Bodies Without Atmospheres in the Outer Solar System
After the invention of the telescope in the 17th century, it was discovered that the Earth was not the only planet to have a satellite. In 1610, Galileo observed the largest satellites of Jupiter and Huygens discovered Titan, the largest of Saturn’s satel
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After the invention of the telescope in the 17th century, it was discovered that the Earth was not the only planet to have a satellite. In 1610, Galileo observed the largest satellites of Jupiter and Huygens discovered Titan, the largest of Saturn's satellites in 1655. Gradually the number of satellites in the realm of the outer planets grew to about a dozen. Not until the 19th century, however, were the Martian satellites, Phobos and Deimos, observed for the first time. It is only very recently, with the Pioneer and Voyager deepspace missions, that numerous new satellites have been discovered, and their main physical properties have been determined. What is striking is that this family of bodies orbiting the outer planets should show such an extraordinary diversity in their surface conditions, degree of internal activity, internal structure and evolution. Nevertheless it has been possible to establish some general laws regarding orbital parameters and macroscopic properties. For example, almost all the satellites have synchronous rotation, with equal periods of orbital revolution and sidereal rotation: they present the same face to the planet throughout their orbits. This implies that tidal forces have stabilized the rotation in an equilibrium state. Our knowledge of the rings surrounding the outer planets has progressed even more slowly. Although the rings around Saturn were discovered in the 17th century, it was not until 1977 that the rings of Uranus were detected. The Voyager probes then found a ring around Jupiter in 1979, then revealed the complexity of Saturn's rings in 1981, followed by those ofUranus in 1986, and finally those ofNeptune in 1989. The rings and satellites associated with each of the outer planets form a set of systems within the Solar System as a whole. Study of the satellites therefore makes constant reference to the evolutionary processes that have affected the inner planets, and the dynamics of the rings are related to that of the protosolar nebula.
10.1 The Satellites of the Giant Planets 10.1.1 The Satellites of Jupiter The Small Satellites At 1 January 2003, forty satellites of Jupiter were known (Table 1.2). Their discovery dates clearly show that there are two distinct families in size, and therefore in their T. Encrenaz et al., The Solar System © Springer-Verlag Berlin Heidelberg 2004
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10 Bodies Without Atmospheres in the Outer Solar System
Fig. 10.1. Amalthea, as seen by Voyager 1 on 1979 March 4 from a distance of 425 000 km. [By courtesy ofNASAj
properties. The satellites discovered by Galileo have diameters in excess of 3000 km, so they are similar in size, or somewhat larger than the Moon. The others discussed here are very small bodies: only Amalthea (Fig. 10.1), discovered a century ago, is more than a hundred kilometres in diameter. So the list of satellites is probably incomplete, because such small bodies are exceptionally difficult to detect. The one characteristic common to all these objects is that of having too low a mass for them to have become spherical und
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