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P is the orbital period, Ag is the geometric albedo, R is the semi-major axis orbital radius, e is the orbital eccentricity, and i is the inclination of orbit to the planet's equator. Only satellites with a mean radius greater than 100 km have been included.

P is the orbital period, Ag is the geometric albedo, R is the semi-major axis orbital radius, e is the orbital eccentricity, and i is the inclination of orbit to the planet's equator. Only satellites with a mean radius greater than 100 km have been included.

Jupiter is accompanied by four large moons, known as the Galilean satellites, after their discoverer Galileo Galilei (1564-1642), which are closely aligned with the planet's equatorial plane. Their alignment and the observed compositional differences (Section 1.3) suggest that these were formed at the same time as Jupiter from the proto-Jupiter accretion disk. Jupiter also has a number of much smaller satellites, which inhabit various eccentric and inclined orbits. These are probably captured planetesimals. The larger satellites of Jupiter, with a radius greater than 100 km, are listed in Table 1.2a. Finally, Jupiter has a small ring that was first observed by Voyager 1 in 1979.

1.2.2 Saturn

Through a telescope, and ignoring Saturn's magnificent ring system, the observable "surface" of Saturn appears much blander than that of Jupiter with less banding and fewer ovals, although the predominant pale ochre color is similar to that of Jupiter. Like Jupiter, Saturn also has a significant internal heat source. However, although this is a greater fraction of the total heat emitted, the combined total is much smaller than that of Jupiter. Hence, the atmosphere is thought to be less meteorologically active than Jupiter's since the heating rate driving it is less. In addition, any con-vective structures that are present are more masked by overlying haze due to the colder temperatures found in the upper observable part of Saturn's atmosphere. These cooler temperatures mean that the expected upper cloud deck of ammonia ice (Chapter 4) occurs deeper in the atmosphere than in Jupiter's. In addition, the lower gravitational acceleration of Saturn means that the atmosphere is more vertically extended, and thus the absorption by upper atmospheric haze is enhanced.

Although generally more quiescent than Jupiter, major convective-type events are occasionally observed in Saturn's brighter zonal regions (outlined in Chapter 5). These are known as "brightenings" or "Great White Spots'' and have been observed intermittently since 1793. Recent "brightenings" occurred in the Equatorial Zone in

1990 and 1994 and were observed with the Hubble Space Telescope (Figure 1.5, see color section). What appears to be happening in these events is that some kind of disturbance deep below the visible cloud/haze top of Saturn triggers rapid, possibly thunderstorm-style, deep vertical convection, and the resultant formation of thick, very high ammonia clouds. These clouds are subsequently torn apart by the zonal wind flow shear and dissipate over a timescale of a few months.

The zonal wind system of Saturn is similar to that of Jupiter's with the wind direction alternating in association with the belts/zones. However, the widths of Saturn's visible belts and zones (but not those of the deeper clouds, as we shall see in Chapter 5) appear greater than those of Jupiter and the equatorial jet is found to be very much stronger and during the Voyager flybys blew at a very rapid 500 m/s in the prograde direction relative to the interior System III rotation rate (although this has since slowed to 400m/s during the Cassini epoch). Thus, the equatorial zone of Saturn is even more super-rotating than that of Jupiter. However, it has been realized recently that the System III rotation rate, as inferred from radio emission at kilometric wavelengths, actually varies with time and is not solely constrained by the bulk rotation rate. Rather, it is determined by the slippage of the magnetosphere relative to the interior (Anderson and Schubert, 2007) and is thus affected by instabilities in Saturn's plasma disk. The latest estimate of Saturn's bulk rotation rate from Cassini observations is 10 h 32min, compared with the 10 h 39min System III value (Anderson and Schubert, 2007), and so Saturn's equatorial winds may not be as super-rotating as was earlier thought.

The satellite system of Saturn is rather different from that of Jupiter and Table 1.2b lists those satellites with a radius greater than 100 km. Most of the satellites are somewhat smaller than the Galilean satellites with the exception of Rhea, Iapetus, and most notably Titan. Titan is the second largest satellite in the solar system (after Ganymede) with a radius of 2,575 km and a substantial atmosphere composed mainly of N2 with a surface pressure of ^ 1.5 bar. However, while its atmospheric composition and surface pressure are similar to the Earth's, its surface temperature of ~90 K is extremely cold. The surface of Titan at visible wavelengths is obscured by atmospheric hazes, although ground-based and orbital telescopes had detected surface features at near-infrared wavelengths prior to the arrival in the Saturnian system of the Cassini/Huygens mission, allowing its rotation rate to be shown to be tidally locked. Instruments on the Cassini orbiter have now mapped the surface of the world and following its aerobraking capture on January 14, 2005, Huygens spent about 2 hours gently descending though Titan's atmosphere, measuring the conditions in its atmosphere and returning breathtaking images of Titan's surface during its descent.

The ring system of Saturn is the most stunning of all the giant planets (Figures 1.6-1.8, see color section for Figures 1.7 and 1.8). It is now known that all the giant planets have ring systems, which may form either from the tidal disruption of captured satellites, which subsequently spread and are dissipated or absorbed by the planet over time, or by the sputtering of particles from the surfaces of satellites. While the very thin ring first observed by the Voyager spacecraft around Jupiter is thought to be due to sputtering, the massive ring system of Saturn can only have

Satellite

Mass (kg)

Radius (km)

Density (gem"3)

P (days)

Ag

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