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[12.13] Calibration of a ground-based technique to measure water on Jupiter using Galileo/NIMS data

G. L. Bjoraker (NASA/GSFC), T. Hewagama (U. Md at GSFC), M. Roos-Serote (Lisbon Astronomical Observatory), G. S. Orton, R. W. Carlson (JPL)

The CSHELL spectrometer at NASA's Infrared Telescope Facility was used in December 1996 to observe Jupiter at nearly the same time as Galileo/NIMS observations of a hot spot in the North Equatorial Belt (NEB). CSHELL spectra of CH₄ at 5.18 microns and NIMS spectra of H₂O between 4.6 and 5.2 microns sound the 3-8 bar region in Jupiter's deep troposphere. CSHELL’s 1-arcsec -wide slit was aligned east-west on Jupiter and stepped from north to south across the NEB. Radiative transfer models indicate that the strength of the CH₄ feature at 5.18 microns is anti-correlated with gaseous H₂O between 3 and 6 bars. The CH₄ feature is predicted to be very strong for H₂O abundances less than 10ppm and it should vanish when H₂O > 300ppm. Roos-Serote et al (JGR 103, 23023, 1998) retrieved a relative humidity of 0.2 of the targeted hot spot (E4FEA). When convolved to the 3800km CSHELL resolution this becomes 0.5 bars. We used the NIMS-derived H₂O abundance in the hot spot to calibrate the ground-based technique. Once calibrated, the CH₄ absorption feature may in turn be used to map the abundance of water in regions that Galileo/NIMS did not observe.