Hydrogen giant planets

Hydrogen is the primary component of Jupiter and the other gas giant planets. At some depth in the... 

There are several basic features to note about the chemical abundances of the solar system. First, the Sun, and thus the solar system, consists dominantly of hydrogen and helium, with these two elements making up >98% of the mass of the solar system. Outside of the Sun, hydrogen and helium are found primarily in the gas-giant planets. 

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. 

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). 

Estimated compositions ofthe giant planets are given in Table 14.3, normalized to the solar composition. The relative proportions of rock and volatiles are estimated from mean densities, the rock compositions are assumed to be chondritic, and the ratios of hydrogen to helium are derived from spectroscopic or spacecraft measurements of atmosphere compositions. 

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. 

Isotopic abundances for hydrogen have been measured in giant planet atmospheres, as shown in Figure 14.11. The D/H ratios in Jupiter and Saturn are similar to those in the Sun, but lower than those in the Earth s oceans or in comets. D/H ratios in Uranus and Neptune... 

How We Know the Giant Planets Contain Hydrogen and Helium... 

Figure 1 Some known giant planets and brown dwarfs, illustrated with a limited azimuthal slice (pie slice) to correct scale. The interiors are color coded according to the principal materials in each zone. Ice and rock refer to elements common in materials that are icy or rocky at normal pressures. Metallic hydrogen indicates ionization primarily through pressure effects. Modeled central temperature in K, and pressure in 10 bar, is shown. The radii of all but G1229b are known directly for G1229b, modeling of the brightness versus wavelength must be used to derive the radius. From left to right, the masses (expressed relative to the mass of Jupiter) are 45,1, 0.7, 0.3, and 0.05...

What are the detailed internal structures of the giant planets and how are the magnetic fields generated While the separation of helium from hydrogen seems to be assured for Saturn, it is unclear to what extent this occurs in Jupiter. Further, the distribution of elements heavier than hydrogen and helium remains unclear in Jupiter and Saturn, in part because of equation-of-state uncertainties. The interiors of Uranus and Nepmne are even less certain. For these reasons, and because of uncertainties in dynamo theory, the specific details of the magnetic field... 

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