Spectroscopic Investigation of Uranian Cloud Structure
Abstract
To constrain the structures of Uranian clouds we are currently using CCD spectra obtained with the Space Telescope Imaging Spectrograph (STIS) in 2002 and CCD spectral samples obtained in 2006 with the ramp filter of the Advanced Camera for Surveys (ACS). We also make use of near-IR grism spectra obtained in 2006 and 2007 with the Keck II telescope and the NIRC2 instrument with Adaptive Optics. The CCD absorption coefficients we used to interpret these observations are based primarily on remote spectral observations of Jupiter and Saturn, while the near-IR coefficients we used are based on laboratory measurements, neither of which reproduce exact conditions in the atmosphere of Uranus and thus require some degree of extrapolation. As a partial test of the validity of our recently improved near-IR coefficient models we compare Uranus cloud structure results based on these two independent spectral ranges. We find that the CCD and Near-IR spectral ranges are in general agreement on a surprising result. Even though high-altitude discrete clouds on Uranus are almost certainly composed of methane ice and methane has a relatively high abundance of 1-4%, there seems to be very little methane ice on average. The bright band prominent in images at deeply penetrating wavelengths appears to be near to 2-3 bars and deeper clouds appear to be present in the 5-10 bar region.To constrain the structures of Uranian clouds we are currently using CCD spectra obtained with the Space Telescope Imaging Spectrograph (STIS) in 2002 and CCD spectral samples obtained in 2006 with the ramp filter of the Advanced Camera for Surveys (ACS). We also make use of near-IR grism spectra obtained in 2006 and 2007 with the Keck II telescope and the NIRC2 instrument with Adaptive Optics. The CCD absorption coefficients we used to interpret these observations are based primarily on remote spectral observations of Jupiter and Saturn, while the near-IR coefficients we used are based on laboratory measurements, neither of which reproduce exact conditions in the atmosphere of Uranus and thus require some degree of extrapolation. As a partial test of the validity of our recently improved near-IR coefficient models we compare Uranus cloud structure results based on these two independent spectral ranges. We find that the CCD and Near-IR spectral ranges are in general agreement on a surprising result. Even though high-altitude discrete clouds on Uranus are almost certainly composed of methane ice and methane has a relatively high abundance of 1-4%, there seems to be very little methane ice on average. The bright band prominent in images at deeply penetrating wavelengths appears to be near to 2-3 bars and deeper clouds appear to be present in the 5-10 bar region.
- Publication:
-
Planetary Atmospheres
- Pub Date:
- 2007
- Bibcode:
- 2007plat.work..113S