The Interstellar Age (18 page)

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?

One answer came from small new moons that were discovered in the images, two of which orbit within a gap in the rings and apparently twist and “shepherd” some of the rings with the push and pull of their combined gravity. If a block of ice in this region starts to wander too far in, one of the shepherd moons will come by and pull it back out; if a house-sized piece of ring in some other region starts wandering away, the other shepherd moon will come by and pull it back in. It was an elegant and completely unanticipated discovery.

Heidi Hammel was an undergraduate at MIT during the
Voyager
Saturn flybys, and she recalls one of her fellow students somehow hacking into NASAs
Voyager
image feed and watching all the images stream by—just like at JPL, but in his dorm room in Boston. When the professors in Heidi’s planetary science class got wind of this, they took the class on a “field trip” to this fellow’s dorm room to
view the images together. “We just sat there, as part of our class, watching the
Voyager
Saturn images come up on this TV screen,” she told me. “I remember when the images of the F Ring, the braided F Ring, first appeared. It was all twisted and strange, and we were just staring at it. And I remember that one of my professors, the astrophysicist Irwin Shapiro—I’ll never forget this—he was looking at that, and he said, ‘Well, that’s just not possible. It’s not possible for rings to do that.’ And we all just started laughing because
there it was
on the TV screen.” It seemed like
Voyager
was making the impossible possible everywhere it went.

Another important finding had to do with the composition of the rings.
Voyager
found them to be made of almost pure water ice, with maybe just the slightest hint of a reddish or pink coloration from some unknown but minor contaminant. Earth-based observations in 1977 had discovered thin dark rings around Uranus, and
Voyager
had discovered thick dark rings around Jupiter, which suggested that icy rings get dark over time, maybe from accumulating dust or dark materials from comet and asteroid impacts with the rings. So why would Saturn alone have a gigantic system of super-bright, ultrapure rings? The question is part of a bigger debate that is still going on among my planetary science colleagues: how old are Saturn’s rings? Their brightness and clean icy nature suggest that they are very young, perhaps having formed from the catastrophic breakup of a large icy satellite just a few tens to hundreds of millions of years ago (that’s young to astronomers). However, their highly ordered structure and the amount of mass contained in the rings suggest that any such doomed precursor moon should have been broken up very early in the history of the solar system, when such giant impacts were more common. That suggests that the rings
are ancient, and that they keep clean by repeatedly clumping and unclumping with their neighbors as they orbit, continually stirring up “
fresh” ice in the process.

Voyager 1
found that not just Saturn’s rings but all its large moons are covered by, and mostly made of, water ice. That was not particularly surprising, given the expectations from previous Earth-based telescope observations and the fact that small bodies like moons and asteroids and comets are expected to get icier the farther out in the solar system they form. What was surprising, however, is that the geology of these moons varies so widely. One of the more distant large moons, Iapetus, has a dark hemisphere (the side always leading or facing forward as it orbits around Saturn) that is five times darker than the other, trailing hemisphere. Several of the closer-in large moons, such as Tethys, have global systems of troughs and fractures, suggesting significant but mysterious past active tectonic stresses in their icy crusts. Some of Saturn’s moons have preserved a history of intense bombardment over time and are literally saturated with the scars of impact craters formed on their icy surfaces (in the cold temperatures of the outer solar system, ice acts like a rock in many ways). But others have some places on their surfaces that are heavily cratered and other places that are not, as if some process had come along and wiped the slate clean, covering or erasing some of the ancient craters but not others.

Some team members speculated that this process may have been volcanism—but in this case a unique kind of outer solar system process called
cryovolcanism
where the “magma” is liquid water, formed by the melting of the “rock” (solid ice), and erupted through cracks and fissures that let it flow across the surface the same way a
rocky lava flow would on Earth. “We certainly did not anticipate the
level and breadth of active geologic processes we would find among the outer solar system satellites,” Larry Soderblom confessed to me. “We were expecting to explore a bunch of ancient cratered balls with not much else on their surfaces. We should have been smarter.” Fortunately, he and the other geologists on the
Voyager
team became smarter very quickly and learned to extend the basic ideas of volcanism—melting of material and transport of the resulting lubricants through cracks or fissures or other underground “plumbing” systems. “Now, because of
Voyager,
we know that no matter where you go in the solar system (or probably in the universe, for that matter) you will always find geological lubricants no matter how cold it gets,” Larry went on. “Nitrogen is the melt on Triton, methane on Titan, and ammonia and water likely provide the geological lubricants for many other icy moons. It turns out that volcanism in some form can occur anywhere—whether it is the traditional rocky kind that we know of on the terrestrial planets, or the ultracold cryovolcanic flows and eruptions that we’ve seen on many of the moons way out there.” The specific internal heat sources that could power such volcanoes is still a mystery, although many of my colleagues, like Larry, suspect that tidal heating—like that which drives Io’s volcanoes—probably plays a role.

One of Saturn’s icy moons proved to be especially enigmatic, as revealed in
Voyager 1
images. Enceladus (pronounced en-CELL-uh-dus) is only about 300 miles across—driving completely around it would be like driving from Boston to Chicago—and yet it is nearly perfectly spherical, has the most reflective surface of all of Saturn’s moons, and relatively few impact craters were detected in distant
Voyager
images. This suggested to
Voyager
team members that the surface of Enceladus could be very young, and that, indeed, active
resurfacing could still be occurring. In addition, the faint outer E ring of Saturn seems to be densest near the orbital distance of Enceladus, suggesting that this moon might be the source of those ring particles. It was exciting and enigmatic and just plain
weird
for such a small moon to show so much evidence of geologic activity. Enceladus and its neighbor Dione orbit in a 2:1 resonance (like Io and Europa in the Jupiter system), so maybe the same kinds of orbit-related tidal forces heat the inside of Enceladus, melting the icy mantle and causing cryovolcanic eruptions? Hopes were high that
Voyager 2
’s much closer encounter with Enceladus nine months later would provide the evidence needed to solve this puzzle.

Voyager 1
’s planetary mission ended at Saturn, as the path required for a close Titan flyby, combined with the other constraints, caused the spacecraft’s trajectory to be bent upward and well away from any known potential future flyby targets. While
Voyager 2
sped on to strange new worlds,
Voyager 1
settled down for the long journey to the edge of the solar system, and beyond.