The Interstellar Age (16 page)

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.

“
By far the simplest explanation for this water vapor is that it erupted from plumes on the surface of Europa,” planetary astronomer and Hubble study lead Lorenz Roth wrote in the official NASA press release. Hubble data appear to show that Europa has plumes or jets coming out of at least some of the long, dark cracks in its icy crust, perhaps like those detected by the
Cassini
spacecraft around
Saturn’s icy moon Enceladus. Roth went on to speculate that “if those plumes are connected with the subsurface water ocean we are confident exists under Europa’s crust, then this means that future investigations can directly investigate the chemical makeup of Europa’s potentially habitable environment without drilling through layers of ice. And that is tremendously exciting.”

The
Voyager
flybys of Jupiter in March and July of 1979 provided a massive shift in our understanding of giant planets and their moons. Once thought to be too cold, too far from the sun, to support life as we know it, the Galilean satellites were found to have significant amounts of internal heating, fueled by tidal forces. Europa, Ganymede, and possibly even Callisto appear to have vast reservoirs of subsurface liquid water—oceans—under relatively thin shells of ice. Astrobiologists—scientists who study the origin, evolution, and fate of life on the Earth and potentially other planets—now think about Europa as one of the leading candidates on the short list of worlds beyond Earth where extraterrestrial life may exist, or may have existed long ago (other places on that short list include Mars, and Saturn’s moons Titan and Enceladus).