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[11.22] Ground-based observations of H2O and NH3 on Jupiter during the Cassini flyby. Part 1: Technique

G.L. Bjoraker, T. Hewagama (NASA/GSFC), G.S. Orton (JPL)

In December 2000, using the CSHELL spectrometer on the IRTF, we mapped the 1928-1932 cm-1 spectral range (5.18 microns) in Jupiter's Great Red Spot and North Equatorial Belt. These observations were acquired at about the same time as Cassini/VIMS and Cassini/CIRS observations of Jupiter. The IRTF spectra are complementary to VIMS spectra at 5 microns which cover a broader bandpass at lower spectral resolution. Both sound the 5-bar region. CIRS spectra between 8 and 11 microns sound the NH3 cloud region near 0.5 bars. Comparisons between ground-based and Cassini data will permit us to map NH3 at 0.5 and 5 bars and to determine whether the H2O abundance is correlated with cloud opacity and NH3 abundance. We will describe a technique using CH4 absorption features at 5.18 microns to measure the spatial variation of water vapor in Jupiter's atmosphere. Radiative transfer models show that the strength of the CH4 absorption feature is anti-correlated with the abundance of gaseous H2O in the 3 to 6 bar region. The ammonia abundance at 5 bars is obtained directly using an NH3 absorption feature within the same bandpass.

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