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Solar composition is assumed to be as estimated by Grevesse et al. (2007) and listed in Table 2.1. Sources of planetary abundances are listed in Chapter 4.

Solar composition is assumed to be as estimated by Grevesse et al. (2007) and listed in Table 2.1. Sources of planetary abundances are listed in Chapter 4.

2006). Hence, these molecules cannot have been accreted from their gaseous phase, but instead must have been concentrated in some other way. Current theories suggest that significant quantities of these molecules were incorporated into water ice, which then accreted in Phase 1 and 2 to produce the observed enhancement. However, there is considerable disagreement as to the form of the ice and its temperature of formation.

A number of laboratory experiments have been conducted to assess how well pre-solar nebula gases might be trapped by water ice. At very low temperatures (T < 100 K), water condenses as amorphous ice; laboratory experiments have shown that to trap the quantities of heavy elements observed in Jupiter's atmosphere the ice formation temperature must have been less than 40 K and may have been as low as 30 K (Bar-Nun et al, 1988; Owen and Encrenaz, 2003, 2006; Owen et al, 1999). This is much lower than the predicted nebula temperatures at 5AU to 6AU during the time that Jupiter formed, assuming it did indeed form at this distance, and suggests that either (i) the pre-solar ice grains (which are almost certainly amorphous) never vaporized in the Jupiter region (i.e., the nebula was much colder than is currently

Table 2.2b. Bulk composition of the Jovian planets (as mole fractions).

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