Laboratory Measurements of Microwave Absorptivity and Refractivity Spectra of Gas Mixtures Applicable to Giant Planet Atmospheres.

Details Laboratory Measurements of Microwave Absorptivity and Refractivity Spectra of Gas Mixtures Applicable to Giant Planet Atmospheres. Laboratory Measurements of Microwave Absorptivity and Refractivity Spectra of Gas Mixtures Applicable to Giant Planet Atmospheres.

Jan 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990phdt........92s&link_type=abstract

Thesis (PH.D.)--STANFORD UNIVERSITY, 1990.Source: Dissertation Abstracts International, Volume: 51-08, Section: B, page: 3994.

Astronomy and Astrophysics

Astronomy

4

Scientific paper

Accurate formulae for predicting microwave absorptivities and refractivities of gas mixtures of hydrogen, helium, methane, and ammonia are vital for interpreting results from radio occultation and radio astronomical investigations of the atmospheres of the giant planets. In those atmospheres ammonia is the source of essentially all microwave opacity at levels accessible by radio methods. Unfortunately, accurate methods for calculating microwave absorptivities of gas mixtures containing ammononia under specified conditions have eluded spectroscopists for more than half a century, partly due to a lack of adequate laboratory data. A microwave spectrometer, based on a cavity resonator, was constructed to measure refractivity spectra of the transparent gases hydrogen, helium, and methane, and to measure both refractivity and absorptivity spectra on gas mixtures containing ammonia. Gas conditions included temperatures from about 210 to 320 K, pressures from 1 to 8.2 atm., and the spectra covered microwave frequencies from 9 to 18 GHz. Data on the transparent gases confirm that their density-normalized refractivities are invariant with temperature, pressure, and frequency within measurement accuracies. Refractivities are consistent with accepted values at optical frequencies: 135 x 10^{-6} for hydrogen, 35 x 10^{-6} for helium, and 440 x 10^{-6} for methane, under standard conditions. Refractivities of mixtures are linear in the partial pressures of constituent gases. Absorptivity data on mixtures containing ammonia show serious disagreements with current prediction formalisms. Van Vleck-Weisskopf theory, and Ben-Reuven theory as modified by Berge and Gulkis (1976), both fail to accurately predict observed temperature, pressure, and frequency dependences. Optimization techniques that fit a parameterized Berge and Gulkis formalism to the data produced a new formalism that more accurately predicts microwave absorption by ammonia. The new formalism predicts that at the low temperatures extant in the atmospheres of the giant planets, ammonia will be less opaque to microwaves than previously thought. This will require revising upward the ammonia abundancies inferred from radio methods, by a few percent to as much as 50%.

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