Predictions for acoustic sensing in Saturn's atmosphere

We investigate using ultrasonic sensing to measure the helium and ortho/para-hydrogen ratios in Saturn's atmosphere, in the context of NASA's New Frontiers Saturn Probe mission concept. We use a theoretical wavenumber model incorporating hydrogen rotational relaxation into a linearized fluid dynamics framework. We assume a windless and cloudless hydrogen-helium environment. Quantitative gas analysis can be achieved using both the frequency dependent sound speed and attenuation coefficient. We present the sensitivity of the two acoustic properties to ortho-H2/para-H2/He ratios at temperatures and pressures typical of Saturn. H2-H2 and He-H2 collisions both affect the hydrogen relaxation times. Ortho/para-H2 ratio changes can be detected from the overall sound speed and attenuation changes and from the corresponding shifts in the characteristic H2 relaxation frequencies (e.g., ∼1 MHz at 100 mbar, ∼10 MHz at 1 bar). Helium abundance should be detectable primarily through the molecular weight difference. The results are encouraging, showing that ultrasonic sensors operating during descent in Saturn's atmosphere from 100 mbar to 10 bar could be able to detect the He content and discriminate between ortho- and para-H2. [Funding was provided by NASA-CIF (Grant No. 80NSSC22M0106)]