The Vertical Structure of Jupiters Atmosphere

Unlike the physical or visual appearance of Jupiter where observations of changes in the longitudinal and latitudinal positions of features has long been an area in which amateurs have been able to participate, observations of Jupiter's vertical structure, at least at the time of this writing, have not. Indeed, even the ability of the professional astronomer to pierce deeply the cloud tops of Jupiter is still severely restricted, as we shall see. However, as amateurs we should seek to understand that knowledge which is available and the processes by which such data is acquired and reduced.

With an understanding of the abundances of elements in Jupiter's atmosphere, and knowing the profile of temperature and pressure with altitude, it is possible to predict the levels at which clouds of various types should form [163]. The depth to which a sensing instrument can view depends upon the opacity of the gas, which is dependent upon the chemical composition. Different gases condense at different temperatures and pressures, both of which increase with depth [164]. Figure 4.1 depicts a simplified model of the vertical structure of Jupiter's atmosphere.

John S. Lewis (1969) first developed the cloud model presently accepted by Jupiter scientists. Lewis predicted three main cloud layers: ammonia ices at the highest level (0.3-0.7 bars), ammonium hydrosulphide below it (NH4SH; 2 bars), and water deeper still (5-6 bars) [165]. Water will condense as it rises within Jupiter's atmosphere and passes through the 4-bar pressure zone. Ammonia condenses at much colder temperatures, and higher in the atmosphere at ~0.7 bars. But, since Jupiter's atmosphere is not cold enough to condense methane, it remains in the gaseous state [166]. The water clouds could include an upper layer of ice crystals and a lower layer of water droplets, like on Earth. The water is not pure, but would have a lot of ammonia dissolved into it (Fig. 4.2). According to Carlson and colleagues (1988), the ammonia clouds are like thick cirrus and are unlikely to precipitate anything. However, the thick water-ammonia clouds below them will probably produce rain [168].

Because the methane is well mixed, and because it has convenient spectral absorption features, it can be used as a 'tracer' for determining the vertical distribution of clouds of other elements. This is why professionals today are so excited that amateurs are now able to image Jupiter in methane wavelengths. The atmospheric pressure of a cloud can be inferred from images taken in the near infrared. Knowing the pressure of a cloud allows us to infer its depth, and knowing its depth allows us to infer its chemical composition. This is because clouds of various types will only form at specific pressures and temperatures. Or, considering this from another point of view, if an observation identifies a cloud of ammonia on Jupiter for example, then we would know the depth of the cloud and its temperature. An image taken at a wavelength which is strongly absorbed by methane can penetrate no deeper than 1 bar; a wavelength which is less strongly absorbed can penetrate a little deeper; and a wavelength which methane does not absorb will penetrate to about 8 bars. If we make assumptions about opacity, then such imagery can be processed to infer the pressure, and thus the depth, at which a feature resides. By 'stacking' the three images it is possible to infer the vertical structure of a specific cloud [169]. This is very useful information.

With a simplified model we can portray the vertical cloud structure of Jupiter's belts and zones, and the convective flow (Fig. 4.3). Note the relative altitudes of the clouds and hazes in the belts and zones. Also note that the top of the GRS resides at the highest altitude.

It may surprise those new to observing Jupiter that we are not able to penetrate very deep into Jupiter's atmosphere with our instruments. Remember, the clouds of the troposphere are optically thick, and we only see the tops of clouds illuminated by sunlight. The stratosphere has lower densities and is mainly gases and aerosols, and is optically thin. In fact, the stratosphere is invisible visually. According to Simon-Miller, the stratospheric haze is also virtually impossible to see, except in the UV, where it is usually featureless, though some features are occasionally seen. (Simon-Miller personal communication). Certainly, getting down to 4 or 5 bars is just scratching the surface. So, what might exist deeper in the planet?

To infer the structure of Jupiter far below the clouds, we must turn to theoretical physics, using what information is available on the behavior of hydrogen and helium at extreme temperatures and pressures [170]. The current accepted theories and models seem to agree on the following main features (Fig. 4.11). Below the visible clouds exists a deep atmosphere/ocean of molecular hydrogen. At the higher altitudes (lower pressure) the hydrogen exists as a gas, but lower down, ~1,000 km w __

below the cloud tops at thousands of bars of pressure, this hydrogen gas turns £ £ ®

into a hot liquid [171]. Even further down, at a depth of 15,000-25,000 km below O «2 §

the cloud tops at temperatures over 15,000 K, at a pressure of 2-4 million bars we CL

should find a mantle of metallic hydrogen [172]. And finally, at the very center of s ^

the planet, at a pressure approaching 1,006 million bars and a temperature over jj g JS

35,000 K, there should exist a rocky core. At this pressure and temperature, most of "o O

the planet's metals should have sunk into this core [173]. U V O

Jupiter's interior generates extreme heat. What causes this tremendous heat? Earth's weather is driven by heat absorbed from the Sun. However, Jupiter's weather is driven more by its own internal heat than from solar heat. Spacecraft

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