37th DPS Meeting, 4-9 September 2005
Session 22 Outer Planets II
Oral, Tuesday, September 6, 2005, 11:00am-12:30pm, Law LG19

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[22.02] Meridional Variations of Acetylene and Ethane On Jupiter

C.A. Nixon (University of Maryland), N.A. Teanby, P.G.J. Irwin (University of Oxford), R.K. Achterberg (SSAI), P.N. Romani (NASA GSFC), Cassini CIRS Team

Hydrocarbons such as acetylene (C2H2) and ethane (C2H6) are important tracers in Jupiter's atmosphere, constraining our models of the chemical and dynamical processes. However, our knowledge of the vertical and meridional variations of their abundances has remained sparse. During the flyby of the Cassini spacecraft in December 2000, the Composite Infrared Spectrometer (CIRS) instrument was used to map the spatial variation of emissions from 10--1400~cm-1 (1000--7~\mum).

We have analyzed a zonally-averaged set of CIRS spectra taken at the highest (0.5~cm-1) resolution, to infer atmospheric temperatures in the stratosphere at 0.5--20~mbar via the \nu4 band of CH4, and in the troposphere at 150--400~mbar, via the H2 absorption at 600--800~cm-1 . Simultaneously, we retrieve the abundances of C2H2 and C2H6 via the \nu5 and \nu9 bands respectively. We find that the column abundance of C2H2 decreases sharply towards the poles by a factor ~4, while C2H6\ is unchanged in the north and increasing in the south, by a factor ~1.8.

An explanation for the meridional trends is proposed in terms of a combination of photochemistry and dynamics. Poleward, the decreasing UV flux is predicted to decrease the abundances of C2H2 and C2H6 by factors 2.7 and 3.5 respectively at a latitude 70\circ. However, the lifetime of C2H6\ in the stratosphere (5\times109~s) is much longer than the dynamical timescale for meridional motions inferred from SL-9 debris (5\times108~s), and therefore the constant or rising abundance towards high latitudes likely indicates that meridional mixing dominates over photochemical effects. For C2H2, the opposite occurs, with the relatively short photochemical lifetime (3\times107~s), compared to meridional mixing times, ensuring that the expected photochemical trends are visible.

The author(s) of this abstract have provided an email address for comments about the abstract: conor.nixon@gsfc.nasa.gov

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