Moons of Neptune and Pluto Help
Moons of Neptune and Pluto
Neptune’s dominant moon has a diameter of 2,700 km (1,680 mi). It orbits at 385,000 km (240,000 mi) from Neptune, just about the same distance as Earth’s moon is from Earth. Figure 10-7 compares Triton in terms of size with Earth and Neptune.
This little world has the distinction of possessing the chilliest surface of any known planet or moon, approximately –235°C (–390°F). Triton is also unique in another way, for it is the only known moon in our Solar System that orbits its parent planet in the opposite direction from that of the planet’s rotation. As if this does not make Triton peculiar enough, its orbit is greatly tilted with respect to the plane of Neptune’s equator.
Measurements of Triton’s density indicate that it is made up of relatively less ice and more rock than the other major moons of outer planets. This fact, along with the retrograde orbit, has given rise to the theory that Triton was not originally a moon at all but instead was a planet in its own right when the Solar System was formed. It ventured too close to Neptune, and the large planet captured it. Strangely, however, Triton’s orbit is essentially a perfect circle, and this fact can be used to argue against the “once it was a planet” hypothesis. Triton keeps the same side facing Neptune all the time, but this generally takes place with planetary moons as the parent planets’ gravitational fields create tidal bulging over millions of years, pulling the rotation and revolution rates into synchronicity.
Triton has an atmosphere, but it is so thin that it would make a good laboratory vacuum for most Earthly purposes. The surface is believed to contain frozen methane along with nitrogen ice because of the pinkish cast to much of its surface. Clouds form occasionally, and these apparently consist of tiny particles of frozen nitrogen. There is evidence of wind erosion on the surface.
Volcanic activity apparently occurs on Triton, but the eruptions are entirely different from the volcanoes we know on Earth or from those that dominate the surface of Jupiter’s restless satellite, Io. Instead of hot lava from the interior, the ejected material is believed to be liquid nitrogen or methane that freezes as soon as it comes into contact with the bitter-cold surface environment.
Charon, with a diameter of 1,190 km (740 mi), is the only known satellite of Pluto. Charon is small in absolute terms, but it is significant compared with Pluto. Figure 7-11 (in Chap. 7) compares Charon for size with Earth and Pluto. Charon orbits Pluto at an average distance of approximately 20,000 km (12,500 mi). Charon is unique not only in that it is the largest moon in size relative to its parent planet, but it is also extremely low in its orbit. In fact, the two bodies tidally affect each other to the extent that they always keep the same sides facing each other.
Charon is believed to be composed of essentially the same stuff as Pluto, a combination of rocks and ices. Charon’s surface, however, is mainly frozen water, unlike that of Pluto, which is largely frozen nitrogen with traces of methane ice. While Pluto has a pinkish or reddish tinge when observed in visible light, Charon appears gray.
Some astronomers think that Charon and Pluto are surviving members of an originally much larger group of icy, comet-like bodies in orbits outside that of Neptune. According to one theory, most of these objects congealed to form Pluto and Charon. During this process, there were countless collisions. Some objects got hurled in toward the Sun and became comets in the classic sense as they got close enough to the Sun to develop tails. The collisions also gave the Pluto-Charon system its eccentric orbit around the Sun. While major collisions of these outer denizens of the Solar System are a thing of the past, smaller collisions and gravitational interactions still take place, and every few years a new comet happens across the watchful eye of some comet-seeking astronomer.
The Pluto-Charon system seems distant and insignificant as you read about it in a book, but Pluto and Charon are relatives of objects that have played crucial roles in the evolution of life on our planet. Some scientists think that a comet brought the first primitive life forms to Earth or produced the energy necessary for amino acids to form. A comet is believed to have struck the Earth in the present-day Gulf of Mexico about 65 million years ago, leading to the extinction of dinosaurs and an upsetting of the terrestrial equilibrium that, had it not been perturbed, would still give sanctuary to the giant ruling lizards today.
Practice problems of this concept can be found at: Major Moons of Outer Planets Practice Problems
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