Friday, 24 February 2012

Uranus – World on its Side

Uranus Data
Mass: 8.68 x 1025 kg or 14.5 times Earth’s
Equatorial diameter: 51 118 km or 4.0 times Earth’s
Surface gravity: 1.17 gees
Axial tilt: 97.9°
Mean surface temperature: -214 Celsius
Rotation period: 17.23 hours
Orbital period: 84.0 years
Inclination of orbit to ecliptic: 0.8°
Orbital eccentricity: 0.046
Distance from the Sun: 18.28–20.08 AU
Sunlight strength: 0.0024–0.0030 of Earth’s
Satellites:  >21
Largest satellite: Titania, diameter 1600 km

Once beyond Saturn, we must again journey nearly twice as far from the Sun to get to the next planet. There, 19 times further out than the Earth, we find Uranus – pale blue-green, remarkably featureless, and bitterly cold. Uranus is a mid-sized giant, roughly half the size of Saturn, a fluid blob of slushy ice with smaller quantities of rock and gas. Strangely, this planet rotates on its side, its spin axis lying almost in the plane in which it orbits. Its large array of satellites and rings, encircling the planet’s equator, are thus similarly inclined, their orbits virtually at right angles to the Solar System. It is almost as if a gigantic collision knocked Uranus sideways in a game of interplanetary billiards. Indeed, just such an event is astronomers’ favoured explanation for Uranus’ weird orientation. And it isn’t just Uranus that has su¤ered. The surfaces of some of its moons paint a similarly brutal history throughout the Uranian system.

Physical Overview
For almost 200 years, little was known about faraway Uranus. But, in 1986, NASA’s probe Voyager 2 arrived at the massive green planet. For the first time, astronomers saw Uranus not as a speck of light visible only to the keenest naked eye; they viewed it as a whole new world.

Uranus is another giant, fully four times larger than the Earth. But it is less than half the size of the gas giants and consequently very different. Uranus is relatively dense, so the materials that make it up must be somewhat heavier than the lightweight hydrogen found throughout Jupiter and Saturn. Most of the planet is almost certainly composed of ices – water, methane and ammonia. Because of the internal pressure and temperature, the ices are not solid. Instead, they surround a suspected rocky core in a deep, slushy ocean that occupies two-thirds of the planet’s interior. Thus, like Jupiter and Saturn, Uranus is a fluid world; but it is an ice giant, not a gas giant. Pure hydrogen (not that locked up in water, methane or ammonia) and helium make up only 15 per cent of its mass, compared with 80 per cent in Jupiter. And unlike on that world, where the hydrogen is almost ubiquitous, in Uranus it exists only in the atmosphere or in a comparatively thin ‘mantle’ region between the icy slush and the atmosphere. The atmosphere, meanwhile, is strikingly bland – a featureless green. The grandiose colours and bands that typify the gas giants are absent. The reason for this is the low temperature. Uranus, twice as far from the Sun as Saturn, is unbearably cold, so frigid that its clouds condense very low down in its atmosphere, where it is warmer. The clouds are so deep that other atmospheric layers hide them. The green colour, meanwhile, comes from a layer of methane high in Uranus’ atmosphere. This gas absorbs red light and reflects primarily blue and green.

Perhaps Uranus’ oddest aspect is its axial inclination. While the Earth is tilted with respect to its orbital plane, the ecliptic, by 23.5 degrees, Uranus lies at an angle of nearly 98 degrees. During summer in the north, the northern hemisphere is pointed within only 8 degrees of the Sun. The south-polar regions then endure a bitter, sunless night that lasts for almost 21 years. After that time, when the planet has completed one-quarter of its 84-year orbit, its equatorial regions then face the Sun as they do on a ‘normal’ planet. Then, 21 years later, the north pole is plunged into darkness while the south pole enjoys its long summer – if you can call it that. Only Pluto matches Uranus for these bizarre seasonal variations. And it is because of Uranus’ strange tilt that the planet resembles – with its system of rings – a vast target in space.

Ring System
Uranus’s rings are different from those of the gas giants, which are in turn different from each other. Its most substantial rings are the so-called ‘classical’ ones – the nine that were discovered from Earth in 1977. The fragments that make up these rings are typically metre-sized boulders, a little larger in size than the inhabitants of Saturn’s great accoutrements. But in stark contrast to the bright, icy particles in Saturn’s rings, those that populate the Uranian versions have exceptionally dim surfaces. They reflect only 4–5 per cent of incident sunlight and are thus about as dark as chunks of coal. In addition to and interspersed with the nine main rings, Uranus has a whole range of others, too transparent to be seen from Earth. These are just as dark as the classical rings but they are made up of far smaller particles – dust grains, like those in the rings of Jupiter.

All of these rings, including the classical ones, are extremely narrow. Most are no more than 10 kilometres in radial extent, and even the widest spans only 100 kilometres – 0.2 per cent of the diameter of the planet. They are kept so narrow because Uranus, like Saturn, plays host to a series of socalled shepherd satellites – tiny moons, mere tens of kilometres across, whose gravitational influences herd the ring particles and prevent the rings from spreading out.

Uranian Satellites
Unlike Jupiter, Saturn and – as we shall see – Neptune, Uranus has no very massive satellites. Its largest five measure between just 480 kilometres and 1600 kilometres across, much smaller than the Earth’s Moon. From the innermost outwards, they are Oberon, Titania, Umbriel, Ariel and Miranda. Umbriel and Oberon are both heavily cratered, and their surfaces appear to have been flooded by icy ‘lava’ long ago in the past. Titania and Ariel are cratered too, but their surfaces are also riddled with vast cracks and faults, evidence of past tectonic activity perhaps brought about by tidal heating, as on the Galilean satellites. Lastly, Miranda, the smallest, has very likely the strangest surface in the entire Solar System. Put simply, it looks like a patchwork. Adjacent areas, separated by sharp boundaries, seem to belong to different worlds. One possible explanation for its jumbled appearance is that Miranda suffered a collision so devastating that the moon shattered and re-formed in orbit. Alternatively its appearance could also have been caused by internal melting. Miranda and its four cousins all have fairly low densities, yet they are a bit denser than Saturn’s moons. They have a bit more rock than ice. But their surfaces are fairly dark because of dirt spread by aeons of impacts.

Aside from these classical satellites Uranus has at least 16 others. Eleven of them are located between Miranda and the rings, and they are regular – that is, they orbit in a similar plane and in the same direction in which the planet rotates. These moonlets truly are puny, between 13 kilometres and 77 kilometres across. They have dark surfaces, and compositions of ice and rock like their large cousins. Meanwhile, Uranus also has five irregular satellites. They orbit the planet at extreme inclinations, in different directions, and are very far from it – many millions of kilometres away. These moons, all of them again small, are all probably captured comets or icy planetesimals, and thus have different compositions from the regular satellites.

History of the Uranian System
As with Saturn and Jupiter, it is not easy to deduce Uranus’ past because the planet has no solid surface, and thus no way of recording the events that may have shaped its evolution. Instead, astronomers rely on their models of the formation of the planets to deduce how Uranus formed and evolved.

The arrangement of Uranus’ regular satellites – those which orbit in the planet’s equatorial plane – hint that the Uranian system formed from a disc of material like the Solar Nebula in miniature, as did Jupiter and Saturn. However, if the Uranian system did form from a disc, how is it that it is now so tilted relative to the plane of the Solar Nebula defined by the orbits of the planets? Nobody has been able to answer this satisfactorily. But the most likely explanation is that Uranus – like Mercury, Venus and the Earth – suffered a shocking cataclysm, very early in its history. So the explanation goes, the icy planetesimal that would one day become Uranus was hit by another planetesimal in its neighbourhood. The crash knocked the Uranus planetesimal on its side, and the debris from the collision formed a disc around the tipped-up planetesimal. Later, when the planetesimal started to suck in gases from the Solar Nebula, these gases joined the sideways disc, and Uranus began to grow at the centre while its regular satellites coalesced in the disc’s outer regions. Its remaining moons were captured at a later date. Uranus was unable to grow as massive as Jupiter because the Solar Nebula, as we have seen, was very sparse at this time. The planet has also lost some of its original hydrogen since it formed, because it has a weaker gravity than Jupiter.

The surfaces of the Uranian moons reveal that, even this far from the Sun, impacts were frequent – and often destructive. These impacts may also have helped to maintain the rings as they do with Jupiter, supplying the rings with dust-sized debris blasted off the moons’ surfaces. The largest ring fragments, meanwhile, could be the remains of moons that got torn apart by Uranus’ gravity. They might be cometary debris instead, but if this is the case then some process in the Uranian system must have darkened their surfaces.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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