JUST a few days from now, one of the Solar System鈥檚 most spectacular sights is set to vanish from the heavens. On Monday, 22 May, Saturn鈥檚 vast, swirling ring system will disappear from sight for an instant as the positions of Earth and Saturn conspire to give an edge-on view. Paradoxically a 鈥渞ing plane crossing鈥� such as this gives astronomers a chance to unlock some of the secrets of the ring system and the moons that chaperone these lanes of ice and dust. And because such an event takes place just once every 14 to 16 years, it will be eagerly watched by astronomers around the world.
Nearly four centuries have passed since Galileo peered through the earliest of telescopes to glimpse faint smudges next to Saturn 鈥� the first clues that the planet had rings. With modern telescopes, astronomers observing Saturn鈥檚 rings from Earth can pick out four distinct broad bands, which have been labelled A, B, C and D in order of decreasing radius. There is an obvious gap, known as Cassini鈥檚 division, separating the A and B rings, about two Saturn radii away from the centre of the planet(see Diagram).
In November 1980 and August 1981, NASA鈥檚 two Voyager spacecraft flew past Saturn to give us the most detailed look at the ring system to date. The best Voyager images delineate structure within the rings as fine as 10 kilometres across. And it is clear that the main rings are very thin 鈥� less than 1 kilometre from one side to the other 鈥� and consist mainly of chunks of water ice typically 1 centimetre to 5 metres across, along with some dust-sized particles. Voyager also revealed a much more complex structure than the four broad bands that can be seen from Earth. In fact there are many thousands of fine rings, some carved into intricate patterns by the gravitational tug of Saturn鈥檚 numerous small moons.
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But much about Saturn鈥檚 rings is still shrouded in mystery. Astronomers do not even know where they come from. One theory, recently investigated by Luke Dones of NASA鈥檚 Ames Research Center in California, suggests that they were formed from the icy debris of a giant comet that passed close to Saturn and was torn apart by planet鈥檚 powerful gravitational forces. Most of the moons of Saturn are predominantly composed of ice, so another possibility is that the rings are the fragments of a large moon that broke up after colliding with a comet or an asteroid.
Glare from the rings usually prevents ground-based observers learning the details of fainter features, such as Saturn鈥檚 many small moons, which could help solve some of these puzzles. That is why the ring plane crossing is so important. With the rings edge-on, the glare practically disappears and small moons and diffuse, extended rings become visible. The last two ring plane crossings, in 1966 and 1980, revealed a whole new ring, now known as the E ring, and five new moons were discovered, including the small satellites Janus and Epimetheus that share almost identical orbits around 15 000 kilometres beyond the edge of the A ring.
Sharper images
Even in 1980, electronic imaging devices were in their infancy. Nowadays these detectors are much more sensitive and precise, and observing capabilities are more sophisticated. For instance, much of the glare from Saturn itself can now be blocked out by observing the rings through special filters. These transmit light only over a narrow band of wavelengths absorbed by methane molecules in Saturn鈥檚 atmosphere. Saturn then appears black, but the icy satellites still shine by reflected sunlight. So this time round, ground-based observations should reveal much more detail.
One important question that these the new observations could help to answer concerns the lifetime of the main rings. The rings are constantly evolving under the gravitational influence of Saturn鈥檚 many moons. Imagine the interaction between a small particle circling the planet and a satellite on an adjacent orbit, slightly further out. Because the particle is nearer the planet, it moves faster than the satellite. Each time the particle catches up, it receives a small gravitational tug into a slightly eccentric orbit. Collisions with other particles in the ring force it back into a circular orbit, but when all these effects are taken into account it turns out that the particle will end up in an orbit slightly closer to the planet than when it started.
At the same time, the satellite receives an even smaller perturbation from the tiny ring particle. This causes it to orbit further away from the planet, so the combined angular momentum of the particle and the satellite remains the same. It seems paradoxical, but this effectively appears to cause the two orbits to repel each other 鈥� the larger the satellite, the larger the effect and the faster the orbits move apart. The difference in mass means that each particle鈥檚 effect on a satellite is tiny. But the satellite continuously receives repeated kicks from millions of particles, so in time it drifts outwards while the particles fall in towards the planet.
Therein lies the problem. Using estimates of the masses of the various rings and satellites taken from the Voyager spacecrafts鈥� data, scientists have calculated that the A ring should collapse into the planet over a very short timescale, astronomically speaking: no more than a hundred million years, or only 2 per cent of the age of the Solar System. This is worrying because it implies that human beings just happen to be around when Saturn has rings. The history of astronomy teaches us not to place ourselves at a special point in space, so astronomers are just as wary of placing us at a special point in time.
This leaves two choices. One is that the rings of Saturn might have been coming and going during the life of the Solar System and we just happen to be around to see the latest version. For this to happen there would have to be some source of material to replenish the rings from time to time 鈥� something like passing giant comets, say. The alternative is that the theory of how the ring particles interact with larger objects is not quite complete, and that there is some as yet undiscovered locking mechanism.
There is a hint that the second answer is the right one. A small satellite called Pandora orbits alongside the narrow F ring, just outside the A ring. Its interactions with particles in the main ring system should be pushing it outwards. But Pandora is also tantalisingly close to 鈥渞esonance鈥� with Mimas, a large satellite 400 kilometre across, with a mass similar to that of the entire ring system, and whose orbit lies beyond Pandora鈥檚. At the resonant radius there would be a permanent, simple numerical relationship between the orbital periods of Pandora and Mimas: Pandora would complete three orbits of Saturn in the time it takes Mimas to do two.
Anchor moon
So if the two moons were in resonance they would repeatedly pass each other at the same relative positions in their orbits, and this periodic force from Mimas, always in the same direction, would slow down the outward movement of Pandora. With Pandora held in a fixed orbit, the ring particles would remain in stable orbits too. Overall, the very massive Mimas would be anchoring the satellites and the particles of the A ring in place.
If Pandora turns out to be in the right position for resonance with Mimas, or even close to it occasionally, this could be the key to the ring system鈥檚 longevity. All indications suggest that it is within 60 kilometres of the resonance orbit, but that it is not quite close enough. During the ring plane crossing, researchers hope to improve our knowledge of Pandora鈥檚 orbit, and so help to find out for certain whether resonance is holding the A ring in place.
In any case, the ring plane crossing will be a useful opportunity to measure the rate at which the small satellites are actually moving outwards. Most of Saturn鈥檚 moons 鈥� and 18 have been found 鈥� orbit in the plane of the ring, and will also be more or less edge-on during the period close to the ring plane crossing. So there will be plenty of opportunities over the next year to catch one passing in front of another, or to see the shadow of one satellite crossing another. At present, the orbital position of most of Saturn鈥檚 large moons is known to only around 500 kilometres. But with the latest detectors, accurate timings of these 鈥渕utual events鈥� could improve the precision for some satellites to around 50 kilometres, allowing their orbits to be calculated much more accurately.
Prometheus is one satellite that astronomers will study. It is the most massive of the three moons closest to the A ring, and so should be moving outwards fastest 鈥� by around 20 metres since the Voyager 2 encounter in 1981. If this is what has happened, Prometheus will have been slowing down slightly and should by now be 1500 kilometres behind the position it would have had if its speed had not dropped. It should be possible to measure its new position during the ring plane crossing. This should, in principle, tell us how fast it is moving outwards from Saturn, and so give a better idea of how long the ring will last.
However, there is a complication that could obscure any such long-term outward drift. According to my own calculations, Prometheus is buffeted by the gravitational pull of other satellites most notably the giant moon Titan. Titan is nine times farther from Saturn but has around 60 000 times the mass of Prometheus. In the long term, the buffeting effect causes no net change to Prometheus鈥檚 orbit. But over several hours it can produce temporary changes in radial distance of almost 1 kilometre, and so slow down or speed up the motion of Prometheus.
There are two ways to get round this, though neither will yield instant results. One relies on the fact that the short-term effects will have no net effect over sufficiently long timescales. So comparing observations taken during several successive ring plane crossings should reveal any underlying outward drift.
The second way round this problem relies on knowing the masses and orbits of all the satellites that exert a tug on Prometheus. Their influence could then be taken into account when interpreting measurements of the position of Prometheus during the ring plane crossing. However, astronomers do not yet know enough about Saturn鈥檚 satellites for this to be possible. There are likely to be numerous small satellites still hidden in the Saturn system (New 杏吧原创, Science, 5 November 1994). Sightings of wavy edges on rings, unexpected voids and a dozen or so unexplained gaps all suggest that there are undiscovered satellites, which have been kept hidden from view by the glare of the rings.
Researchers hope to build up a complete picture of all the satellites and their motions, and the best time to search for them will be between 22 May and 10 August, and again between 19 November and 11 February next year, when the Earth and the Sun will be on opposite sides of the ring plane. But even then, the chances of detecting such small objects 鈥� which are probably less than five kilometres across 鈥� are remote. Definitive evidence for their existence must await the arrival of the Cassini spacecraft in 2004 (see: 鈥淐assini aims to fill in the gaps鈥�). Over the course of its four-year orbital tour of the Saturnian system, Cassini will provide a complete inventory of satellites, monitor the changes in their orbits, and observe how they interact with the rings.
As the glare from Saturn鈥檚 main rings dims, one faint ring will become much easier to see. This is the nebulous E ring discovered during the 1980 ring plane crossing, which extends from 3 to 8 Saturn radii, dwarfing the main ring system. It is also very much thicker: around 6000 kilometres thick near its inner edge rising to 30 000 kilometres thick near the outer edge. Data from the Voyager craft suggest that it is composed almost entirely of ice particles approximately one micrometre across.
Curiously, this ring is brightest at the orbital radius of the satellite Enceladus, which suggests that this icy moon might somehow be producing the particles that make up the E ring, though nobody knows how. Voyager images show that parts of the surface of Enceladus have relatively few craters and appear much smoother than others. This means that it must recently have acquired a new surface. It is possible that this happened when some kind of impact melted the moon鈥檚 surface, releasing a cloud of debris that formed the ring. An alternative explanation is that the particles could have spurted from active geysers of some kind that are now no longer visible.
Whatever the original source of E ring material, Mihaly Horanyi of the University of Arizona, and Joe Burns and Doug Hamilton of Cornell University showed in 1992 that the rings should continually be replenished due to impacts. Small, charged E ring particles must experience electromagnetic forces from Saturn, in addition to its gravitational pull. This combination of forces should lead to highly eccentric orbits, causing the particles to collide with fragments in the main rings as well as the surfaces of other satellites. This would produce yet more icy particles which would be injected into the E ring.
This model also explains the consistent size of E ring particles. Larger or smaller ones would not move on the right orbits to explain the observed ring structure. During the ring plane crossing astronomers will be testing another prediction of this model, which is that there should be faint extensions to the ring and various asymmetries in its structure.
Observations of the E ring structure will be closely analysed by researchers at NASA鈥檚 Jet Propulsion Laboratory in California, who are planning the Cassini spacecraft鈥檚 orbital tour. There are no plans to subject the spacecraft to the hazards of the main ring system, but it cannot avoid passing through the very extensive E ring. The ring plane crossing will be the last chance before the launch to check whether there are any fragments lurking within it that are big enough to damage the spacecraft.
Measurements made at ring plane crossing will also improve our estimates of the thickness of the main rings. It may be that the rings are essentially just a single layer of particles side by side, much slimmer even than the 1 kilometre thickness suggested by observations so far. If this is the case, the observed thickness could be the result of warping in the rings, something that is known to occur as a result of 鈥渂ending waves鈥�. These corrugations of the rings are produced by the gravitational pull of satellites outside the plane, on orbits that are slightly inclined to the rings. Voyager images revealed bending waves at certain locations in the ring system by Mimas, and the entire ring system should also have a more gradual warp caused by the combined gravitational effect of all the other satellites.
The ring plane crossing could even help to uncover new information about Saturn鈥檚 interior. There is every reason to believe that Saturn鈥檚 pole, like the Earth鈥檚, does not point in a fixed direction in space, but slowly wobbles or 鈥減recesses鈥� under the influence of the gravity of Titan and the Sun. Knowing the precise time of crossing will help to refine our measurements of the current position of the pole. Keeping track of how the pole moves over the next few decades will provide telling clues about the planet鈥檚 moment of inertia and the mass distribution deep within its core(see Diagram).
The list of unresolved Saturnian puzzles goes on. But as the great ring plane twists out of view next week, and with Cassini set to depart in 1997, there are golden opportunities on the horizon to find the answers.
Cassini aims to fill in the gaps
IN mid-2004, the Cassini-Huygens mission will begin a four-year orbital tour of the Saturn system. This US-European collaboration is scheduled for launch in October 1997, and will then swing past Venus, Earth and Jupiter before reaching Saturn. It is named after the Italian-French astronomer Giovanni Domenico Cassini and the Dutch scientist Christiaan Huygens. During the 17th century, both made important discoveries connected with the Saturn system.
The exact path of the tour has yet to be finalised, but it will be designed with a number of different goals in mind. Researchers studying Saturn鈥檚 rings hope Cassini will give them detailed views of the ring system from outside the equatorial plane. Geologists hope to get close-ups of the large satellites that orbit within the plane. The itinerary is sure also to include a detailed look at Titan, Saturn鈥檚 largest moon, which is almost half the size of the Earth. Cassini will use Titan鈥檚 gravity to alter its flightpath, and on its first approach it will deploy the Huygens probe, which will descend by parachute into Titan鈥檚 atmosphere and land on its surface. From there Huygens will relay information about the satellite鈥檚 atmosphere and surface conditions back to the main spacecraft.
Titan will also be studied with the main spacecraft鈥檚 radar mapper and other cameras, the most sophisticated ever sent to the outer planets. Titan has a thick atmosphere composed mainly of nitrogen and methane which instruments aboard the Voyager probes could not penetrate. But observations at near infrared wavelengths, made using the Hubble Space Telescope (New 杏吧原创, Science, 26 November 1994), show that Titan sports a mysterious feature about the size of Australia.
Cassini will also have the chance to test the idea that conditions on Titan鈥檚 surface are right for the existence of oceans of liquid ethane. Another feature suggested for Titan is that its upper atmosphere is a complex chemicals factory, in which ultraviolet radiation from the Sun acts on hydrocarbons in the atmosphere to produce complex organic molecules that rain down on the satellite鈥檚 surface.
The great advantage of being in orbit around a planet for several years is that phenomena that vary with time can be studied as they evolve. Saturn鈥檚 atmosphere produces ever changing storm systems, for instance, which Cassini will be able to track, and there are known to be seasonal changes in Titan鈥檚 atmosphere. Voyager images of Titan showed that the northern hemisphere was darker, whereas recent ground-based observations have shown that half a seasonal cycle later, the southern hemisphere is darker.
Even Saturn鈥檚 ring system shows detectable changes over a period of months. For instance, the faint F ring has an intricate, braided appearance, which changed in the nine months between the Voyager 1 and 2 encounters. Disturbances caused by the gravity of small satellites are thought to be responsible for this.
The satellites Janus and Epimetheus, which share approximately the same orbit and almost meet every four years, are due for an encounter in February 2006. Their gravitational interaction will shift the orbit of Janus inwards by about 10 kilometres, while Epimetheus moves outwards by about 40 kilometres. From the response of features in the A ring to this event, researchers will be able to deduce some of the physical properties of the ring system, such as its effective viscosity. From mesurements of the distance between the satellites at their closest approach, it should also be possible to calculate the mass of both satellites more accurately than ever before.
Details of the ring plane crossing are available from Rings Node of NASA鈥檚 Planetary Data System at