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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[22.09] Mid-Infrared Ethane Emission on Neptune and Uranus

H. B. Hammel, M. L. Sitko (Space Science Institute), D. K. Lynch, R. W. Russell (The Aerospace Corporation), T. Hewagama (U. Maryland), L. Bernstein (Spectral Sciences, Inc.)

We report spectroscopy of Neptune at 8-13 microns using the NASA Infrared Telescope Facility spanning 19 years. The data showed an increase in Neptune's atmospheric 12-micron ethane emission through 2003. The simplest explanation is an increase in stratospheric effective temperature from 155 K (1985) to 178 K (2003), followed by a slight decrease in 2004; the temperatures represent a combination of cold background and warm regions. The ethane band shape also exhibited variation that could not be reproduced by temperature changes alone. We interpret this variation as evidence for stratospheric ethane ice particulates, which were sensed by emission that propagated downward before reflecting off a lower layer to re-emerge. No ice was required to fit the 1985 and 1991 spectra, but after 2002 an ice column abundance of order 0.23 cm-atm was required. In 2002, the ice absorption fell at a short-wavelength position. From 2002 to 2003, the ice absorption shifted to a long-wavelength position as the effective temperature increased. From 2003 to 2004, the only difference was a drop in effective temperature. The long-term mid-IR variations may be correlated with increasing visible-wavelength brightness, possibly indicating hydrocarbon creation associated with cloud activity. We plan coordinated mid-IR spectroscopy and adaptive-optics imaging with Gemini and Keck in July 2005 to explore this. We also report a possible detection of ethane emission on Uranus in 2002, but the signal levels are more than two orders of magnitude lower than those of Neptune. The deduced mole fraction and temperature are consistent with past upper limits. Additional observations are warranted to confirm this. This work was supported in part by NASA grants NAG5-10451 and NAG5-11961. DKL and RWR acknowledge The Aerospace Corporation's Independent Research and Development program. HBH, DLK, and RWR were visiting astronomers at the IRTF, operated by U. Hawaii under contract from NASA.

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