23 August 2011

Dance of the rings

Saturn's rings are shedding their cloak of mystery, and may owe their very existence to the chaotic death of an ancient moon.

Saturn eclipsing the Sun, seen from behind from the Cassini orbiter. The colour is exaggerated in this version. Credit: Wikimedia

WHEN GALILEO FIRST looked at Saturn in 1610, he saw a planet with a blurry brim he interpreted as two large moons, one on each side. But two years later, the ‘moons’ had vanished … only to reappear again, two years after that.

Galileo was puzzled. The best explanation he could come up with was that Saturn had some sort of wings or arms, sometimes spread wide, sometimes tightly folded. It was a brilliant deduction – and it immediately marked Saturn as different from the other heavenly bodies.

Today, we know the rings he saw had ‘disappeared’ because Saturn passes through phases during which they are nearly edge on (as seen from Earth) and therefore invisible to his crude telescope.

And yet, 400 years later, there is still plenty of mystery – even though the high-resolution cameras of NASA’s Cassini spacecraft, which has been orbiting Saturn since 2004, have photographed them from nearly every angle.

Not that we haven’t learned a lot. We know, for example, that the rings are extremely hin – only about 10 m thick. “That’s like a two- or three-floor building,” says Phil Nicholson, a planetary scientist at Cornell University, in Ithaca, U.S. Furthermore, from one side of Saturn to the other, the rings are more than 350,000 km wide. “That’s a ratio of 30 million to one,” Nicholson says. “It makes a sheet of paper look pretty thick.”

The rings are so thin, in fact, that if the paper you’re holding in your hands (assuming you’re reading the print edition) had the same relative thickness as Saturn’s rings, you would be able to fit thousands of copies of the magazine in a 1 cm stack. “The rings,” says Nicholson, “are the most two-dimensional structure we know of in the universe.”

But they are also bright enough to easily see and dense enough to block starlight. That means the ring particles, which range in average size from golf balls to refrigerators, must be very tightly packed. “They’re only a shoulder-width apart,” says Nicholson. “They’re having collisions all the time.”

These collisions damp out differences in their motions, causing nearby particles to orbit at nearly the same speed. Nicholson says the relative velocities are millimetres per second. “Imagine a bunch of horses running around a racetrack at almost the same speed. One jockey can reach out and pass his whip to another even though the horses are going [55 to 65 km/h].”

Officially there are seven rings, known simply as the A, B, C, D, E, F, and G rings. The names might be the only simple things about the ring system – and even they aren’t as straightforward as they sound. They’re named more or less in order of discovery (approximately the same as order of brightness), which is not the same as their order from Saturn. The innermost ring is the D ring. From there, it goes C, B, A, F, G, E. And the G ring is so faint it wasn’t even seen on the first Saturn fly-by by Pioneer 11 in 1979. Rather, it took the better cameras of the Voyager 1 spacecraft to discover the ring a year later.

But these are merely the official divisions. Cassini’s best photos reveal hundreds, even thousands, of ringlets within each major one. The closer we look, the more complex the rings become. Scientists have known for some time, for example, that small moons several kilometres in diameter, orbiting within them, can gravitationally sweep ring particles aside, creating gaps such as the A ring’s 325 km wide Encke Gap, patrolled by the 28 km moon Pan.

The gravitational effects of more distant moons create other gaps, such as the huge Cassini Division (4,800 km wide, separating the A and B rings). Closer examination reveals these gaps to have wondrously filigreed edges, sculpted by even more complex interactions with the moons’ gravities.
There are also radial stripes, called spokes, first seen by Voyager 1 in 1980, but – in an echo of Galileo’s vanishing ‘arms’ – they were not spotted again until September 2005 when Cassini saw them reappear. It had been on the lookout for them since entering Saturn’s orbit.

Scientists now think that, like Galileo’s ‘arms’, the spokes too might be linked to Saturn’s 29.7-year orbit. But in this case, it’s not the tilt of the rings as viewed from Earth that makes them come and go. Rather, it’s the slow parade of Saturn’s seasons.

The hypothesis is that the spokes are formed by microscopic particles suspended above the rings by static electricity – bright in some lighting conditions, dark in others.

The static electricity, this theory holds, comes from the interaction of giant thunderstorms with Saturn’s magnetic field, and the come-and-go nature of the spokes simply represents Saturn’s changing weather as it moves toward its equinoxes, one in 1979, and another in 2009. (There was another equinox in 1994, but there was no spacecraft there at the time to take pictures.)

Also included in Saturn’s ring zoo are ‘propellers’, so named because they look like old-fashioned airplane props – only they’re a few kilometres wide and tens of thousands of kilometres long.

Some of the largest have even been unofficially named for famous aviators. History buffs will recognise Blériot, Earhart and Santos-Dumont among others, says Carolyn Porco, head of the Cassini imaging team.

Porco believes the propellers are created by moonlets – or even wannabe moonlets called ‘clumps’ – too small to open full, 360˚ gaps. Rather, they open partial gaps, which eventually fill in, both in front and behind, around the curve of the rings.

Not surprisingly, they lie in the densest part of the rings, where clump formation is gravitationally easiest. The biggest are in the outer sections of the A ring, but enough others have been found that there are unofficial ‘propeller zones’ elsewhere in the A ring, where hundreds of smaller propellers mark even smaller moonlets.

In the years since 2005, when the first propeller was seen, scientists have even been able to track them, watching how their movements change in the rings’ complex gravity. That, Porco says, allows them to track moons too small to see (perhaps 1 km in diameter), letting astronomers use the rings as a laboratory for watching, on a smaller scale, the evolution of the early Solar System.

“Scientists have never tracked disc-embedded objects anywhere in the universe before; all the moons and planets we knew about before orbit in empty space,” says Matthew Tiscareno, also of Cornell University. Porco adds: “It allows us a glimpse into how the Solar System ended up looking the way it does.”

Yet another intrigue was discovered in 2009, when Saturn passed directly through its equinox. At that time, the rings were almost perfectly edge-on to the Sun, and for a brief interlude, unexpected vertical features rising above the plane of the rings were thrown into sharp relief, much as the setting Sun on Earth casts large shadows from every tree, every blade of grass.

The biggest (as in tallest) of these surprises appeared along the outer edge of the B ring, which turned out to have peaks rising as high as 3.5 km above the ring plane. It was a totally unexpected find, but a team led by Joseph Spitale from the Space Science Institute in Boulder, Colorado, believes they have found the answer. That part of the rings, the scientists surmise in a study in the 1 November 2010 issue of the Astrophysical Journal, probably contains an entire population of moonlets that migrated through the ring to its edge, then became trapped by a gravitational resonance with the moon Mimas, which lies around 68,000 km farther out.

Such resonances occur where the relative orbital positions of ring particles and a moon repeat, orbit after orbit, allowing gentle tugs to accumulate until they have warped the rings out of their natural shape. At the same time, Spitale’s team found, the rings can produce their own oscillations, somewhat like water sloshing in a bathtub. Just like the water in the tub, these oscillations reflect off the sides (in this case, the gap at the outer edge of the B ring), producing startlingly large sloshes.

Or, if you prefer a musical analogy, it’s like plucking a guitar string. Like the guitar, Spitale says, “the ring has its own natural oscillation frequencies. That’s what we’re observing”.

Vertical oscillations are also studied by watching changes in the light of stars passing behind the rings. “A reasonable analogy is a person walking behind a picket fence, watching the light from the setting Sun come through,” Nicholson says.

In this way, he has found a narrow gap in the C ring, with a raised edge on one side and a dipped edge on the other, each about 1.5 km high. “It’s like a snapped-up hat rim, Humphrey Bogart style,” he says.

This feature and adjacent ripples seem to have been set off by a resonance with the gravity of the giant moon Titan, Saturn’s largest with a 5,100 km diameter. “It’s yanking the ring particles up due to the inclination of its orbit,” he says.

The waves propagate by gravity, as rising particles first tug up on their neighbours, and are then pulled back into the plane of the rings by its overall gravity field. Nicholson compares the result to a slow-motion tsunami. “The amplitude is about 500 m,” he says, noting that this would be huge for a real tsunami. “But it’s moving pretty slowly, about 250 m a day.”

Other wave-forming processes are even more exotic. In a 2010 study of Jupiter’s rings (far less prominent than those of Saturn), Mark Showalter of the SETI Institute, Mountain View, California, and Joseph Burns of Cornell University, concluded that mysterious ripples in these rings – first observed when the Galileo spacecraft passed by in 1996 – were caused by gravitational disruptions from something passing through the rings en route to an impact with Jupiter.

Since then the ripples have been winding tighter and tighter, like coiling rope. By mathematically turning back the clock, they uncovered not one, but two sets of ripples, one tracing to 1994 and the other to 1990.

The simple passage of an asteroid through the rings, like a giant bullet, couldn’t have produced such widespread disruption, they add. Rather, they reported in Science on 1 April 2011, it would have required a shotgun blast of smaller particles, such as would be produced by a broken-up comet. In other words, one set of ripples was created by the impact of comet Shoemaker-Levy 9 into Jupiter in 1994, and the other by an unknown impact four years earlier.

And on Saturn? Similar ripples, they say, have been discovered in the planet’s C and D rings, where they appear to reflect an event in 1983 or 1984. “The rings are witness plates for cometary impacts, giving their frequencies and sizes,” Burns says. If comet strikes are making the ripples, it means comet impacts are more common than previously thought, suggesting there are likely to be more comets hurtling through the outer Solar System.

But the rings’ greatest intrigue is their most fundamental. Where do they come from? They contain enough mass to form a moon about 400 km across. So, not surprisingly, traditional theory says that’s how they started: as a 400 km moon that strayed too close and was torn apart by Saturn’s mammoth gravity.

There’s just one problem: the rings are made almost entirely of ice – 90 to 95% pure, says Robin Canup, a planetary scientist from the Southwest Research Institute in Boulder, Colorado. And the primordial Solar System is believed to have been composed of about equal parts rock and ice. So, if the rings are formed from a smashed-up moon, what happened to the rock?
Some scientists have attempted to get around this by positing that the rings’ progenitor was a giant comet, rather than a moon.

That would have meant a higher fraction of ice, perhaps, but Canup doesn’t think it’s enough. “It’s not clear that that would give you a pure ice ring, either,” she says. And, she notes, Saturn is the least likely of the giant planets to have had such a comet-bursting encounter. Neptune and Uranus lie closer to distant zones where comets originate, and Jupiter’s larger gravity most easily draws such bodies into it, such as Shoemaker-Levy 9.

“[Such an encounter] is 10 times more likely at Jupiter, Uranus, and Neptune than at Saturn,” Canup says. Instead, she thinks the rings came, not from the demise of a small moon, but a large one – perhaps Titan’s size.

In a paper in the 16 December 2010 issue of Nature, she suggests that such a moon could have spiralled into the infant Saturn, shedding an icy mantle while its rocky core, made of tougher stuff, remained intact until its final plunge into the planet. “If the core hits the planet’s surface before it disrupts, the end result is a pure ice ring,” she says.

It’s a very clever, new idea,” says Burns. “One of the things it can do is produce rings made out of quite pure water ice, which has been a problem in the past.”

Furthermore, he says, it makes sense that the outer shell would be fairly pure water, because the doomed moon would have heated up from tidal friction as it moved inward. That would have softened it enough, he says, for the “heavy stuff” to settle to the centre.

Computer models show that large moons, if they form early in the evolution of a gas/dust cloud such as that believed to orbit the infant Saturn (or the early Sun, for that matter), easily migrate inward due to gravitational interactions with remaining gas in the inner parts of the disc. It’s a process called Type 1 migration, and its speed is strongly dependent on the moon’s size. “We think that when they reach a mass comparable to Titan, they start to migrate,” Canup says.

The mantle of a Titan-sized moon would contain a lot of ice; about 1,000 times more than presently remains in the rings, Canup says. But as the ice spread, much would have fallen into Saturn. Some would also have condensed into new moons.

And, gravitational interactions would have had the opposite effect on these moons as on the giant moon that preceded them, according to a study led by Sébastien Charnoz of Paris Diderot University, France, in the June 2010 issue of Nature. Rather than spiralling into Saturn, Canup says, these “spawned moons” migrate outward, as though recoiling from their birthplaces.

WHICH SOLVES ANOTHER riddle. These “spawned moons,” would include Enceladus, Dione, and Tethys, all of which have densities low enough that they must be composed primarily of ice. Scientists had been puzzled by how such icy moons could form, but Canup’s theory answers the question, almost by accident. “The advantage of [her theory] is that it occurs very naturally as part of another origin process – the process by which the rings themselves were made,” Burns says. “She has a pretty convincing story, I would say.”

There’s no reason this entire process need only to have happened once. Currently, Saturn only has one large moon. Jupiter has four. But perhaps Saturn, like Jupiter, once started with several, each suffering the same fate, one after the other until, eventually, the gas cloud dissipated and the moon system settled into its present configuration. Under Canup’s hypothesis, it’s possible to imagine a chain of Titan-sized moons spiralling to their deaths, until only one remained when the system stabilised, with the present rings formed by the last big one to die.

Ultimately, however, the rings’ greatest contribution to science is as a close-at-hand laboratory for studying other astronomical processes. Spitale’s team compares them to a spiral galaxy; Nicholson compares them to protoplanetary discs. “The theoreticians assume there are gaps in these discs as well,” he says, referring to his Humphrey Bogart fold in the C ring. And once these gaps form, he says, computer models show that the growth of planets can grind to an abrupt halt. “Understanding the physics is important,” he says. “Saturn’s rings are probably the laboratory for studying this issue.”

The next step, he hopes, is a not-yet-formally-proposed mission called the Saturn Ring Observer, in which a spacecraft would be placed in an orbit close to the rings – perhaps only 3–5 km above them – so that it would travel with the ring particles, “a bit like the pick-up [truck] that precedes harness-track horses,” Nicholson says, where it can take high-resolution pictures of the particles’ collisions. “The craft would slowly drift across the rings, like a laser playing a CD or an old-fashioned phonograph, building up a map of the entire system over several months.”

“To some of us,” he adds, “this is the ultimate in cool.”

Rick Lovett is a science writer from Portland, Oregon. He enjoys running and has a degree in astrophysics.

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