Raman Spectroscopy of Salt Deposits from the Simulated Subsurface Ocean of Enceladus

TL;DR

The paper investigates whether Raman spectroscopy can be used to infer Enceladus's subsurface ocean pH from salt deposits formed by plumes. Using a flight-like Raman instrument, the authors prepared two brine chemistries at $pH=9$ and $pH=11$, freeze-dried them to salts, and analyzed the products; they detected NaHCO3 at $pH=9$ and Na2CO3 at $pH=11$, with Na2CO3 also present at $pH=9$. The results show that Raman signatures can distinguish pH-dependent carbonate phases in plume-derived salts, enabling a non-invasive method to constrain whether the ocean is weakly alkaline or strongly alkaline. The work supports deploying Raman spectrometers on Enceladus landers to assess ocean chemistry and habitability.

Abstract

Saturn's ice-covered moon Enceladus may host a subsurface ocean with biologically relevant chemistry. Plumes released from this ocean preserve information on its chemical state, and previous analyses suggest weakly to strongly alkaline pH (approximately pH 8--12). Constraining the pH requires identification of pH-sensitive minerals in plume deposits. Several analytical techniques could provide such mineralogical information, but few are practical for deployment on planetary missions. Raman spectrometers, which have recently advanced for \textit{in situ} exploration and have been incorporated into flight instruments, offer a feasible approach for mineral identification on icy moons. However, their applicability to pH estimation from plume-derived minerals has not been investigated. In this study, we evaluate whether Raman measurements of plume particles deposited on the surface of Enceladus can be used to distinguish between weakly and strongly alkaline subsurface ocean models. Fluids with pH values of 9 and 11 were frozen under vacuum conditions analogous to those on Enceladus. The resulting salt deposits were then analyzed using a flight-like Raman spectrometer. The Raman spectra show pH-dependent carbonate precipitation: NaHCO$_3$ and Na$_2$CO$_3$ peaks were detected at pH 9, whereas only Na$_2$CO$_3$ peaks were detected at pH 11. These findings demonstrate that Raman spectroscopy can distinguish pH-dependent carbonate phases. This capability allows us to constrain whether the pH of the subsurface ocean is weakly alkaline or strongly alkaline, which is a key parameter for assessing its chemical evolution and potential habitability.

Figures (5)

• Figure 1: Cooling experimental apparatus. (a) Photograph and (b) schematic diagram of the dry freezing system. (c) The sample chamber with a sample (i.e., a frozen solution) in the chamber. Above this is a cooler filled with liquid nitrogen. (d) Photograph of the Raman spectrometer.
• Figure 2: Photographs of the samples at (a) pH = 9 immediately after starting vacuum pumping and (b) pH = 9 after 18 h of vacuum pumping, (c) pH = 11 immediately after starting vacuum pumping, and (d) pH = 11 after 18 h of vacuum pumping. The sample holder is 5 cm in diameter.
• Figure 3: Raman spectra of samples under different pH conditions. (a) Spectra in the 400–1300 cm$^{-1}$ range; measured spectra of standard $\mathrm{NaHCO_{3}}$ and $\mathrm{Na_{2}CO_{3}}$ powders are shown at the bottom for reference. Raman peaks observed at approximately 686, 1045, and 1267 cm$^{-1}$ are assigned to vibrational modes of the bicarbonate ion (HCO$_3^-$).Raman peaks observed at approximately 686 and 1081 cm$^{-1}$ is assigned to vibrational modes of the carbonate ion (CO$_3^{2-}$). (b) Spectra in the 3200-4000 cm$^{-1}$ range.
• Figure 4: Example of a spectrum with the emission lines of Na atoms. Accumulation count = 100 times, grating = 150 lpm, and exposure time = 10 ns.
• Figure 5: Sublimation time as a function of temperature for different coating suppression factors ($f = 1.0$, $0.5$, and $0.1$). Salt coatings substantially increase the survival time of icy grains under Enceladus-like conditions.