by Jim Bell
I wasn’t really doing any of the important work. Rather, I was a gofer (go for this, go for that) for Ed or other team members who needed copies made, a print retrieved from the image-processing lab, a cup of coffee, or a pizza delivery. I tried to avoid being tossed out for bumping into something, or even for breathing too loudly, or too much. I’m sure I had a dopey grin on my face most of the time.
We all knew that Uranus didn’t have strong, high-contrast cloud bands like Jupiter, or bright rings like Saturn, but I think many of us ended up being quite surprised at just how bland the atmosphere turned out to be, even at high resolution, when Voyager 2 sped past. The blue-green color was strikingly different, to be sure—caused by methane in the upper atmosphere that absorbs mostly red light from the sun and reflects the rest. Weak, broad bands of slightly brighter or darker tones could be seen at some latitudes, and some occasional small white clouds—almost like smeared-out thunderheads—would pop into view and zip past now and then. But this giant planet was definitely a different flavor, a different beast, from the ones we had seen before.
Ed Stone and his “fields and particles” colleagues were delighted (though, he confessed, “not surprised”) to discover that Uranus does indeed have a strong magnetic field, and were even more delighted when that magnetic field turned out to be pretty weird. The fields that Voyager had found around Jupiter and Saturn were much stronger, and sort of what the science team had expected. Deep within the interiors of those giant planets are mega-Earth-sized cores of hydrogen compressed to such high pressures and temperatures that the gas acts more like a metal, easily conducting electricity. We know from our own planet’s interior that spinning, electrically conducting cores of planets (such as Earth’s partially molten iron core) can generate a magnetic field. And the shape of that field is sort of like the shape that iron filings take when they are exposed to a regular bar magnet—the field lines orient along north-south magnetic “poles” that come close to lining up with the north-south spin axis of the planet. That’s why a compass can tell you which way is north.
At Jupiter and Saturn, the magnetic fields looked basically as expected—like the fields that would come from giant bar magnets placed at their centers. The field lines get warped and bent and “swept back” by the solar wind, streaming out almost cometlike into a long magnetic “tail” (called a magnetotail) that points away from the sun. In fact, Jupiter’s magnetic field and magnetotail are so enormous that if we could see it with our naked eye it would be five times larger than the full moon in our night sky—making it the largest single structure in the solar system except for the sun’s own magnetic field. Saturn’s field is not as large but is similarly impressive. By comparison, Voyager measurements showed that the magnetic field of Uranus is quite different, probably because of the crazy tilted way the planet spins. It’s almost like there’s a giant bar magnet down below the clouds—meaning that there is a rapidly spinning, conducting material of some kind—but Uranus’s is not located in the center of the planet. Instead, it appears to be offset to one side about one-third of the way to the cloud tops. And unlike Earth’s, the magnetic-field axis is not even close to being lined up with the planet’s edge-on spin axis; instead, it’s tilted at an angle of about 60 degrees from the orientation of the planet’s spin. Your compass won’t do you much good there.
“The combination of the planet’s spin axis tilt, and the offset, tilted nature of the magnetic field, combined to make the angle of the field relative to the solar wind really not much different than all the other magnetic fields that we’ve encountered,” says Ed Stone. “But what was different was that, unlike the others, the tail end of the Uranian magnetic field is a spiral, because it’s being wound up by the planet’s tilted-over spin.” The solar wind warps and sweeps this bizarre and unexpected structure downwind in a unique way.
One of the possible implications of the planet’s strange magnetic field is that the inner core of Uranus, which Voyager gravity data showed is rocky and icy and about the size of the Earth, is not hot and dense enough to be electrically conductive. Instead, the magnetic field might be offset from the center because the electrically conductive layer that is causing the magnetic field might be one of the layers above the core, in the planet’s mantle.
“The electrical current system inside the planet, the circulation of ionized particles, is clearly not a simple, global thing,” offered Ed Stone. “That may well have to do with the way the interior is differentiated.” Voyager results and theoretical models appear to show that the mantle of Uranus is rich in water ice and other kinds of “volatile” molecules that started out as ices. At high enough pressures and temperatures, many of these ices, especially if they have hydrogen in their structure (such as H2O), can become electrically conductive when vaporized and compressed. Some planetary scientists thought that the strange, offset, tilted magnetic field discovered by Voyager observations could be telling us that Uranus is really not like Jupiter and Saturn but is instead a completely different kind of giant planet. But it was hard to know, partly because the physical characteristics of Uranus are so different from the other giant planets.
Tilted on its side, not generating its own internal heat . . . “Clearly, something strange had happened to that planet,” says Heidi Hammel. “It was hard to generalize about how Uranus-like planets could be so different from Jupiter and Saturn, because Uranus itself was just personally so screwed up.”
During the days leading up to Voyager 2’s eventual closest approach, we all witnessed the five large Uranian moons go from mere specks of light to small disks to fully resolved worlds of their own. This was an especially exciting process for me. I hadn’t been through the Jupiter and Saturn flybys in this way; I hadn’t seen those dots slowly revealed as distinct worlds. Rather, like most people, the first time I saw Io or Europa or Titan revealed for what it truly is, was via one of the “greatest hits” close-approach photos that was published in the newspaper or shown on the evening news after each flyby. It was just—bam! Io has volcanoes! Or—bam! Mimas has a giant crater that makes it look like the Death Star! During Voyager 2’s Uranus approach, however, there were no such moments. Rather, the reality of these new worlds came into view slowly, over many weeks, with a grace and air of anticipation that more accurately reflected the gentle gravity assist that we were all actually going through, riding along with the spacecraft.
During those last ten hours, though, as Voyager plunged deeper into the gravity well of the seventh planet, we all experienced plenty of breathtaking moments. One by one the five large icy moons were revealed to us from Voyager’s high-resolution images, and a total of ten new, smaller moons would eventually be discovered lurking in the images. All the large moons are heavily cratered, attesting to their generally ancient ages. Oberon and Umbriel are the most cratered, suggesting that they have changed little during their more than 4-billion-year histories. It was a mystery to us why Umbriel is so dark compared to the other four large moons—indeed, it’s still a mystery today. Perhaps its surface contained a higher fraction of carbon-bearing ices that have been darkened more over time by the constant irradiation from the solar wind. The other moons show more geologic diversity. Fractures of 1 to 3 miles deep as well as cliffs on Titania suggest a past active interior. Similarly large rifts on Ariel, as well as evidence for some sort of icy, perhaps cryovolcanic, flows, suggest past tectonic activity and internal heating on that world. But the most diversity, and the most vexing mysteries, came from Voyager’s high-resolution images of tiny Miranda, the innermost of the large Uranian moons.
Miranda is a small world, only about 300 miles across, comparable in size to Saturn’s moon Enceladus. The expectation for such small worlds is that they are too small to have had active interior heating or large-scale geologic processes—too little heat inside, and what little heat there would have been at the beginning would have dissipated quickly. The reality for both has turned out to be dramatically different
from the expectations. Miranda has been far and away the most geologically active moon in the Uranian system. The surface is a mishmash of heavily cratered, relatively bland terrain adjacent to patchwork patterns of bright and dark curving grooves and ridges that look like strange alien racetracks. One of the patchwork terrains has sharp corners shaped like a giant V or chevron. And scattered around the boundaries of some of these patchy terrains are enormous steep-sided cliffs of ice. In some places, if you were to fall off the edge, you would fall 6 to 10 miles before you’d hit the bottom. The 50,000-foot-tall ice cliffs of Miranda are on my bucket list of the most spectacular places in the solar system that I’d like to go photograph someday.
It was exhilarating watching these pictures coming in along with other members of the Voyager imaging team. Each photo would flash on the screen for a few minutes, and then the next one would replace it. I remember a stunned crowd of planetary geologists sitting around one of the worktables and watching the Miranda flyby image playbacks. A chorus of “Oooh!” “Ahhh!” “Wow!” and “What the heck is going on there?!” It was like watching a fireworks show that just kept getting better and better. People were giddy, and the geologists were deeply puzzled. “These objects are tiny—Miranda is only 1/100,000th the mass of the Earth. Yet this tiny world has giant ridge structures like racetracks curving across its surface,” Voyager imaging team member Larry Soderblom said, clearly recalling being astounded by the diversity of the Uranian satellites.
Mission architect Charley Kohlhase once told me that in the beginning of the Voyager Project some people were worried about whether a lot of the moons that would be revealed would end up being sort of like the Earth’s moon, heavily cratered and not particularly different from one another. “Would it be ‘Once you’ve seen one moon, you’ve seen them all’?” he said they were asking. Happily, though, “that did not happen! And that was one of the great surprises of Voyager. There was no uninteresting moon. They were all interesting—from the volcanoes of Io, the cracks on Europa, the haze on Titan to the Death Star of Mimas. . . .”
Voyager imaging team member Rich Terrile was also both puzzled and delighted by how different the many worlds of the outer solar system turned out to be. “Before Voyager, we were kind of used to seeing a lot of craters, and a lot of ‘boring’ things,” he told me. “Mars was only just starting to get interesting, with some evidence of streambeds and the like coming in from the Viking missions. But we really hadn’t yet had that experience of seeing something for the first time and just immediately having your mind blown by something that flashes up on the screen. That had just not happened before. Voyager just turned the tides on everything. The outer solar system was so different than what we had expected. The joke was, the only thing you can expect from Voyager was to be surprised.”
What did happen on Miranda? How could such dramatically different and bizarre kinds of geologic features coexist in such proximity? It was almost as if Miranda were a giant 3-D jigsaw puzzle that had been taken apart and then put back together, but with a bunch of the pieces twisted around or put back inside out. Indeed, some sort of massive but relatively gentle breakup of that moon, perhaps by a low-velocity giant impact or a tidal encounter with some other large moon, followed by reassembly, seems to be a leading hypothesis for what happened. Maybe Miranda was ripped apart and then poorly sewn back together long ago. Or maybe there are some kinds of geologic processes on small, icy, far outer solar system bodies that we simply do not yet understand. The feeling as Voyager sped on was partly exhilarating—no one ever expected to see such wonders—but also partly wistful and melancholy. The geology is so strange, so unexpected, and the encounter was so short . . . it could easily be many, many decades before we’re able to go back and get a better look.
Still, no one could focus too much on the distant future, for there was science still to be done as the spacecraft glided past its closest approach and then through the shadow of Uranus to study the planet’s atmosphere and rings. The rings had been discovered nine years earlier by a team of planetary astronomers led by the late Jim Elliot of MIT. Jim, his then grad student Edward “Ted” Dunham, and other colleagues were “occultation hunters,” astronomers who could very accurately predict when a planet or moon or asteroid would pass in front of a bright star, allowing them to study the object’s surface or atmosphere by the way that star’s light was blocked, or occulted, as it passed behind. It was a neat trick, but it meant being nimble and flexible, because such occultations occur only rarely, and are visible only from certain very specific places on Earth—and almost never where a telescope has been built. So occultation hunters had to take their telescopes to the event, not the other way around. Jim and Ted and their colleagues built an occultation-chasing system that they could fly on NASA’s Kuiper Airborne Observatory, a modified C-141A jet that flew missions in the stratosphere, above most of our atmosphere’s clouds and water vapor. From there, they could chase occultations over a wide range of the Earth and be guaranteed good weather because the airplane flies above the clouds.
Ted was aiming to do his PhD dissertation research project on the composition of the atmosphere of Uranus by watching events like the March 10, 1977, passage of the bright star SAO 158687 behind the planet. As the starlight passed through the upper atmosphere before slipping completely behind, he’d be able to watch for telltale changes in the color and intensity of the star’s light that would provide clues about the density, temperature, and composition of the gases that the light was passing through. Everything was set up perfectly for the experiment and as Uranus slowly moved closer to the star, they started recording data. At first, they thought that five little blips in the starlight that they saw well before the occultation with the planet were glitches in their setup, or maybe noise from the airplane or other systems. But then, after successfully recording the occultation, they saw the same five little blips just as far away from Uranus on the other side of the planet. It was as if the starlight had been blocked by five narrow rings around the planet. Wait—Uranus has rings! Subsequent observations revealed them to be much darker than Saturn’s rings and confirmed the discovery of four more rings, making this what was then only the second known ring system in the solar system.
Motivated by this Earth-based discovery, Voyager imaging scientists were keen to find out if all the giant planets had ring systems, which led to the specific imaging sequences that would enable Voyager 1 to discover the faint, dark rings of Jupiter in 1979. The best opportunity to study the rings of Uranus would come after Voyager 2 passed the planet and was looking back toward the sun, using the same kind of light-scattering trick that had been used to study the Jovian rings. The planning paid off and Voyager 2’s images and its own stellar occultation data showed not just the nine previously known main rings around Uranus, but two more thick rings and a thin, dark, dusty sheet of ring material filling the dark bands in between (later, Hubble Space Telescope images would lead to the discovery of two more main rings, bringing the total rings around Uranus to thirteen). The Uranian rings are dark as charcoal and likely made of centimeter- to meter-sized blocks of icy, carbon-bearing materials that have been darkened by radiation from the solar wind and from the planet’s magnetic field. Neptune, too, was later discovered to have dark rings like Uranus and Jupiter, further adding to the debate over the young versus old age of these kinds of ring systems, as compared to the brighter, “cleaner” ring system that the Voyagers studied around Saturn. One clue that suggested to Voyager scientists that the Uranian rings are young is the fact that they vary in width and thickness around their circumference, including some places where some of the thinner rings seem to disappear completely. While not definitive, this kind of variability suggests a young, evolving system that has not settled down into the kind of orderly, stable state that would be expected if they were ancient survivors from the formation of Uranus itself.
The year after Voyager flew by Uranus, I graduated from college and was acce
pted to graduate school in the Planetary Geosciences Program at the University of Hawaii in Honolulu. I was very lucky to get into grad school at all. Partly because I spent way too much time doing research rather than homework and studying, my grades at Caltech were awful, and I scored horribly on the physics part of the GRE test, so I’m sure I was a particularly weak applicant on paper. But luckily, I had spent a summer fellowship the year before working with colleagues from the University of Hawaii on some planetary astronomy research at Mauna Kea Observatory on the Big Island, and they thought I might be a bit more useful than my five pages of crappy grades and test scores suggested (the six other grad schools that I applied to didn’t see it that way). Who am I to judge? I now often think to myself as I sit with other professors and read through applications for our own graduate program at Arizona State University. . . .
One of the opportunities that came up while I was in graduate school was a chance to finally try to do something useful scientifically with the Voyager images. NASA announced a program called the Uranus Data Analysis Program, which would enable researchers outside of the Voyager team to compete for funding to do new and different kinds of analyses of the data. I had no idea that grad students weren’t allowed to submit such proposals, so I wrote up my project idea, estimated how much of my time I’d spend on it, put together a budget, and mailed in the proposal to NASA headquarters in Washington. About six months later, to my amazement, and to the surprise and consternation of my advisor and department administrators, the university got a letter back from NASA saying that they’d be happy to fund my research but wanting to know if there was a faculty member who could help to oversee the work. They were chuckling in DC (according to the NASA official who had helped to select my proposal), but they were discombobulated in Honolulu.