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The tropospheric components are, in descending order of abundance: H2, He, CH4, and NH3, as well as certain hydrogenated components (PH3, GeH4, AsH3, H2O, and H2S) as shown in Fig. 4.20. For Uranus and Neptune, these products (with the exception of H2, He, and CH4) are not detectable because they condense at levels that are too deep to be accessible for observation.

The stratospheric species may be of two different origins. Some of them arise through the photodissociation of methane, the most abundant being C2H6 and C2H2. Others, discovered at the end of the 1990s by the ISO satellite (Fig. 4.21), have an external origin. These are H2O and CO2, whose presence betrays the existence of an oxygen flux, which may be of local (from the rings or satellites), or interplanetary (meteoritic, micrometeoritic, or cometary) origin. The collision of Comet Shoemaker-Levy 9 with Jupiter in July 1994 showed that such events could actually occur, despite their low probability.

A certain number of isotopic species have also been detected in the giant planets: HD, CH3D, 13CH4, 12C13CH2, and 15NH3. These measurements have been used to determine the D:H, 15N:14N, and 13C:12C ratios. Together with the element ratios, these factors provide important constraints on models for the formation and evolution of the giant planets.

Fig. 4.20 Spectra of Jupiter at 5 |m as recorded by the Voyager probe. This spectral region enables the tropospheric layers of the planet to be examined (After Drossart et al., 1982)

Fig. 4.20 Spectra of Jupiter at 5 |m as recorded by the Voyager probe. This spectral region enables the tropospheric layers of the planet to be examined (After Drossart et al., 1982)

5 -j.-T——r—T—~T-'-I-r-i-1-|-1-1--1-[-1-1-1-1-1-1-i-]-1-1—T

5 -j.-T——r—T—~T-'-I-r-i-1-|-1-1--1-[-1-1-1-1-1-1-i-]-1-1—T

Fig. 4.21 The infrared spectra of the giant planets, Jupiter (top), Saturn (middle) and Neptune (bottom) as recorded by the SWS instrument on the ISO satellite. Stratospheric lines appear in emission, and tropospheric lines in absorption (After Encrenaz et al., 2004)

Wavelength (Mm)

Fig. 4.21 The infrared spectra of the giant planets, Jupiter (top), Saturn (middle) and Neptune (bottom) as recorded by the SWS instrument on the ISO satellite. Stratospheric lines appear in emission, and tropospheric lines in absorption (After Encrenaz et al., 2004)

4.4.4.3 Element and Isotopic Ratios in the Giant Planets a. Element ratios

In the nucleation model that we have described earlier, the giant planets formed initially as a core of heavy elements (A > 10), with a mass of 10-12 Earth masses. This mass was sufficiently large to cause the collapse of the surrounding nebula. According to cosmic abundances, the mass fraction of heavy elements is about 2 per cent. Assuming that all the heavy elements are equally trapped in ices, and that the elements became homogenized within the planets' interiors through the heating caused by the accretion phase, it is possible to calculate, from the total masses of the giant planets, their enrichment in heavy elements relative to hydrogen, by comparison with solar values. In the absence of any initial core, the enrichment would be 1. For a planet consisting solely of its initial core of heavy elements, the enrichment would be 50. Table 4.2 lists the enrichments predicted for the four giant planets, assuming a mass of 12 Earth masses for the initial core of heavy elements.

Table 4.2 shows a remarkable agreement between predictions and the observational measurements that are available. The most striking example is that of Jupiter, where precise measurements have been obtain by the mass spectrometer on the

Table 4.2 Enrichment in heavy elements predicted (using the nucleation model, assuming an initial core of Mc of 12 Earth masses) and observed in the giant planets (After Encrenaz, 2005)

Planet Mr Mps Mtps E E0bs

Table 4.2 Enrichment in heavy elements predicted (using the nucleation model, assuming an initial core of Mc of 12 Earth masses) and observed in the giant planets (After Encrenaz, 2005)

Planet Mr Mps Mtps E E0bs

Jupiter

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