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a 1AU is one astronomical unit, the mean distance of the Earth from the Sun equal to 1.496 x 108 km. b The equatorial radii quoted refer to the 1 bar pressure level. References are given in Table 2.4. c Recently revised figure from Cassini observations for bulk rotation rate is 10 h 32min (Anderson and Schubert, 2007).

d The bolometric temperature is the temperature of a black body sphere which would radiate heat to space at the same rate as that observed for the planet.

a 1AU is one astronomical unit, the mean distance of the Earth from the Sun equal to 1.496 x 108 km. b The equatorial radii quoted refer to the 1 bar pressure level. References are given in Table 2.4. c Recently revised figure from Cassini observations for bulk rotation rate is 10 h 32min (Anderson and Schubert, 2007).

d The bolometric temperature is the temperature of a black body sphere which would radiate heat to space at the same rate as that observed for the planet.

Jupiter at 696,265 km. The mass of Uranus and Neptune are similar to each other at around 15 Me (where Me is the mass of the Earth), while the masses of Jupiter (318 Me) and Saturn (95 Me) are substantially different indicating that Jupiter is much more compressed than Saturn, given their similar size. Although the giant planets are truly massive, their combined mass is still only 0.1% of the solar mass, which is estimated to be 1.9891 x 1030 kg. However, while most of the solar system mass is accounted for by the Sun, its combined spin angular momentum and orbital momentum about the solar system barycenter account for only 1% of that of the total solar system. Instead, most of the solar system angular momentum is accounted for by the giant planets, with the orbital angular momentum of Jupiter and Saturn contributing 85%. This counter-intuitive observation provides major constraints on models of solar system formation as we shall see in Chapter 2.

The atmospheres of the giant planets are found to be meteorologically active and highly convective with the notable exception of Uranus, which seems normally to have a more sluggish atmospheric circulation, except during equinox periods. A clear indication of convective activity can be seen from observing the mean temperatures of the giant planets. Solar irradiance drops as the inverse square of distance from the Sun, and thus the calculated effective radiating temperature (the temperature at which absorbed solar radiation is balanced by the emitted thermal radiation of the planet, discussed in Chapter 3) also decreases with distance from the Sun. While the observed mean bolometric temperatures (the mean temperatures at which the planets actually radiate) are indeed found to decrease with distance, the bolometric temperature exceeds the effective radiation temperature for all the giant planets except Uranus. Hence, all the giant planets except Uranus radiate significantly more radiation than they receive from the Sun indicating a substantial source of internal heat. The absorbed solar fluxes vary from a maximum at the subsolar point to zero at the limb and on the night side while the emitted thermal fluxes are found to a first order to be independent of latitude and longitude and are shown as a function of latitude in Figure 1.2. The ratios of emitted to absorbed fluxes are listed in Table 1.1. As can be seen, both Jupiter and Saturn have significant internal heat sources. However, Uranus and Neptune, which are otherwise so similar, are found to be very different in this regard with Uranus having almost no internal heat source and Neptune a very large supply giving it the highest emitted/absorbed flux ratio of all the giant planets. As we shall see in Chapter 2, this internal heat is thought, for all the giant planets except Saturn, to be residual heat left over from the formation of the planet, which is slowly radiating away to space as the planets contract via the Kelvin-Helmholtz mechanism (e.g., the radius of Jupiter is estimated to be currently shrinking by approximately 1 mm/yr). The emitted flux of Saturn is thought to be too high for the source to be just residual formation heat since this is estimated to have radiated away almost 2 billion years ago. Instead, the source is thought to arise due to internal differentiation of helium. We will return to this topic in Chapter 3.

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