The Fate of Frozen Carbonated Water at Europa-like Conditions

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 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 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.

Pressure–temperature (P–T) conditions of our experiments compared to phase diagram of the CO₂–H₂O system. Red dots correspond to actual P–T conditions at various stages of ice preparation. Dashed lines are only indicative and do not correspond to measured variations in pressure. Paths labeled in red, yellow, and magenta correspond to ice retaining CO₂. Paths labeled in black correspond to ice devoid of CO₂. Lowercase letters show the stages of the experiment described in the text, with subscripts showing the applicable experiment number. The stars near the bottom left denote the P–T conditions during acquisition of spectra. — The Planetary Science Journal

The Fate of Frozen Carbonated Water at Europa-like Conditions, The Planetary Science Journal (open access)

Astrobiology,