Ammonia, Phosphine, And Cloud Structure On Saturn Derived From 5-micron Spectra

Details Ammonia, Phosphine, And Cloud Structure On Saturn Derived From 5-micron Spectra Ammonia, Phosphine, And Cloud Structure On Saturn Derived From 5-micron Spectra

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.0504b&link_type=abstract

American Astronomical Society, DPS meeting #38, #05.04; Bulletin of the American Astronomical Society, Vol. 38, p.488

Computer Science

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Scientific paper

The CSHELL spectrometer on NASA's Infrared Telescope Facility was used to observe Saturn at several settings between 5.07 and 5.27μm beginning in 2004 February through 2006 January at about 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 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. These lines are very broad due to collisions with 3 to 5 bars of hydrogen. The Saturn spectrum exhibits several strong NH3 and PH3 lines, as well as Fraunhofer lines due to CO in the Sun.
The spatial variation of this spectrum is dominated by the variable opacity of Saturn's 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-μm 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 work was supported by NASA's Planetary Astronomy program.

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