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274 SATURN AND ITS MOONS
A Cassini radar image of lakes around Titan’s north pole
Hyperion
Orbiting Saturn some way beyond Titan, Hyperion is a misshapen moon some 360 km long and 266 km wide (224 × 165 miles). This oval shape is unusual for such a large body whose gravity should have pulled it into a spherical shape, so many astronomers suspect that Hyperion is just a large surviving fragment of a much larger moon that was broken up in an ancient collision. The unusual shape may also be related to Hyperion’s curious ‘chaotic’ rotation – it spins in unpredictable directions and at varying rates, in contrast to the orderly synchronous spin of neighbouring moons.
Hyperion’s appearance is also bizarre – a spongelike structure of bright, razor-sharp ridges surrounding dark-floored pits. This strange landscape is a result of slow erosion, where the weak heat of the Sun causes ice to sublimate from darker patches of the surface, leaving the surviving rocky component to crumble inwards slowly. This unique process may only be possible because the moon’s ancient interior has been exposed at the surface.
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276 SATURN AND ITS MOONS
Iapetus
Saturn’s third-largest moon, Iapetus is a world unlike any other in the solar system. Following its discovery in 1671, astronomers were puzzled by marked changes in brightness when seen at different positions in its orbit. The first space-probe flybys confirmed the long-held theory that the moon’s two hemispheres have very different colours; the satellite’s ‘leading’ face, which points along its orbit, is as black as coal, while its backward-facing or ‘trailing’ face is as bright as snow. Cassini images of Iapetus’s surface showed the boundary between light and dark terrains in much more detail. The dark surface seems to overlay the brighter one, with no shades of grey between them. One popular explanation suggests that the dark material is rocky ‘lag’ left behind by the slow evaporation of ice from parts of a generally bright surface. Since dark surfaces absorb more heat than bright ones, the process would ‘snowball’ once it was begun, but how did it start? The best current theory is that the leading hemisphere picks up a thin film of dark dust from the Phoebe Ring (see page 250) as it moves along its orbit.
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The equatorial ridge
Iapetus’s starkly contrasting hemispheres are not its only curious feature – images from the Cassini space probe revealed a long, straight ridge that runs around much of the equator, giving the moon an overall shape similar to a walnut. This equatorial ridge is sharpest where it crosses a dark terrain called Cassini Regio. This central section is some 1,300 km (800 miles) long (about one-third of the moon’s circumference), 20 km (12 miles) wide and up to 20 km (12 miles) high. In brighter regions, the ridge breaks down into smaller outcrops and mountains up to 10 km (6 miles) tall.
The origin of this remarkable feature is still hotly debated. One theory is that the ridge originated as an upwelling of ice from beneath the crust, and that tidal forces pulling on its extra mass slowly changed the moon’s orientation to put it on the equator. Another is that the ridge is a remnant of a period when Iapetus rotated much faster and bulged out around its equator. A third alternative is that the ring is the remnant of a ring system that once orbited Iapetus and subsequently collapsed.
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280 SATURN AND ITS MOONS
Phoebe
large gap separates Iapetus, the outermost of Saturn’s
original satellites (which formed alongside the planet) from Phoebe, the largest and innermost of the many ‘irregular’ satellites that have been captured throughout the planet’s long history. Phoebe’s orbit, an average of almost 13 million km
(8.1 million miles) from Saturn, is distinctly elliptical and also retrograde, orbiting the ‘wrong way’ around the planet.
When the Cassini probe flew past Phoebe in 2004, its images revealed an extremely dark, cratered world, roughly spherical and about 220 km (137 miles) across on average. Initial theories suggested that it was a captured asteroid, but relatively bright crater floors hint at the presence of ice just beneath the surface, and many astronomers now suspect the moon actually began life as a centaur (see page 26). Dark material that has been knocked off Phoebe’s surface by small meteorite impacts now forms the huge outer Phoebe ring around Saturn, spiralling inwards where some of it is swept up by Iapetus.
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282 SATURN AND ITS MOONS
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Moons of Saturn
Name
Diameter
Orbital period (days)
Eccentricity (circular = 0)
S/2009 S1
0.6 km (0.4 miles)
0.47
0
Pan
17x16x10 km (11x10x6 miles)
0.58
0
Daphnis
4.3x4.1x3.2 km (2.7x2.5x2.0 miles)
0.59
0
Atlas
20x18x9 km (12x11x6 miles)
0.60
0
Prometheus
68x40x30 km (42x25x19 miles)
0.61
0.00204
Pandora
52x41x32 km (32x25x20 miles)
0.63
0.0042
Epimetheus
65x57x53 km (40x35x33 miles)
0.69
0.009
Janus
102x93x76 km (63x58x47 miles)
0.69
0.007
Aegaeon
1 km (0.6 miles)
0.81
0.0002
Mimas
396 km (246 miles)
0.94
0.0202
Methone
3.2 km (2 miles)
1.01
0.0001
Anthe
2 km (1.2 miles)
1.04
0.001
Pallene
2.9x2.8x2.0 km (1.8x1.7x1.2 miles)
1.14
0.004
Enceladus
504 km (313 miles)
1.37
0.0045
Calypso
15x12x7 km (9x7x4 miles)
1.89
0.001
Telesto
16x12x10 km (10x7x6 miles)
1.89
0.001
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Tethys
1062 km (660 miles)
1.89
0
Helene
22x19x13 km (14x12x8 miles)
2.74
0.005
Dione
1124 km (698 miles)
2.74
0.0022
Polydeuces
1.5x1.2x1.0 km (0.9x0.7x0.6 miles)
2.74
0.0192
Name
Diameter
Orbital period
Eccentricity
Rhea
1526 km (948 miles)
4.52
0.001
Titan
5150 km (3200 miles)
15.95
0.0292
Hyperion
180x133x103 km (112x83x64 miles)
21.28
0.1042
Iapetus
1469 km (913 miles)
79.33
0.0283
Name
Diameter
Period *
Ecc.
Name
Diameter
Period *
Ecc.
Kiviuq
~14 km (~8.7 mi)
449
0.334
S/2004 S07
~6 km (~3.7 mi)
1140 (R)
0.58
Ijiraq
~10 km (~6.2 mi)
451
0.316
S/2006 S3
~6 km (~3.7 mi)
1227 (R)
0.471
Paaliaq
~20 km (~12.4 mi)
687
0.364
Kari
~6 km (~3.7 mi)
1234 (R)
0.478
Albiorix
~26 km (~16.2 mi)
783
0.469
Fenrir
~4 km (~2.5 mi)
1260 (R)
0.136
Bebhionn
~6 km (~3.7 mi)
835
0.469
Surtur
~6 km (~3.7 mi)
1298 (R)
0.451
Erriapus
~8 km (~5 mi)
871
0.474
Ymir
~18 km (~11.2 mi)
1312 (R)
0.335
Tarqeq
~6 km (~3.7 mi)
888
0.16
Loge
~6 km (~3.7 mi)
1313 (R)
0.187
Siarnaq
~32 km (~19.9 mi)
896
0.295
Fornjot
~6 km (~3.7 mi)
1491 (R)
0.206
Tarvos
~14 km (~8.7 mi)
926
0.531
Phoebe
109x109x102 km (68x68x63 mi)
548 (R)
0.164
Narvi
~6 km (~3.7 mi)
1004 (R)
0.431
Skathi
~6 km (~3.7 mi)
728 (R)
0.27
Bergelmir
~6 km (~3.7 mi)
1006 (R)
0.142
S/2007 S2
~6 km (~3.7 mi)
808 (R)
0.218
S/2006 S1
~6 km (~3.7 mi)
1015 (R)
0.13
Skoll
~6 km (~3.7 mi)
878 (R)
0.464
Suttungr
~6 km (~3.7 mi)
1017 (R)
0.114
Greip
~6 km (~3.7 mi)
921 (R)
0.326
Hati
~6 km (~3.7 mi)
1039 (R)
0.372
Hyrrokkin
~8 km (~5 mi)
932 (R)
0.333
S/2004
S12
~6 km (~3.7 mi)
1046 (R)
0.401
S/2004 S13
~6 km (~3.7 mi)
933 (R)
0.273
Bestla
~6 km (~3.7 mi)
1084 (R)
0.521
Mundilfari
~6 km (~3.7 mi)
953 (R)
0.21
Farbauti
~6 km (~3.7 mi)
1086 (R)
0.206
Jarnsaxa
~6 km (~3.7 mi)
965 (R)
0.216
Thrymr
~6 km (~3.7 mi)
1094 (R)
0.47
S/2007 S3
~6 km (~3.7 mi)
978 (R)
0.13
Aegir
~6 km (~3.7 mi)
1117 (R)
0.252
S/2004 S17
~4 km (~2.5 mi)
986 (R)
0.259
* R = Retrograde orbit
Uranus
Roughly twice as far from the Sun as Saturn, Uranus is the first of the solar system’s two ‘ice giant’ worlds – a distinct class of planets intermediate in size between rocky worlds and gas giants, and dominated by chemicals with low melting points, such as water and ammonia. With a diameter of 50,724 km (31,518 miles), Uranus is four times larger than Earth but still less than half the size of its inner neighbour, Saturn.
The 1986 flyby of the Voyager 2 space probe revealed Uranus as a near-featureless turquoise ball, its distinctive colour created by a small amount of methane in the planet’s outer atmosphere (2.3 per cent) that absorbs the red component of sunlight. However, studies with giant Earth-based telescopes have shown Uranus becoming far more active in the decades since Voyager’s flyby, with obvious storm features erupting in its atmosphere. This suggests that the probe just happened to encounter Uranus during a particularly placid phase in its
84-year orbit around the Sun.
286 URANUS AND ITS MOONS
Inside the ice giants
Astronomers classify planets such as Uranus (opposite)
and Neptune (shown in cross-section on page 319) as ‘ice giants’. While the gas giants Jupiter and Saturn consist almost entirely of hydrogen and helium, in Uranus and Neptune these elements only dominate the outer atmosphere. About 5,000 km (3,100 miles) beneath the surface, they give way to a mantle dominated by chemicals such as water, ammonia and methane. These compounds (known in chemical terms as ‘ices’ because of their relatively low melting points) form a churning, slushy mantle around a rocky, roughly Earth-sized core. Changing temperature and pressure at different levels in the mantle can produce unusual chemical changes. For example, in Neptune at least, methane is thought to disintegrate at great depths, releasing heat and pure carbon that crystallizes into a ‘rain’ of diamonds. Swirling electric currents moving through the ices produce strong magnetic fields, but these are not symmetrical – instead (in both Uranus and Neptune), they are not only tilted sharply to the axis of rotation, but also offset from the planet’s centre.
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Upper atmosphere with weather features
Mantle of slushy ices
Silicate rock and nickel-iron core
Atmosphere becomes richer in water, ammonia and methane with depth.
Tilted planet
Uranus’s most striking feature is its bizarre tilt, discovered
in 1977 through studies of the orbits of its moons, and confirmed by the discovery of a ring system that makes the planet resemble a bullseye. While most planets have their axis of rotation tilted only moderately (if at all) from ‘upright’, Uranus’s tips over at 98 degrees so that its north pole points slightly ‘downwards’ relative to its orbit. Astronomers suspect the planet’s strange tilt is due to gravitational interactions with the other giants early in its history (see page 42). Today, however,
it gives Uranus the strangest seasons of any planet. Regions close to the poles experience endless darkness througout
a 40-year winter followed by 40 years of endless daylight in summer. Equatorial regions, in contrast, experience a day-night cycle more in tune with Uranus’s rotation period of 17 hours 14 minutes. Around midwinter and midsummer, it’s thought
that the transport of heat around the planet, to balance out temperatures, suppresses the development of atmospheric features such as bands and storms.
290 URANUS AND ITS MOONS
page 42
A Hubble Space Telescope image of Uranus from 1999 reveals the planet’s tilted orientation and bright surface storms.
The ring system and inner moons
Uranus’s ring system is very different from the broad, bright planes seen around Saturn. Instead, the planet is encircled by 13 well-defined rings. The nine innermost of these are narrow but densely packed streams of orbiting particles; the next two are little more than dust trails; and the outer pair are diffuse and faint. Most of the ring particles have a distinctly reddish and unreflective coating, thought to be methane ice.
As in other ring systems, the streams of particles orbiting Uranus are influenced by nearby moons. The comparatively bright ε (epsilon) ring, for instance, is hemmed in by the influence of the shepherd moons Cordel
ia and Ophelia, while the outermost μ (mu) ring surrounds the orbit of another small moon, Mab. The rings show slight inclinations from the planet’s equator, and along with the small but measurable eccentricity of the ε ring, these suggest the system is not entirely stable.
Scientists are, therefore, fairly confident that the Uranian rings are less than 600 million years old.
292 URANUS AND ITS MOONS
An enhanced view of the inner rings as seen during the Voyager 2 flyby. The bright epsilon ring is on the right.
Miranda
Uranus’s family of 27 known satellites is dominated by five larger worlds. With a diameter of just 472 km (293 miles), Miranda is the smallest of these, and also the closest to Uranus. Its surface is a mishmash of different terrains, formed by a variety of processes at different times in the moon’s history. Verona Rupes in Miranda’s southern hemisphere, for example, is one of the tallest cliff escarpments in the solar system, towering up to 5 km (3 miles) high. Elsewhere, there are racetrack-like structures of parallel grooves, ancient and heavily cratered plains, Ganymede-like sulci (see page 224), and even regions where icy eruptions seem to have obliterated earlier craters in the recent past. All of this suggests a long history of geological activity and cryovolcanism that may still be ongoing, but is hard to explain in terms of the ‘tidal heating’ effect found on moons such as Io. One possibility is that Miranda’s orbit was once considerably more eccentric than it is now, driven by the influence of the outer moon Umbriel. This would have generated sufficient tidal heating to warm the moon’s icy core, perhaps so much that it still hasn’t entirely cooled down.
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Ariel
The second major moon of Uranus, Ariel is considerably larger than Miranda, with a diameter of 1,158 km (720 miles). It has the brightest surface of any Uranian moon, generally grey in colour, but with a slight reddening on its leading hemisphere due to bombardment by particles trapped in the magnetosphere or by its parent planet. Based on estimates from crater counts, Ariel’s surface is also the youngest of any Uranian moon, suggesting that cryovolcanic activity has resurfaced much of the moon with a fresh coating of ice since its early days. In places, deep canyons cut across the landscape – the largest, Kachina Chasmata, is 622 km (386 miles) long and up to 50 km (31 miles) wide. As with Miranda, Ariel’s active history is hard to explain in terms of the tidal forces acting on the moon today, but computer models suggest that the satellite went through a period of orbital resonance with the larger Titania early in its history. After this produced significant tidal heating, Ariel’s unusually high rock content may have been sufficient to trap heat that persists to the present day.
Solar System in Minutes Page 11