Laboratory Measurements of the Microwave Opacity of Phosphine: Opacity Formalism and Application to the Atmospheres of the Outer Planets

Abstract

Preliminary room-temperature measurements of the microwave opacity of phosphine (Hoffman and Steffes 1999). Icarus 140, 235–238, suggested that phosphine (PH 3) may contribute significantly to the microwave emission spectrum of Neptune, and to the centimeter-wavelength opacity measured by the Voyager 2 spacecraft at Neptune and Saturn. As a result, new laboratory measurements of the microwave opacity of PH 3 in an H 2/He atmosphere have been conducted at frequencies (wavelengths) of 1.5 GHz (20 cm), 2.2 GHz (13.3 cm) 8.3 GHz (3.6 cm), 13.3, GHz (2.3 cm), 21.6 GHz (1.4 cm), and 27 GHz (1.1 cm) over a range of temperatures (175–298 K) and pressures (1–6 bars). A measurement-based centimeter-wavelength opacity formalism for phosphine has been developed and provides an order of magnitude improvement over previous models. The resulting model is applied to results from the Voyager 2 radio science experiments at Saturn and Neptune and to disk-averaged, centimeter-wavelength observations of Neptune and Saturn. Future uses will include application to spatially resolved microwave images of the outer planets from radio telescopes and spacecraft radiometry and to future radio occultation measurements, including Cassini–Saturn.