The Satellites of Uranus

The two largest satellites of Uranus, Titania and Oberon, were already discovered in 1787 by W. Herschel. Later Ariel, Umbriel and Miranda were found. The remaining ones were discovered by the Voyager 2 space probe in 1986. The smallest moons move on strongly perturbed orbits and collisions are likely to happen, e.g. Desde-mona may collide either with Cressida or Juliet within the next 100 million years. 

Name Mean diameter (km) Mass (xlO kg) Semi-major axis (km) Orbital period (day) 

Up to now 27 satellites of Uranus are known. The properties of Uranus satellites with a diameter 100 km are listed in Table 4.3. 

The cratering history on the moons of Uranus was investigated by Plescia, 1987 [265]. The surfaces of Oberon and Umbriel are interpreted to be the oldest, that of Titania intermediate and those of Ariel and Miranda the youngest. 

Umbriel (Fig. 4.20) is the darkest Uranian satellite, its mean density is 1.4 g/cm. Its surface is more heavily cratered than that of Ariel and Titania, so its is older and there are no signs of geologic activity. Near its equator, a crater like bright structure, called Wunda is found. The crater Skynd possesses a huge bright central peak but no rim. 

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Effects of condensation are also seen in the bulk compositions of the planets and their satellites. The outer planets, Uranus and Neptune, have overall densities consistent with their formation from icy and stony solids. The satellites of Uranus have typical densities of 1.3g/cm which would tend to indicate a large ice com-... 

Precise measurements of the orbital periods of Charon (6.387 223 0.000017 day) and the difficult measurement of the orbital wobble of both bodies with respect to their common center of mass lead to estimates of the masses of both (Tholen Buie, 1997). Together with a measurement of the radii an estimate of the mean densities can be derived. The density of Pluto was found to be between 1.92 and 2.06 and that of Charon between 1.51 and 1.81 gcm , indicating a substantial fraction of rocks in addition to ices for Pluto and a smaller amount of hydrated rocks below water ice for Charon. Consequently, Pluto is much more similar in composition to Neptune s retrograde satellite Triton (density = 2.043 0.0121, radius = 1352.6 2.4 km see McKiimon et al., 1995), while Charon seems to be more similar to the smaller, icy satellites of Neptune and Uranus. For a discussion of Triton and the smaller satellites of Neptune see the book edited by Cmikshank (1995) and the paper by Quirico etal. (1999). The satellites of Uranus are discussed in the book edited by Bergstralh et al. (1991). 

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