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Over the past decade, several extensive laboratory studies have been conducted of the microwave opacity of ammonia and water vapor in preparation for interpretation of the precise measurements of jovian microwave emission to be made with the Microwave Radiometer (MWR) instrument aboard the NASA Juno Mission. (See, e.g., Hanley et al. [2009] Icarus, 202, 316–335; Karpowicz and Steffes [2011a] Icarus 212, 210–223; Karpowicz and Steffes [2011b] Icarus 214, 783; Devaraj et al. [2014] Icarus, 241, 165–179) These works included models for the opacity of these constituents valid over the pressure and temperature ranges measured in the laboratory experiments (temperatures up to 500 K and pressures up to 100 bars). However, studies of the microwave emission made using these models indicate that significant contributions to the emission at the 24-cm and 50-cm wavelengths to be measured by the Juno MWR will be made by layers of the atmosphere with temperatures at or exceeding 600 K. While the ammonia opacity models described by Hanley et al. (2009) and Devraj et al. (2014) give consistent results at temperatures up to 500 K (within 6%), they diverge significantly at temperatures and pressures exceeding 550 K and 50 bars, respectively. Similarly, at temperatures above 500 K, the model for water vapor opacity developed by Karpowicz and Steffes (2011a,b) exhibits non-physical attributes. To resolve these ambiguities, we have conducted laboratory measurements of the microwave opacity of ammonia at temperatures up to 600 K and that for water vapor at temperatures up to 600 K. Additionally, since the microwave opacity of ammonia is influenced by pressure-broadening from methane (a significant constituent in jovian atmospheres), measurements of the effects of methane on the ammonia absorption spectrum have also been conducted. These measurements have resulted in updated models for the opacities of ammonia and water vapor under conditions of the deep jovian atmosphere.

Over 130 laboratory measurements of the 2–4 mm wavelength opacity of sulfur dioxide in a carbon dioxide atmosphere under simulated conditions for the upper Venus troposphere (temperatures between 308 and 343 K and pressures between 0.03 and 2 bar) have been made. These measurements along with the centimeter wavelength measurements by Steffes et al. (Steffes, P.G. et al. [2015]. Icarus 245, 153–161) have been used to empirically assess existing formalisms for sulfur dioxide opacity in a carbon dioxide atmosphere (Fahd, A.K., Steffes, P.G. [1992]. Icarus 97(2), 200–210; Suleiman, S.H. et al. [1996]. J. Geophys. Res.: Planets 101(E2), 4623–4635). The Van Vleck and Weisskopf Model (VVW) used by Fahd and Steffes with the JPL rotational line catalog (Pickett, H. et al. [1998]. J. Quant. Spectrosc. Radiat. Transfer 60(5), 499–890) was found to fit 85.88% of all 500 measurements within the 2-sigma uncertainty. This work will improve the confidence in retrievals of the atmospheric abundance of sulfur dioxide from millimeter-wavelength observations of the Venus atmosphere.

In the past two decades, multiple observations of Venus have been made at X-Band (3.6 cm) using the Jansky Very Large Array (VLA), and maps have been created of the 3.6 cm emission from Venus (see, e.g., Devaraj, K. [2011]. The Centimeter- and Millimeter-Wavelength Ammonia Absorption Spectra under Jovian Conditions. PhD Thesis, Georgia Institute of Technology, Atlanta, GA). Since the emission morphology is related both to surface features and to deep atmospheric absorption from CO2 and SO2 (see, e.g., Butler, B.J., Steffes, P.G., Suleiman, S.H., Kolodner, M.A., Jenkins, J.M. [2001]. Icarus 154, 226–238), knowledge of the microwave absorption properties of sulfur dioxide in a carbon dioxide atmosphere under conditions for the deep atmosphere of Venus is required for proper interpretation. Except for a single measurement campaign conducted at a single wavelength (3.2 cm) over 40 years ago (Ho, W., Kaufman, I.A., Thaddeus, P. [1966]. J. Geophys. Res. 71, 5091–5108), no measurements of the centimeter-wavelength properties of any Venus atmospheric constituent have been conducted under conditions characteristic of the deep atmosphere (pressures from 10 to 92 bars and temperatures from 400 to 700 K). New measurements of the microwave properties of SO2 and CO2 at wavelengths from 3.7 to 20 cm have been conducted under simulated conditions for the deep atmosphere of Venus, using a new high-pressure system. Results from this measurement campaign conducted at temperatures from 430 K to 560 K and at pressures up to 92 bars are presented. Results indicate that the model for the centimeter-wavelength opacity from pure CO2 (Ho, W., Kaufman, I.A., Thaddeus, P. [1966]. J. Geophys. Res. 71, 5091–5108), is valid over the entire centimeter-wavelength range under simulated conditions for the deep atmosphere of Venus. Additionally, the laboratory results indicate that both of the models for the centimeter-wavelength opacity of SO2 in a CO2 atmosphere from Suleiman et al. (Suleiman, S.H., Kolodner, M.A., Steffes, P.G. [1996]. J. Geophys. Res. 101, 4623–4635) and from Fahd and Steffes (Fahd, A.K., Steffes, P.G. [1992]. Icarus 97, 200–210) can reliably be used under conditions of the deep atmosphere of Venus.