual delights. Waves, swirls, spirals, and streaks waft across the photos like the mad flicks of Van Gogh’s brush, painting a cosmic canvas scene like none before. Clouds that Voyager scientists later found to be made of ammonia, methane, hydrogen sulfide, phosphine, and even plain old water vapor dance upon a palette of reds, browns, yellows, and whites—swirling gracefully but at speeds of more than 200 miles per hour. The pressure and the turbulence would certainly shake to pieces any modern jetliner trying to cruise above those storms. It’s a landscape that elicits awe and wonder.
Since the Voyagers flew past in 1979, three more robotic space probes have visited Jupiter. The Galileo orbiter arrived in 1995 and, despite a stuck radio antenna that severely limited the amount of data that could be sent back to Earth, it successfully explored the
system until it nearly ran out of maneuvering fuel and was commanded to plunge into the clouds of Jupiter in 2003 (in NASA parlance, the spacecraft was “disposed of” within Jupiter, to avoid an accidental crash with and possible contamination of the potentially life-bearing ocean on Europa). As it descended deeper into the giant planet’s endless crushing pressures, Galileo was eventually completely vaporized (atoms and molecules from our planet, and our handiwork, are now freely floating through those beautifully colored, gracefully swirling clouds). Since then, the Cassini mission flew past Jupiter in late 2000–early 2001, getting a gravity-assist kick on its way to Saturn; and the New Horizons mission flew past Jupiter in 2007, also getting a gravity-assist kick from the giant planet, helping to propel that spacecraft to higher speeds for a quicker flight time to Pluto, which it will fly past in 2015. Both the Cassini and New Horizons flyby missions took photos and other measurements of Jupiter and its moons, in a sort of modern redo of the Voyager flybys, but with more high-tech instruments and data-storage capability. More recently, NASA’s Juno mission was launched in 2011 and is en route to a 2016 encounter with Jupiter. Once there, it will spend an Earth year orbiting the giant planet and studying its magnetic field, radiation environment, and gravity, building on the earlier Voyager and Galileo observations by providing new clues about the planet’s deep interior and core.
Even very recently, there’s been new excitement in the Jovian system: plumes of water vapor have been discovered emerging from the south pole of Europa. Over the past few years, astronomers have been using the Hubble Space Telescope to make a sensitive search (much more sensitive than had been possible with Voyager’s technology and trajectory) for water vapor near Europa. Several “puffs” of water vapor were seen in the Hubble data, coming and going in time with the gentle tidal stretching (puffs seen) and contracting (no puffs seen) of Europa’s cracked surface ice.
