Saturn

Composition profiles

Tables 2.2a and 2.2b list the "deep" composition of the Saturnian atmosphere. Like Jupiter there is good evidence that Saturn (Figure 4.15) initially formed from icy planetesimals before reaching sufficient mass to collapse and condense the solar nebula in its feeding zone. The fact that Saturn is much less massive than Jupiter suggests that it was able to attract a much smaller mass of H2 and He from the nebula, and thus the mixing ratios of the heavier elements (X/H) are expected to be correspondingly higher. The observed estimated value of the deep C/H ratio, from Cassini CIRS observations, of —11 x the solar value (Fletcher et a/., 2008b) is thus entirely consistent with this expectation. Similarly, the deep abundance of ammonia has been estimated from ground-based microwave observations to be approximately 4-5 x the solar value (de Pater and Massie, 1985), although 5 ^m observations suggest a lower value. The estimated abundance of phosphine from submillimeter observations suggest that the deep P/H ratio is 7-14x the solar value (Orton et a/., 2000, 2001) and this is consistent with the figure estimated from Cassini CIRS observations (Fletcher et a/., 2007b, 2008b) which is roughly 13 x the solar value.

The troposphere of Saturn is colder than that of Jupiter and thus the condensation levels of different tropospheric gases is correspondingly lower. Figure 4.16 shows the results of calculation of a Saturn ECCM. The deep abundances of O, N, S, and C (relative to H) are assumed to be 10x, 10x, 14x, and 11x the solar value, respectively. It can be seen that water vapor should start to condense near 18 bar and thus the composition of this molecule falls rapidly with height and has a v.m.r. of only 2 x 10-7 at a pressure of 3 bar. The ammonia v.m.r. remains fixed until approximately 5 bar, where it is partially depleted by the formation of a putative

Figure 4.15. Image of Saturn recorded by the HST/WFPC-2 instrument in 1990. Courtesy of NASA.

Figure 4.16. Equilibrium cloud condensation model of Saturn's atmosphere (as Figure 4.7). Calculated cloud layers: H20 cloud (water, then ice) at ~18 bar, NH4SH at ~5bar, andNH3 ice at ~1.8 bar. Assumed composition: O/H, N/H = 5x the solar value, S/H = 14x the solar value, and C/H = 11 x the solar value.

Figure 4.16. Equilibrium cloud condensation model of Saturn's atmosphere (as Figure 4.7). Calculated cloud layers: H20 cloud (water, then ice) at ~18 bar, NH4SH at ~5bar, andNH3 ice at ~1.8 bar. Assumed composition: O/H, N/H = 5x the solar value, S/H = 14x the solar value, and C/H = 11 x the solar value.

NH4SH cloud. Just as for Jupiter, the ammonia v.m.r. in this model is expected to remain fixed until the ammonia cloud condensation level of approximately 1.8 bar, and to then fall rapidly above this due to the same combination of condensation, photolysis, and mixing which defines Jupiter's ammonia profile.

Estimates of the Saturn composition profiles (and references thereto) are listed in Table 4.7 and best-fit profiles are shown in Figure 4.17. The abundance of water at 3 bar is found to be similar to that calculated from the ECCM. Ammonia, however, is again found to be less abundant than predicted by the ECCM with an estimated deep mole fraction of only x the solar value from ISO 5 ^m observations (de Grauuw et al., 1997), although earlier 5 ^m observations put the figure at less than 2.5x the solar value, while microwave observations (de Pater and Massie, 1985) suggest a value as high as 5 x the solar value. The variation of ammonia above the condensation level is more difficult to detect than for Jupiter since the spectral absorption features in the mid-IR are swamped by those of phosphine, which has a much higher abundance in Saturn's atmosphere than it does in Jupiter's. An ammonia profile with approximately 50% humidity is consistent with ISO measurements (de Graauw et al., 1997). The phosphine profile appears to be fixed up to a pressure level of ^600 mbar and falls rapidly above this due to photodissociation and mixing. Significant levels of CO are detected in the troposphere (of the order of 10 —9, if uniformly mixed, according to

Table 4.7. Composition of Saturn.

Gas

Mole fraction

Measurement technique

Reference

He

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