Evolution Of The Giant Planet Atmospheres Jupiter

The bolometric temperature of Jupiter (124 K) is consistent with that expected by the planet radiating its primordial heat to space continually since formation via the Kelvin-Helmholtz mechanism. The interior heat flux remains sufficiently high to keep the liquid metallic interior highly convective with the result that most of planet is expected to mix thoroughly on a timescale of the order of 100 years. As we saw in Chapter 2, this vigorous convection of the metallic-hydrogen interior easily accounts for Jupiter's very high magnetic field.

The phase transition between molecular hydrogen and metallic hydrogen may, however, have an effect upon the atmospheric abundances in the observable atmosphere. Some calculations suggest that solar abundances of helium cannot be dissolved in metallic hydrogen under current Jovian temperature conditions at the lowest pressure where metallic hydrogen exists. Droplets of liquid helium would form in these regions and drop towards the center of the planet converting their gravitational energy into thermal energy and reducing the abundance of helium in the observable atmosphere. However, these solubility calculations are highly model-dependent and thus it is not clear whether or not there has been significant depletion of helium. The current helium mass fraction of 0.238 (von Zahn et al., 1998) is almost the same as the current observable solar value, although the primordial helium fraction is believed to have been slightly larger, perhaps 0.25 (Grevesse et al., 2007). Hence, Jupiter's atmosphere could be depleted if the Sun underwent a similar process of helium separation over time with the Sun fusing helium in its core. The atmospheric depletion of neon observed by the Galileo entry probe would be consistent with helium differentiation since neon is highly soluble in liquid helium. However, the low abundance of neon may also be explained by the clathrate-hydrate theory of formation outlined in Chapter 2, since neon is not easily trapped in clathrates.

The deep circulation of the interior is not well understood. Most models assume that the molecular and metallic regions are homogeneous (i.e., well-mixed) since Jupiter (and Saturn) are still emitting more energy than they receive from the Sun, which implies active convection at great depth. This is a good assumption provided that the hydrogen-helium mix is sufficiently opaque such that convective heat transfer is more efficient than radiative heat transfer. This is almost certainly true in the metallic-hydrogen region, which is thought to be highly opaque to thermal photons, but not particularly conductive for a metal. However, in the molecular-hydrogen region some models suggest that opacity may be sufficiently small at kilobar pressure levels to allow the presence of a thin radiative zone, which would serve as a barrier to convection. This may also occur in Saturn. Likewise the molecular-metallic interface (Figure 2.9), if it exists as a discrete phase boundary, may also act as a barrier (Hubbard, 1997a), although recent laboratory experiments (Nellis, 2000) suggest that the molecular-metallic phase boundary is in fact somewhat smooth as was noted earlier in Chapter 2. In these "barrier" regions, stable to convection, the fluid interior would be stably stratified. If there were negligible turbulent eddy mixing (see Chapter 4) then gravitational settling of the heavier molecules towards the base of the layer may occur in these regions. Hence, a composition gradient would be set up leading to a net transfer of molecules between the well-mixed convective layers above and below the stable region and thus lead to long-term evolution of the composition of the observed atmosphere. In reality, interior models suggest that it is very unlikely that eddy mixing is so low that gravitational settling occurs. However, these "barrier"

regions may have an effect on the observed composition in another way. Eddy mixing is not as efficient at transporting material as convective mixing and thus if the material above and below the convective "barrier" initially has a different composition then the barrier would greatly inhibit the transfer of material across it, thus maintaining compositional differences over long periods of time. During the formation of the giant planets deeper material is thought, from the core accretion model, to have been more icy than material accreted later, which contained proportionally more and more nebula gas. If the whole interior was convectively unstable then these differences would be rapidly eliminated. However, the presence of con-vective barriers may mean that the material above and below the barriers has still not come to equilibrium today and thus that the composition of the observable atmosphere may still be evolving.

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