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G. L. Bjoraker, D. A. Glenar (NASA/GSFC), N. J. Chanover (New Mexico State U.), T. Hewagama (U. Maryland)
The CSHELL spectrometer on NASA's Infrared Telescope Facility was used to observe Saturn at 5.07 \mum on 2005 March 7 at about the same time as Cassini/VIMS was mapping the planet. The slit was aligned north-south on Saturn's Central Meridian. At this wavelength thermal radiation originates from the deep atmosphere (5 bars) and it is attenuated by both NH4SH and NH3 clouds. In addition, there is a component of sunlight reflected from the NH3 cloud that varies spatially on Saturn. CSHELL is an appropriate tool for this study because it can spectrally resolve profiles of absorption lines of phosphine (PH3) and ammonia (NH3) on Saturn. These lines are very broad due to collisions with 4 bars of hydrogen. The Saturn spectrum exhibits several strong PH3 lines, as well as a weak NH3 line and several Fraunhofer lines due to CO in the Sun. The strength of all of these features varies spatially on Saturn. We interpret the spatial variation of this spectrum as primarily due to the variable opacity of Saturn's cloud structure rather than due to changes in the mixing ratio of PH3. The 5.07-micron spectrum of Saturn's Equatorial Zone (10 South) is significantly different from a region near 60 South. The PH3 and NH3 lines are weaker and narrower in the EQZ, while the Fraunhofer lines are stronger. This can be explained by thicker clouds in the EQZ. Preliminary models indicate that opacity changes in the NH3 cloud alone cannot fit the spectra. Both spatial regions can be fitted with ~4ppm PH3, but the EQZ requires a deeper cloud with significant opacity (tau ~8) between 3 and 4 bars and a temperature near 200K. This is near the predicted level of the NH4SH cloud in chemical equilibrium models such as those developed by P. N. Romani (1986 PhD dissertation, U. of Michigan.) The region between 50 South and 65 South on Saturn is also of interest. Both CSHELL data and SpeX observations that we acquired the next night exhibit strong PH3 and NH3 absorption here. This area appears bright in images at 5.2 \mum (Yanamandra-Fisher et al 2001, Icarus 150, 189-193). A combination of imaging and spectroscopy demonstrates that this is the closest saturnian analog to Jupiter's 5-micron hot spots; however, the morphology is different because this region extends over a wide range of longitudes.
This work is being supported by the NASA Planetary Astronomy Program.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.