Standard Interpretation Method for Methane Band Imaging Disproven for Uranus

Details Standard Interpretation Method for Methane Band Imaging Disproven for Uranus Standard Interpretation Method for Methane Band Imaging Disproven for Uranus

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.5008k&link_type=abstract

American Astronomical Society, DPS meeting #40, #50.08; Bulletin of the American Astronomical Society, Vol. 40, p.489

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Methane band imaging has been successfully applied to all jovian planets and Titan to infer the vertical aerosol structure. The standard qualitative interpretation is simple: bright and dark areas in methane bands indicate areas of increased and decreased opacity, respectively.
Voyager 2 found one main feature in methane band images of Uranus, a brighter south polar region versus a darker low-latitude region with a transition near -45 deg latitude. The interpretation was a south polar haze or cloud somewhere between the 1 and 2 bar level.
During the following two decades, this main feature has shown up in most methane band imaging of HST and ground-based observations, and the interpretation has never been challenged.
In our HST-STIS data set from 2002, we obtained images of Uranus in 1750 wavelengths between 300 and 1000 nm. We confirm the same main feature in methane bands. However, in hydrogen absorptions probing the same altitudes, the main feature disappears. Our conclusion is that the main feature is not an opacity feature, but a variation in the methane mixing ratio.
The main feature has been also visible in some 30 filters of HST since 1994, but not visible in one filter (NICMOS-F108N) in 1997 and 1998, which is dominated by hydrogen absorption and probes similar altitudes.
Since most of our information about aerosol opacity in the atmosphere of Uranus comes from interpretation of observations in methane absorptions, and since the standard method seems to be flawed, we may need to reconsider previous results. This may possibly apply to Neptune also.
A methane mixing ratio varying with latitude has profound implications on our understanding of the workings of the atmosphere for Uranus. It offers a probe to its interior dynamics.
This research was supported by NASA grants NAG5-12014 and NNX08AE74G and by STScI grant HST-GO-09725.01-A.

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