Neptune is somewhat smaller in size (equatorial radius of 24,750km) than Uranus (25,560km) but more massive (17.2 vs 14.4 times the mass of Earth). Neptune's bulk density of 1.64 g/cm3 suggest that its interior is similar to that of Uranus, a molecular hydrogen atmosphere over a rock and ice core. The core contains about half of the total mass. Neptune resembles Jupiter and Saturn in that it emits substantially more energy than it absorbs from the sun. It also has the highest velocity winds of the the giant planets, up to 2000km/s, driven by the heat carried upwards from the interior by convection. The convection also drives large storms. When Voyager flew by Neptune in 1989 it photographed a huge storm which came to be known as the Great Dark Spot, in analogy with the Great Red Spot on Jupiter.
Triton is Neptune's largest moon. It orbits at a distance of seven planetary radii in a circular retrograde orbit. Like the Moon, Triton raises a tidal bulge on its primary (Neptune). However, because the orbit of Triton is retrograde the tidal bulge on Neptune is behind the line joining the centers of the two bodies. This differs from the Earth's tidal bulge, which is ahead of the line joining the Earth and Moon. As a result of this trailing bulge, the tidal interaction between Neptune and Triton is extracting energy from Triton's orbit. In a few billion years Triton will approach the Roche limit and be torn apart.
The orbit of Triton is currently circular, but it probably was not so early on. The fact that the orbit is retrograde suggests that Triton did not form in a disk around Neptune. Rather, it is speculated that Triton was originally a small planet similar to Pluto. Triton was captured by collision with a primordial regular moon of Neptune several billion years ago. The collision would have extracted a small amount of energy from the orbit of Triton, leaving it on a large, highly eccentric orbit around Neptune rather than around the sun. The high eccentricity was then rapidly reduced by tides on Triton raised by Neptune. The energy extracted from the orbit was dumped into Triton, melting it. Voyager found evidence for such a process in the form of a very young surface on Triton.
Triton has a thin N2 atmosphere, with a pressure about 10-5 that of Earth's atmosphere. It also possess a polar cap, and shows quasi-volcanic activity in the form of N2 geysers. The bulk density of 2.02 g/cm3 suggests the presence of a substantial amount of silicates; the ratio of ices to silicates might be about 1/2. Triton appears to be differentiated, with an icy mantle surrounding a silicate core.
Voyager discovered eight regular moons well inside the current orbit of Triton. The outermost is at 4.6 planetary radii. All these moons are on prograde circular orbits nearly in the plane Neptune's equator; the innermost is inclined by about 4 degrees. These moons are unlikely to be primordial; if Triton was captured onto a highly eccentric orbit, it would have forced large eccentricities in the orbits of the current moons, causing them to collide with each other.
The outermost known moon is tiny Nereid, the most eccentric satellelite in the solar system. It may have been forced onto this unusual orbit by the rampages of Triton soon after that body's capture.
Comets are often referred to as dirty snowballs. Their primary constituent is ice, forming a nucleus typically 10 km across. The orbital periods are typically tens to thousands of years (see below). For most of each period a comet is far from the sun, so the ice is stable. As a comet approaches the sun however, the ice begins to vaporize, forming the spectacular tails familar to amature astronomers. The sketch to the right (from the astronomical society of Hawaii web site ) shows the components of a generic comet (not all comets display all the components). The dirty snow ball is called the nucleus. Surrounding the nucleus is the coma, a roughly spherical region of volatilized ice from 100,000 to 1,000,000km in size. Largest of all (and containing the least mass) is the tail, which may have up to three components; a bluish ion tail, which streams directly away from the sun, an envelope of hydrogen gas, and a (typically yellow) micron sized silicate dust particle tail. The ion tail always points away from the sun, carried along by the solar wind. In fact, it was the observation of comet tails that first led astronomers to postulate the existance of a solar wind. In contrast the dust particles tend to follow an orbit similar to that of the comet as a whole, except that they are more strongly affected by radiation pressure exerted by light from the sun than is the nucleus.
In the picture of comet West above, the ion tail is blue, while the dust tail (above the ion tail) is slightly yellow.
The nucleus does not follow a Keplerian orbit either. Gas boils off the comet with sufficient force to affect the orbit. In addition, most comets cross the orbit of Jupiter, and are subject to strong perturbations from that planet, which also alter the orbit on time scales much shorter than the lifetime of the solar system. The result is that comets that enter the inner solar system have short lives. This suggested to Jan Oort that there must exist a reservoir of comets lying at large distance from the sun; this is now known as the Oort cloud.
At the time, the distribution of known comet inclinations was isotropic, so Oort suggested that the cloud was spherically symmetric. In the last twenty years or so, a second class of comet was recognized. These comets had short (<1000 year) periods. The distribution of orbital inclinations of these short period comets was highly non-isotropic; most orbited nearly in the plane of the Earth's orbit. Scott Tremaine and Martin Duncan, among others, suggested the existence of a second reservior, just beyond the orbit of Neptune. This reservior should have a flattened distribution, to produce the observed low inclination of the short period comets. The suggested the name Kuiper for this belt.
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