Microwave opacity of phosphine: Application to remote sensing of the atmospheres of the outer planets

The pressure-broadened absorption of gaseous phosphine was measured in the laboratory under simulated conditions for outer planet atmospheres. Phosphine absorption was shown to be stronger than theoretical calculations indicate by more than an order of magnitude at long centimeter wavelengths. A new laboratory measurement-based formalism was developed for computation of absorptivity of gaseous phosphine in a hydrogen-helium atmosphere. Application of this formalism has shown that, in equal abundance, phosphine is a stronger absorber at long centimeter wavelengths than is ammonia, contradicting the widely held assumption that ammonia is the single dominant microwave absorber in outer planet atmospheres.; Re-examinations of the Voyager radio occultation experiment results at Saturn and Neptune revealed that the inferred ammonia abundance for both planets requires supersaturation if ammonia is assumed to be the only major source of microwave opacity. The new formalism for phosphine opacity has been applied to a reinterpretation of these results at Saturn and Neptune. Results indicate that phosphine mixing ratios of 3–12 ppm and 1–3 ppm for Saturn and Neptune, respectively, account for the additional opacity over ammonia and hydrogen sulfide saturation.; An existing disk-average radiative transfer model has been updated to include the new formalism and has been applied to the Saturn and Neptune atmospheres. Results from the updated radiative transfer model indicate best-fit deep abundances that are consistent with those from the re-interpretation of Voyager radio occultation experiments and with those from ground-based radio telescope observations of the microwave emission spectra of those planets.; Also, a new ray-tracing-based elliptical-shell local radiative transfer model has been developed to aid in prediction and, eventually, interpretation of measurement results from the Cassini RADAR/radiometer. The ability of the Cassini radiometer to detect phosphine has been investigated and results indicate that Cassini will detect phosphine at Saturn and will be capable of mapping phosphine variations on the order of 0.6–1.2 ppm. This sensitivity will likely be limited primarily by uncertainties in ammonia abundance. Suggestions for improvements and extensions of laboratory studies of phosphine and of the local radiative transfer model are discussed.