Arrival of CO2 from the ocean to the surface of Europa: A laboratory study

Observations of the leading side of Europa by NIRSpec aboard the JWST reveal a doublet profile of the absorption attributed to solid CO₂, centered at 4.249 and 4.269 µm (Trumbo & Brown 2023; Villanueva et al. 2023). A second absorption at ~2.695 µm is also observed, which is close to the absorption at 2.71 µm of crystalline CO₂. At the ultra-high vacuum conditions prevailing over the surface of Europa, the stability of solid CO₂ at temperatures exceeding 70 K is intriguing (Bryson et al. (1974)). Therefore, the operation of a trapping mechanism for CO₂ is considered despite the similarity of absorptions at 4.269 and 2.695 µm to that of crystalline CO₂. Mapping the integrated area of the doublet across the surface conveyed increased abundance of solid CO₂ at lower latitudes closer to the equator than at the poles, despite the former being warmer (Trumbo & Brown 2023; Trumbo et al. 2018). Furthermore, increased concentrations are seen at chaos terrains, with Tara Regio having the maximum concentration. All proposed mechanisms for the formation of geologically young chaos terrains involve exchange of material between ice shell and the subsurface ocean (Anderson et al. 1998; Kivelson et al. 2000; Schilling et al. 2007). This forms the basis of CO₂ being endogenous, from the ocean.

Substances with carbon in their chemical composition, sourced from the ocean, could be processed by radiolysis and/or chemical reactions at the surface to generate CO₂. Alternatively, CO₂ could be sourced from the ocean in its native form during the migration of water ice between the ocean and surface. Distinguishing CO₂ formed via these pathways given their plausible co-existence, forms another query.

We performed experiments attempting a qualitative replication of the pressure-temperature conditions surrounding CO₂ retained in water ice, as the latter migrates from the ocean to the surface. Water ice and frozen NaCl brine containing CO₂ were produced which were then ground at liquid N₂ temperature. The diffused infrared reflectance spectra of these samples were recorded at 100 K and evacuated conditions. We observe the appearance of a doublet absorption at 4.258 and 4.278 µm and a weak absorption at 2.706 µm, characteristic of clathrate hydrates of CO₂ (Oancea et al. 2012). However, these absorptions do not coincide with those observed on Europa. A separate batch of ices produced by flash freezing at temperatures below 90 K also retained CO₂. The corresponding absorption also forms a doublet with blue-shifted band centers at 4.251 and 4.272 µm, while the 2.706 µm feature is absent. The doublet absorptions in both batches of ice remain stable up to 150 K for prolonged durations.

Therefore, given the mismatch of the band centers with those observed on Europa and their stability at the pressure – temperature conditions expected on Europa, we conclude that the endogenous CO₂ observed at the chaos terrains is not sourced directly from the ocean. It must be the result of transformation of carbon-based materials, sourced from the ocean, driven by radiolysis and/or chemical reactions at the surface.

References

Anderson, J. D., Schubert, G., Jacobson, R. A., et al. 1998, Science, 281, 2019,

doi: 10.1126/science.281.5385.2019

Bryson, C. E. I., Cazcarra, V., & Levenson, L. L. 1974, Journal of Chemical & Engineering Data, 19, 107,

doi: 10.1021/je60061a021

Kivelson, M. G., Khurana, K. K., Russell, C. T., et al. 2000, Science, 289, 1340,

doi: 10.1126/science.289.5483.1340

Oancea, A., Grasset, O., Le Menn, E., et al. 2012, Icarus, 221, 900,

doi: 10.1016/j.icarus.2012.09.020

Schilling, N., Neubauer, F. M., & Saur, J. 2007, Icarus, 192, 41,

doi: 10.1016/j.icarus.2007.06.024

Trumbo, S. K., & Brown, M. E. 2023, Science, 381, 1308,

doi: 10.1126/science.adg4155279

Trumbo, S. K., Brown, M. E., & Butler, B. J. 2018, The Astronomical Journal, 156, 161,

doi: 10.3847/1538-3881/aada87

Villanueva, G. L., Hammel, H. B., Milam, S. N., et al. 2023, Science, 381, 1305

doi: 10.1126/science.adg4270