Formation and Trapping of CO2 from Cryogenic Irradiation of Carbonate

Abstract

The detection of CO2 on the Jovian satellite Europa by Galileo NIMS and recent mapping of the leading side by JWST has revealed that it is most concentrated in geologically young terrains, and its v3 asymmetric stretch appears as a spectral doublet centered at 4.25 and 4.27 um. Since crystalline CO2 is unstable at Europan surface conditions, this observation implies an active source and a trapping medium, which may be separate. To this end, several hypotheses have been proposed, but no laboratory work has successfully reproduced the spectral features of CO2 on Europa so far. Radiolyzed carbonates have also been discussed as plausible precursors and host materials for CO2, though their role has not been experimentally validated in a Europa-like environment. Here, we report the first laboratory experiments investigating CO2 production from carbonate salts exposed to 10 keV electron irradiation at 50, 100, and 120 K in ultrahigh vacuum. Using diffuse reflectance FTIR spectroscopy, we observe the emergence, growth, and saturation of an absorption doublet centered near 4.25 and 4.27 um, consistent with the CO2 v3 band. Postirradiation thermal desorption studies using residual gas analysis reveal that the radiolytically formed CO2 is stable at temperatures beyond Europa's surface. This work provides the first experimental evidence that low-energy electron irradiation of carbonates in cryogenic, vacuum conditions can produce and retain CO2, and suggests that carbonates can serve as endogenous reservoirs of CO2 on irradiated icy bodies in the outer solar system.