11. Uranus and Neptune
The large moons of Uranus and Neptune
Five major moons of Uranus
Uranus possesses five major satellites discovered telescopically from Earth before the space age, and named Miranda, Ariel, Umbriel, Titania and Oberon. As a group, they are similar in size to the mid-sized satellites of Saturn. The two largest and outermost Uranian moons, Titania and Oberon, are roughly half the size of the Earthís Moon; the smallest and innermost, Miranda, is about one-seventh the lunar size. Infrared spectroscopy from Earth indicated that they all have water ice on their surfaces, but their icy surfaces are darker and less reflective than Saturnís moons.
Accurate masses for the largest moons of Uranus were obtained by observing their gravitational effects on the trajectory of the Voyager 2 spacecraft during its flyby on 24 January 1986. Combined with the size of the moons, these masses yield mean mass densities for the four largest ones of between 1400 and 1700 kilograms per cubic meter, higher than their Saturnian counterparts, between about 1100 to 1400 in the same units.
The high mass density implies that the large moons of Uranus are about half rock and half water ice. Thus, with the exception of Miranda, the major moons of Uranus are rocky on the inside, as well as dirty on the outside. Their dark surfaces and rocky interiors may be related to an ancient collision that might have knocked Uranus on its side before the satellites were fully formed.
The landscape on Miranda is one of the most amazing yet observed in the solar system. It includes old cratered plains, bright younger terrain, and an eclectic mixture of ridges, grooves, mountains, valleys, fractures and faults.
The four largest moons of Uranus can be divided into two pairs, Ariel and Umbriel and Oberon and Titania. The members of each pair have similar mass and size, but very different surfaces. The reason why they have similar bulk properties and look so different may be related to the differences in the way that the moons produced internal heat and eventually froze inside, but the details remain a perplexing mystery.
Neptuneís Triton, a large moon with a retrograde orbit
Jupiter, Saturn and Uranus have a flock of satellites whose orbits mimic those of the planets around the Sun. Their larger moons revolve in regularly spaced; circular orbits in the same direction as the rotation of the planet and close to the planet's equatorial plane, presumably because they share the rotation of the nebular disk from which the planet and its satellites formed. The radii, orbital distances and other characteristics of these regular satellites also tend to differ in smooth progression.
In sharp contrast to the other major outer planets, Neptune lacks a system of regular larger satellites. Its largest moon, Triton, is the only large satellite in the solar system to circle a planet in the retrograde direction, opposite to the planetís direction of rotation. This oddity is compounded by the high orbital inclination. The satelliteís orbital plane is titled at an enormous 157 degrees from the planetís equator. The tilted orbit gives rise to dramatic seasonal variations, for each pole of Triton in turn faces the Sun for nearly half of Neptune's 165-year orbit about the Sun, and the planet appears to move along the satelliteís horizon.
Nereid, the outermost moon of Neptune, adds to the mayhem, with the most elongated orbit of any planetary satellite, seven times as distant from the planet at its farthest compared with its closest approach.
Tritonís frozen surface, thin atmosphere and geyser-like eruptions
Measurements from Voyager 2 indicated that Triton is the ultimate icebox, with a daytime surface temperature of 38 degrees kelvin at the time of encounter. That is only about three dozen degrees above absolute zero, when nothing can move, not even an atom. In fact, Triton has the coldest measured surface of any natural body in the solar system! It is so cold because it is so far away from the Sun, therefore receiving little sunlight, and also because Triton reflects more of the incident sunlight than most satellites - only Enceladus and Europa are comparable. As a result, the total amount of sunlight absorbed by Triton's surface is less than that of any other planet or satellite.
A frosty coating of nitrogen and methane ices overlies all of the surface features, reflecting the incident sunlight. The brilliant ice has a salmon-pink tint with peach hues, possibly due to organic compounds derived from methane by the bombardment of energetic particles from the solar wind and Neptune's radiation belts.
Although nitrogen and methane frosts are apparently the dominant constituents of Triton's visible disk, water ice is needed to support and preserve the observed topography, including cliffs and ridges that exceed one kilometer in height. At the frigid temperature of Triton, water ice is as strong as steel, and behaves like hard rock on Earth; but the methane and nitrogen ice do not have sufficient strength to support the elevated features, which would deform and collapse under their own weight. Thus, thin, brilliant veneers of nitrogen and methane ice apparently overlie a rigid crust of water ice.
Even in the middle of southern summer, a bright ice cap extends from the south pole three-quarters of the way to Triton's equator. It is so cold that some of Tritonís air freezes out at its poles, coating them with a huge ice cap of frozen nitrogen. By way of comparison, the Earthís polar caps contain frozen water ice, for it is too warm for nitrogen to freeze at our planetís poles, and it is too cold for water ice to vaporize from Tritonís surface and enter its atmosphere.
On a world that is literally frozen solid, astronomers were amazed to find at least four erupting plumes near the center of Tritonís sunlit polar cap. These plumes rise in straight columns to an altitude of 8 kilometers, where dark clouds of material are left suspended and carried downwind for over 100 kilometers, like smoke wafted away from the top of a chimney. Since the active plumes occur where the Sun is directly overhead, the solar heat might energize them.
Scientists have not reached a consensus about what produces the plumes, but one likely explanation is that geysers are sending up plumes of nitrogen gas laced with extremely fine dark particles. Triton is far too cold for geysers to spout steam and water, like geysers on Earth, but Sun-powered geysers might expel dark material when pent-up nitrogen gas becomes warm and breaks through an overlying seal of ice.
Long cracks or faults on Triton seem to have been partially filled with oozing ice, as they are on Ariel, moon of Uranus. Vast frozen basins found within Triton's equatorial regions have apparently been filled by icy extrusions flowing out from the warm interior, like a squeezed slush cone. These frozen lakes of ice look like inactive volcanic calderas, complete with smooth filled centers, successive terraced flows and vents. The warm liquid or slushy ice on Triton apparently acted like lava on Venus, resurfacing the globe and filling low flat areas with smooth deposits that subsequently froze.
Origin and evolution of Triton
Tritonís retrograde and inclined orbit suggests that it is not a true satellite of Neptune, dating back to the planetís formation, but that Triton was once a separate world, which was captured into an eccentric, backwards and tipped orbit in the remote past. Neptune's lack of an ordered family of large satellites might be also be explained if Triton was born in its own independent orbit around the Sun, and was subsequently captured by Neptune, destroying any regular satellite system the planet may have had in the process.
All in all, Voyager 2's journey past Neptune was a brilliant success. As it drew away, the intrepid explorer took one last picture of Neptune and Triton as neighboring crescents; continuing out to where Triton might have originated and billions of comets now hibernate.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University