THE GIANT PLANETS

The giant planets, Jupiter, Saturn, Uranus, and Neptune, occupy the 5th through 8th planetary orbits, eounting outward from the Sun. They seem to form pairs Jupiter and Saturn are similar in size and other properties, and so are Uranus and Neptune (see Appendix 3). All giant planets have deep atmospheres, composed mainly of 

Measured radiation from planetary objects up to Neptune 

As seen in the figures the brightness temperature levels of the spectra decrease from Jupiter (5.2 AU), to Saturn (9.5 AU), and to Uranus (19 AU). Neptune (30 AU), however, is slightly warmer than Uranus. As will be shown later, the effective temperatures, that is, the temperatures of blackbodies that emit the same spectrally integrated energy as the planets, are 124.4 K, 95.0 K, 59.1 K, and 59.3 K, respectively. Although Neptune is still further away from the Sun, it contains a substantial 

The spectra of Uranus and Neptune have been measured by Voyager 2 over only a small spectral range (Fig. 6.3.3). Comparison with the spectra of Jupiter and Saturn identifies the measured feature as part of the broad S(0) line of molecular hydrogen. With the data analysis methods discussed later, the atmospheric temperature profiles were obtained. However, little can be deduced from a visual inspection of the spectra shown in Fig. 6.3.3, except that large quantities of hydrogen must be present and that the temperatures increase with pressure in the tropospheres of both planets. Voyager results from Uranus and Neptune systems can be found in the books edited by Bergstralh et al. (1991) and Cmikshank (1995), respectively. 

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The giant planets possess low surface temperatures and have atmospheres that extend several thousand miles. The markings on Jupiter, the largest planet, consist of cloud formations composed of methane containing a small amount of ammonia. The atmosphere of Jupiter absorbs the extreme red and infrared portions of the spectrum. These absorptions correspond to the absorption spectra of ammonia and methane, suggesting the presence of these gases in Jupiter s... 

Binzel et al. (1991) give an account of the origin and the development of the asteroids, while Gehrels (1996) discusses the possibility that they may pose a threat to the Earth. The giant planets, and in particular Jupiter, caused a great proportion of the asteroids to be catapulted out of the solar system these can be found in a region well outside the solar system, which is named the Oort cloud after its discoverer, Jan Hendrik Oort (1900-1992). Hie diameter of the cloud has been estimated as around 100,000 AU (astronomic units one AU equals the distance between the Earth and the sun, i.e., 150 million kilometres), and it contains up to 1012 comets. Their total mass has been estimated to be around 50 times that of the Earth (Unsold and Baschek, 2001). 

One more important property of Jupiter must be mentioned the Earth owes its relatively quiet periods (in geological terms) to the huge gravitational force of the giant planet. Jupiter attracts most of the comets and asteroids orbiting in its vicinity, thus protecting the Earth from impact catastrophes ... 

The detection of light from an extrasolar planet was reported by A. C. Cameron, K. Horne, A. Penny, and D. James, Probable Detection of Starlight Reflected from the Giant Planet Orbiting t Bootis , Nature, 402 (1999), 751. 

Ice mantles are important constituents of interstellar grains in molecular clouds, and icy bodies dominate the outer reaches of the solar system. The region of the solar system where ices were stable increased with time as the solar system formed, as accretion rates of materials to the disk waned and the disk cooled. The giant planets and their satellites formed, in part, from these ices, and probably also from the nebular gas itself. 

In contrast to the terrestrial planets, the giant planets are massive enough to have captured and retained nebular gases directly. However, concentrations of argon, krypton, and xenon measured in Jupiter s atmosphere by the Galileo spacecraft are 2.5 times solar, which may imply that its atmosphere preferentially lost hydrogen and helium over the age of the solar system. 

In this chapter we will consider the cosmochemistry of ice-bearing planetesimals. We will focus first on comets, because more is known about their chemistry than of the compositions of objects still in the Kuiper belt and Oort cloud. We will then explore asteroids whose ices melted long ago, and we will briefly consider some larger icy bodies, now represented by satellites of the giant planets. The importance of ice-bearing planetesimals to cosmochemistry stems from their primitive compositions, which have remained largely unchanged because of hibernation in a frozen state. 

Pluto and some moons of the giant planets contain considerable amounts of ices and deserve special mention. In some cases they even exhibit active or recent processes that liberate liquids and gases derived from ices, suggesting a tentative link with cometary activity. These bodies, which are too large to be called planetesimals, include former KBOs now relocated into the planetary region, as well as objects that probably accreted in the giant planet region. 

The giant planets are composed mostly of hydrogen and helium. Uncompressed mean densities provide constraints on the proportion of rock to ice or gas, although the enormous internal pressures in some of these planets produce phase changes in hydrogen that complicate this determination (discussed below). 

Table 14.3 Abundances of major elements and molecules in the atmospheres of the giant planets, relative to solar abundances (Lunine, )...

Gravitational stirring of icy planetesimals by the giant planets could have sent many comets careening into the inner solar system, providing a mechanism for late addition of water to the terrestrial planets. Comets impacting the Earth and the other terrestrial planets would have delivered water as ice (Owen and Bar-Nun, 1995 Delsemme, 1999), whereas the accretion of already altered carbonaceous chondrite asteroids would have delivered water in the form of hydroxl-bearing minerals (Morbidelli el al., 2000 Dauphas et al., 2000). 

To the accuracy of the measurement of molecular weights for the giant planets, only hydrogen and helium have significant abundances. The relative proportions of these elements, expressed as the molar fraction He/H, are 0.068+0.002 for Jupiter, 0.068+0.013 for Saturn, 0.076+0.016 for Uranus, and 0.100+0.016 for Neptune (Lunine, 2004). None of these ratios are like those of the nebula (0.085, Table 4.1). 

Models of the interiors of the giant planets depend on assumed temperature-pressure-density relationships that are not very well constrained. Models for Jupiter and Saturn feature concentric layers (from the outside inward) of molecular hydrogen, metallic hydrogen, and ice, perhaps with small cores of rock (rocky cores are permissible but not required by current data). Uranus and Neptune models are similar, except that there is no metallic hydrogen, the interior layers of ice are thicker, and the rocky cores are relatively larger. 

How do the chemical compositions of the giant planets differ from those of the terrestrial planets, and from each other ... 

Lunine, J. I. (2004) Giant planets. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 623-636. This chapter nicely summarizes how the physical and chemical properties of the giant planets are determined and discusses models for the origin of these bodies. 

The recent advances in modem technology continue to open new opportunities for the observation of chemical reactions on shorter and shorter time scales, at higher and higher quantum numbers, in larger and larger molecules, as well as in complex media, in particular, of biological relevance. As an example of open questions, the most rapid reactions of atmospheric molecules like carbon dioxide, ozone, and water, which occur on a time scale of just a few femtoseconds, still remain to be explored. Another example is the photochemistry of the atmospheres of nearby planets like Mars and Venus or of the giant planets and their satellites, which can help us to understand better the climatic evolution of our own planet. 

West, R.A. 1999. Atmospheres of the giant planets. Encyclopedia of the Solar System. Academic Press, New York. 

The lifetime of protoplanetary disks determines the time available for planet formation with the loss of the dusty gas disks no raw material is left to form planetesimals or giant planets. Thus, disk mass as a function of time is perhaps the single most important constraint on the formation of both the rocky and the giant planets. The most readily observable, albeit imperfect, indicator of disks is the presence of excess emission above the stellar photosphere, emerging from small, warm dust grains. 

The effect of the giant planets and the formation of the Asteroid Belt... 

The giant planets, especially Jupiter and Saturn, significantly influenced accretion in the inner Solar System, with important consequences for the properties of the terrestrial planets, described in Section 10.4.1. The influence of the giant planets is especially strong in the Asteroid Belt. Given that meteorites are our primary samples of primitive Solar System material, understanding the role of dynamical and collisional processes in the formation and evolution of the Asteroid Belt is of fundamental importance for theories of planet formation (Section 10.4.2). 

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