Production and detection of carbon dioxide on Iapetus

摘要

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.