The Chemical Composition

As mentioned previously, spectra of Jupiter's clouds have not provided many answers regarding the chemical composition of the planet, and all of the major compounds predicted to form clouds are chemically simple and would appear white. However, because of Jupiter's size and mass, it is expected to have retained chemical elements similar to the cosmic proportions when it formed - that is, the proportions found in the Sun and the interstellar gas [142]. Therefore, as we understand the formation of our solar system, we would expect hydrogen and helium to make up the greatest part of Jupiter's mass, or 99%. The hydrogen-dominated atmosphere is chemically 'reducing' - that is, the other elements should exist mostly as hydrogen compounds [143]. Over the years ground based, and more recently, spacecraft observations have contributed significantly to our knowledge of Jupiter's chemistry. We know that different elements exist in Jupiter's clouds at different levels or bars of pressure, since their formation could only occur at specific pressures. Therefore, as different wavebands are emitted from different depths, a spectral line reveals the concentration of a gas at and above the corresponding depth at that wavelength [144]. We shall see later how this property helps to characterize the vertical structure of Jupiter's atmosphere.

The first molecules that were discovered on Jupiter were methane and ammonia. Rupert Wildt (1931, 1932) identified them from several absorption bands that had long been known in the red-light spectrum [145]. After the Voyager spacecraft missions, and prior to the Galileo era, we knew that the following molecules were present in Jupiter's atmosphere [146].

Methane (CH4) is the most abundant gas after hydrogen and helium. It exists at all levels of the atmosphere [147].

Ammonia (NH3) is the next most abundant, existing at 1 bar or deeper. Ammonia levels seem to be affected by Jupiter's weather. For example, there is less ammonia in the belts than in the zones. This pattern is consistent with the theory that belts are regions of sinking gas or downwelling, and the ammonia poor gas drawn down from higher altitudes explains why the ammonia clouds are lower and thinner in the belts [148].

Water (H2O) is much less abundant than ammonia. There is none at 1 Bar and only a little at ~4 Bars [149].

Hydrogen sulfide (H2S) has not been definitely detected, but it is expected to be present deeper, below the clouds [150].

Phosphene (PH3), germane (GeH4), and arsine (AsH3) ( Noll and colleagues, 1990), have been detected in proportions similar to those which are predicted from cosmic abundances. Since they should be unstable in the troposphere, their presence may be due to updrafts from much deeper levels. However, so far there is no definite sign of variations in their abundances between the belts and zones [151].

Carbon monoxide (CO) is even more surprising as it is an oxidized compound that must be unstable in reducing atmospheres. However, the observations show it to be well mixed throughout the troposphere. Therefore, it must also be brought up from deeper regions [152]. According to Lewis and Fegley (1984), the abundances of phosphene, germane, and carbon monoxide can all be explained by updrafts from the level of Jupiter's atmosphere where the temperature is -800-1,300 K, a depth well below any direct probing [153].

Hydrogen cyanide (HCN) was first definitely detected by Tokunaga and colleagues (1981), and is not in chemical equilibrium. Its origin is uncertain, its most likely source being photochemical (light-induced) reactions in the upper troposphere [154].

Acetylene (C2H2) and ethane (C2H6) have also been detected, but in infrared emission, indicating they are in the stratosphere. It is believed that acetylene and ethane are produced photo-chemically by irradiation of methane in the stratosphere [155].

Since the Voyager missions, the Galileo spacecraft has orbited Jupiter and the Cassini spacecraft has made its historic flyby on its way out to Saturn. As a result of these two missions, we now have evidence of additional molecules. We also know that the origins of these gas molecules are linked to internal thermo-chemistry, photo-chemistry, and impactor chemistry; and these processes are now more fully understood with respect to Jupiter.

According to Kunde et al., "down in the deep atmosphere where pressures and temperatures are high, Jupiter's thermo-chemical furnace processes the approximately solar elemental composition by converting H (hydrogen) atoms into molecular forms (H2) and reactive atoms (e.g., C, N, and O [carbon, nitrogen, and oxygen]) into saturated hydrides (methane, Ch4; ammonia, NH3; water, H2O). Convection transports these molecules upward into the cooler regions, where H2O, NH4SH, and NH3 condense to form clouds" [156]. Thus, Jupiter is a very good example of thermo-chemical processes.

Photo-chemistry relates to the changing of molecules due to the interaction of UV photons with molecules in Jupiter's upper atmosphere, or bombardment of the surfaces of asteroids and meteors by solar UV.

The composite infrared spectrometer (CIRS) on board the Cassini spacecraft also detected two new hydrocarbon species in Jupiter's stratosphere, the methyl radical CH3 and diacetylene C4H2. Both of these elements contribute to Jupiter's stratospheric photo-chemistry. They were detected in Jupiter's north and south auroral infrared hotspots [157]. The polar auroral stratosphere is driven by the deposit of energetic magnetospheric electrons and ions that heats the atmosphere, enhances the abundances of some hydrocarbons by ion-induced chemistry, and increases the visibility of all stratospheric species in the thermal infrared by elevating the ambient temperatures. CIRS measurements of Jovian auroral regions show that the emissions of many hydrocarbons within the auroral infrared hotspots are enhanced compared to the surrounding ambient polar atmosphere; that is, there So®

is a distinct difference in temperature and/or composition between the hotspot <D «jg §

relative to its surroundings [158]. y E

Impactor chemistry relates to the influence of chemical composition by external s ^

sources of material. The most dramatic illustration of this was the multiple impacts ¡5 g of comet Shoemaker-Levy 9 (SL9) into Jupiter in July 1994. CIRS observed spatial "5 O m-

distributions of carbon dioxide and hydrogen cyanide and both are considered to CC

be by products of the SL9 impacts. SL9 injected large quantities of nitrogen, oxygen, and sulfur bearing molecules into Jupiter's stratosphere. Substantial amounts of hydrogen cyanide, carbon monoxide, and carbon monosulfide were produced in

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