The Fate of Frozen Carbonated Water at Europa-like Conditions

Abstract

We present the results of experiments probing the retention of CO2 in crystalline water ice, frozen sodium chloride (NaCl) brines, and flash-frozen carbonated water using diffuse reflectance infrared spectroscopy. Characteristic absorptions alluding to the formation of clathrate hydrates in crystalline ices and frozen brines are observed. NaCl in frozen brines does not affect qualitatively affect the formation of clathrate hydrates. Generation and stability of clathrates in crystalline ice transiently subjected to pressure-temperature (P-T) conditions in the stability region is observed, despite conditions being unviable at the onset of freezing. Retention of CO2 in flash-frozen carbonated water is observed to be dependent on the temperature of the substrate during freezing. The state of CO2 retained in the resulting ices differs from clathrate hydrates, as inferred from the respective infrared spectra. Both mechanisms of CO2 retention are stable up to 140 K and under evacuated conditions. In the context of Europa, the P-T states traversed by the samples plausibly represent the typical conditions around endogenous CO2 if it is indeed transported from the subsurface ocean to the surface while being retained in ice/frozen brines and/or liquid emerging on the surface. However, the absorptions of CO2 in the laboratory infrared spectra do not match those detected on the leading side of Europa by the NIRSpec instrument on board JWST. Therefore, it is unlikely that the endogenous CO2 observed at the surface of Europa is sourced directly from the ocean, unless additional processes affect the observed bands of CO2 on Europa.