The stability and transport of carbon dioxide on Iapetus

Carbon dioxide has been detected associated with Iapetus' dark material by the Cassini spacecraft. This CO₂ may be primordial and/or resulting from ongoing production by photolysis of water-ice in the presence of carbonaceous material Allamandola, L.J., Sandford, S.A., Valero, G.J., 1988. Icarus 76, 225-252]. Although any primordial CO₂ would likely be complexed with the dark material and thus stable against thermal transport to Iapetus' poles Buratti, B.J., and 28 colleagues, 2005. Astrophys. J. 622, L149-L152], active production of CO₂ would result in some fraction of the CO₂ being mobile enough to allow the accumulation of CO₂ at Iapetus' poles. We develop a computer model to simulate ballistic transport of CO₂ ice on Iapetus, accounting for Iapetus' gravitational binding energy and polar cold traps. We find that the residence time of CO₂ ice outside the polar regions is very short; a sheet of CO₂ ice near the equator of Iapetus decreases in thickness at a rate of 50 mm year⁻¹. The sublimated CO₂ will ballistically move around Iapetus until it reaches the polar cold traps where it can be sequestered for up to 15 years. If the total surface inventory of CO₂ exceeds 3×¹⁰⁷ kg, the polar ice cap will be permanent. While CO₂ is moving around the surface, a small percentage will eventually reach escape velocity and be lost from the system. As such, a seasonal polar cap is lost at rate of 12% every solar orbit as the CO₂ moves between the two polar cold traps.