Mid-Infrared Ethane Emission Mid-Infrared Ethane Emission on Neptune and Uranuson Neptune and Uranus

Heidi B. Hammel, M. Sitko (Space Science Inst.)

D. Lynch, R. Russell (The Aerospace Corp.), L.S. Bernstein (Spectral Science), T. Hewagama

(Goddard)

Variation of ethane (C2H6) emission on Neptune at 12.2 microns

Variation in the methane and ethane emission band at 7.8 and 12.2 microns respectively. The increase indicates an increase in the ethane abundance by about 15% from 1985 to 1991 (Hammel et al. 1992, Icarus, 99, 347). This motivated further observations of Neptune.

Obtained new spectra using the BASS spectrometer on the IRTF. Spectra obtained from 3-13 microns with a single exposure.

One can see significant changes in the ethane emission at 12.2 microns as well as changes in the methane emission at 7.8 microns. Note the unexpected dip at the peak of the ethane emission.

The ethane band profile was fitted with a model that includes ethane ice absorption that might exist in a high-altitude cloud. The fits are shown above and show the ethane ice absorption for the first time.

Uranus has a much colder stratosphere and therefore the ethane emission is much lower. In fact, ethane emission has not been observed until now. It is possible that as Uranus approaches its spring equinox that the stratosphere is getting warmer.

BASS detection of ethane and acetylene emission on Uranus. Yellow line: ethane emission with ethane ice absorption. Blue: H2 continuum emission. Red: Acetylene emission. Grey: model spectrum.

Summary

Neptune• Rise in stratospheric temperature over past 20 yrs• Ethane variation intrinsic in addition to apparent ("seasonal")• Line shape variations indicative of ice component

Uranus• Ethane and acetylene likely detected• Abundance similar to Neptune with colder stratosphere