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H. B. Hammel, M. L. Sitko (Space Science Institute), D. K. Lynch, R. W. Russell (The Aerospace Corporation), T. Hewagama (University of Maryland), L. S. Bernstein (Spectral Sciences, Inc.)
We report 8- to 13-micron spectral observations of Neptune and Uranus from the NASA Infrared Telescope Facility (IRTF) spanning more than a decade. The spectroscopic data indicate a steady increase in Neptune's mean atmospheric 12-micron ethane emission from 1985 to 2003, followed by a decrease in 2004 to near 1991 levels. The simplest explanation for the intensity variation is an increase in stratospheric effective temperature from 154 +/- 3 K in 1985 to 178 +/- 3 K in 2003 (an average rate of 1.3 K/year), and subsequent decrease to 166 +/- 3 K in 2004. We also detected variation of the overall spectral structure of the ethane band, specifically an apparent absorption structure in the central portion of the band. Some of this structure is created by a non-uniform response function within detector diodes, or alternatively, by absorption due to ethane-containing ice particulates in the lower stratosphere. We also report a probable direct detection of ethane emission on Uranus. The deduced peak mole fraction is approximately an order of magnitude higher than previous upper limits for Uranus; the model fit suggests a slight warming of the globally-averaged stratosphere of Uranus and hints at the presence of ethane ice. HBH acknowledges support for this work from NASA grants NAG5-11961 and NAG5-10451. DKL and RWR acknowledge support at The Aerospace Corporation by the Independent Research and Development (IR&D) Program. LSB acknowledges the support of Spectral Sciences, Inc. IR&D funding. HBH, DLK, and RWR were visiting astronomers at the IRTF, operated by U. Hawaii under contract from NASA.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.