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# Saturn: The Unfinished Symphony

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Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1230-1232
DOI: 10.1126/science.304.5675.1230

The 4-year Cassini-Huygens mission to Saturn will probe the lingering mysteries of a “mini solar system,” much of whose evolution seems to have been arrested in its earliest days

Move over, Mars, Saturn is up next. And what it reveals promises to be on a much grander scale than the eons-old remains of a salty martian sea and blocks of blasted lava being captured this year by the twin rovers.

The $3.3 billion Cassini orbiter and attached Huygens probe, which will arrive next month after a 7-year trip, will be taking the pulse of the entire Saturn system, replete with grand rings and a bevy of satellites. It's “the most complex interplanetary spacecraft ever built,” according to NASA's press information, and is NASA's largest. And scientists are particularly keen to apply Cassini's powerful instrument complement to a surprisingly young and dynamic ring system. “We're going to get a close-up, time-evolving picture” of the rings, says Larry Esposito of the University of Colorado, Boulder. “It will knock your socks off.” Beyond the rings, debris from the earliest days still litters the landscape. Tiny satellites have yet to settle into fixed orbits, and some still share orbits with larger companions. Modest-sized Enceladus shows signs of rejuvenation even though it should have long ago run out of the energy to redo its surface. And little Phoebe, Cassini's target in an 11 June close flyby, may be a planetary building block left behind after the rest got swept up or simply flung away. Saturn and company resemble a miniature solar system with the litter of construction lingering and the finishing touches still under way, says Cassini imaging team member Torrence Johnson of NASA's Jet Propulsion Laboratory in Pasadena, California. ## Flagship of planetary probes Ironically enough, Cassini-Huygens arose from—“survived” might be more accurate—an early NASA exercise in cost containment. A Saturn orbiter-plus-probe concept came out of a joint NASA and European Space Agency (ESA) study in 1982. In the mid-1980s NASA studied designs for a standardized spacecraft for the outer planets that, with ESA participation, became the separate Cassini mission and the Comet Rendezvous/Asteroid Flyby (CRAF) mission. But after Congress mandated a cost cap in 1992, CRAF was cancelled and the Cassini- Huygens design was made much simpler. Still, it ended up at a total mass of 5636 kilograms, nearly double the weight of the Galileo orbiter-probe that ended its 8-year tour of Jupiter in 2003. Launched in 1997 on a circuitous, fuel-efficient route to Saturn, Cassini-Huygens will fire its rocket to settle into orbit on 1 July. Total mission costs come to about$3.3 billion, with ESA chipping in \$500 million for the Huygens probe.

The budget cuts did little to shrink the science that Cassini will attempt at Saturn. The 12 instruments on the orbiter total 262 kilograms, compared to the 142-kilogram, 11-instrument payloads of the two Voyager spacecraft that flew through the Saturn system in 1980 and 1981. Galileo's orbiter instrument total was 103 kilograms. Cassini's added instrument mass, incorporating vastly more capable electronics and computing capacity, makes for a powerful science package. For example, the five instruments measuring charged and neutral particles and electromagnetic fields are “far more capable than anything flown before,” says the Cassini team's particles and fields interdisciplinary scientist Tamas Gombosi of the University of Michigan, Ann Arbor, “including around Earth.” Four remote-sensing instruments image and chemically map everything from wispy rings to satellites and Saturn itself at wavelengths from the extreme ultraviolet to the far infrared. The radio used for data transmission doubles as a probe of rings and atmospheres. And Cassini carries two instruments the Voyagers lacked: a radar, for penetrating Titan's clouds, and a dust analyzer.

Earlier visitors Pioneer 11 and Voyagers 1 and 2 blew through the Saturn system in a matter of days, but Cassini will orbit Saturn 74 times over 4 years while dropping the Huygens probe into Titan's atmosphere next January and getting a bead on a still-evolving planetary system. “It'll be like 74 Voyager encounters with instruments an order of magnitude better than Voyager's,” says rings interdisciplinary scientist Jeffrey Cuzzi of NASA's Ames Research Center in Mountain View, California. The Voyagers provided their share of surprises, says Johnson, but “I wouldn't be surprised to be surprised again.”

## Rings within rings

All Cassini's instruments, even the particles-and-fields instruments and perhaps the radar, will probe the crown jewels of Saturn, its rings of innumerable icy bits, blocks, and boulders. Researchers will need all the data they can get. “There's so much to explain,” says imaging team member Carl Murray of Queen Mary, University of London.

The Saturn ring system has the look of a work in progress. Four and a half billion years after the planet's formation, the main rings don't seem to be more than a few hundred million years old, and some of the more bizarre features, such as braided rings, are still changing from year to year. The Voyagers revealed thousands of “rings” embedded within the broad A, B, and C rings, like grooves on a 270,000-kilometer-wide phonograph record. They also found impossibly narrow rings, broad, imperceptibly faint rings, and a fuzzy outer ring with a heart of rock. In Saturn, “you've got examples of all the types of ring systems you see in the rest of the solar system,” says Murray, who was a postdoc studying planetary rings when the Voyagers passed Saturn in 1980 and 1981.

More than 2 decades of theoretical work since the Voyagers passed Saturn have taken some of the mystery out of its rings. Gravity, it turns out, can make ring particles behave as weirdly as any subatomic particle. Most of the “rings” within the A ring, for example, are triggered by the periodic gravitational tug of moons orbiting just outside the main rings. A moon such as Mimas can set off a wave that spirals inward like the groove of a phonograph record.

In rings, gravity can also repel. The 700-kilometer-wide F ring just outside the A ring should have long ago spread out, but two satellites orbiting just inside and outside the F ring shepherd its particles back into a tight bunch. As a moon and ring particles pass in adjacent orbits, the moon raises a bulge in the ring that allows an exchange of orbital energy, driving the particles into the ring. But before the bulge comes around again, when the orbital energy exchange would be in the opposite direction, jostling among the ring particles destroys the bulge. The net effect can therefore be a push into the ring.

Plenty of ring mysteries remain, however. “I don't think we have any idea what is causing the structure of the B ring,” says theoretician Scott Tremaine of Princeton University. Moons beyond the main rings or even moonlets embedded in the B ring aren't responsible for most of the finely detailed grooves there. Researchers aren't even sure why the faint, diffuse E ring is there at all; its micrometer-sized particles should have been swept up by Enceladus and other moons long ago. Perhaps E renews itself as its particles collide with Enceladus and splash off fresh particles, or maybe Enceladus is spewing particles from icy volcanoes.

The biggest remaining mystery is the age of the rings. Voyager observations strongly suggest that they do not date from the formation of the planet. If they did, the rain of dark interplanetary debris that pelts them would have blackened them over the eons. Also, if they were primordial, they would have pushed the tiny satellites fringing them farther outward. Some of the finer ring features are clearly of recent origin. The Voyagers found year-to-year changes as the F ring broke into strands that braided and unbraided themselves. “Most likely we're looking at the latest version of rings” as they continue to evolve and age, says Murray.

A dynamic ring system makes a particularly inviting target for Cassini. The processes aging and reshaping the rings of Saturn are likely to be the same ones that operated in the disk of gas and growing particles that gave rise to the planets. “This is the only disk we're going to see close up,” says Murray. “It will be a great test” of ideas about how particles in a preplanetary disk interact. Cassini has an advantage over the Voyagers because it will spend such a long time in the Saturn system, notes Cuzzi: “We'll actually have the chance to see the rings evolve.”

Cassini will bring all the Voyager-type instruments to bear on the rings: imagers, spectrographs for composition, particles and field instruments for ring effects on the magnetosphere, and radio propagation experiments for probing the rings—but at a whole new level. The Ultraviolet Imaging Spectrograph (UVS), for example, is “immensely superior” to Voyager's UV instrument, says UVS principal investigator Esposito. It has 50 times the sensitivity of Voyager's. When it records the light of a star twinkling through the rings, it will reveal 10 times more structural detail than did Voyager. Unlike Voyager, which recorded just one stellar occultation, UVS will observe more than 60 of them, resolving ring structure down to a scale of 10 to 20 meters.

## Fuzzy satellites

At Saturn, “we're going in with a lot less background information [on the satellites] to guide us than when Galileo entered the jovian system,” says Johnson, a former Voyager and Galileo team member. The eight sizable icy moons of Saturn appear fuzzy to varying degrees in Voyager images, thanks to their small size (110 to 764 kilometers) and considerable distances from the passing Voyagers. Still, planetary geologists have seen enough of them to suspect that they all had “interesting histories,” says Johnson.

In approaching the icy satellites, Cassini will not have the freedom to roam the Saturn system the way Galileo jumped among the four big Galilean satellites of Jupiter; of Saturn's moons, only Titan is massive enough to gravitationally sling the spacecraft into an entirely new orbit. Even so, Cassini and its more powerful instrument set will make a half-dozen close passes of four of the eight icy satellites and at least two dozen passes at the range of typical Voyager encounters.

The first of several weird objects on Cas-sini's itinerary is 110-kilometer Phoebe, which the spacecraft will pass on its way toward Saturn for the first time. Phoebe's highly inclined and “backward” orbit mark it as a captured object, one slowed into orbit by the drag of the gas that enveloped the still-forming Saturn 4.5 billion years ago. That would make it a surviving building block of the outer planets, a sort of object never seen up close before.

Perhaps weirdest of all the icy satellites is 718-kilometer Iapetus. Even astronomer Giovanni Domenico Cassini, who discovered it in 1672, recognized that one side is bright and the other dark. Post-Voyager opinion is pretty evenly split on how to explain the pitch-black stuff, says Johnson. Some planetary scientists say the dark material oozed from the interior as some sort of volcanic outpouring, whereas others say it fell as dust from some external source, such as dark Phoebe or even Titan's atmosphere.

Most intriguing to geologists is 250-kilometer Enceladus. Voyager got close enough to reveal both ancient, heavily cratered terrain and much younger, ridged plains with practically no impact craters. Apparently Enceladus summoned enough heat in the geologically recent past to melt some of its ice and resurface itself. Many of the other icy satellites show some signs of resurfacing, but how Enceladus did it so recently and extensively remains a mystery. “The common view is that [melting] is hard to do if you only have water ice,” says planetary physicist David Stevenson of the California Institute of Technology (Caltech) in Pasadena. A dollop of another, more volatile compound, such as ammonia, would lower water's melting point and ease the creation of water-ammonia lavas. Erupting water-ammonia geysers on Enceladus might solve the mystery of the E ring's origin as well.

## Saturnian wind and rain

Gas giant Saturn certainly wasn't too small or distant for the Voyagers to study, but “we in fact know very little about Saturn,” says atmospheres interdisciplinary scientist Tobias Owen of the University of Hawaii, Manoa. The second largest planet hides many of its secrets far below its cloud tops, which even Cassini will not penetrate. The coming mission may yet pick up some clues, however.

For one, scientists hope to learn why Saturn radiates so much more heat than was generated from the planet's formation. Far out of reach in Saturn's interior, theoreticians say, helium should be separating from the planet's dominant constituent hydrogen and forming droplets that fall like so much rain, giving off heat as they go. The Voyagers misread the extent of helium separation by incorrectly measuring the ratio of hydrogen to helium, says Owen. Cassini's Composite Infrared Spectrometer should do the trick.

The heat welling up from Saturn's interior helps drive the visible eastward circulation of the atmosphere, but it doesn't explain another saturnian mystery. “Why is Saturn the windiest planet in the solar system?” asks imaging team member Andrew Ingersoll of Caltech. Saturn's broad equatorial jet stream blows at more than 1400 kilometers per hour, almost 10 times faster than on Earth.

Ingersoll suspects the answer is that Saturn has no rough surfaces and little turbulence to slow down the winds. “We're going to get up close with a high-resolution camera and measure the small-scale turbulence,” says Ingersoll, who hopes to see whether an especially placid planetary visage can explain the record winds.

## The saturnian milieu

The entire Saturn system—planet, rings, and satellites—is immersed in an electrically neutral sea of charged particles, called a plasma, that's confined in the magnetic bubble of Saturn's magnetosphere. And Saturn's “is the perfect intermediate example between the extreme of Jupiter and the home planet,” says Michigan's Gombosi. Comparing the three magnetospheres should help clear up some questions about the jovian magnetosphere, he says. For example, at Earth, plasma normally drags magnetic field lines along wherever it goes. Not at Jupiter. Somewhere, somehow, plasma and magnetic field lines have become decoupled and the plasma is lost from the magnetosphere. Having Cassini follow the plasma for 4 years will make an “unbelievable difference” addressing such problems, says Gombosi.

For all its observational power and its protracted tour of the Saturn system, Cassini will leave much still shrouded in mystery. Some Cassini team members hope against hope for a return visit. In the meantime, however, Gombosi says he's “absolutely determined to have fun” exploring a still-ragged solar system in miniature.

### What's to Come …

11 June

Phoebe flyby at 2000 kilometers

26 October

Titan flyby at 1200 kilometers

13 December

Titan flyby at 2300 kilometers

15 December

Dione flyby at 81,000 kilometers

24 December PDT

(25 December UTC)

Release of the Huygens probe

1 January 2005

Iapetus flyby at 63,000 kilometers

14 January 2005 PDT

(15 January UTC)

Descent of the Huygens probe

15 February 2005

Titan flyby at 950 kilometers

17 February 2005