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Rings of revelation.

Saturn's rare display of light and shadow brought planetary astronomers some unexpected surprises.

Right on cue, in 1995 and 1996 Saturn once again tipped the edge of its magnificent ring system our way. The planet's ring plane has slipped across the Earth and Sun 82 times since Galileo was first perplexed by the phenomenon in 1612. But astronomers never tire of watching the planet's geometric spectacle, always alert to the discoveries that come when the beautiful rings vanish from view. Thirteen of Saturn's 18 known moons were found during these special times. "It's ironic," notes observer Andrew S. Rivkin (University of Arizona), "but the best time to learn about Saturn's rings is when they're not actually there."

Such disappearances occur when Saturn reaches one of its equinoxes, which occur twice during the planet's 29 1/2-year orbit. The circumstances in 1995-96 offered a boon to observers: Earth was in the ring plane three times (May 22nd, August 10th, and February 11th), and we viewed the side of the rings in shadow from the Sun's plane crossing on November 19th through early February.

Armed with fortuitous geometry and an array of new imaging technologies, astronomers anticipated a big payoff. During the disappearance in May, Amanda S. Bosh (Lowell Observatory) and Rivkin spied four moonlet candidates in Hubble Space Telescope images (S&T: October 1995, page 11). The one designated 1995 S1 appears to have been the known moon Atlas, even though it was found 25 [degrees] ahead of the predicted position. Dynamicists believe they can accommodate the misfit, since Atlas - 40 kilometers long and circling just outside the A ring - has not been seen since Voyager 2 bade Saturn adieu in 1981. An error of just 0.4 second in its presumed orbital period, compounded over 14 years, would account for the shift in location.

The HST object dubbed 1995 S3 was initially touted as a newly discovered moon. However, at magnitude 17.5 it should have been seen - but wasn't - by one of the Voyagers or by HST's camera during the later plane crossings. Bosh concedes that S3 was likely a large but temporary clot of particles on the outer edge of the F ring, a thin band just beyond the wreath of Saturn's more familiar rings. A second false alarm was raised by the candidate 1995 S4, which was glimpsed briefly in May but not seen again.

PROBLEMATIC PROMETHEUS

The brightest of the blips seen by HST, 1995 S2, made itself known in both May and August. It moves in the same orbit as Prometheus, the inner of two satellites discovered by Voyager that straddle the F ring. But it lagged the moonlet's predicted location by 20 [degrees] in May and 19 [degrees] later in 1995. Gravitational interactions with the A ring are gradually pushing Prometheus outward and thus lengthening its orbital period. However, the accumulated lag in the years since the Voyager 2 flyby amounts to a mere 0.2 [degrees] - a hundredth of what was observed. "We really didn't expect this Prometheus problem," says Bosh.

According to investigator Philip D. Nicholson (Cornell), who coordinated the HST camera observations in August and November, three ideas might explain Prometheus's wandering. First, its movement could be more eccentric than thought, such that at times the moonlet scoots along faster or slower than its mean rate. But this dynamical fiddling is probably unrealistic. Measuring a respectable 140 by 80 km, Prometheus was easy prey for the Voyagers, and its orbit is known too precisely to permit 20 [degrees] of positional slop. In fact, by running the Voyager-derived orbit in reverse, dynamicists have tracked the moon down in images taken during the ring-plane crossings of 1980 and even 1966.

A second possibility is that Prometheus collided with something massive enough to retard its orbital motion. However, in that case the lag should be gradually increasing with time. For example, a hit just after Voyager 2's 1981 flyby would cause the lag to grow by 0.11 [degrees] per month. But just the opposite was observed after May 1995.

Although cleanly separated most of the time, Prometheus and the F ring occasionally do come into intimate contact. Carl D. Murray (Queen Mary and Westfield College, London) and Silvia M. Giuliatti Winter (UNESP, Brazil) have deduced that collisions occur every 19.1 years; the last one began in 1990 and might still be going on. If Prometheus got a jolt 5 years ago its orbital lag must have accumulated since then and should have grown another 1 [degrees] between August and November, whereas HST data show a change of just 0.08 [degrees] . Murray still thinks Prometheus gets bumped around during its forays into the F ring, and that such collisions might play a role in shaping the moon's orbit. But he admits the evidence does not favor his model.

The best suggestion to date, most agree, is that a small companion moon shares Prometheus's orbit. The two objects would occasionally crowd one another but never actually collide, instead exchanging a bit of angular momentum before moving apart. (Saturn's small moons Janus and Epimetheus have precisely this kind of orbital cohabitation.) To induce the observed shift in the motion of Prometheus, a hypothetical companion would need 1/30 its mass, making it roughly the size of Atlas.

Coincidently, last August three groups of observers did spot something trailing Prometheus in its orbit by about 15 [degrees]. Designated 1995 S7, the object had a magnitude of 17.9 and an estimated size in Atlas's range. But it wasn't seen in May or November (Saturn was near the Sun during February's ring-plane crossing). Nor was the putative moonlet obvious to either of the Voyagers, which scrutinized the F ring and its surroundings with scores of images. "It's a nice explanation," observes theorist Luke Dones (NASA/Ames Research Center). "But it predicts a satellite that's pretty large."

Nicholson still holds out some hope that Prometheus has a coorbiting companion, noting that its key attribute must be mass, not size or brightness. But he is bothered by 1995 S7's apparent stealthiness and concedes it was probably a large clump in the F ring. "We don't have a real consensus yet on the lag mechanism for Prometheus," Nicholson says. "We're unhappy with all of them."

One of last year's disappointments was the failure to recover Pan, a 20-km moonlet trapped within the Encke gap in the outer A ring. Some observing teams suspect that Pan is lurking in their images, but a traffic jam of other objects has made its positive identification impossible.

Sorting out the goings-on would be easier if there weren't so many decoys in and near the F ring. Hubble investigators conclude that 1995 S3, S5, S6, and S7 are probably all clumps of F-ring particles. These "sandbank satellites" appear to be short-lived - they are obvious in some images but absent in others taken a few months earlier or later. Similar bunchings in the F ring likely fooled many would-be moon discoverers during the edge-on events in 1966 and 1980.

The F ring has been an enigma ever since Pioneer 11 chanced upon it in 1979. Its discovery elated celestial mechanicians Peter Goldreich and Scott D. Tremaine, who a year before had predicted that gravitational forces from small satellites could confine particles into narrow rings. Voyager 1 completed the picture with the discovery of the F ring's "shepherds," Prometheus and Pandora, in 1980.

But satellite confinement is about the extent of what dynamicists think they understand about this unpredictable band. Whereas Voyager 1 spotted a bizarre series of kinked and contorted "braids," Voyager 2 saw nothing but smooth symmetry when it arrived 9 1/2 months later. "The F ring is strange," Murray admits, "even for a planet surrounded by strange rings."

Now observers suspect that the F ring may be throwing off their attempts to gauge the thickness of Saturn's rings. The true vertical depth is only about 10 to 100 meters, as determined by Voyager probings. However, from Earth the rings have always looked much thicker: 2.4 km when edge on in 1966 and 1.1 km in 1980. Most researchers have ascribed the thickening to minor warps in the ring plane, levitated "spokes" of fine dust, and embedded satellites.

The presence of the F ring may dominate all of these effects. In May the time of minimum light during the ring-plane crossing occurred 23 minutes later than predicted. In August the west ansa's crossover preceded the east's by 50 minutes, and in November's HST images much of the F ring lay hidden in the A ring's shadow. All this evidence suggests that the F ring is very slightly inclined - mere thousandths of a degree - with respect to the main ring system. Bosh adds that minute changes in Saturn's pole position or precession rate may have influenced the crossing time too.

THE OTHER RING SYSTEM

Even though the F ring stole the show last year, investigators found plenty to look at elsewhere in the planet's ring system. Since the rings consist largely or wholly of water ice, they must be enshrouded in a tenuous amount of water vapor. The water itself is undetectable, but not so the hydrogen atoms and hydroxyl (OH) molecules formed when water breaks down.

About a week before the August ring-plane crossing, Doyle T. Hall (Johns Hopkins University) and three collaborators exploited HST's ultraviolet sensitivity to probe the gas hovering near the ring system. They detected OH emission at 3085 angstroms just above the A and F rings and in three locations extending away from the B ring.

Based on this feeble fluorescence, the Saturnian ring system must be losing about 3 tons of water per second. That's too much gas to be created by the impact of interplanetary dust alone. Surprisingly, the cloud is densest, several hundred OH molecules per cubic centimeter, above the F ring. "I've convinced myself we're seeing a torus of gas, with a peak outside the main rings," says Doyle. He suspects that most of the gas is being launched from the outer A ring, the location most susceptible to bombardment by magnetospheric ions and flecks of ice imported from the faint E and G rings farther out.

Discovered in 1966, the E ring is more tenuous than any of Saturn's other bands yet was spotted by both HST and large ground-based telescopes. It extends from near the orbit of Mimas to beyond that of Dione - three times the breadth of rings A, B, and C. Last year observers confirmed its very blue color, first noted in 1980 by observer Stephen M. Larson (University of Arizona).

The quirky blue hue probably has nothing to do with composition. Instead, modelers argue that the E ring's icy particles are almost all 1 micron across, too small to scatter the long, red wavelengths of visible light very efficiently. These flecks of ice come predominantly from the large moon Enceladus; once lofted into space they are smeared into a huge but sparse ring by a fortuitous combination of sunlight's radiation pressure, gravitational tugs from Saturn, and electromagnetic charging.

Observers recorded both E and G using an infrared camera on the Keck I telescope in Hawaii. Team leader Imke de Pater (University of California) had expected to see only the E ring, blue color notwithstanding. In Keck's images its faint smear can be traced to the orbit of Tethys, about 250,000 km from Saturn.

Glimpsing the G ring, which had never been seen from Earth, was a pleasant surprise for the Keck team. "It's much brighter than predicted," de Pater says, "probably half as bright as the E ring." She explains that to be seen at 2.2 microns its color must be rather neutral, an indication that the ring contains larger particles than the E ring. Some rock-size pieces must be in there too; they absorb high-energy protons in the vicinity, a reduction that betrayed the ring's presence to Pioneer 11 in 1979.

Managers of the forthcoming Cassini mission are following the ring-plane results closely. They are especially eager to understand the G ring, which will be crossed by the spacecraft when it reaches Saturn early next century. And in turn Earth-based observers await Cassini's findings, with the hope that those data will finally explain the mysterious workings of Prometheus, the F ring, and much, much more.
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Title Annotation:rings of the planet Saturn
Author:Beatty, J. Kelly
Publication:Sky & Telescope
Date:Aug 1, 1996
Words:2060
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