DPS 35th Meeting, 1-6 September 2003
Session 50. Outer Planets/Gas Giants III
Oral, Chairs: G. Orton and K. A. Rages, Saturday, September 6, 2003, 3:30-5:40pm, DeAnza I-II

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[50.03] The Abundance of Carbon Monoxide in Neptune’s Atmosphere

B.E. Hesman (University of Saskatchewan), G.R. Davis (University of Saskatchewan/Joint Astronomy Centre), D.A. Naylor (University of Lethbridge)

The unexpected discovery of carbon monoxide in the stratosphere of Neptune in 1991 and its absence in Uranus have important implications for the origin of the planet. The two potential sources of CO in Neptune are infalling material from the interplanetary medium, and convection due to Neptune’s stronger internal heat source. The tropospheric CO abundance would be a key discriminator between these two possibilities. The CO J=3-2 absorption line is strongly pressure-broadened and ideally suited for measurement using a Fourier transform spectrometer (FTS), which offers moderate spectral resolution over a wide spectral range.

Observations in 1993 using the University of Lethbridge FTS at the James Clerk Maxwell Telescope (JCMT) resulted in a marginal detection of the CO absorption line (Naylor et al., 1994, A & A 291, L51-L53). The accuracy of the abundance was limited by channel fringing, which is the result of multiple reflections from parallel surfaces within the detector optics and produces a high-frequency modulation in the data. To improve upon this measurement, the instrumentation was upgraded including a completely new FTS in the Mach-Zehnder configuration, a new detector system with better optical design to minimize the effect of channel fringing, and an enhanced data processing pipeline. Analysis of the data obtained during an observing run in September 2002 has confirmed the existence of tropospheric CO in Neptune. These results will be presented.

This research is supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.