Jupiter

Composition profiles

The bulk composition of Jupiter (Figure 4.6) was discussed in Chapter 2. Of all the giant planets it is the one that most closely approximates a proto-solar composition, but even here the abundance of heavy elements such as carbon, sulfur, and nitrogen is found to be three to five times greater than would be expected in a purely solar composition atmosphere. This is consistent with the core accretion theory (Mizuno, 1980; Pollack et al., 1996) that Jupiter initially formed from icy planetesimals, which then formed an embryo big enough to gravitationally attract a very large quantity of the surrounding nebula gas.

When we talk about "deep" compositions, which are listed in Tables 2.2a and 2.2b, it is worth clarifying how "deep" we really mean. Jupiter is the only giant planet from which in situ measurements of the atmospheric composition have been made by the Galileo entry probe in 1995. These measurements extend down to pressures of approximately 20 bar. While this is very high compared with remotely sensed infrared and microwave observations, which extend down to 10 bar at most, it is still only scraping the surface of the enormous Jovian atmosphere. Hence, all we can really measure is the composition of the top of the molecular-hydrogen region. Any composition gradients that may occur in radiative zones, or at the metallic-

Figure 4.6. Voyager 1 image of Jupiter. The Great Red Spot (GRS) is clearly visible near the center of the image. Courtesy of NASA.

hydrogen/molecular-hydrogen phase boundary are very difficult to detect and may only be inferred from interior models matching the observed oblateness, rotation rate, and gravitational /-coefficients.

Figure 4.7 shows the results of a calculation using a simple equilibrium cloud condensation model (ECCM) of Jupiter. In these models (e.g., Atreya, 1986; Lewis, 1995), as described in Section 4.3.3, a parcel of air is raised upwards and if the partial pressure of a gas exceeds the saturated vapor pressure (s.v.p.), the excess is assumed to condense as cloud droplets and be lost from the parcel. Here, the temperature profile observed in the upper troposphere has been extended downwards towards the interior along the SALR, consistent with the condensation of water, NH4SH, and

Figure 4.7. Equilibrium cloud condensation model of Jupiter's atmosphere. (1, top left) Temperature profile follows SALR below radiative-convective boundary. Dotted line follows DALR below 1 bar level. (2, top right) Variation of calculated lapse rates with height. Dotted line is DALR, solid line is SALR. (3, bottom left) Composition profiles: H20 (solid), H2S (dotted), NH3 (dashed), CH4 (dot-dashed). (4, bottom right) Cloud densities: H20 cloud (water, then ice) at ~ 7 bar, NH4SH at ~2 bar, and NH3 ice at ~0.7 bar. Assumed composition: O/H, S/H = 5x the solar value, N/H = 5.5x the solar value, C/H = 4.8x the solar value.

Figure 4.7. Equilibrium cloud condensation model of Jupiter's atmosphere. (1, top left) Temperature profile follows SALR below radiative-convective boundary. Dotted line follows DALR below 1 bar level. (2, top right) Variation of calculated lapse rates with height. Dotted line is DALR, solid line is SALR. (3, bottom left) Composition profiles: H20 (solid), H2S (dotted), NH3 (dashed), CH4 (dot-dashed). (4, bottom right) Cloud densities: H20 cloud (water, then ice) at ~ 7 bar, NH4SH at ~2 bar, and NH3 ice at ~0.7 bar. Assumed composition: O/H, S/H = 5x the solar value, N/H = 5.5x the solar value, C/H = 4.8x the solar value.

NH3 clouds at the different levels shown in the figure. The temperature profile that is calculated assuming a DALR is shown as the dotted line for comparison. The deep abundances of O, N, S, and C (relative to H) are assumed to be 5x, 5.5x, 5x, and 4.8 x the solar value, respectively. The lapse rates calculated at different altitudes in the troposphere are also shown in Figure 4.7. The DALR can be seen to increase with height, due almost entirely to the decrease of molecular-hydrogen heat capacity with height as the temperature falls. The slight reduction in gravitational acceleration with height also tends to increase the lapse rate, but this effect is small. When condensation is included, the SALR can be seen to be smaller than the DALR near the bases of the main clouds due to the release of latent heat, although the resultant differences in the calculated dry and saturated temperature profiles is small since the main condensates have low deep abundances. The abundance profiles of NH3, H2S, H2O, and CH4 are calculated by limiting the partial pressures to the s.v.p. when condensation occurs (and for the NH4SH cloud via Equation 4.49), and the associated cloud densities are also shown. We will discuss the calculated cloud profiles in the next section ("Clouds and hazes").

Mole fraction

Figure 4.8. Observed and modeled abundance profiles in the atmosphere of Jupiter.

Mole fraction

Figure 4.8. Observed and modeled abundance profiles in the atmosphere of Jupiter.

While such ECCM models are useful for making an initial estimate of the basic tropospheric abundance profiles of condensing molecules, observations of the composition profiles are rather different as can be seen in Figure 4.8 where the observed and best-modeled composition profiles of condensable gases and other species are shown. Sources of estimated Jovian composition data and references are listed in Table 4.6. In particular, analysis of the 5 ^m part of the infrared spectrum measured by Voyager, and more recently Ga/i/eo, has found water to be significantly less abundant than expected. This is probably due to this spectral region being most sensitive to cloud-free areas, which appear to be volatile-depleted regions either due to subsidence or column-stretching (as we shall see in Chapter 5). Typical levels of saturation in the 5 bar region are found to be of the order of 10%. It was hoped that the question of the deep abundance of water would have been answered by the Ga/i/eo entry probe. However, the probe unfortunately descended through just such a cloud-free 5 ^m bright region and so its estimates of volatile abundances are similarly depleted. The probe Mass Spectrometer found that the abundance of water increased with depth and had reached a value of — 0.5 x the solar value at 19 bar. A similar depleted profile was inferred from the probe's Net Flux Radiometer. Presumably the water vapor profile in upwelling, cloudy regions is similar to the calculated ECCM case, but until estimates of water vapor can be made in such regions we cannot be sure.

Table 4.6. Composition of Jupiter.

Gas

Mole fraction

Measurement

Reference

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