The relative abundance of ethane to acetylene in the Jovian stratosphere

Abstract

The observed ratio of C_2H_6 to C_2H_2 in the Jovian stratosphere increases from ∼55 at 2 mbar to ∼277 at 12 mbar. In current photochemical models this ratio typically increases between 2 and 12 mbar by a factor of ≤3. Recent laboratory kinetics studies on the reaction between C_2H_2 and H_2 to form C_2H_4 suggest an efficient chemical mechanism for hydrogenation of C_2H_2 to C_2H_6. Inclusion of this scheme as part of a comprehensive updated model for hydrocarbon photochemistry in the atmosphere of Jupiter provides an explanation of the altitude variation of the C_2H_6/C_2H_2 ratio. The sensitivity of these results to uncertainties in the key rate constants at low temperatures is illustrated, identifying needs for additional laboratory measurements. Since the key reaction rate constants decrease with decreasing temperature, the hydrogenation of C_2H_2 as proposed predicts a qualitatively decreasing trend in the C_2H_6/C_2H_2 value with decreasing distance from the Sun. The observed variation between Jupiter and Saturn is consistent with this prediction.

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