17:15 〜 18:45
[PPS01-P08] Raman spectroscopy of salt deposits from the simulated subsurface ocean of Enceladus
キーワード：エンケラドス、ラマン分光法、氷衛星、pH、宇宙探査、カッシーニ探査機
Introduction
Enceladus hosts a global subsurface ocean and emits plumes from its south polar region. The plume-derived ice deposits on the surface and allows for indirect analysis of compositions of the subsurface ocean. Fox-Powell & Cousins (2021) JGR [1] investigated the ionic compositions of oceanic liquid based on integrating Cassini Dust Analyzer (CDA) observation with theoretical calculations. They conducted freezing experiments with simulated liquid and showed that the produced minerals were consistent with the CDA data, suggesting that the detection of specific minerals can provide insights into pH conditions within the subsurface ocean and the cooling rate of the plume particles. However, the method employed in the previous study was unsuitable for in situ observation of Enceladus because XRD analysis and SEM observation of microstructure were used to characterize the frozen liquids in each pH and cooling rate. Flight-ready techniques, including Raman spectroscopy, are preferable for such measurements.
Here we conducted freezing experiments of droplets of the simulated subsurface ocean of Enceladus and subsequent Raman spectroscopic analysis to find any spectral differences characteristic to the pH conditions of the frozen liquid.
Methods
The liquid compositions were taken from [1] (both pH 9 and 11, the values considered possible for the subsurface ocean). The amount of frozen droplets of the simulated subsurface ocean was 0.5 mL each and frozen to 100 K by liquid N2. The water ice was sublimated by keeping a cryostat vacuum chamber at a condition comparable to the Tiger stripes (100 - 101 Pa and 200 K) for 2 hours. Then the salt deposits from the frozen droplets were measured with a time-gated Raman spectrometer, which was built to simulate the performance of SuperCam aboard NASA’s Perseverance rover. We also produced salt deposit samples from the liquid concentrated tenfold in salts, simulating possible conditions near Enceladus’ surface, where the ice from subsurface ocean is potentially more enriched in salts due to water sublimation.
Results & Discussion
Initial spectral analyses of the salt deposits did not reveal any notable Raman peaks or differences between the two pH conditions. This lack of differentiation could stem from either the current signal-to-noise ratio which might be too low to detect peaks from the crystallized salts, or the possibility that the salts became amorphous, making their peaks undetectable.
Further analyses were conducted on the salt deposits derived from the tenfold enriched droplets. We observed Raman peaks at 1055 cm-1 (pH 9) and 1081 cm-1 (pH 11), corresponding to the symmetric stretch in sodium carbonate. At pH 11, the peak shift towards a higher frequency suggests the presence of thermonatrite (Na2CO3 H2O). Only at pH 9, nahcolite (NaHCO3) was identified at 1270 cm-1. In both pH vlaues, hydrohalite (NaCl 2H2O) was identified at 1659 cm-1. The peaks at 3448 cm-1 (pH 9) and 3434 cm-1 (pH 11) were observed and attributed to symmetric -OH stretching modes, indicating the deliquescence of sodium carbonate during the Raman measurements. The mineral species observed in the 10 times concentrated compositions were consistent with those in the original compositions reported by [1].
Conclusion
Our results suggest that a Raman spectrometer can be a valuable instrument for future in situ geochemical analysis on the surface of Enceladus. Our Raman spectroscopic analysis revealed that it is possible to detect carbonate and chloride minerals on the surface if the salt deposits are derived from the droplets of 10 times enriched in salts than subsurface ocean, even if the amount of droplet was only 0.5 mL. Identifying the carbonate minerals such as nahcolite may allow for constraining the pH of the subsurface ocean. The pH constraints, related to the solubility of ions and even habitability, are important for understanding the subsurface ocean and may give us clues to the interior of Enceladus.
Enceladus hosts a global subsurface ocean and emits plumes from its south polar region. The plume-derived ice deposits on the surface and allows for indirect analysis of compositions of the subsurface ocean. Fox-Powell & Cousins (2021) JGR [1] investigated the ionic compositions of oceanic liquid based on integrating Cassini Dust Analyzer (CDA) observation with theoretical calculations. They conducted freezing experiments with simulated liquid and showed that the produced minerals were consistent with the CDA data, suggesting that the detection of specific minerals can provide insights into pH conditions within the subsurface ocean and the cooling rate of the plume particles. However, the method employed in the previous study was unsuitable for in situ observation of Enceladus because XRD analysis and SEM observation of microstructure were used to characterize the frozen liquids in each pH and cooling rate. Flight-ready techniques, including Raman spectroscopy, are preferable for such measurements.
Here we conducted freezing experiments of droplets of the simulated subsurface ocean of Enceladus and subsequent Raman spectroscopic analysis to find any spectral differences characteristic to the pH conditions of the frozen liquid.
Methods
The liquid compositions were taken from [1] (both pH 9 and 11, the values considered possible for the subsurface ocean). The amount of frozen droplets of the simulated subsurface ocean was 0.5 mL each and frozen to 100 K by liquid N2. The water ice was sublimated by keeping a cryostat vacuum chamber at a condition comparable to the Tiger stripes (100 - 101 Pa and 200 K) for 2 hours. Then the salt deposits from the frozen droplets were measured with a time-gated Raman spectrometer, which was built to simulate the performance of SuperCam aboard NASA’s Perseverance rover. We also produced salt deposit samples from the liquid concentrated tenfold in salts, simulating possible conditions near Enceladus’ surface, where the ice from subsurface ocean is potentially more enriched in salts due to water sublimation.
Results & Discussion
Initial spectral analyses of the salt deposits did not reveal any notable Raman peaks or differences between the two pH conditions. This lack of differentiation could stem from either the current signal-to-noise ratio which might be too low to detect peaks from the crystallized salts, or the possibility that the salts became amorphous, making their peaks undetectable.
Further analyses were conducted on the salt deposits derived from the tenfold enriched droplets. We observed Raman peaks at 1055 cm-1 (pH 9) and 1081 cm-1 (pH 11), corresponding to the symmetric stretch in sodium carbonate. At pH 11, the peak shift towards a higher frequency suggests the presence of thermonatrite (Na2CO3 H2O). Only at pH 9, nahcolite (NaHCO3) was identified at 1270 cm-1. In both pH vlaues, hydrohalite (NaCl 2H2O) was identified at 1659 cm-1. The peaks at 3448 cm-1 (pH 9) and 3434 cm-1 (pH 11) were observed and attributed to symmetric -OH stretching modes, indicating the deliquescence of sodium carbonate during the Raman measurements. The mineral species observed in the 10 times concentrated compositions were consistent with those in the original compositions reported by [1].
Conclusion
Our results suggest that a Raman spectrometer can be a valuable instrument for future in situ geochemical analysis on the surface of Enceladus. Our Raman spectroscopic analysis revealed that it is possible to detect carbonate and chloride minerals on the surface if the salt deposits are derived from the droplets of 10 times enriched in salts than subsurface ocean, even if the amount of droplet was only 0.5 mL. Identifying the carbonate minerals such as nahcolite may allow for constraining the pH of the subsurface ocean. The pH constraints, related to the solubility of ions and even habitability, are important for understanding the subsurface ocean and may give us clues to the interior of Enceladus.