Moons of Neptune

Prior to the Voyager 2 encounter, only two moons were known, Triton and Nereid. Today we know Neptune has thirteen moons, following the discovery of six new moons by Voyager 2 and later observations. William Lassell discovered the largest moon, Triton, in 1846. Triton is spherical in shape and has a nearly circular orbit. The other moons are small, irregularly shaped objects with highly elliptical orbits, suggesting Neptune has captured them. One of the interesting things about Triton is that it has a retrograde orbit; that is, it orbits in the opposite direction to Neptune's rotation. Some astronomers believe Neptune may also have captured Triton some three to four billion years ago.

Triton has a diameter of 2700 km and orbits Neptune every 5.88 days at a distance of 354 800 km. Its surface contains many interesting features, including fault lines, cracks and ice (water, methane and ammonia) flows. There are not many impact craters, an indication that ice flows from the interior may have caused extensive resurfacing. The equatorial region has a wrinkled terrain that resembles the skin of a cantaloupe or rock melon. Long narrow valleys rimmed by ridges cross the area. Such a region may have formed from repeated episodes of melting and cooling of the icy crust. Triton also has a few frozen lakes that may be the calderas of extinct ice volcanoes. In other areas, dark features surrounded by bright aureoles or rings are visible - these may be some of the geyser-like plumes detected by Voyager 2. The pinkish South Polar Region is covered by a cap of frozen methane and nitrogen, and temperatures there are around -245°C. The pinkish colour of the polar cap is probably due to frozen nitrogen.

The very thin atmosphere of Triton contains nitrogen and methane.

Triton's density was found to be a little over 2 g/cm3, roughly twice the density of water. This suggests that Triton is made up of a mixture of rock and icy material.

Figure 12.9 The South Polar Region of Neptune's largest moon, Triton. (Photo: NASA)

Triton has a tidal effect on Neptune that is tending to pull Triton towards Neptune. In about a quarter of a billion years, Triton may be pulled apart by Neptune's gravitational pull.

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