James P. Hoffman, Paul G. Steffes and David R. DeBoer
Georgia Institute of Technology
School of Electrical and Computer Engineering
Atlanta, Georgia 30332-0250
It has been suggested that phosphine (PH3) may contribute significantly to the microwave emission spectrum of Neptune, and to the centimeter-wavelength opacity measured by the Voyager 2 spacecraft at Neptune (DeBoer and Steffes, Icarus 123, 324, 1996a). As a result, laboratory measurements of the microwave opacity and refractivity of PH3 in an H2/He atmosphere have been conducted at 1.5 GHz (20 cm), 2.2 GHz (13.3 cm), 8.3 GHz (3.6 cm), 13.3 GHz (2.3 cm), and 21.6 GHz (1.4 cm) at pressures of 1, 3, and 5 bars at room temperature. Results indicate that current theories significantly understate the centimeter-wavelength opacity of PH3 . This higher-than-expected opacity is likely due to a molecular inversion spectrum, of which no measurements have been reported. These results will elucidate the interpretation of centimeter-wavelength microwave observations from radio telescopes and spacecraft radio occultation measurements of the atmospheres of all four Jovian planets.
Keywords: Abundances, Atmospheres; Atmospheres, Composition; Jovian Planets; Radio Observations; Spectroscopy