CO2 Production from Cryogenic Irradiation of Calcite

NIRSpec observations of Europa’s leading hemisphere have revealed that CO₂ appears as a spectral doublet centered at 4.25 and 4.27 µm. Given that crystalline CO₂ sublimes at 80 K in UHV and Europa’s surface reaches temperatures up to 120 K, the presence of CO₂ implies an active source and a stable trapping material—both of which remain unidentified. Characterizing these processes is essential for constraining Europa’s surface chemistry and its interaction with Jupiter’s magnetosphere. Laboratory investigations so far have focused on electron irradiation of carbonic acid, C-bearing minerals, and mixtures of ice and organics, and uncovered multiple CO₂ trapping mechanisms, including clathrate formation, physisorption onto minerals such as Ca-montmorillonite, and entrapment within non-ice materials. The idea that carbonate salts could be a plausible source and host material for CO₂ has also been discussed, and a tentative 3.9 µm absorption feature characteristic of carbonates has been reported. However, no experimental work has directly examined irradiated carbonates as a CO₂ source under conditions relevant to Europa. To address this gap, we irradiated calcite with 10 keV electrons at 50 K, 100 K, and 120 K in a vacuum chamber and monitored spectral changes and gaseous release using FTIR and mass spectroscopy. We found that CO₂ is produced during irradiation; it exhibits absorption features consistent with those observed on the Galilean satellites and remains stable at temperatures beyond 100 K. Our work provides the first experimental evidence that carbonates may be a plausible source of CO₂ on the Galilean moons.