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

We use thermal infrared data from the Composite Infrared Spectrometer, which was on board the Cassini orbiter, to retrieve the helium abundance in Saturn’s visible atmosphere. We find that the data is most consistent with a helium-to-hydrogen mole ratio in the range between 0.04 and 0.075, near the lower end of previous measurements, and implying a significant sequestration of helium in Saturn’s interior. The primary sources of uncertainty are in the spectroscopic parameters for H 2 –H 2 collision induced absorption and the nonuniqueness of the spectral inverse problem.