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G. S. Orton, E. Serabyn (JPL), Y. T. Lee (Caltech)
Far-infrared Fourier-transform spectrometer measurements of the 1-0 and 3-2 PH3 transitions in Saturn's disk near 267 and 800 GHz (8.9 and 26.7 cm-1), respectively, were analyzed simultaneously to derive a global mean profile for the PH3 vertical mixing ratio between 100 and 800 mbar total pressure. The far-infrared spectrum is relatively free from spectral interlopers, suffers minimal absorption or scattering by atmospheric particulates, and contains fairly weak PH3 lines that are sensitive to a range of atmospheric depths. The combined spectra are inconsistent with a constant tropospheric mixing ratio, even with a stratospheric cut-off. They are consistent with a volume mixing ratio of PH3 that drops from 1.2 x 10-5 at 800 mbar pressure to a value of 1.7 x 10-7 at 100 mbar pressure, connected by the expression 1.913 x 10-5 p2.063, for p in bar. This expression is consistent with the data, even when extended to pressures higher than 800 mbar and pressures lower than 100 mbar. The mixing ratio could drop even more quickly at atmospheric pressures below 100 mbar and still be consistent with the data, and the mixing ratio could also remain constant with depth at a value near 8 x 10-6 for pressures above 630 mbar. If the mixing ratio does not decrease in value with depth, then the highest values in the best fit model imply a [P]/[H] ratio that is well above the solar ratio. The falloff feature of the PH3 mixing ratio in the upper troposphere is sensitive to the eddy diffusion coefficient above the 1-bar level. Photochemical models of PH3 provide an estimate of eddy diffusion coefficient in this region of the atmosphere to be somewhere between 105 to 106 cm2 sec-1.
The author(s) of this abstract have provided an email address for comments about the abstract: go@orton.jpl.nasa.gov