31st Annual Meeting of the DPS, October 1999
Session 69. Outer Planet Chemistry
Contributed Oral Parallel Session, Friday, October 15, 1999, 10:30-12:00noon, Sala Kursaal

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[69.05] Evidence for lightning-production of acetylene in Jupiter's equatorial troposphere.

Y. B\'{e}tremieux (Boston University), R.V. Yelle (Northern Arizona University)

Acetylene absorption features overlapping with ammonia features have been detected for the first time longward of 190.0~nm in Jupiter's equatorial spectrum obtained with the Hubble Space Telescope's (HST) Faint Object Spectrograph (FOS). A detailed analysis of this spectrum with a Rayleigh-Raman radiative transfer model shows that the column-averaged acetylene mole fraction decreases from 3-4\times10-8 below 20~mbar to 1-1.5\times10-8 at 20-60~mbar, consistent with past observations. However, the depth of two prominent acetylene features around 207.0~nm reveals that the mole fraction then increases to about 1.5\times10-7 in the upper troposphere.

A photodestruction transport model was developed to explain the acetylene vertical profile. The photodestruction efficiency for acetylene is assumed to be 2.5%, while the eddy diffusion coefficient, and sources of acetylene in the stratosphere and troposphere are left as free parameters. According to the model, the increase of acetylene with pressure in the lower stratosphere can only be explained by a sudden increase in the mixing rate of the atmosphere around 110 to 170~mbar. The eddy diffusion coefficient rises from 3-6\times102~cm2~s-1 in the lower stratosphere to more than 1\times104~cm2~s-1 in the upper troposphere.

The abundance of acetylene in the upper troposphere requires an upward flux at 700~mbar which is most likely created through lightning-induced chemistry deep in the troposphere. This source supplies about an order of magnitude more acetylene than does methane photolysis in the stratosphere. These results constitute the first compelling evidence that lightning-induced chemistry plays a dominant role in the compositional structure of Jupiter's upper atmosphere.

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