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[30.06] Is C₂H₂ Present Deep in the Jovian Atmosphere?

Y. B\'{e}tr\'{e}mieux, R. V. Yelle (Center for Space Physics, Boston University)

On Jupiter, acetylene (C₂H₂) is widely accepted to be produced predominantly by methane photolysis by UV photons and hence should be found high in its stratosphere. Moreover, observations indicate that the C₂H₂ abundance decreases rapidly with increasing pressure throughout the stratosphere. However, spectroscopic observations conducted by HST's Faint Object Spectrograph (FOS) suggest that C₂H₂ is present in significant abundance in the upper troposphere. The FOS spectra exhibit C₂H₂ features shortward of 190.0~nm that are consistent with C₂H₂ located in the upper stratosphere above 50~mbar. Also detected are pronounced C₂H₂ features overlapping with ammonia (NH₃) features longward of 200.0~nm. Using a radiative transfer model that incorporates Raman scattering by molecular hydrogen reveals that these features can only be explained with a C₂H₂ mixing ratio of 1~\times~10^-7 or more below the 150~mbar level. Either the C₂H₂ in this region is produced through some unknown chemical cycle or it is convecting from deeper regions of the atmosphere. Although the latter is less probable on thermodynamical grounds, since the available C₂H₂ cross section cuts off longward of 210.0~nm, it is difficult to say if C₂H₂ is present at the 350~mbar level or deeper and hence distinguish between the two possibilies.