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 profi le for the PH3 vertical mixing ratio between 100 and 600 mbar total pressur e, The far-infrared spectrum is relatively free from spectral interlopers, suffers minimal absorption or scattering by atmospheric particulates, and c ontains intrinsically weak PH3 lines that are sensitive to a range of atmos pheric depths. The combined spectra are inconsistent with a uniform troposp heric mixing ratio, even with a stratospheric cutoff. They are consistent w ith a volume mixing ratio of PH3 that drops from 1.2 x 10(-5) at 645 mbar p ressure 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 consisten t with the data. The mixing ratio may well remain constant with depth for p ressures above 630 mbar, The maximum PH3 mixing ratio in this model is cons istent with a [P]/[H] ratio in the deep atmosphere that is about a factor o f 10 higher than solar composition. Such a model is consistent with rapid m ixing up to the radiative-convective boundary and transport by, for example , vertical waves just above this boundary, In the best fitting model, the e ddy diffusion coefficient is similar to 10(4) cml near 630 mbar, and it mus t increase with altitude, The predominant PH3 loss mechanisms are direct ph otolysis by UV radiation and scavenging by H atoms produced by the photolys is. (C) 2000 Academic Press.