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The Detection, Production and Stability of Carbon Dioxide on Iapetus and other Saturnian Moons

Wednesday, January 28, 2009
Palmer (LPL)

Carbon dioxide has been found by Cassini VIMS throughout the Saturnian system in locations such asIapetus' equator where the temperature is too high for it to remain as free ice for more than a few hundred years. We suggest that the 4.26-micron absorption feature found on Iapetus and many other moons is theresult of either UV photolysis or ion bombardment driving chemistry between the carbon rich layer and thewater ice regolith. The form of the CO2 appears to be complexed, a form where there is interaction between the surface and the component, such as gas or fluid inclusions, trapped in amorphous solid water, oradsorption.

We conducted experiments to simulate the generation of CO2 by UV radiation under conditions similar to those on the surface of Iapetus. A simulated icy regolith was created in an argon atmosphere using flash-frozen, degassed water crushed into sub-millimeter sized particles. Isotopically labeled amorphous carbon (13C), which was ground into a fine dust, was mixed into the regolith allowing for extensive grain contact.This sample was placed in a vacuum chamber and cooled to temperatures as low at 60K. The sample wasirradiated with UV light, and the products were measured using both a mass spectrometer to identify freemolecules and an IR spectrometer for molecules that remained trapped on and in the simulated regolith.

We report on the production and reaction rates of CO2 and CO, as well as the generation of free hydrogen and oxygen as detected by an SRS-100 mass spectrometer. We also identify residual products that eitherfreeze on the surface or become entrained by or adsorbed onto the ice grains. We attempt to match the CO2absorption feature found on Iapetus with that seen in our simulation, perhaps identifying a possible source of CO2 in the Saturnian system.

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