Production and detection of carbon dioxide on Iapetus

Cassini VIMS detected carbon dioxide on the surface of Iapetus during its insertion orbit. We evaluated the CO₂ distribution on Iapetus and determined that it is concentrated almost exclusively on Iapetus' dark material. VIMS spectra show a 4.27-μm feature with an absorption depth of 24%, which, if it were in the form of free ice, requires a layer 31nm thick. Extrapolating for all dark material on Iapetus, the total observable CO₂ would be 2.3×10⁸kg.Previous studies note that free CO₂ is unstable at 10 AU over geologic timescales. Carbon dioxide could, however, be stable if trapped or complexed, such as in inclusions or clathrates. While complexed CO₂ has a lower thermal volatility, loss due to photodissociation by UV radiation and gravitational escape would occur at a rate of 2.6×10⁷kgyear^-1. Thus, Iapetus' entire inventory of surface CO₂ could be lost within a few decades.The high loss/destruction rate of CO₂ requires an active source. We conducted experiments that generated CO₂ by UV radiation of simulated icy regolith under Iapetus-like conditions. The simulated regolith was created by flash-freezing degassed water, crushing it into sub-millimeter sized particles, and then mixing it with isotopically labeled amorphous carbon (¹³C) dust. These samples were placed in a vacuum chamber and cooled to temperatures between 50K and 160K. The samples were irradiated with UV light, and the products were measured using a mass spectrometer, from which we measured ¹³CO₂ production at a rate of 2.0×10¹²mols^-1. Extrapolating to Iapetus and adjusting for the solar UV intensity and Iapetus' surface area, we calculated that CO₂ production for the entire surface would be 1.1×10⁷kgyear^-1, which is only a factor of two less than the loss rate. As such, UV photochemical generation of CO₂ is a plausible source of the detected CO₂.