5-µm Spectroscopy of Saturn with the IRTF

1. 5-µm Spectroscopy of Saturn with the IRTF Nancy Chanover (New Mexico State U.) Gordon Bjoraker & David Glenar (NASA/GSFC) Tilak Hewagama (U. Maryland) Kevin Baines (JPL)

2. Observed Saturn with SpeX and CSHELL at the same time that the Cassini spacecraft has targeted observations of Saturn’s atmosphere. • The spectral coverage afforded by SpeX and CSHELL fills in an important gap between the wavelength sensitivity of Cassini’s VIMS and CIRS spectrometers • Saturn’s 5.0-5.4 µm spectrum contains absorption features of NH3 and PH3 that are indicative of cloud structure

3. CSHELL Results At 5 µm thermal emission originates from Saturn's deep atmosphere (4-bar level) and is attenuated by cloud layers of varying thicknesses.

4. SpeX Results • SpeX data were used to build up an image cube between 2.5 and 5.5 µm • Both SpeX and Cassini/VIMS images at 5.1 µm show interesting cloud structure in Saturn’s southern hemisphere that is not evident at shorter wavelengths. Image of Saturn’s southern hemisphere at 5.05 µm derived from SpeX observations

5. Interpretation • The pressure level of Saturn’s clouds was derived from CSHELL spectra by modeling the strength and width of ammonia (NH3) and phosphine (PH3) absorption lines • Saturn’s Equatorial Zone appears to have a massive cloud near 3 bars in addition to the NH3 cloud at 0.5 bars that is evident at optical wavelengths. • The SpeX data show that the physics changes abruptly between 4.7 µm, which is dominated by reflected sunlight, and 5.1 µm, which is primarily thermal emission.