Uranus Cloud Layers As Constrained By HST STIS Spectra

Astronomy and Astrophysics – Astronomy

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Scientific paper

Space Telescope Imaging Spectrograph (STIS) observations of Uranus were obtained in 2002. We analyzed observations taken with the slit parallel to Uranus' spin axis and positioned on the central meridian, combining 430L and 750L grating observations to obtain a rectified spectrum spanning the wavelength range of 290 nm to 1050 nm. At the time of these observations the subearth planetocentric latitude was -20.5 degrees, making latitudes of 43 S and 7.6 N latitudes of approximately equal view angle. Comparing wavelengths that probe different depths of the Uranian atmosphere, controlled mainly by Rayleigh and Raman scattering at short wavelengths, and by Methane absorption at longer wavelengths, we are able to estimate the pressure levels at which cloud bands reside in the Uranus atmosphere and identify asymmetries in cloud and haze properties. At 399 nm we find that the southern hemisphere is darker than the northern hemispheres at comparable view angles, providing evidence of stratospheric haze absorption. At 467 nm there is nearly perfect symmetry about the center of the disk, with Rayleigh scattering obscuring views of deeper cloud bands. At 590 nm, which is more deeply penetrating, there appears a strong asymmetry in which the southern hemisphere is brighter than corresponding view angles in the northern hemisphere. Wavelengths of 725 nm and 789 nm imply that the bright band near seen at 45 S at 789 nm but not seen at 725 nm lies between about 1.7 bars and 3-4 bars. Quantitative radiation transfer models of these spectra are currently stymied by calibration issues identified by comparison of central disk spectra with central disk I/F values obtained from WFPC2 bandpass filter images.
This research was supported by the Outer Planets Research Program.

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