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A and B rings
Saturn’s rings mostly bear alphabetical letters, in a scheme
that is partly based on their distance from the planet, but also affected by their order of discovery. The A ring is the largest, easily seen from Earth with an outer edge 136,800 km (85,000 miles) from the centre of Saturn; the equally bright B ring lies just inside it, with an inner edge about 92,000 km (57,100 miles) from the centre. The rings are separated by the Cassini Division, an apparent ‘gap’ in the system some 4,700 km (2,920 miles) wide, while the narrower Encke Division subdivides the A ring itself. Such gaps are not entirely empty, but the thinner material orbiting within them appears much darker due to the contrast with its surroundings.
The A and B rings are bright because they are dominated by house-sized chunks of highly reflective water ice. Astronomers still aren’t entirely certain where these particles came from, but one plausible theory is that they were created when a large outer moon, spiralling in towards Saturn, was broken up by tidal forces and stripped of its outer layers.
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C and D rings
Orbiting closer to Saturn than the bright A and B rings lie two much fainter structures. The C or ‘crepe’ ring directly adjoins the B ring on its inner edge, while the tenuous D ring stretches down to a few thousand kilometres above Saturn’s cloud tops. Particles in these two rings are much smaller and more scattered than those in their outer neighbours, and both rings are remarkably thin (as little as 5 m/16.4 ft deep on average), rendering them partially transparent.
Exactly why different rings have different-sized particles
is still uncertain – one theory is that collisions between ice boulders in the brighter rings create grains of debris that slowly spiral inwards and, ultimately, sift down into Saturn’s upper atmosphere. Certainly, structure in this region evolves over time – the Cassini probe tracked the motion of wavelike ‘corrugations’ at 30-km (19-mile) intervals across the inner rings, a pattern that may have been created by an injection of debris from a disrupted comet in the early 1980s.
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The F ring and Prometheus
The fine structure of Saturn’s rings is defined by a huge variety of gravitational influences. While the giant planet’s gravity is dominant, the gravitational tug of war created by the influence
of Saturn’s many moons and countless ‘moonlets’ orbiting among the rings produces distinctive gaps and ringlets. In particular, gaps are found where the orbital periods of particles would ‘resonate’ with those of certain moons, in a similar process to the formation of the asteroid belt’s Kirkwood gaps (see page 162).
The effect is at its most obvious in the F ring – a sharply defined ring just a few tens of kilometres wide that orbits just beyond the A ring. Here, particles are ‘shepherded’ into a single long-lived structure by the influence of the moon Prometheus orbiting on its inner edge. However, particles in the ring are subjected to constantly changing gravity from more distant moons, creating short-lived ripples and wavelike patterns. Prometheus is one of several known ‘shepherd’ moons that help maintain structure in both Saturn’s ring system and those of other planets.
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Outer rings
Beyond the F ring, Saturn’s rings are looser and more diffuse. The first of these outer structures, known as the E ring, was discovered around the orbit of Enceladus in the 1960s, but many more have been found by space probes. Incomplete ‘ring arcs’ close to the planet often follow the orbits of very small moons and comprise microscopic particles of dust and ice blasted from the surface of the moons by ‘micrometeoroid’ impacts. The E ring, in contrast, is thought to be linked to geyser activity on Enceladus (see page 258). The curious G ring has a bright core (a cloud of metre-scale boulders centred on the small moonlet Aegeon), coupled with a dusty arc to complete the rest of the ring.
In 2009, astronomers using NASA’s infrared Spitzer Space Telescope discovered the largest ring of all, a huge dust cloud produced by impacts on Saturn’s outer moon Phoebe. Tilted at 27° to the other rings, it extends up to 12.4 million km (7.7 million miles) from the planet itself, and may be linked to the curious features of Phoebe’s inner neighbour, Iapetus (see page 278).
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page 258
page 278
Epimetheus and Janus
Lying just beyond the major rings, Epimetheus and Janus are a unique pair of moons that effectively ‘timeshare’
their orbits around Saturn. Heavily cratered and oval in shape, Janus has an average diameter of 179 km (111 miles), while the 145-km-diameter (90 mile) Epimetheus is more sharply chiselled – a clue that both worlds may be the broken remnants of a larger predecessor. The moons were both spotted for the first time in 1966, but were at first mistaken for a single object – an error that was not corrected until 1978.
The two moons coexist by a unique orbital dance – at times, Janus orbits about 50 km (31 miles) closer to Saturn than Epimetheus, but this means that it completes its orbit slightly faster. Every four years, as the two moons close in on each other, their mutual gravity slows the inner moon down and pulls it outwards, while accelerating the outer moon and causing it to drift inwards. Eventually, the two swap orbits and begin to move apart.
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A Cassini image shows Epimetheus against the larger and more distant Titan, above Saturn’s rings.
Mimas
The moons of Saturn grow larger out to the orbit of Titan, then smaller once again. The first of the ‘mid-sized’ moons (with orbits of a few hundred kilometres), is Mimas. At just 400 km (250 miles) across, it is the smallest known world in the solar system with sufficient gravity to pull itself into a sphere. This is largely due to composition – since Mimas is made mostly of ice, it is far easier to pull into shape than a rockier world.
Mimas is covered in craters; the largest, called Herschel, is some 130 km (80 miles) wide and 10 km (6 miles) deep. Herschel dominates an entire hemisphere, while on the moon’s opposite side, huge fractures have formed in line with the crater rim. Yet despite a location directly in the firing line for objects drawn in by Saturn’s gravity, Mimas’s surface is not entirely saturated with craters. This suggests the moon was once resurfaced
by a form of icy geological activity known as ‘cryovolcanism’– eruptions of ice or water (perhaps mixed with ammonia to lower its freezing point) that wiped away many of the earliest craters.
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Enceladus
Beyond Mimas, and past a handful of recently discovered small moonlets, Saturn’s next mid-sized satellite is the intriguing Enceladus. Slightly misshapen, but with an average diameter of 504 km (313 miles), Enceladus has a brilliant-white surface (the most reflective in the solar system) and far fewer craters than might be expected.
Enceladus’s landscape is blanketed in snow, created as water bursts in geyser-like eruptions from just beneath the surface, and freezes on exposure to the cold of space. Some areas are distinctly more cratered than others, suggesting that snows have fallen on different parts of Enceladus at different times in its history, while smooth plains may be a result of widespread cryovolcanic eruptions overwriting earlier terrain. Enhanced-colour images, meanwhile, reveal distinct variations in colour across the landscape, the most obvious of which are the bluish ‘tiger stripes’ near the moon’s south pole, associated with the geyser outbursts.
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Geysers of Enceladus
The geysers of Enceladus, like those on other planets, form when a reservoir of liquid heated to above its natural boiling point escapes through cracks in the surface, boiling suddenly and violently. In this case, the geysers’ eruptions are powerful enough to fling material free of the moon’s weak gravity, where it fuels Saturn’s tenuous E ring. When NASA’s Cassini probe flew through a geyser ‘plume’ in 2008, it discovered that it was mostly composed of pure water (with some traces of organic chemicals), escaping from a global ocean layer.
Enceladus’s activity is thought to be driven by the same tidal heating seen among the satellites of Jupiter; in this case, Enceladus is caught in a gravitational tug-of-war between Saturn and the outer moon Dione. Currently, geyser activity
is concentrated around the tiger stripes of the southern hemisphere, but it may migrate over time, and there are still some puzzles – principally, Enceladus seems to be warmer than the Saturn–Dione mechanism alone can explain.
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Tethys
With a diameter of 1,062 km (660 miles), Tethys represents a step up in size from Saturn’s innermost moons, and is a near-twin of Dione, the next moon out. Made almost entirely from ice, its density is actually less than that of water.
Tethys’s surface is brilliant white, particularly on the leading hemisphere that faces ‘forward’ along its orbit. Here, the landscape is covered in a thin layer of ‘snow’ swept up from the outer edge of the E Ring (itself fuelled by the geysers of Enceladus). A broad swathe of terrain with fewer craters and a slightly darker colouring suggests that early in its history, icy eruptions wiped away some of Tethys’s older craters. One hemisphere is dominated by the 400-km (250-mile) Odysseus crater. This vast but shallow impact basin has flattened over time as its ice has slumped back towards the average surface level at glacial speed. Elsewhere, faults and cracks in the surface show where the crust expanded as the interior froze. The longest of these is the 2,000-km (1,250-mile) Ithaca Chasma.
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Telesto and Calypso
Remarkably, Saturn’s mid-sized satellite Tethys shares its orbit with two other, much smaller worlds. Telesto and Calypso orbit Saturn precisely 60 degrees in front of and behind Tethys, respectively (in a rare example of a truly circular orbit). These positions put them at two of the Saturn–Tethys system’s Lagrangian points, where the gravitational influence of the
two larger worlds balances in such a way as to permit a stable orbit. Quite how these moons ended up in their current orbit
is uncertain – they may be fragments of a larger satellite that broke up and were pushed towards their present locations by tidal forces, or they might even have formed alongside Tethys.
Physically, both moons resemble small misshapen asteroids, with average diameters of 25 km (15 miles) for Telesto (shown opposite) and 21 km (13 miles) for the slightly more elongated Calypso. Their surfaces are bright and unusually smooth, with traces of all but the largest craters smothered in a blanket of icy dust that they accumulate from Saturn’s E ring.
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Dione
With a diameter of 1,123 km (698 miles), Saturn’s moon Dione seems an obvious twin to its inner neighbour Tethys.
It even has a similar pair of ‘co-orbital’ satellites, known as Helene and Polydeuces. However, while Tethys and Dione appear similar at first glance, closer inspection reveals some important differences. Space-probe flybys have confirmed that Dione is denser and considerably rockier than Tethys (although it still contains substantial ice). This may have allowed Dione to remain warmer and geologically active for longer, since it shows signs of more sustained icy cryovolcanism.
Like Tethys, Dione shows a history of heavy cratering, especially on its leading hemisphere. The trailing hemisphere, however, is criss-crossed by bright streaks of so-called ‘wispy terrain’. In reality, these are towering cliffs up to several hundred metres high, with a reflective face of brilliant ice. They show that, at some point in its history, Dione endured huge tidal stresses that warped and broke its crust.
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Rhea
Saturn’s second-largest satellite, with a diameter of 1,528 km (949 miles), Rhea bears a striking resemblance to Dione. It, too, has a heavily cratered forward-facing surface and a darker trailing hemisphere criss-crossed by bright wispy terrain created by geological faulting. Craters on Rhea, however, are rather better defined than those on either Dione or Tethys, suggesting that its ice is less prone to slumping over time, perhaps because it is compressed to a greater density. Crater sizes vary between different parts of the moon, with some areas showing a mix of sizes and others only smaller impacts – evidence that Rhea was not always so deeply frozen, and that ice once erupted to wipe away some of the larger, earlier craters.
Scientists analysing space-probe images also identified another curious feature – a faint line of brighter material dotted around the planet’s equator. One possible explanation is that Rhea once had a tenuous ring system, whose particles have since fallen to the surface, but the theory remains unproven.
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Titan
The largest satellite in the Saturnian system, Titan has a diameter larger than the planet Mercury, and dwarfs all of its neighbours. It circles Saturn every 15.9 days, and spins on its axis in the same period.
Titan’s strong gravity, coupled with the cold conditions some
10 AU from the Sun, allows the moon to hold onto a substantial atmosphere (something that the more massive Ganymede cannot do because of its warmer surface). As a result, Titan
is shrouded in a dense orange haze that conceals its surface.
It was only when the Cassini space probe arrived in the
mid-2000s that near-infrared cameras finally pierced the atmosphere to reveal an eerily Earth-like world beneath. Titan’s landscape mixes elevated continent-like regions with low-lying basins, river- and lakebeds and very few craters. While the moon appears to be made of a similar rock-ice mix to Saturn’s other satellites, it clearly has a complex layered structure that gives rise to a variety of surface activity.
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The atmosphere of Titan
Titan’s atmosphere, like Earth’s, is dominated by the unreactive gas nitrogen. However, a small amount of methane (CH4), amounting to just 1.4 per cent by volume, renders the atmosphere opaque by producing clouds and a high-altitude haze that leaves the surface in a permanent orange twilight. Overall, the dense atmosphere exerts a pressure 1.45 times greater than Earth’s own air at the satellite’s surface.
The source of Titan’s methane is something of a mystery, since unstable molecules in the gas break down when bombarded by solar radiation. Even at this distance from the Sun, the effect should be enough to remove Titan’s methane completely within about 50 million years, so the gas must be replenished. As with the methane on Mars (see page 154), there are two plausible sources familiar from Earth – volcanic activity (most likely ongoing low-temperature cryovolcanism) or methane-producing microorganisms. A third possibility is that fresh methane somehow ‘seeps’ into the atmosphere from Titan’s icy interior.
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This Cassini image captures complex haze layers in Titan’s upper atmosphere.
Titan’s surface
The first close-up views of Titan’s surface came from the European Space Agency’s Huygens lander, which was released from the Cassini probe on its
arrival at Saturn and parachuted into the moon’s atmosphere in January 2005.
Huygens landed just off the ‘coast’ of one of Titan’s elevated land masses. The probe was designed to float, in case it
found itself landing in a liquid ocean, but in fact the landing site turned out to be a wide, flat plain littered with pebbles between 5 and 15 cm (2 and 6 in) wide. The underlying ground had the consistency of an ice-covered snowfield, but both
soil and pebbles were tinted various shades of brown, most likely by hydrocarbon chemicals coating underlying water ice. The surface temperature was –179°C (–290°F), there was a gentle wind, and although there was no rainfall during the brief period that Huygens was able to send back data, the ground was damp – strong evidence to support the idea of a ‘methane cycle’ dominating Titan’s weather.
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A wide-angle view of Titan’s surface, captured during the descent of the Huygens lander
Titan’s methane cycle
Methane on Titan is thought to play a similar role to that of water on Earth. Conditions on the icy moon allow this simple hydrocarbon (chemical formula CH4) to exist in liquid, solid and gaseous forms. Methane frosts coat deep-frozen water ice in the surface soil and rocks (alongside a mix of other hydrocarbons), while atmospheric methane condenses to form clouds, which precipitate methane rain and snow back to the surface, gradually eroding and softening the landscape as they run downhill.
Although scientists were somewhat disappointed when Cassini and Huygens data showed that Titan lacked widespread oceans, radar mapping subsequently revealed substantial lakes of liquid methane around the moon’s north pole. Subsequent studies have suggested that the methane circulates in a climate cycle as complex as anything on Earth, accumulating in polar lakes during a particular hemisphere’s seven-year ‘autumn’, returning to the atmosphere in ‘spring’, and being transported to the opposite hemisphere on prevailing winds.
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