Using the estimated solar system elemental abundances given in Table 2.1, we can calculate the expected composition of an atmosphere of solar abundance assuming a cold dense atmosphere where all elements appear in their fully hydrogenated form. The expected solar composition abundances of the most abundant molecules (relative to H2) are compared with those actually estimated in the giant planet atmospheres in Table 2.2a. The sources of these data will be reviewed in Chapter 4. The same abundances, expressed as volume mixing ratios (or mole fractions), are listed in Table 2.2b. As can be seen the atmospheres of the giant planets have compositions rather different from a pure solar mixture, and it can also be seen that the proportion of heavy elements increases as we move outwards through the solar system.
2.6.1 Constraints on formation: bulk composition X/H
In addition to accumulating hydrogen and helium, the outer planets are also observed to have accumulated significant quantities of other "heavy" elements such as carbon, nitrogen, and sulfur. The predominant pre-solar nebula form of these elements was in uncondensed, gaseous molecules (C as CO or CH4, N mainly as N2 with perhaps 10% as NH3/HCN, S as H2S, O as H2O, etc.) and thus a solar composition abundance of these constituents might be expected. However, the ratio of these elements with respect to hydrogen for the giant planets is found to be significantly supersolar with values of approximately 3-5 x the solar value for Jupiter for C/H, N/H, S/H, Ar/H, Kr/H, Xe/H. The C/H ratio is estimated to increase to 5-11 x the solar value for Saturn and of the order of 40 x the solar value for Uranus and Neptune. Although there remain gaps in the data, at first glance it appears that the other heavy molecules are enriched to approximately the same degree on Jupiter (Table 2.2a) and it is suspected that this is the case for all the other giant planets (Owen and Encrenaz,
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