Methane Absorption Spectra by the Descent Imager / Spectral Radiometer

Erich Karkoschka, Martin Tomasko (Lunar and Planetary Lab)

In the visible and near-infrared, the spectra of the Jovian planets
and Titan are dominated by methane absorption features, which allow
probing a variety of levels of their atmospheres.  Interpretation of
most data in this wavelength range requires an understanding of methane
absorption features.  Methane absorption data have provided the best
constraints on the vertical distribution of aerosols in the Jovian
atmospheres.  Methane plays an important role in the heat balance in
the Jovian atmospheres.  Measured methane mixing ratios have given
important clues about the history of the solar system.

Above 5000 nm wavelength, methane lines are theoretically understood
sufficiently well to predict methane absorption for every temperature,
pressure, methane abundance, and gas composition.  Toward lower
wavelengths, theoretically methane data are more and more incomplete.
Below 1600 nm, our knowledge of methane absorption is based on
laboratory measurements.

Because the size of laboratories on Earth are measured in meters, but
the scale heights of the atmospheres are measured in kilometers,
laboratory measurements have to be extrapolated to be applied to
planetary conditions which is intrinsically unreliable.

On January 14, 2005, we set up a kilometer-sized laboratory
in the atmosphere of Titan.  The Huygens probe measured temperature,
pressure, methane mixing ratio, and the transmission of the
atmosphere between Huygens and the sun at more than 300 wavelengths
and at many altitudes.  The data revealed that some of the
extrapolations from laboratory conditions to Titan's conditions were
quite accurate while others were off by up to a factor of 100.

The Huygens data confirmed our knowledge about methane absorption up to
850 nm wavelength.  Between 850 and 1050 nm, the strongest absorptions
of methane were some 30 percent weaker than the extrapolations from
laboratory data.  From 1050 nm to 1600 nm, the upper wavelength
limit of the Huygens data, strong methane absorptions were almost
as strong as expected, but the weak absorptions were up to 100 times
stronger than expected.

We investigated these differences and found that the previous model
of temperature dependence of methane absorption was too simple.
We altered the model and eventually could fit all laboratory data
and all Huygens data fairly well.  We estimate that the Huygens data
revealed the major flaw of previous methane band models, and that our
new model provides reliable methane absorption coefficients for the
conditions in the Jovian planets and Titan probed by most observations.

Some previous investigations based on the data before the Huygens
descent have to be reevaluated.  For example, some estimated cloud
altitudes in Titan atmosphere were too low.  Some estimated albedos
of Titan's surface need to be revised up by 50 percent or more.
Some aerosol optical depths for the atmospheres of Uranus and Neptune
need to revised up by large factors.

This work was supported by NASA grant NNX08AP81G.