Laboratory Measurements of the Millimeter Wavelength Opacity of Phosphine (PH3) and Ammonia (NH3) Under Simulated Conditions for the Cassini-Saturn Encounter

Abstract

The molecular compositions of the atmospheres of the giant planets (Jupiter, Saturn, Uranus and Neptune) are fundamental to understanding the processes which formed these planets and the solar system as a whole. Microwave observations of these planets probe regions in their atmospheres from approximately 0.1 to several bars, a process otherwise unachievable by visible and infrared means. Many gases and various cloud layers influence the millimeter wave spectra of the outer planets; however phosphine and ammonia are the main microwave absorbers at Saturn at pressures less than two bars. Understanding the pressure induced absorption of both constituents at observational frequencies is therefore vital to the analysis of any observational data. Laboratory measurements have been conducted to measure the microwave absorptivity and refractivity of phosphine and ammonia at Ka-band (32-40 GHz) and W-band (94 GHz), under conditions characteristic of the atmosphere of Saturn. The results were used to verify the accuracy of the phosphine formalism created by Hoffman et. al (2001) for use at millimeter wavelengths. Based on the laboratory measurements conducted, new formalisms were also created to express the opacity of ammonia at the measured frequencies. An important method for the study of planetary atmospheres is the radio occultation experiment ??method that uses radio links between Earth, and the spacecraft which passes behind the planet. The Cassini mission to Saturn, which will be conducting such experiments at Ka-band as well as S (2.3 GHz) and X (8.4 GHz) bands, has prompted the development of a radio occultation simulator used to calculate excess Doppler shifts and attenuation profiles for Saturn, utilizing the newest formalisms for phosphine and ammonia. The results indicate that there will be unambiguous detection and profiling of phosphine and ammonia, and predictions are made for the pressures at which loss of signal is anticipated.