Mid-Infrared Ethane Emission on Neptune and Uranus

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Mid-Infrared Ethane Emission on 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)

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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.

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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.

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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.
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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.
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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.
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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