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M.H. Wong, P.R. Mahaffy (NASA GSFC)
The Galileo Probe Mass Spectrometer (GPMS) measured counts at mass-to-charge ratio 18 in three pressure intervals during the probe descent: 0.6 to 3.8 bar (DL1), 9.0 to 11.7 bar (DL2a), and 16.4 to 20.9 bar (DL2b). Water mixing ratio upper limits, for all three intervals, were presented in Niemann et al.\ 1998 (JGR 103:E10 22,831-22,845; hereafter N98), and the DL1 result in N98 is still the best available.
We have completed a detailed analysis of the DL2 calibration experiments conducted in 1997 using the duplicate GPMS experimental unit at NASA's Goddard Space Flight Center. We report an average water mixing ratio in DL2a of (5.4±.3)\times 10-5, consistent with the previous N98 upper limit of (5.6±.5)\times 10-5. The average DL2b mixing ratio is (7.5±.4)\times 10-4, slightly larger than but still consistent with the N98 upper limit of (6±)\times 10-4. Uncertainties in the calibration constants dominate the many sources of error included in the quoted mixing ratio uncertainties.
Based on the chi-squared goodness of fit test, the DL2a data are marginally consistent with a linear increase in water mixing ratio up to the final water mixing ratio measurement at 20.9 bar. The DL2b data themselves are statistically consistent with a linear increase in water that extrapolates to solar abundance at 20 to 40 bar, depending on the calibration constant pressure dependence used. However, the GPMS data do not actually constrain water mixing ratios at pressures greater than the last measurement of mass 18 at 20.9 bar. Our revised background correction to the DL2b data removes the strong decrease in mixing ratio with increasing pressure that was presented in N98.
The author(s) of this abstract have provided an email address for comments about the abstract: mike.wong@gsfc.nasa.gov