Saturn's Deep Cloud Structure Derived From 5-Micron Spectra

Details Saturn's Deep Cloud Structure Derived From 5-Micron Spectra Saturn's Deep Cloud Structure Derived From 5-Micron Spectra

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p31a0184b&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P31A-0184

Computer Science

Sound

5210 Planetary Atmospheres, Clouds, And Hazes (0343), 5704 Atmospheres (0343, 1060), 5709 Composition (1060), 6275 Saturn

Scientific paper

The CSHELL and SpeX spectrometers on NASA's Infrared Telescope Facility were used to observe Saturn between 4.5 and 5.4 microns on several occasions from 2004-2007 at the same time as Cassini/VIMS and CIRS were mapping the planet. At these wavelengths thermal radiation originates from the deep atmosphere (5 bars) and it is attenuated by two cloud layers considered in equilibrium models to be composed of NH4SH and condensed NH3. In addition, there is a component of sunlight reflected from the upper (NH3) cloud that varies spatially on Saturn. CSHELL can spectrally resolve profiles of absorption lines of ammonia (NH3) and phosphine (PH3) on Saturn at selected wavelengths. These lines are very broad due to collisions with 3 to 5 bars of hydrogen. The Saturn spectrum exhibits numerous strong NH3 and PH3 lines, as well as Fraunhofer lines due to CO in the Sun. SpeX observations cover the entire 5-micron window sampling both thermal emission and reflected sunlight. Image cubes were obtained by stepping the slit across the planet. The best contrast in reconstructed images occurs at 5.05 microns, which coincides with the wavelength where VIMS sees spectacular structure on Saturn. The spatial variation of Saturn's 5-micron spectrum is dominated by the variable opacity of its deep cloud structure. Superimposed on this are smaller variations in the mixing ratios of NH3 and PH3. The abundances of these gases can be retrieved reliably in relatively cloud-free regions between 50 South and 65 South, which are analogous to Jupiter's belts and 5-micron hot spots. Elsewhere, it is more difficult to separate changes in cloud opacity from gas abundances. We use near-simultaneous CIRS observations which sound the ~500-mbar level to provide an upper boundary condition to PH3. The 5-micron spectrum of Saturn's Equatorial Zone (10 South) is significantly different from a region near 60 South. The NH3 and PH3 lines are weaker and narrower in the EQZ, while the Fraunhofer lines are stronger. We will present synthetic spectra calculated from models which fit both regions and which explore the tradeoffs between gas mixing ratios and cloud opacity. This will constrain the pressure level of the deep clouds and extend our knowledge of gas abundances to deeper levels than is possible using Cassini data alone. This work was supported by NASA's Planetary Astronomy program.

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