Vertical distribution of PH3 in Saturn from observations of its 1-0 and 3-2 rotational lines

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 600 mbar total pressure, The far-infrared spectrum is relatively free from spectral interlopers, suffers minimal absorption or scattering by atmospheric particulates, and contains intrinsically weak PH3 lines that are sensitive to a range of atmospheric depths. The combined spectra are inconsistent with a uniform tropospheric mixing ratio, even with a stratospheric cutoff. They are consistent with a volume mixing ratio of PH3 that drops from 1.2 x 10(-5) at 645 mbar pressure to a value of 4.1 x 10(-7) at 150 mbar pressure, a decrease that is linear in log abundance vs log pressure, The mixing ratio could drop even more quickly at atmospheric pressures below 150 mbar and still be consistent with the data. The mixing ratio may well remain constant with depth for pressures above 630 mbar, The maximum PH3 mixing ratio in this model is consistent with a [P]/[H] ratio in the deep atmosphere that is about a factor of 10 higher than solar composition. Such a model is consistent with rapid mixing up to the radiative-convective boundary and transport by, for example, vertical waves just above this boundary, In the best fitting model, the eddy diffusion coefficient is similar to 10(4) cml near 630 mbar, and it must increase with altitude, The predominant PH3 loss mechanisms are direct photolysis by UV radiation and scavenging by H atoms produced by the photolysis. (C) 2000 Academic Press.