The Interstellar Age (26 page)

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 shifted 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.

Voyager 2
flew right through the gap between the two outermost rings but was still impacted by hundreds of tiny dust particles per second for several minutes. Luckily, it emerged unscathed, perhaps because the dusty ring particles are so small—only about 1/100th the width of a human hair. It’s still not known what makes the rings clumpy, though planetary scientists suspect that Neptune’s ring clumps may be getting “shepherded” in their orbits, like some of the
thin rings of Saturn, by little moons that were too small and faint for
Voyager
to see. The rings were eventually named after early astronomers who were instrumental in the initial discovery and characterization of Neptune, including Le Verrier, Galle, and Adams, and the thickest clumps in the outermost Adams ring have been named Liberté, Égalité, and Fraternité, in honor of the fact that it was France that in 1846 took victory in the race to discover Neptune.

I spent the week surrounding the
Voyager
Neptune flyby back in Pasadena, having been granted a magic access badge through Fraser Fanale’s invitation, and maybe also owing to the fact that enough of the team knew and remembered me from the Uranus flyby that they figured I wouldn’t cause too much trouble. And this time, as a graduate student in the field, I might even be useful. Upon setting foot once again in those rooms where, brand-new, the images were streaming in from the farthest reaches of our solar system, I happily filled the role of gofer and errand boy for the real members of the imaging team. I was dutifully taking lunch orders, making copies, and otherwise trying to stay out of the way but still soak it all up, as if through some sort of academic osmosis. This would be
Voyager
’s last stop on the way to the stars.

THE NON-PLANETARY SOCIETY

By Neptune,
Voyager
team members like Rich Terrile and others had become very good at hunting for small new moons or strange patterns in the rings around giant planets, with the team having so far discovered twenty moons around Jupiter, Saturn, and Uranus,
and thus having increased the number of then-known moons in
the entire solar system by nearly 70 percent.

“That was a lot of fun,” recalls Rich. “As an astronomer, you’re trained in trying to pull some useful signal out of noisy data, and hunting for new small moons was exactly that kind of problem.” One of his
Voyager
team colleagues challenged him at one point, stating flatly that “it’s no big deal to find these things; they’re just in the data.” So Rich challenged that person back, giving them some images with recently found moons to look through as a test—which they promptly failed. The reason why imaging team members like Rich Terrile could spot these subtle little moons so quickly was because they’d had years and years of experience studying all the quirks and characteristics of stars, cosmic-ray hits, and camera artifacts in the
Voyager
images, and they could tell when a new little dot in an image wasn’t any of those—especially if it was moving from image to image.

True to form, as the final pre-flyby
Voyager
approach images were streaming in, Rich and other team members were busily starting the process of discovering six small new moons of Neptune that hadn’t been seen or confirmed from previous telescopic observations. Candy Hansen recalls the lighthearted teasing that Rich got from the team about his prowess for discovering new moons or features in
Voyager
images. “Like the time we told Rich that he couldn’t come drinking with us at the Loch Ness Monster [a bar nearby the lab] until he discovered something,” she recalled, laughing. “And then he showed up at that seedy, seedy bar with a hard copy of a
Voyager
photo showing an elliptical ring!” Rich earned his drink that night, and many others.

“That really happened,” Rich confirms. “I was perfectly OK to
stay at work and find more things, though. I was on a roll!” Combined with the giant moon Triton (about the size of Europa or our own moon), and the smaller, elliptically orbiting moon Nereid, these new moons took Neptune’s then-known moon count up to eight. Since
Voyager,
six more smaller, fainter moons have been discovered by more sensitive ground-based and space-based telescopes.

If the moons of Neptune were to be the last planetary hurrah for
Voyager
, it would head for the stars with fanfare. After swinging just 3,000 miles above the cloud tops of Neptune (closer than to any planet that the spacecraft visited since leaving Earth),
Voyager 2
’s trajectory would take it past one final, glorious, unknown destination: Triton. The moon was discovered shortly after Neptune itself in 1846 (because it is so big and so bright), and
Voyager
imaging team members figured it would be an oddball of some kind because of its unusual, backward orbit. The spacecraft would fly within 25,000 miles of Triton, taking pictures of surface features as small as 5 to 10 miles across, and no one really knew what to expect. Because it is so bright and so far from the sun, Triton’s is among the coldest natural surfaces in the solar system, with an average temperature only about 38 degrees above absolute zero (or an incomprehensible –391°F). Triton’s brightness suggested that there would be relatively clean ice on the surface, perhaps even including exotic, low-temperature ices other than water ice. And its strange backward orbit suggested that it may have been through some sort of planetary-scale trauma, such as being captured by Neptune, or had its course changed by some sort of giant impact. It was a great way to end the surface-imaging phase of a great mission—with an encounter that would be surprising no matter what was revealed.

Last Port of Call.
Voyager 2
flyby trajectory past Neptune.
(NASA/JPL)

About five hours after closest approach to Neptune,
Voyager 2
flew past Triton. Several days later, I remember being in the JPL workroom with imaging team member Larry Soderblom, looking over the first high-resolution images of Triton that had come in. Larry is a friendly, outgoing, sometimes mischievous, and highly respected member of the planetary science community who works at the US Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona (I’ve always been confused about the name of that group, as
astrogeology
technically means “the geology of stars,” but stars don’t have geology—planets and moons and asteroids and comets do. But
then again, “astronauts” don’t travel to the stars . . . at least, not yet . . .). Larry was one of the
Voyager
imaging team members who would occasionally give me a nod and a wink and beckon me away from some dark, out-of-the-way corner to come sit at the big table and look at some images. How could I resist?