The Giant Planets

We have seen that the giant planets may be divided into two classes, the gas giants (Jupiter and Saturn) and the ice giants (Uranus and Neptune), and that this classification has a simple explanation in terms of the formation of the giant planets by nucleation. We will now summarize the principal physical and chemical properties of these four planets.

The giant planets have no surfaces in the sense that we understand the term as applied to the terrestrial planets and the asteroids. Measurements of density and gravity, coupled with theoretical models, suggest the existence of a central core consisting of heavy elements, surrounded by a mixture of hydrogen and helium under great pressure, which is probably metallic in Jupiter and Saturn, and molecular in Uranus and Neptune (Fig. 4.17). The pressure at the boundary would be about

Fig. 4.17 The mass:radius relationship for a self-gravitating body, for different compositions. After Stevenson, 1982

Fig. 4.17 The mass:radius relationship for a self-gravitating body, for different compositions. After Stevenson, 1982

40 Mbars in Jupiter, 13 Mbars in Saturn, and 6 Mbars in Uranus and Neptune, with temperatures - still according to the models - of 23 000 K, 12 000 K, 3000 K, and 3000 K, respectively (see Chap. 7). Only the outermost layer of the atmospheres of the giant planets is accessible to observation. Remote sounding measurements have enabled the tropospheres to be studied down to a depth of a few bars; in addition, in December 1995, the Galileo descent probe provided measurements of Jupiter's troposphere down to a level at which the pressure was 20 bars. The chemical composition of the giant planets is dominated by hydrogen and helium, with any other elements bound to hydrogen in a reduced form (CH4, NH3, PH3, H2S, etc.).

With the exception of Uranus, the giant planets have an internal energy source. The ratio between the energy radiated to space and the amount of solar energy received is 1.7, 1.8, and 2.6, for Jupiter, Saturn and Neptune, respectively. It is less than 1.1 for Uranus. The origin of this energy is probably the contraction and cooling of the planets, following the initial collapse of the gas onto the core, and the heating this produced. Another possible contribution in the case of Jupiter and Saturn comes from the predicted condensation of helium within the metallic hydrogen phase (see Sect. 7.1.4.2). With Uranus, the absence of an internal source of heating remains an enigma.

4.4.4.1 The Thermal and Cloud Structure of the Giant Planets

A feature of the thermal profiles of the four giant planets (Fig. 4.18) is a troposphere where the gradient is adiabatic (about 2K km-1) below the tropopause, which in each case lies at a pressure level that is about 0.1 bar. This similarity is explained by the generally similar composition of the four planets, and which is dominated by hydrogen and helium. In contrast, however, the temperature at the tropopause decreases as the heliocentric distance increases (110 K for Jupiter, 90 K for Saturn, and about 50 K for Uranus and Neptune). The tropopause therefore acts as a very

Temperature (K)

Fig. 4.18 The thermal structure of the giant planets (Encrenaz et al., 2004)

Temperature (K)

Fig. 4.18 The thermal structure of the giant planets (Encrenaz et al., 2004)

Fig. 4.19 The cloud structure of the giant planets (After R.A. West, 1999)

efficient cold trap, where the products of the dissociation of methane, produced at a higher altitude, condense. In the stratosphere, the temperature again increases with altitude in a manner that varies greatly from one planet to another, which is a sign of different heating mechanisms in each case. (The heating being caused by the absorption of solar radiation by the aerosols and the dissociation products.) These differences increase in the upper stratosphere, where other heating mechanisms (gravity waves, and energetic particles) intervene.

There is a striking contrast between our excellent knowledge about the thermal structure of the giant planets and current uncertainties as to their cloud structure. The basic reason for this is undoubtedly because the spectroscopic determination of the nature of the condensates is far more difficult that for gases. The spectral signatures of solids are often ambiguous. So, for example, we still do not know with any degree of certainty the nature of the components of the Great Red Spot, a large-scale dynamical structure, discovered three centuries ago, and which has proved to be stable since then. It is possible to derive the structure of an atmosphere theoretically, assuming a given chemical composition, by determining the photochemical equilibrium of the predicted molecular compounds, and by determining the condensation sequence associated with each molecule (Fig. 4.19).

4.4.4.2 Chemical Composition

The chemical composition of the atmospheres of the giant planets is shown in Table 4.1. The components may be grouped into two major categories: tropospheric components and stratospheric components.

Table 4.1 The atmospheric composition of the giant planets (After Encrenaz et al., 2004)

Species Jupiter Saturn Uranus Neptune

Table 4.1 The atmospheric composition of the giant planets (After Encrenaz et al., 2004)

Species Jupiter Saturn Uranus Neptune

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