Impact of species diffusion on the attenuation of acoustic waves in multi-component planetary atmospheres

Acoustic waves in planetary atmospheres are attenuated by dissipative processes such as viscous stresses and heat conduction, particularly in the rarefied upper layers where multiple species exist in diffusive equilibrium. While the roles of viscosity and thermal conduction in wave attenuation are well understood, species diffusion-the relative motion of molecular species different from the bulk gas driven by gradients in concentration, pressure, and temperature-has received less attention. This study investigates species diffusion as an additional attenuation mechanism in dilute, multi-component gas mixtures, using generalized macroscopic transport equations derived from kinetic theory that reduce to the classical Navier-Stokes equations in the single-species limit. Using a multiple-scales approach, we derive a dispersion relation for linear acoustic and gravity waves, from which an expression for attenuation in multi-species atmospheres is obtained. We apply this framework to the upper atmospheres of Earth, Venus, Mars, Titan, Uranus, and Neptune. Results show that species diffusion-primarily via barodiffusion (diffusion driven by pressure gradients)-can significantly enhance acoustic attenuation on Earth, Venus, and Mars, reaching up to 16%, 45%, and 17%, respectively. On Earth, the effect is most pronounced above 150 km, where light and heavy species such as molecular nitrogen and atomic oxygen coexist in appreciable concentrations. In contrast, species diffusion plays a minor role-contributing less than ≤ 5%-on Titan, Uranus, and Neptune, with bulk and shear viscosity effects dominating wave attenuation in these atmospheres. These findings expand existing models of planetary wave propagation and have implications for planetary diagnostics, remote sensing, and space missions.