296 URANUS AND ITS MOONS
Umbriel
Anear twin of Ariel in terms of size, Umbriel orbits somewhat further away from Uranus, circling the planet and spinning on its axis in 4.14 days. In contrast to bright Ariel, Umbriel is the darkest of the major satellites, with a slightly blue tint to its cratered surface. Broad canyons and variations in average brightness between different areas of the crust testify to activity shortly after the moon’s formation, but the density of craters suggests that Umbriel has been geologically dead since the Late Heavy Bombardment. With an icier composition than Ariel, it was too small to retain much heat from its birth, while its orbit ensured it never experienced tidal heating.
A few large craters stand out from the rest due to their bright walls or central peaks; the most prominent of these, Wunda, is 131 km (81 miles) in diameter and sits on the moon’s equator. The bright areas are probably freshly exposed water ice, but the absence of icy ejecta forming bright rays around these craters remains a mystery.
298 URANUS AND ITS MOONS
Titania
The largest Uranian satellite, Titania has a diameter of
1,578 km (981 miles) and resembles a bigger version of Ariel, with a relatively bright surface scored by deep canyons, and signs of widespread resurfacing once the worst of the early bombardments had subsided. Titania’s density suggests it also has a similar composition to Ariel, with an equal mix of rock and ice. Combined size and rocky composition probably allowed Titania to retain heat from its formation and sustain geological activity, such as cryovolcanism, for some time (it orbits too far from Uranus to have ever experienced tidal heating). If the water ice is mixed with ammonia (as many think is possible given the chemical composition of the ice giants themselves), then its freezing
point could be lowered considerably – perhaps enough for a subterranean ocean to survive today. Further questions surround the detection of carbon dioxide frosts alongside water ice on the surface – it’s unclear whether the gas is produced by the action of solar radiation on surface minerals or whether it somehow escapes from reservoirs locked within the interior.
300 URANUS AND ITS MOONS
Oberon
The outermost major moon of Uranus, Oberon is barely
50 km (31 miles) smaller than Titania, and has a similar
rock-ice composition. Images obtained by Voyager 2 suggest that it shares many of the features common among Uranian moons, including broad ‘chasmata’, heavy cratering and a roughly equal rock-ice composition. In terms of overall brightness, Oberon is the second-darkest moon after Umbriel, but its surface displays considerable variation. Bright rays, most likely formed as subsurface ice sprayed out during impact, blanket the landscape around many of the largest craters, but the crater floors are noticeably darker than other parts of
the crust. The true nature of these dark patches is unknown – they may represent upwellings of material from cryovolcanic eruptions that followed major impacts or be due to the impacts themselves exposing a darker interior layer as they burrowed deeper. A prominent mountain seen on the moon’s edge towers some 11 km (7 miles) high; it is probably the central peak of an unseen impact basin several hundred kilometres in diameter.
302 URANUS AND ITS MOONS
Moons of Uranus
Name
Diameter
Orbital period (days)
Eccentricity (circular = 0)
Cordelia
40 km (25 miles)
0.34
0.0003
Ophelia
42 km (26 miles)
0.38
0.0099
Bianca
54 km (34 miles)
0.43
0.0009
Cressida
82 km (51 miles)
0.46
0.0004
Desdemona
70 km (43 miles)
0.47
0.0001
Juliet
106 km (66 miles)
0.49
0.0007
Portia
140 km (87 miles)
0.51
0.0001
Rosalind
72 km (45 miles)
0.56
0.0001
Cupid
18 km (11 miles)
0.61
0.00007
Belinda
90 km (56 miles)
0.62
0.0001
Perdita
26 km (16 miles)
0.64
0.0012
Puck
162 km (101 miles)
0.76
0.0001
Mab
24 km (15 miles)
0.92
0.0025
304 URANUS AND ITS MOONS
Name
Diameter
Orbital period (days)*
Eccentricity (circular = 0)
Miranda
240x234x233 km (149x146x145 miles)
1.41
0.0013
Ariel
581x578x578 km (361x358x359 miles)
2.52
0.0012
Umbriel
1169 km (727 miles)
4.14
0.0039
Titania
1578 km (980 miles)
8.71
0.0011
Oberon
1523 km (946 miles)
13.46
0.0014
Francisco
22 km (14 miles)
267 (R)
0.1324
Caliban
72 km (45 miles)
580 (R)
0.1812
Stephano
32 km (20 miles)
677 (R)
0.2248
Trinculo
18 km (11 miles)
758 (R)
0.2194
Sycorax
150 km (93 miles)
1283 (R)
0.5219
Margaret
20 km (12 miles)
1695
0.6772
Prospero
50 km (31 miles)
1977 (R)
0.4445
Setebos
48 km (30 miles)
2235 (R)
0.5908
Large Blackboard
Ferdinand
20 km (12 miles)
2823 (R)
0.3993
* R = Retrograde orbit
Neptune
The solar system’s outermost major planet, Neptune orbits the Sun at an average distance of 30.1 AU. Slightly smaller than Uranus, with a diameter of 49,244 km (30,600 miles), it
is also a distinctly deeper shade of blue. This colour is in part due to the light-absorbing effects of methane, but since
the proportion of the gas in each planet’s atmosphere is the same, some other chemical must also play a role in generating Neptune’s deeper colour.
Neptune’s axial tilt of 28.3 degrees creates an Earth-like, though extremely elongated, cycle of seasons as the planet orbits the Sun every 164.8 years. In the absence of extreme temperature differences between the poles (thought to suppress activity on Uranus, see page 290), Neptune’s ‘normal’ weather patterns were on clear display during Voyager 2’s 1989 flyby. These range from deep, dark storms to fast-moving white clouds, formed by assorted chemicals condensing at different levels in the atmosphere.
306 NEPTUNE AND ITS MOONS
page 290
Neptune’s storms
Despite the feeble effects of heat from the Sun, Neptune is a surprisingly active planet, with violent weather powered in large part by heat escaping from the planet’s interior. The slow contraction of slushy ices in Neptune’s mantle triggers changes in the chemistry of compounds such as methane, releasing huge amounts of energy so that the planet radiates about 2.6 times more heat than it receives from the Sun.
Wind speeds of up to 600 metres per second (1,300 miles per hour) wrap narrow bands of wispy, high-altitude cloud around
Neptune, and also propel compact storms of white cloud known as scooters. Larger storms take the form of dark oval spots – the so-called Great Dark Spot seen by Voyager 2 was originally assumed to be similar to Jupiter’s long-lived Great Red Spot (see page 204). But these storms seem to last for just a few years at most, and instead of being high-altitude features, they actually seem to be clearings into the deeper, darker layers of Neptune’s atmosphere.
308 NEPTUNE AND ITS MOONS
page 204
Rings of Neptune
Like all the giant planets, Neptune’s gravity traps small particles in orbit around it, to form a system of rings.
Narrow, dark and elusive, Neptune’s rings have a reddish hue that hints at the presence of organic chemicals, such as methane ice. Early Earth-based attempts to spot occultations (dips in the light of distant stars) caused by intervening rings often produced contradictory results, and many scientists suspected that any rings might take the form of incomplete arcs. Voyager 2’s flyby proved this was not too far from the truth – while Neptune’s rings can be traced all the way around the planet, much of their material seems to clump together in certain regions, influenced by the gravity of shepherd moons. Five distinct rings are now recognized, each named after an astronomer who made an important early contribution to studies of Neptune. However, as with the rings of Uranus, the system seems to be relatively young and unstable – images taken in the decades since the Voyager flyby have shown a dramatic deterioration in their brightness and consistency.
310 NEPTUNE AND ITS MOONS
A backlit image from Voyager 2 reveals the uneven brightness of Neptune’s rings.
Triton
In contrast to the other giant planets, Neptune’s satellite system is dominated by a single giant world that dwarfs 13 other known moons. Furthermore, the lone giant Triton (some 2,700 km or 1,680 miles wide) follows a retrograde orbit, circling the planet the ‘wrong way’. This strongly suggests that it is a once-independent ice dwarf world that was captured into orbit around Neptune with catastrophic consequences for the planet’s original satellites. With a surface temperature of around –235°C (–391°F), Triton is one of the coldest places in the solar system and we might expect it to be a deep-frozen, ancient world. In fact, its surface shows great variety, with few craters, a bright ‘polar cap’ at the south pole and a curious, pitted, ‘cantaloupe terrain’ covering much of one hemisphere. The 1989 Voyager 2 flyby even revealed active geysers spewing dust-laden nitrogen gas into a thin atmosphere. Triton’s surprisingly active geology is probably linked to the events surrounding its capture – tidal heating that melted its interior allowed its heavier elements to form a core, from which heat is still escaping to this day.
312 NEPTUNE AND ITS MOONS
Proteus
Neptune’s second-largest moon, Proteus has an average diameter of 420 km (261 miles), although its shape is distinctly elongated along an axis that points towards its parent planet. The moon’s dark surface is pitted with countless craters, the largest of which, known as Pharos, is itself about 240 km (150 miles) across. Such a large impact should have been enough to shatter a solid world, and this fact, along with Proteus’s shape, offers another clue to the moon’s true nature.
Proteus is thought to be a second-generation moon that coalesced out of debris created in the chaos of Triton’s arrival in the Neptunian system. Computer models suggest that even close to the planet, Neptune’s original moons were unlikely to have survived intact (most likely falling victim to collisions with other satellites), but once the havoc subsided and Triton’s orbit developed its current circular shape, new moons could form
at a safe distance. Loosely structured and shaped by the tidal influence of Neptune itself, Proteus is the largest of these.
314 NEPTUNE AND ITS MOONS
Nereid
The third-largest moon of Neptune (and the second to be discovered), Nereid follows an elliptical path that ranges between 1.37 million km and 9.64 million km (851,300 and 5.99 million miles) from Neptune. This kind of extreme orbit is typical of the outer solar system’s ‘irregular’ moons (asteroids and comets captured into orbit around the giant planets). However, Nereid’s diameter of roughly 340 km (211 miles) would be unusually large for such a captured world. Furthermore, the properties of sunlight reflected off its surface are distinctly different from those seen in centaurs (see page 338). In fact, Nereid’s surface composition seems to be a closer match
for Uranus’s moons Titania and Umbriel. A smaller irregular satellite called Halimede, with a similar surface to Nereid, may be a fragment of the larger moon broken off during a collision. Coupled with the traumatic history of the Neptune system, all this evidence raises the intriguing possibility that Nereid is a surviving member of Neptune’s original satellite family, ejected to its current orbit by the arrival of Triton.
316 NEPTUNE AND ITS MOONS
page 338
Even Voyager 2’s most detailed image of Nereid shows it as little more than a blurry dark shape at the limits of Neptune’s gravitational grasp.
Moons of Neptune
Name
Diameter
Orbital period (days)*
Eccentricity (circular = 0)
Naiad
48x30x26 km (30x19x16 miles)
0.29
0.0003
Thalassa
54x50x26 km (34x31x16 miles)
0.31
0.0002
Despina
90x74x64 km (56x46x40 miles)
0.33
0.0001
Galatea
102x92x72 km (63z57x45 miles)
0.43
0.0001
Larissa
108x102x84 km (67x63x52 miles)
0.55
0.0014
S/2004 N1
20 km (12 miles)
0.95
0
Proteus
1.12
0.0004
220x208x202 km (137x129x126 miles)
Triton
2707 km (1682 miles)
5.88 (R)
0.000016
Nereid
340 km (211 miles)
360
0.7512
Halimede
60 km (37 miles)
1880 (R)
0.571
Sao
40 km (25 miles)
2914
0.293
Laomedeia
40 km (25 miles)
2168
0.424
Psamathe
40 km (25 miles)
9116 (R)
0.45
Neso
60 km (37 miles)
9374 (R)
0.495
* R = Retrograde orbit
Blackboard label
Interior of Neptune
Mantle of water, methane and ammonia ices
Silicate rock and nickel-iron core
Upper atmosphere with storms and clouds
Comet orbits
The vast majority of comets orbiting the Sun reside far beyond the realm of the planets, following more or less circular paths within the enormous spherical halo of the Oort Cloud (see page 370). Those that enter the inner solar system, however, follow highly elongated ellipses that bring them
close to the Sun at one end (perihelion), and are grouped into several families depending on their orbital characteristics. Those with orbits of less than 200 years (known as ‘short-period’ or ‘periodic’ comets) have their aphelion – the furthest point in their orbit – among the giant planets or just beyond
in the Kuiper Belt. ‘Long-period’ comets reach aphelion much further out, perhaps returning to the Oort Cloud from whence they came. ‘Hyperbolic’ comets are flung out of the solar system completely after their single encounter with the Sun. The gravitational influence of the giant planets, particularly Jupiter, can transform a long-period comet into a short-period one, and even circularize its orbit, making it virtually indistinguishable from
a centaur (see page 336) or asteroid.
320 COMETS AND CENTAURS
page 370
page 336
Comet Halley (orbit 76 years)
Typical long-period comet (orbitc.17,000 years)
Comet Tempel 1 (orbit 5.6 years)
Comet Ikeya-Seki
The most spectacular comet of recent decades, Comet Ikeya-Seki was discovered in September 1965, as it was already about to cross inside Earth’s orbit bound for a rendezvous with the Sun. Like all comets, it spends most of its orbit in a dormant state, and only becomes active when, heated by the Sun, ice in its solid nucleus vaporizes to form a huge gas halo called a ‘coma’. Escaping jets of vapour are caught up on the solar wind and blown away from the Sun to form an elongated tail. Ikeya-Seki’s activity was particularly impressive – the comet rapidly grew bright enough to see in daylight, and its tail extended across more than 113 million km (70 million miles) of space. On 21 October, as the comet passed within 450,000 km (280,000 miles) of the Sun, it broke into three fragments that were tracked on their retreat into the outer solar system on orbits that will see them return in about 1,000 years. Curiously, astronomers think something like this happened to the comet once before – it is a member of the ‘Kreutz sungrazers’, a family of comets in similar orbits, thought to originate from the break-up of an earlier ‘great comet’ seen by skywatchers in 1106.
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