Helium giant planets

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). 

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

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... 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

A problem with adopting the solar wind He/" He ratio as representative of the solar nebula is the production of He from deuterium very early in solar system history consequently, the solar wind value (—4.4 X 10 " ) is too high by a factor between —2.5 and —3 relative to the proto-Sun (Geiss and Reeves, 1972). To circumvent this difficulty, recourse has been made to analyzing the giant planets whose atmospheres are expected to reflect proto-solar values (Wieler, 2002). Jupiter is the only giant planet whose atmospheric He/" He ratio has been determined (Mahaffy et al, 1998). Its value of 1.66 Xl0 (—120Ra where Ra = air He/" He)—remarkably similar to measurements on primitive meteorites (see below), is now adopted as most representative of primordial (proto-solar) helium. 

The four giant planets have hydrogen- and helium-rich compositions reminiscent of the Sun, but all of them clearly depart from strict solar composition in that their densities are too high and the few heavier elements whose tropospheric abundances can be measured all show clear evidence of enrichment. For all four giant planets we have spectroscopic compositional data on the few compounds that remain uncondensed in the visible portion of their atmospheres, above their main cloud layers. These include ammonia, methane, phosphine, and germane. For Jupiter, these volatile elements (C, N, S, P and Ge) are enriched relative to their solar abundances by about a factor of five. For Saturn, with no detection of germane, the enhancement of C, N, and P is about a factor of 10. For Uranus and Neptune the methane enrichment factor is at least 60, consonant with their much higher uncompressed densities. 

The outer or giant planets - Jupiter. Saturn, Uranus, and Neptune - are massive low-density bodies with a rocky core surrounded by deep layers consisting mainly of solid, liquid, and gaseous hydrogen and helium. They are much further from the sun and therefore much cooler. All have large numbers of satellites Jupiter has at least 63 Saturn at least 61 Uranus 27 and Neptune 13. The outer planets also have ring systems composed of smaller bodies, rocks, dust, and ice particles. 

Hydrogen and helium are the most abundant elements in the universe, accounting for more than 99% of all atoms in stars, the interstellar medium, and the giant planets Jupiter and Saturn. Of the heavier elements, the most abundant in... 

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