The Interstellar Age (23 page)

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 accepted 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.

With my wrists duly slapped but the study green-lighted, I went to work on a project to map the different kinds of materials on the five major icy satellites of Uranus using not just the high-resolution black-and-white photos that
Voyager 2
had taken, but also the lower-resolution (more distant) color photos that were taken on approach. In retrospect, it was actually a really dopey and naïve kind of project, because the moons are very gray and show only small and not particularly diagnostic color variations on their surfaces.
Voyager
team members recognized this quickly, which is why (I believe) no one had yet gone off and worked on and published the project that I had proposed to do. Maybe NASA officials and my proposal reviewers realized this too, but since my proposed budget was so small, it was treated as a worthy student training project rather than the path to the next major discovery in planetary science. I had to learn, and in some cases create, image-processing software to work with the images, to do the color analysis, and to make the resulting maps. My results were essentially null—no major color variations
and thus no new clues to the moons’ compositions—but I found somewhere to publish my null result and in the process learned how to write
a peer-reviewed scientific journal article. It felt good to have an official scientific connection to
Voyager
, albeit a weak one.

In the two decades since
Voyager 2
flew by Uranus, we’ve learned a lot more about the place from a variety of ground-based and space-based telescopic observations of the planet and its moons and rings. Perhaps the most striking discovery has been that we were fooled by the
Voyager 2
images into thinking that the planet’s atmosphere is always boring. It turns out that
Voyager
happened to fly by at an anomalously boring time.

“We flew by at the peak of the summer solstice,” Heidi Hammel says, “when the whole polar region was enveloped in haze. You couldn’t see discrete cloud features in the
Voyager
images, and we didn’t have an infrared camera that would let us see through that haze to the cloud layers below.” In Hubble Space Telescope images as well as those from one of the giant 10-meter Keck telescopes on Mauna Kea taken since the
Voyager
flyby, many more visible bright and dark bands, clouds, and small storm systems have been tracked, revealing
more complex and dynamic weather than originally thought.
Voyager 2
had flown by Uranus when the southern hemisphere was nearly continuously sunlit and the northern hemisphere was dark. Since then, the seasons have advanced through the southern fall/northern spring equinox (in 2007), and telescopic observations have revealed some significant seasonal changes. “It’s a very different kind of planet now than the one
Voyager
saw, because of its dramatic seasonal changes,” says Heidi Hammel. “Uranus has now traveled a quarter of the way around the sun since 1986, and now almost the entire planet is in sunlight, rather than just the south pole.
If we were to fly by Uranus with
Voyager
now, we would see a much more active planet than we did back then. It’s kind of neat to watch those changes.”