Tuesday, 6 March 2012

Neptune – Last Giant Outpost

Neptune Data
Mass: 1.02 x 1025 kg or 17.1 times Earth’s
Equatorial diameter: 49 532 km or 3.9 times Earth’s
Surface gravity: 1.77 gees
Axial tilt: 29.6°
Mean surface temperature: -220 Celsius
Rotation period: 16.12 hours
Orbital period: 164.8 years
Inclination of orbit to ecliptic: 1.8°
Orbital eccentricity: 0.009
Distance from the Sun: 29.79–30.32 AU
Sunlight strength: 0.0011 of Earth’s
Satellites: >8
Largest satellite: Triton, diameter 2706 km

Perched almost on the edge of the realm of the planets, the serene blue Neptune is the last giant outpost. Another colossal world of slushy ice and gas, Neptune has many features in common with the green planet before it. But its hydrogen–helium atmosphere is one of constant turmoil, in contrast to that of the pallid and unchanging Uranus. Like the other giants, Neptune is graced by rings and satellites. Triton, its largest moon, is surprisingly active. It has a fresh, young surface. But something is very wrong with Neptune’s comparatively small retinue of eight moons. If astronomers are correct, Triton is not a Neptune native – it was gravitationally captured. The process all but destroyed Neptune’s original satellite system. And now, Triton aside, only haphazard shards of ice and rock remain in orbit about the blue giant.

Physical Overview
Taking almost 165 years to complete its orbit of the Sun, Neptune is extremely distant. From Earth there is not even the faintest chance of viewing it without binoculars or a telescope. And so, although it was discovered in the 1840s, virtually nothing was known about this fascinating blue gem for well over 100 years. Astronomers had to wait, twiddling their thumbs, until 1989 before they had any real data with which to work. This was the year in which the Voyager 2 probe made its last planetary encounter, before heading into the depths of interstellar space.

What Voyager 2 found was an apparent twin of Uranus, at least in terms of appearance. Neptune is slightly smaller than Uranus, by about 3 per cent, but it has a similar colour – again due to the presence of redlight- absorbing methane in its atmosphere. Even internally, Neptune is a lot like its green-faced partner. It has a rocky core – perhaps a bit larger than that of Uranus – surrounded by a vast ocean of icy slush, a thin shell of liquid hydrogen and other materials, and a hydrogen–helium atmosphere. But there is a surprise. While Uranus’ atmosphere is bland and featureless, Neptune’s is banded and stormy. This is perhaps odd. The blue planet is 1.5 times further from the Sun than its green neighbour, so its clouds ought to condense very low down, well out of sight. The reason for the difference is that Neptune, unlike its twin, has an unknown source of internal heat. This keeps the planet warmer than it would otherwise be and drives the atmospheric activity so clearly evident on the planet’s face. When Voyager 2 arrived at Neptune, the largest atmospheric feature on display was the Great Dark Spot. Scientists immediately likened it to a storm system, similar to the Great Red Spot on Jupiter. Scooting around this storm, meanwhile, were a number of wispy blue-white clouds of methane crystals. These floated some 50 kilometres above the bulk of the atmosphere, high enough to catch the sunlight. And yet, Neptune’s great storm has since vanished. There was no sign of it in 1994 when the Hubble Space Telescope was swung towards the distant blue giant. Other cyclones, too, have come and gone. Evidently Neptune’s weather patterns are not as long-lived as Jupiter’s.

Not surprisingly Neptune has rings, just like the other three giants. And again, they are different from those we have looked at so far. As usual, Neptune’s rings cannot compete with those of Saturn. They are very dim. Either they are made of rocky rather than icy fragments that reflect little light, or they are icy particles with a dark coating of organic molecules.

In total, Neptune has five rings. The most well defined, Adams and LeVerrier, are narrow, 50 kilometres and 110 kilometres in extent respectively. Still, these are generally wider than most of the rings of Uranus. The particles in these two rings are most likely metre-sized boulders. Two other rings are found between these two. One is again faint and narrow, while the other, though still faint, is fairly broad. Called Lassell, this ring stretches for about 4000 kilometres in radial extent. Lastly, the ring closest to the planet, Galle, is also fairly extensive, stretching for some 2500 kilometres. As well as the boulders present in some of these rings, all contain large amounts of dust, as found in the rings of Jupiter and Uranus. One thing that makes Neptune’s rings stand out, though, are the so-called ring arcs. Most of the material in the rings is evenly distributed around their circumference, as it is in all other ring systems. But in three places, Neptune’s outermost ring, Adams, is brighter than elsewhere. Apparently these arc-shaped segments contain more than their fair share of particles. At first this was a mystery. Why didn’t these arcs spread out along their orbit and repopulate the rest of the ring? The answer, astronomers think, is that the arcs are caused by the gravitational pull of a small moon just inside the orbit of the Adams ring. This moon, called Galatea, is only about 160 kilometres across, with a puny gravity. But because it is so close to the Adams ring it is able to herd the ring fragments together and prevents them from spreading out along their orbit. Galatea is one of eight satellites around Neptune – fewer than the other giant planets.

Triton and Other Satellites
Like Saturn, Neptune has only one large satellite. It is called Triton, and it is about 78 per cent the size of our own Moon. Its density indicates a roughly 50 : 50 mixture of rock and ice. The surface is dominated by frozen nitrogen, methane, carbon monoxide and carbon dioxide, with water ice underneath.

Triton is an exceptional world. Perhaps most surprising of all is that this frigid snowball is volcanically active – even though it possesses the coldest surface temperature ever measured in the Solar System, at -235 Celsius. But its volcanoes do not spout lava. Sunlight penetrates Triton’s transparent icy surface and heats up underground nitrogen. As the gas boils and its pressure rises, it escapes through cracks in the surface ice and is squirted several kilometres above the moon’s surface into its atmosphere. This is another surprise, that Triton should have an atmosphere. It is one of very few moons that do. Like Saturn’s Titan, Triton holds onto its gas shroud – nitrogen with traces of methane – because the cold gases move too sluggishly to escape the feeble gravity. But this primitive sky is a far cry from the atmospheres of the terrestrial planets. Some 100 000 times thinner than the air we breathe, Triton’s gaseous envelope is almost a vacuum. Most likely, Triton’s atmosphere derives from the surface, and the reason has to do with the moon’s orbit. Triton encircles Neptune backwards, in a path that is highly inclined to its planet’s equatorial plane – it is the only major satellite with such an irregular orbit. This means that Triton’s poles, like those of Uranus, endure long seasons when first one, and then the other – more than 80 years later – is turned towards the Sun. This sets up a cycle on Triton in which surface gases evaporate in the summer and help maintain the planet’s atmosphere, then freeze out in the winter. This also explains some of Triton’s surface features. There are few craters, probably because they are eradicated when liquid rises from the interior and later
While Triton is impressive, Neptune’s other satellites are all little more than broken lumps of ice and rock. The largest of these worldlets is Proteus. It measures 400 kilometres by 440 kilometres, and so is not a perfect sphere. Meanwhile the smallest moons, closest to the planet, have sizes measured in just tens of kilometres. All but one of these moonlets lie well inside the orbit of Triton and, unlike that satellite, more or less in Neptune’s equatorial plane. But one of the small moons chooses not to conform: Nereid. Some 340 kilometres across, Nereid has a highly inclined orbit, like Triton, and a very elongated one – its distance from Neptune varies by an astonishing 8 million kilometres, from 9.5 million to 1.3 million kilometres. Such an orbit is evidence, as is that of Triton, that something very dramatic must have happened long ago in the history of the Neptunian system.

History of the Neptunian System
So why is Neptune’s satellite system in such disarray? Why is Triton’s orbit so bizarre? The answer seems to be that Triton did not form around Neptune in a disc, as did the regular satellites of the other giant worlds. Neptune itself did grow from a disc of gas and ice just as Jupiter, Saturn and Uranus did. It is also possible that a system of regular satellites formed around Neptune as the planet was developing. But at some later time, Triton – a massive, icy planetesimal in a similar orbit to Neptune’s – ventured too close to the blue giant and got captured into its retrograde and highly inclined orbit. During the event, Triton may well have collided with any regular satellites that had already existed. Either they were ejected from their orbits, or they were slingshotted into their parent world and destroyed. Now, aside from Triton itself, only shards remain. A few still orbit in Neptune’s equatorial plane; perhaps these are the remains of its original satellites. Nereid, meanwhile, was almost ejected from the system – but not quite. Evidently it did not acquire enough speed to escape entirely, and instead settled into its very elongated elliptical meander.

As with the rings of Uranus, Neptune’s are very likely recent. They probably did not form alongside their planet, for they would long ago have disappeared. Again, they could be fragments of comets or small moons that were torn apart by Neptune’s gravity.

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



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