by Jim Bell
“We had a huge number of antennas online,” Charley Kohlhase recalls, “and that allowed us to get respectable data rates—tens of kilobits per second!” While still much slower than good old dial-up computer modem speeds, this was not a bad data rate for an interplanetary Internet connection that had to span a distance of 2.7 billion miles. Voyager science sequence coordinator Randii Wessen, thinking back to the impressive collection of nearly thirty giant radio telescopes from the West Coast of the United States to Australia to Japan that were all linked together or “arrayed” to pick up the spacecraft’s faint signal at Neptune, told me, “We joked that we were listening to Voyager 2 with the entire Pacific Basin!”
There was also extensive speculation among planetary scientists about what Voyager would find at Neptune. Sunlight at Neptune is a mere 3 percent as intense as sunlight at Jupiter, so some colleagues figured that Neptune would have a relatively bland and featureless atmosphere, like Uranus, because of the lack of solar energy to power the kinds of intense storms seen in the gas giants closer to the sun. Others speculated that Neptune could have substantial reserves of internal energy, like Jupiter and Saturn, which could power significant atmospheric activity. If the pattern held, Neptune would have a powerful magnetic field, and the field would be more like Jupiter’s and Saturn’s because Neptune’s spin axis is also hardly tilted at all. The radiation from that magnetic field, along with the solar wind, would probably darken the rings (rings that were discovered earlier in the decade from ground-based telescopic occultations, the same method that had led to the discovery of the Uranian rings) and any small satellites in the vicinity of the planet. Many of the predictions were fairly safe to make, as they reflected a newfound understanding—made possible by the Voyagers—of some of the basic properties and processes that occur on, in, and around gas giant planets.
More speculative, however, were the predictions of what Neptune’s large moon Triton (not to be confused with Saturn’s moon Titan . . .) would be like. Triton, which was discovered in 1846, just a few weeks after Neptune itself was discovered, is an oddball because it is one of the largest moons in the solar system (a fact known well before the Voyager flyby) and it orbits Neptune backward relative to Neptune’s spin direction, and at about a 25-degree tilt relative to Neptune’s equator. Triton is the only large, planet-sized moon in the solar system that orbits its planet backward relative to the spin direction of its parent. Partly because of this, astronomers believe that Triton was formed elsewhere and was then somehow “captured” by Neptune, as no one’s computer model or thought experiment can come up with a good way to explain how it could have formed in a backward, tilted orbit. But where would it have been formed? How did it get captured, and why backward? Would it be anything like any of the other big icy moons seen previously by Voyager? Fortunately, since Triton would be Voyager 2’s last port of call on its Grand Tour, no one had to worry about exactly where the spacecraft needed to be “next.” So mission planners could target Voyager’s aim point very close to and through the shadow of Neptune (passing high enough above the planet’s north polar atmosphere to avoid any potential drag on the spacecraft or electrical arcing from friction with upper-atmosphere ions), so that the planet’s gravity would divert the spacecraft toward a close flyby of enigmatic Triton about five hours later. Charley Kohlhase has estimated that the accuracy needed to deliver Voyager 2 to its aim point above Neptune to within about 62 miles after traveling on an arc more than 4.4 billion miles long is roughly equivalent to a golfer sinking a putt from Washington, DC, to Phoenix, Arizona. Although he did acknowledge that a few “fine adjustments” (a.k.a. cheats) might be needed along the way. Amazingly, after the flyby, the actual navigation errors were found to be roughly ten times smaller than the requirement!
Voyager science team members Candy Hansen and Torrence Johnson recall some particularly anxious moments as they were trying to plan the photographs of Triton. No one had ever seen the place up close, of course, and so there was significant uncertainty about how best to take the photos. From Earth-based telescopic observations, Triton was just a point of light near Neptune. “By Saturn we felt more comfortable making exposure-time estimates from ground-based observations, but the scariest was at Neptune with Triton,” recalled Johnson. It reflected a certain overall amount of sunlight that could be very accurately measured, but was it as bright as observed because it was very reflective (maybe icy) and small, or less reflective (like rock or soot) and large? Team members took some images of Triton with the Voyager cameras during the cruise from Uranus to Neptune to try to get more information. It was still just a point of light, but by photographing it from different angles compared to Earth-based telescopes, they hoped to gain some insight on what to choose for the exposure times. This kind of building up a set of images from different angles is part of characterizing what is esoterically called the phase function of a planet or moon or asteroid, and it is a common way for astronomers to obtain information remotely on what kind of materials—for example, icy or rocky or metallic—a surface is made of.
Even while still very far away, as Voyager 2 approached the Neptune system, the phase function data were suggesting that Triton was smaller and brighter (with a very reflective, more icy, surface) than their initial estimates. “We switched to shorter exposures on this basis, just weeks before encounter,” Johnson said. “We were changing exposure times in every iteration of the sequence,” Candy Hansen added, “until finally the sequence was on the spacecraft and we just couldn’t change it anymore. Fortunately by then we had nailed it!” Time to bake more cookies!
The summer months leading up to the Voyager 2 flyby of Neptune in 1989 were filled with all kinds of anticipation and excitement as the planet slowly grew in the images from a point of light to a resolvable orb. I was following along remotely from Honolulu for most of the approach, using my new e-mail account to receive occasional images from Ed Danielson or other colleagues on the imaging team. Neptune was clearly going to be different from Uranus. First of all, the color of the planet was different—Neptune’s strikingly blue color was apparent even from images taken months before the flyby. At Uranus, the planet’s aquamarine color was a telltale sign of the presence of methane, which absorbs red light scattered through the atmosphere. In advance of the flyby, it was theorized that Neptune’s azure hue was also due to methane, but at a colder temperature and higher pressure than at Uranus, causing it to absorb not just the red colors in the sunlight but some of the green colors as well. Voyager’s instruments would eventually provide flyby data that would prove this to be the case, but in the meantime, we were all just marveling at the fact that our solar system had a second blue planet.
And it wasn’t just the color of Neptune that was causing surprise and delight. Folks like Andy Ingersoll and others on the imaging team had planned another approach movie for this flyby—a series of snapshots taken at least every day, and sometimes many times per day, during the long approach to the planet. Even from more than 60 million miles away in June of 1989, more than two months before the flyby, cloud features could be clearly seen and tracked in Neptune’s atmosphere. Darker and lighter bands could be seen at different latitudes on the disk of the planet—not as colorful and artistic as the belts and zones of Saturn or, especially, Jupiter, but more dramatic than the relatively bland atmosphere of Uranus. Candy Hansen and brand-new Voyager imaging team member Heidi Hammel spent a lot of late-night time doing science at the JPL browse workstations. They were among the first to notice a large, dark, oval-shaped feature in Neptune’s southern hemisphere. It had a similar shape to Jupiter’s Great Red Spot, and so it was quickly dubbed the Great Dark Spot.
Soon, small white features began to be resolved in the images, spinning around the edges of the Great Dark Spot. Those features eventually came into view as clouds, cementing the idea of the Great Dark Spot being a giant (Earth-sized!) storm system, probably much like Jupiter’s Great Red Spot. As the spacecraft got even
closer, smaller white clouds and other dark spots came into view, and calculations by Heidi and Candy and others on the team showed that some of them appeared to scoot around the planet at much faster speeds than the Great Dark Spot—indeed, Neptune’s features have some of the fastest wind speeds ever clocked in the solar system (up to 1,300 miles per hour!). There was clearly a lot going on.
“Every day when we came in, it was like the veil of mystery around Neptune was getting thinner and thinner and thinner,” Heidi says, reflecting back on that pre-encounter approach phase when Neptune went from the small, fuzzy blob that she knew as a telescopic observer to the richly detailed world that Voyager observations would reveal it to be. “It was just incredible,” she mused. “Every time a new series of images would come down, there was sort of a gasp, and a moment of excitation, and we would say, ‘Oh wow—that’s amazing. . . .’ The giant fuzz blobs that I’d been tracking from Mauna Kea turned out to be just one feature out of many features.” In what she amusingly describes as her “great tragedy of the Neptune encounter,” she recalled how she was sent back to Mauna Kea to take photos of Neptune at exactly the same time as Voyager during the closest approach. “‘We’re going to need ground truth,’” she recalls Brad Smith telling her, “so we know how to compare our spacecraft data with previous and future ground-based data.” She remembers saying, “Yeah, that makes a lot of sense, Brad,” to which he replied, “And you’re the world’s expert. . . .” So she spent the entire flyby on the summit of Mauna Kea, hungrily devouring fax messages from imaging team colleague Andy Ingersoll telling her what they were seeing in the Voyager images back at JPL. “I missed the whole thing,” she lamented. “But I guess that’s what you do for science.”
While dazzling, the higher level of atmospheric activity on Neptune compared to Uranus was also extremely puzzling. Neptune gets 40 percent less solar heating than Uranus, and so if sunlight is what is driving the energy of these giant-planet atmospheres, then Neptune should be even less active than the relatively bland Uranus. Indeed, the pattern observed by Voyager of the decreasing numbers of clouds, belts, and storms as the mission traveled from Jupiter, out to Saturn, then farther out to Uranus, was consistent with this idea of solar energy mostly powering the weather on these worlds. But Neptune proved that simple explanation wrong, dramatically.
One of the things that Voyager was able to measure during the flybys was the total amount of heat energy coming out of each giant planet. If they were in balance with the heat energy provided by the sun, then the total amount of solar energy going in would equal the total amount of thermal (heat) energy coming out. This kind of balance is a fundamental feature of the terrestrial planets Venus, Earth, and Mars, where the temperatures of the surfaces and atmospheres of those worlds are very different but, ultimately, driven by solar heating. At Jupiter, however, Voyager measured almost twice as much heat energy coming out of the planet compared to the solar energy going in. The same was true at Saturn. Clearly, for those giant planets, there must be additional internal heat sources that contribute to the energy in their atmospheres. Planetary scientists speculate that the extra heat could be the result of the enormous amount of gravitational energy stored in the high-pressure, high-temperature deep interiors of these planets, or possibly from heat released by the decay of radioactive elements within the rocky cores thought to exist deep inside them, or even from heat released during chemical reactions within each planet as materials change from one phase (such as ice) to another (such as vapor) with rising pressure and temperature.
At Uranus, however, the amount of heat energy coming out was essentially equal to the amount of solar energy going in. This makes Uranus fundamentally different from Jupiter and Saturn. Aha! figured the science community: smaller gas giant planets don’t have internal heat sources like the much larger Jupiter and Saturn. But then, Voyager 2 measured three times as much internal heat energy coming out of Neptune compared to the solar energy going in. Another monkey wrench thrown into the gears of understanding—to the delight of many of my colleagues. New discoveries lurk within the details of the unexpected.
Just like for Uranus, telescopic observations from the ground and from space since the Voyager 2 flyby have revealed major changes in the atmosphere of Neptune over time. First, the Great Dark Spot in the southern hemisphere disappeared. Then a different Great Dark Spot formed in the northern hemisphere, along with a second northern dark spot. White clouds and smaller spots fade in and out, and the belts at different latitudes brighten and darken over time.
“We seem to see a new Great Dark Spot form and then dissipate about every five years, but we don’t know why,” Heidi Hammel says. “But we aren’t able to look very often with high-resolution tools like HST or Keck. We see one, then we don’t, but we don’t know what happens in between. We really need more continuous coverage of the planet’s weather to be able to track the features and figure this place out.” It is a dynamic atmosphere, and still largely mysterious, as we’ve been studying the place at high resolution for only a small fraction of Neptune’s 165-Earth-year trip around the sun (indeed, we’ve only known about the place at all for just one Neptune year). And even today, the reasons for Neptune’s strong internal heating, and for the lack of strong internal heating on Uranus, are not clearly understood.
Voyager’s measurements of the overall chemistry and interior structure of Uranus and Neptune have led to a transformation in the way we view these giant planets as compared to their larger Jovian-class cousins. Voyager helped us look inside these worlds, revealing that while the outer layers and the visible “surfaces” of all four of the giant planets are made of clouds and gases, deep in their interiors they differ from one another in ways that are not obvious from our vantage point on Earth. As Jupiter and Saturn were forming some 4.5 billion years ago, they captured huge amounts of hydrogen and helium in the cloud of gas and dust (called the solar nebula) from which our sun and the rest of our solar system was then forming. This gaseous envelope surrounds and dominates the (relatively) tiny, Earth-sized, rocky/metal cores of Jupiter and Saturn, meaning that they have essentially the same hydrogen-rich composition as the sun. Deep inside, that hydrogen acts like a metal at super-high pressures and temperatures, conducting electricity and powering those planets’ giant magnetic fields. Jupiter and Saturn are, truly, gas giants.
In contrast, while Uranus and Neptune were forming early in the history of the solar system, there was not as much gas available farther away from the sun. At the colder temperatures of the far outer solar system, a lot more ice was condensing out of the solar nebula than in the warmer regions closer to the sun. The end result appears to have been that the interiors of Uranus and Neptune are made of a relatively larger fraction of vaporized ices (like water ice, methane ice, ammonia ice, and other volatiles) than the interiors of Jupiter and Saturn are. These smaller worlds also each have an Earth-sized rocky/metallic core, but it is surrounded by a deep mantle of high-pressure, high-temperature vaporized ices, which is then surrounded by a relatively thinner, though still hydrogen-rich, gassy atmosphere. Uranus and Neptune aren’t really gas giants, then: they are ice giants, dominated by a larger fraction of initially icy materials than their classical gas-giant cousins Jupiter and Saturn. Voyager observations had revealed an entirely new and unanticipated class of planet.
“I think the idea of ice giants as distinct kinds of planets from gas giants was developed during that year leading up to the Neptune encounter,” Heidi Hammel recalled. Neptune was different. It has a perfectly normal tilt, and an internal heat source like Jupiter and Saturn. “But as Voyager drew closer, it became clear that even though it is a giant, this planet is not like Jupiter or Saturn. It didn’t have the swirling cloud patterns that Jupiter and Saturn did. Even though it had this big Great Dark Spot, the more we saw of it, the less it looked like the Great Red Spot. It wasn’t stable and round, but had a weird oval shape. It had all these bright companion-cloud features that s
hifted all around, sometimes under it, sometimes across it. . . . Even the smaller features were just very different than the small features on Jupiter and Saturn. The way the clouds were forming was entirely different. The closer you got, the more they resolved into tiny spots, like tiny clusters of connected thunderheads.” The idea of Uranus and Neptune as fundamentally different beasts in the planetary zoo was part of an evolving understanding rather than an instant realization.
The prediction that Ed Stone and others had made about Neptune’s magnetic field came true: it is strong and behaves in some ways like the fields around Jupiter and Saturn. But like the field inside Uranus, it is offset relative to the center of the planet, and tilted relative to Neptune’s spin axis. Maybe, then, the strange tilt and offset of the field at Uranus is not a feature of a strangely tilted planet but is instead a feature of any planet with an electrically conductive middle layer or mantle of high-pressure vaporized ices. Maybe all ice giant planets have tilted, offset magnetic fields. Certainly all the ones in our solar system do.
“Uranus and Neptune are not just like Jupiter and Saturn, except blue,” said Heidi Hammel. “The processes going on there are fundamentally different.”
Voyager 2 made other exciting discoveries in the Neptune system. Based on the earlier Earth-based telescopic discovery of at least partial rings around the planet, Voyager imaging team planners were able to design special imaging observations that took advantage of being able to look back toward the sun at the areas where the rings should be, enhancing the ability to see super-fine particles in the rings and to tell if they were complete rings or just partial arcs of material orbiting the planet (a question that was driving the mathematicians crazy, because such structures were predicted to spread out and turn into full rings in only a few years). The imaging worked beautifully and revealed a curiously stunning system of at least five separate rings around Neptune that are complete but clumpy, with the thickest, coarsest clumps corresponding to the ring arcs that had been seen from Earth, and the thinnest parts consisting of dark, fine-grained, very dusty materials that are too faint to have been seen from Earth.