Saturn's Atmospheric Helium Abundance from Cassini Composite Infrared Spectrometer Data

The ratio of helium to hydrogen in Saturn's atmosphere provides an important constraint on models of Saturn's formation and evolution, but has been poorly constrained by available data. Existing published measurements give He/H₂ mole ratios varying from 0.034 ± 0.024 (Conrath et al. 1984, Ap. J., 282:807-815) up to 0.135 ± 0.025 (Conrath and Gautier 2000, Icarus, 144:124:134). Because of this discrepancy, estimating Saturn's atmospheric helium abundance has been a major goal of the Composite Infrared Spectrometer (CIRS) instrument on the Cassini mission to Saturn.

We have used a retrieval method similar to Conrath and Gautier (2000) to retrieve the He/H₂mole ratio in Saturn's atmosphere using a series of observations in which the CIRS

far-IR (20 - 600 cm^-1; 16-500 µm) detector is held at a fixed latitude and emission angle for several hours. The retrieved He/H₂mole ratio is in the range from 0.04 - 0.06, near the lower end of the published values and consistent with results from the Cassini Visual and Infrared Mapping Spectrometer (Sromovsky et al., 2016, Icarus, 276:141-162), but significantly lower than recent results from combining data from CIRS and the Cassini Ultraviolet Spectrometer (Koskinen and Guerlet, 2018, Icarus, 307:161-171) of 0.11 ± 0.02 . The reasons for the large variation in Saturnian atmospheric helium measurements are still not understood.