Uranus and its Cloud Layers: Expected Theoretically but Contradicted by Observations

Details Uranus and its Cloud Layers: Expected Theoretically but Contradicted by Observations Uranus and its Cloud Layers: Expected Theoretically but Contradicted by Observations

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.0906k&link_type=abstract

American Astronomical Society, DPS meeting #39, #9.06; Bulletin of the American Astronomical Society, Vol. 39, p.424

Astronomy and Astrophysics

Astronomy

Scientific paper

The standard model for the haze and cloud structure of Uranus has been a high-altitude haze above a methane condensation cloud near 1.4 bar pressure and one or more clouds below the 3 bar level. This type of model has been based on theoretical expectations. The model parameters have been adjusted to fit available observations. The observations have been either high spatial resolution but poor spectral coverage, such as Voyager 2 and HST imaging, or low spatial resolution but good spectral coverage, such as ground-based spectroscopy.
Observations of Uranus in 2002 by the STIS spectrograph of HST are unique since they combine high spatial and spectral resolution. The image cube consists of 2000 wavelengths and 80x40 pixels covering the complete afternoon hemisphere of Uranus. Unlike previous data, these observations can distinguish between vertically confined cloud layers and extended haze layers.
Radiative transfer calculations show that each cloud layer produces a specific feature in the change of the center-to-limb variation with wavelength, and the location of the feature in the spectrum indicates the pressure level of the cloud. No such features were detected. The observations fit best to models with no cloud layer at all, but an optically thin haze of small aerosols (compared to visible wavelengths) above the 1.4 bar level and optically thick haze of large aerosols below. The observations are especially sensitive to altitudes above the 2-bar level where even thin clouds would have been detectable. This result holds for all observed latitudes, except for a few discrete spots which may be condensation clouds.
The theoretically expected condensation cloud layers had to be optically thinner at latitudes where the air flow is believed to be upwelling, contradictory to expectations. The absense of cloud layers solves this problem.
This research was supported by NASA/STScI.

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