A JWST study of CO$_2$ on the satellites of Saturn

Abstract

Solid state CO$_2$ has been detected throughout the outer solar system, even at temperatures where crystalline CO$_2$ is unstable, requiring that the CO$_2$ be trapped in a separate host material. The Saturnian satellites provide an ideal laboratory for the study of this trapped CO$_2$, allowing us to examine objects with identical insolation, but with a range of environments, ice exposure, organic abundance, and formation locations. Here, we present JWST spectra of 8 mid-sized satellites of Saturn, including Mimas, Enceladus, Tethys, Dione, and Rhea interior to Titan, and Hyperion, Iapetus, and Phoebe exterior. The $\sim$4.26 $\mu$m CO$_2$ $\nu_3$ band is detected on each satellite, and the $\sim$2.7 $\mu$m $\nu_1+\nu_3$ band is detected on all but Phoebe and the leading hemisphere of Iapetus. Based on the wavelength shifts of these bands, we find four separate types of trapped CO$_2$ on the satellites. On the inner satellites, CO$_2$ appears trapped in amorphous ice sourced from Saturn's E-ring, and a second component of CO$_2$ is associated with the dark material most prominent on the trailing hemispheres of Dione and Rhea. On the outer satellites, CO$_2$ appears to be produced by irradiation of organics on Phoebe, which are then transported to the dark leading hemisphere of Iapetus and onto the dark regions of Hyperion. CO$_2$ is also trapped by water ice on the trailing hemisphere of Iapetus and on Hyperion. These observations point to the continued need for laboratory studies to better understand the sources and trapping mechanisms of CO$_2$ throughout the outer solar system.