by Jim Bell
“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).
Indeed, just in the last few years, support for a NASA mission back to the Jupiter system that would focus on Europa (and that would complement the Europeans’ JUICE mission to Ganymede) has grown substantially. This support comes partly from the scientific community, which ranked a return mission to Europa as an extremely high priority for NASA in the most recent National Academies of Sciences “Decadal Survey of Planetary Science”; partly from extraordinary public and media interest in astrobiology and the search for life elsewhere in the universe; and partly (and unexpectedly) from the individual, personal interest of US congressman John Culberson, who represents Texas’s Seventh District in the suburbs west of Houston. For reasons that I can describe only as remarkable and supremely fortunate for my line of work, Mr. Culberson just loves Europa. He is fascinated by the possibility of life on this ocean world. I’ve visited him in his office in Washington (he is perhaps the only person out of 535 members of Congress with photos of Europa on his office walls!) and, along with such Voyager team members as Candy Hansen (who recently served a term as the chair of the American Astronomical Society’s Division for Planetary Sciences), have helped keep him updated on the latest findings about Europa from NASA’s missions and telescopes. He is an educated and engaged advocate for space exploration—a rare and delightful occurrence in Congress, to be sure. But perhaps most important, he is also a high-ranking member of the US House of Representatives Committee on Science, and so he takes it as a personal goal and passion to try to convince his colleagues in Congress to authorize funds for NASA to go back to Europa and find out what it’s really like. Mr. Culberson has been successful in the past few years in getting some funds allocated for the development of new technologies needed to operate spacecraft for long periods of time in Jupiter’s high-radiation environment. Sometimes, even in rocket science, a personal touch makes all the difference.
5
Drama within the Rings
EVEN WITH TWO spectacular flybys of Jupiter in the bag, and a trove of planetary science discoveries and puzzles opened in the process, the Voyager team didn’t feel like they had much time to rest and reflect on their good fortune. Both probes were speeding on to encounters with the giant ringed planet Saturn, with Voyager 1’s Saturn flyby set to occur only sixteen months after Voyager 2’s at Jupiter. That might seem like a lot of time, but every working hour was spent on the planning that needed to be done to optimize the trajectories of the spacecraft past the planet’s moons and rings.
I remember the first time I saw Saturn through a real astronomical telescope. I must have been around ten years old, and I think it was during a Boy Scout field trip to a small rural observatory run by a local amateur astronomy club (I think it was the SkyScrapers, the club I would later join). It was a clunky old refractor—the kind of long telescope that uses lenses instead of the more modern shorter kind that uses mirrors—with a tube like an iron water main and giant counterweights and rivets that could have come right out of a 1930s WPA construction project. Still, it was a real telescope, capable of large magnification and good image quality, especially from a rural site on a clear summer night. The telescope operator was hopping around the sky, manually slewing the tube to focus on many of the greatest hits (which he knew by heart) for our viewing pleasure. Double stars, nebulae, star clusters, a faint, fuzzy comet . . . Each of us had a few seconds of viewing time, perched up on a step stool so we could reach the eyepiece. When it was my turn to see Saturn, highly magnified, in steady skies, with rings tilted gloriously in view, I remember going into an almost trancelike state, hypnotized by the elegance of what I was seeing. The next kid in line started pushing me to step away so he could have his turn, which I did only reluctantly.
What I didn’t realize at the time was that professional astronomers in the 1970s weren’t really that far ahead of the amateurs in terms of knowing what Saturn was really like. It’s funny how we always think that the experts are leagues beyond the rest of us, members of a secret club that we can never enter. Knowledge of the chemistry of Saturn’s atmosphere at the time was based on some good measurements from big telescopes, some assumed similarities with the chemistry of Jupiter, and the assumption that this gas giant, too, was likely mostly made of hydrogen and helium, like the sun (after all, it formed from the same cloud of gas and dust that formed the sun itself). Even less was known about the planet’s famous rings and its collection of ten then-known moons. It was understood that the rings could not be a solid ring of material (the British physicist James Clerk Maxwell showed in the 1850s that such a ring would break apart from the stresses of the inner and outer parts orbiting at different speeds). It was also known that some of the moons were quite large, perhaps even planet-sized bodies. The first hints of the icy composition of these worlds were only just starting to come in from the latest high-tech spectrometers mounted on large Earth-based telescopes.
While the Pioneer 11 flyby of the Saturn system in September 1979 helped us understand the gravity and magnetic fields of Saturn in much more detail, even that flyby encounter, with its limited imaging capabilities, didn’t dramatically increase our knowledge of what Saturn and its rings and moons looked like in detail. Besides showing that Saturn’s largest moon, Titan, is an extremely cold world—at only maybe 100 degrees or so above absolute zero, probably too cold for life as we know it—a key facet of Pioneer 11’s trip through the Saturn system was to simply demonstrate that it could be done.
“Pioneer 11 was targeted to fly through Saturn’s ring plane near where Voyager 2 had to fly through the ring plane to go on to Uranus,” says Ed Stone. Pioneer was only about a year or so ahead of the Voyagers, and so Ed and Charley Kohlhase and the rest of the Voyager team were paying careful attention to Pioneer’s mission, which was run by colleagues from NASA’s Ames Research Center, just south of San Francisco. Among the questions scientists had for Pioneer: Could a spacecraft pass unscathed through the plane of Saturn’s rings, not that far from the dense rings themselves? Would there be unanticipated radiation or magnetic field effects close to Saturn that could be more dangerous than what was encountered at Jupiter?
From below and beyond the rings, Pioneer 11’s farewell pictures of Saturn foretold of the even more spectacular sights and perspectives to come from the Voyagers that would follow. Pioneer had passed through the ring plane of Saturn without incident—but perhaps only just barely. Later analysis of the Pioneer 11 trajectory showed that the spacecraft may have just missed (by about
2,500 miles—a near miss in astronomy) crashing into a small moon only later discovered orbiting near Saturn’s rings. The experience caused some concern for Voyager 2, but mission planners were not particularly concerned.
“We thought, So it passed within 2,500 miles of something. So what?” Charley Kohlhase says. “Space is big!”
Saturn Swingbys. Voyager 1 (top) and Voyager 2 (bottom) flyby trajectories past Saturn. (NASA/JPL)
Regardless, because of the importance of the Titan flyby, Voyager 1 had to be targeted to cross the plane of Saturn’s rings much farther from the planet itself than Pioneer had. Voyager 2, however, would have to take a deeper plunge through the ring plane—closer-in to the planet—if it was going to use Saturn’s gravity to slingshot ahead to Uranus and Neptune. Just like at Jupiter, Voyager mission planners tried to time each spacecraft’s two-day trip through the heart of the Saturn system to get as close to the planet, rings, and as many moons as possible, albeit with some important constraints.
An important constraint was imposed by Ed Stone and the Voyager science team: the trajectory had to take the spacecraft behind and into the shadow of the planet as seen from both the Earth and the sun, so that both sunlight traveling to the spacecraft, and the spacecraft’s radio signal traveling to Earth, would pass through Saturn’s upper atmosphere. Such an event is called an occultation, because the planet blocks or occults (obscures) the sun (or the Earth) from the viewpoint of the observer, which in this case is the Voyager spacecraft. An eclipse is a kind of occultation. Occultations of sunlight passing through a planetary atmosphere (which Voyager could observe) or Voyager’s radio signals passing through a planetary atmosphere (which the DSN could observe from Earth) provide a way for scientists to probe the details of the pressure, temperature, and chemistry of gases in that planet’s atmosphere. As sunlight passes through the upper atmosphere, it is absorbed by gases deeper and deeper in the atmosphere until it is blocked entirely by the increasing density of the gas (for a giant planet), or by the surface itself (for a moon or terrestrial planet). Instruments on Voyager could measure the patterns of that absorbed sunlight and use those patterns as fingerprinting tools for the identification of specific atoms and molecules. The same is true of the DSN antennas watching the pattern of Voyager’s radio signal slowly fade from view as it went behind the planet from the Earth’s perspective. It’s a powerful scientific trick to exploit and so it couldn’t be passed up when designing the optimum orbit trajectory. I can imagine that it was just another source of headaches sometimes for Charley Kohlhase and his mission-design team, however.
Even more important, perhaps, was the flyby of the moon Titan. What little information there was about Titan up until that point seemed to suggest that its environment could be similar to that of the early Earth. The flyby was a possible way to get in touch with our primordial past! That called for close-up imaging and other measurements of Titan, including a pass behind the moon as seen from the Earth and the sun. This requirement would almost single-handedly define the eventual trajectory and fate of Voyager 1 past Saturn. Moreover, the possibility of Voyager 2 continuing on to Uranus and Neptune would depend entirely on whether Voyager 1’s Titan encounter was successful.
Prior to the space age, Titan had been discovered to have an atmosphere (the only moon known to have one), consisting of at least methane and perhaps some other complex hydrocarbons. Pioneer 11’s low-resolution flyby images showed just an orangey sphere, bland but suggestive that the atmosphere was likely thick and hazy. Despite having extremely low temperatures, the evidence that Titan was a model of what the early Earth’s atmosphere may have been like was significant. Before life began adding oxygen to our planet’s atmosphere, Earth’s atmosphere was also rich in hydrocarbon (and nitrogen), what chemists call a reducing environment (as opposed to an oxidizing environment).
In some pioneering experiments in the 1950s, biochemists Stanley Miller and Harold Urey did a famous set of experiments to demonstrate how adding water and energy (like lightning) to a reducing environment containing simple hydrocarbon gases could lead to the formation of even more complex organic molecules, including some simple amino acids. Biologists, and now astrobiologists as well, believe that this kind of chemistry could have led to the formation of life on Earth. Voyager scientists wondered if it could have led to similar kinds of biogeochemical magic on Titan. Maybe the oldest single-celled organisms in the solar system were swimming around in Titan’s primordial soup.
So Voyager simply had to encounter this special, one-of-a-kind, Mercury-sized moon called Titan, up close, to measure its atmospheric pressure, temperature, and chemistry in more detail and to try to glimpse its frigid surface. Indeed, based partly on the Pioneer 11 results from a year earlier, Voyager 1 was targeted for a very close pass just 4,000 miles—or a little over one Titan diameter—above the haze. To help increase the odds of a successful flyby, Charley Kohlhase and the other mission designers also timed the Titan flyby to occur before Voyager 1 passed through the plane of Saturn’s rings and its closest approach to the planet. This “Titan before” approach also had the benefit of bending Voyager 1’s trajectory away from a potentially more risky dive deeper through the heart of Saturn’s rings while still allowing encounters with many of the planet’s other large moons. It was a hopefully safe course through poorly charted territory, focused on the prize: Titan.
The flyby went well. Titan’s size and mass were measured directly, leading to an estimate of its density that implies that Titan is a world of rock and ice, like Europa and Ganymede, rather than ice alone. Atmospheric measurements found that Titan’s air is indeed highly reducing—mostly made up of nitrogen, but also with significant amounts of hydrocarbon gases like methane, ethane, acetylene, and ethylene, as well as hydrogen cyanide. Many of these same kinds of gases may have dominated the early reducing atmosphere of our own planet. The surface temperature was found to be only around 90 degrees above absolute zero, but when combined with the high surface pressure found by Voyager 1—about 50 percent thicker than Earth’s atmosphere—this led to one of the most surprising discoveries of the encounter: at those pressures and temperatures, methane, ethane, and many other hydrocarbons found on Titan can be gaseous, liquid, or solid. The ubiquitous haze covering Titan is thus likely due to thick, exotic clouds made out of methane and ethane!
And this is what led to the one and only disappointing part of Voyager 1’s encounter with the solar system’s second-largest moon. That thick haze completely obscured the surface from view, covering up what many scientists believed could be spectacular vistas of rivers, lakes, even oceans of liquid hydrocarbons on the surface below. River valleys carved by liquid ethane! Waterfalls of methane! What sights were hidden from view by that blasted haze?! Without the capability to look through the clouds, for example by using radar like the meteorologists do on the evening weather report, Voyager’s cameras were blind to the surface itself.
If the spacecraft didn’t have to aim for such a close Titan encounter, Voyager 1 could have used a gravity assist to continue to travel on to Uranus and Neptune (as Voyager 2 did). Voyager 1 gave up a lot for that close flyby, and the fact is that Voyager 2 might have skipped Uranus and Neptune if anyone had thought it could get a better look at Titan. That is how much emphasis had been put on finding out what Titan was really like.
I asked mission architect Charley Kohlhase what he thought of the official Voyager Project policy of giving up on Voyager 2’s Grand Tour if the Voyager 1 Titan flyby had not been successful. He replied in a millisecond: “I hated it.” There was a lot of unofficial support for the Grand Tour option (which helped Charley and the other mission designers keep that possibility on the table as they were developing their Voyager 2 Jupiter and Saturn flyby scenarios), but also a lot of scientific interest in Titan as a potential model for the early Earth. Both Charley and I agreed that it’s impossible to know which way that decision would have gone.
Ed Stone is more certain: “If Voyager 1 had not worked, Voyager 2 would have gone the same way, as a Jupiter-Saturn-Titan mission.” Interestingly, an opportunity had been identified relatively early in Voyager’s mission planning to use the Saturn swingby to propel Voyager 1 to a later encounter with Pluto—at the time the solar system’s most distant known planet. However, the need for the close pass by Titan took that option off the table. Fortunately, Pluto still garners significant public and scientific interest, and so even though it has since been officially demoted from planethood, it was still judged worthy of a flyby mission of its own, and so the New Frontiers probe will give us, finally, a first glimpse of that distant former planet in the summer of 2015.
ICE VOLCANOES
Titan was just the first step in Voyager 1’s path of discovery through the Saturn system. The team planned and acquired the first high-resolution images and other measurements of all of Saturn’s other large moons—Tethys, Mimas, Enceladus, Dione, Rhea, and Iapetus—as well as the first detailed images of Saturn’s famous rings, viewed on approach from above, then from edge-on, and then from below and behind. Before Voyager, the rings were thought to consist of just three major sections (imaginatively dubbed A, B, and C, and so on, from outermost to innermost), with empty gaps between them. But all that changed after the Voyagers passed by. High school students in the ’70s could study posters of Saturn and its rings (mine were made from Voyager images by the staff at the former Hansen Planetarium in Salt Lake City) and gaze at the thousands and thousands of rings that orbit Saturn. Each of the bright segments of the rings seen from Earth can be broken down into smaller and thinner rings—some of them like finely braided strands of hair; some of them, as Rich Terrile would discover, with strange radial “spokes” apparently embedded within them; and some significantly oval-shaped, or eccentric, rather than perfectly circular shapes, seen in more and more detail as the resolution of the images improved. The way the spacecraft’s radio signal blinked on and off as it passed through the rings was used to discover that each ring is made up of countless individual blocks of ice, ranging from dust-sized to the size of a house, all orbiting in lockstep with their neighbors. What kept them marching in such orderly fashion?