Absorption of infrasound in the lower and middle clouds of Venus

The paper presents predictions for the infrasonic attenuation coefficient in the clouds of Venus at altitudes of 50-60 km, where instrumented balloons will likely be deployed. The acoustic wavenumber is obtained by modifying the model of Baudoin, Coulouvrat, and Thomas [J. Acoust. Soc. Am. 130(3), 1142-1153 (2011)] to account for typical Venus cloud composition. A two-phase system, consisting of polydisperse aqueous-H₂SO₄ liquid droplets with a trimodal size distribution and their vapors is considered. Assuming sulfuric acid as the main condensable species, the low-frequency cloud attenuation coefficient is dominated by the evaporation/condensation of H₂SO₄. It ranges from 2×10^-5 dB/km at 10 mHz to 0.1 dB/km at 10 Hz, exceeding that of the dry atmosphere by up to 2 orders of magnitude. Varying the cloud density by ±50% changes the attenuation by -35%/+100% at 1 mHz and ±50% at 10 Hz. The same variation in the acid vapor diffusion coefficient causes attenuation changes from -20%/+70% at 1 mHz to +25%/-40% at 10 Hz. As the evaporation coefficient of H₂SO₄ (presently poorly constrained) is varied from 0.01 to 1, the attenuation drops from 10^-4 dB/km to 4×10^-6 dB/km at 10 mHz and increases from 10^-3 dB/km to 2×10^-2 dB/km at 10 Hz.