Phosphine

Photolysis of phosphine may lead to the formation of diphosphine P2H4 via the reactions:

The main pressure level of photolysis is again around 100mbar and thus the abundance of phosphine is expected to decrease throughout the upper troposphere and lower stratosphere of all the giant planets with a scale height governed by the vertical eddy-mixing coefficient and solar insolation.

If temperatures are low enough for the diphosphine to condense, then this may provide an extra source of haze materials in the upper tropospheres of Jupiter and Saturn. However, if the diphosphine does not condense then further reactions are possible which, provided that the levels of scavenging molecules such as acetylene (C2H2) and ethylene (C2H4) are not too high, give rise to the production of the solid phosphorus allotrope P4(s). The P4(s) allotrope is bright red and it has been suggested that this product might be able to explain the red coloration of Jupiter's GRS. However, this identification is highly speculative and unproven.

Figure 4.4. UV cross-sections of different gases relevant to giant planet atmospheres. From Atreya (1986).

Figure 4.4. UV cross-sections of different gases relevant to giant planet atmospheres. From Atreya (1986).

Unfortunately, insufficient thermodynamic data exists for diphosphine to predict whether or not it does actually condense in the upper tropospheres of the giant planets.

Methane

Methane photochemistry is highly complicated with a number of possible branch reactions shown in Figure 4.5.

From Table 4.4, we can see that the UV photons capable of initiating methane photolysis (mainly Lyman-a at 121.6 nm) may penetrate to a depth of only lOmbar in the atmospheres of all the giant planets due to Rayleigh scattering. However, the photoabsorption cross-section of methane is very high and thus the peak level of methane photodissociation is actually at somewhat higher levels (depending on the methane abundance) and for Jupiter extends between O.l ^bar and O.l mbar. While this is the main region of methane photodissociation, to combine the resulting products into hydrocarbons requires a number of two-body and three-body reactions, which are only efficient at higher pressures (O.l mbar and higher). The

Figure 4.5. Methane photochemistry paths. From Moses et a/. (2000). Reprinted with kind permission of Elsevier.

main resultant products of methane photodissociation are acetylene (C2H2), ethane (C2H6), and polyacetylenes (C2„H2). The intermediate molecule ethylene (C2H4) is more unstable, but has been detected in the atmosphere of Jupiter. Further reactions are possible leading to more complicated molecules such as methyl acetylene (CH3C2H) and benzene (C6H6), which have been detected in some of the giant planet atmospheres. The intermediate product CH3 has now also been detected in all the giant planet atmospheres except Uranus.

Haze particles, similar to terrestrial "smog", are probably produced at these altitudes by further complex chemical reactions that produce long-chain polymers that ultimately form aerosol particles. In addition, the lower stratospheres of Uranus and Neptune are so cold that some of the photochemically produced hydrocarbons freeze out directly. Both the hydrocarbons and the hazes spread vertically to other pressure levels through eddy mixing. At deeper pressures, the haze particles may also start to coagulate and settle gravitationally through the atmosphere. Eventually the photochemical products will reach the deep warm atmosphere where they are pyrolyzed back into methane again.

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