Modeling of methane photolysis in the reducing atmospheres of the outer solar system

Abstract

Abstract. It is important that methane photolysis is accurately treated in models of

Photochemistry of Saturn's Atmosphere I. Hydrocarbon Chemistry and Comparisons with ISO Observations

“…12 show how the abundances of the major observable hydrocarbons vary with changes in the methane photolysis branching ratios at H Ly-α. The results from our nominal model lie somewhere between the two suggested cases of Mordaunt et al (1993), while the branching ratios suggested by Smith and Raulin (1999) result in abundances of complex hydrocarbons that are measurably lower than our nominal model.…”

“…Sensitivity of the CH 3 , C 2 H 6 , C 2 H 2 , C 2 H 4 , CH 3 C 2 H, and C 4 H 2 mixing ratios to changes in the CH 4 photolysis branching ratios at H Ly-α. The solid curves represent our nominal model, the dashed curves incorporate branching ratios from Model 2 of Mordaunt et al (1993), and the dotted curves incorporate the branching ratios suggested by Smith and Raulin (1999). See Table IV for a description of the CH 4 photolysis branching ratios for these different models.…”

“…Unfortunately, as mentioned in Section 3.5, the branching ratios of CH 4 at Ly-α are not well known due to the high reactivity of some of the photolysis products and to other experimental difficulties. For a good discussion of the current state of knowledge regarding this problem, see Smith and Raulin (1999), Brownsword et al (1997), Heck et al (1996), and Romani (1996). The channel producing CH 3 (R5) is clearly important (Heck et al 1996, Mordaunt et al 1993, but the sometimes contradictory data regarding the quantum yields of H, H 2 , and CH make branching-ratio assignments difficult.…”

Atomic and molecular hydrogen budget in Titan’s atmosphere

“…In our photochemical model, we used the second scheme from Mordaunt et al (1993) for the photodissociation of methane (see, e.g., Lebonnois et al, 2001). We also tested the influence of this scheme on H and H 2 production by using either the first scheme from Mordaunt et al (1993), or the more recent scheme published by Smith and Raulin (1999). The main impact of these two schemes is to favor 1 CH 2 over CH in the photodissociation of methane.…”

Cross Sections and Reaction Rates for Comparative Planetary Aeronomy

“…Only in recent years have the primary products and H and H 2 quantum yields been well characterized (e.g. Cook et al 2001;Wang et al 2000;Smith and Raulin 1999;Brownsword et al 1997;Heck et al 1996;Mordaunt et al 1993); however, the quantum yield of CH, which is of critical importance for the production of unsaturated hydrocarbons, is still uncertain. In addition, the branching ratios from photolysis of other major hydrocarbons (e.g., C 2 H 2 , C 2 H 4 , C 2 H 6 , CH 3 C 2 H, C 3 H 8 ) need to be worked out in better detail.…”