SATURN - The Sixth Planet
Its rings make Saturn, sixth planet from the sun, one of the most remarkable objects in the heavens. Brighter than all the stars except Sirius and Canopus, it shines with a steady ashen color. Saturn is second among the planets in mass and size. The mean diameter of Saturn minus its ring system, is almost 10 times that of the earth, and its mass is about 100 times greater. Its density is the lowest of any planet, 0.7 times that of water. The small mean density leads to the often-quoted observation that, if you could find a lake large enough, Saturn would float in it, being lighter than water. Rapid rotation (a rotation period of a little over 10 hours) and an unusually low density give it more polar flattening than any other planet, about 11 percent.
Saturn is twice as far from us as jupiter, but the markings that we can see on the noticeably flattened disk of Saturn faintly resemble the banded cloud structure of jupiter's atmosphere. The coloration is more restrained, and the details are less distinct. On rare occasions a bright spot may appear. Thus, as in the case of jupiter, Saturn is a fluidlike body rather than a solid like a terrestrial planet. As for jupiter, astronomers have detected weak radio emission in low-frequency bursts that are synchronized with Saturn's 10.2-hour rotation period.
Satu rn's axis of rotation is inclined by 29° to its orbital plane. Since the plane of its rings is perpendicular to its rotation axis, the rings do not lie in the orbital plane and therefore present a varying aspect to Nearly 3 billion kilometers from the earth, Uranus presents an almost featureless appearance. Although a few atmospheric features have been reported, none have been confirmed. As with Jupiter and Saturn, we are probably seeing clouds in its atmosphere rather than a sofid surface.
Uranus's rotation has a peculiarity. Its axis is tilted 98° to the perpendicular of its orbital plane-that is, it lies on its "side," so that we see it rotate in the reverse direction barely. For Uranus the retrograde rotation is due to the peculiar inclination of the axis, while for Venus it is a true reverse rotation. When its axis is in our line of sight every 42 years (half the sidereal period), we observe either its sunlit northern or southern hemisphere, while the opposite hemisphere is dark. One-quarter or three-quarters of its period later (21 years or 63 years), its axis is at right angles to our line of sight, and we observe both the northern and southern hemispheres.
The two brightest satellites of Uranus, Titania and Oberon, were discovered by William Herschel in 1787, only 6 years after he discovered the planet itself. In all, the planet has five known satellites, which are visible in the infrared photograph in Figure 6.10. All the satellites move in nearly circular orbits that lie close to the equatorial plane of Uranus, the same plane as the ring system, and well outside the rings. In these respects Uranus is similar to Saturn. The ring system was accidentally discovered in 1977 from observations from an airborne telescope that was being used to remeasure Uranus's diameter and study its atmosphere as the planet passed over a background star. The expectations of future findings produced by such accidental events as the discovery of Uranus's the earth as the planet goes through its roughly 30-year orbital period. Figure 6.8 also shows geometrically how this occurs, along with photographs of the planet with its rings seen at fou r different angles to the line of sight. When seen almost edgewise, every 15 or so years, the rings almost disappear from sight, indicating that they are very thin compared to their radius. Most of Saturn's satellites orbit in the same plane as the rings, the planet's equatorial plane, and orbit outside the rings. Titan is one of the most massive satellites in the solar system, and it is one that has been known for some time to possess an atmosphere.
