Probing Europa's subsurface ocean composition from surface salt minerals using in-situ techniques
Introduction
The global ocean believed to exist beneath Europa's ice shell (Carr et al., 1998; Kivelson et al., 2000) is often regarded as one of the most likely places in the Solar System to find evidence of extraterrestrial habitable environments or even extant life (Des Marais et al., 2008; Hendrix et al., 2019). NASA's upcoming Europa Clipper mission (Phillips and Pappalardo, 2014; Pappalardo et al., 2017) is designed to investigate its habitability, for which knowledge of the ocean's chemical composition is a critical determining factor. A follow-on Europa Lander mission concept (Hand et al., 2017) is also being considered to further examine Europa's habitability in situ and seek direct evidence for present or past life.
The surface of Europa is very diverse and consists of icy bright plains crisscrossed with fracturing and dark linea features (Dalton et al., 2012). Recently, evidence for periodic plume activity has been tentatively detected in Europa's southern region (Roth et al., 2014; Sparks et al., 2016; Jia et al., 2018), giving rise to the prospect that subsurface liquids may be ejected out to space and onto the surface. However, until direct sampling of these plumes is possible in future missions, efforts to constrain the composition of subsurface liquids will, in the near term, rely heavily on the analysis of icy materials expressed on the surface.
Europa exhibits one of the youngest surfaces in the Solar System with a crater-derived average surface age of ~60–100 Myr (Zahnle et al., 2008; Bierhaus et al., 2009), implying active geological resurfacing processes in recent past. The chemical makeup of these surface materials is expected to reflect those of the subsurface ocean (Fanale et al., 2001; Zolotov and Shock, 2001; Zolotov and Kargel, 2009). A number of mechanisms have been proposed for the emplacement of frozen ocean materials on the surface, including cryovolcanism (Fagents, 2003), diapirism (Rathbun et al., 1998; Nimmo and Manga, 2002), tidal pumping (Barr and Showman, 2009), and subduction (Kattenhorn and Prockter, 2014). In contrast to cryovolcanism where ocean fluids would likely experience flash freezing prior to deposition on the surface, the latter mechanisms would correspond to slow, near thermodynamic equilibrium freezing over geologic timescales. An example of this is the recent work by Howell and Pappalardo (2018) which suggests that ocean materials, originally frozen into the base of the ice shell, can be transported onto the surface through convection within the ice shell.
Previous data from the Galileo Near Infrared Mapping Spectrometer (NIMS) have indicated the presence of hydrated salt minerals, such as Mg and Na sulfates, at various abundances across Europa's terrains (McCord et al., 1998; McCord et al., 1999; Dalton et al., 2005; Dalton, 2007). In particular, epsomite has been tentatively identified in ground-based infrared spectra of Europa's trailing edge, possibly via radiolytic processing of MgCl2 (Brown and Hand, 2013). In addition, NaCl has been hypothesized as one of the dominant surface components on Europa, and is thought to be responsible for the colors captured in Galileo images of Europa's surface (Hand and Carlson, 2015; Trumbo et al., 2019).
Using these ionic constituents as a guide, Vu et al. (2016) experimentally investigated the freezing of several Na-Mg-Cl-SO4 brines and found that these solutions preferentially crystallize into hydrated Na2SO4 and MgCl2 minerals, even at elevated Na+/Mg2+ ratios. Johnson et al. (2019) subsequently developed flowcharts to establish the mineral formation sequence in this four-component system at different pH's using chemical divide modeling verified by experimental data. This modeling approach relies upon the phase diagrams of the salt-water systems, whereby species would precipitate out of solution in the order of their aqueous solubilities (from lowest to highest) (Hardie and Eugster, 1970; Eugster and Jones, 1979). For the Na-Mg-Cl-SO4 mixture (which primarily exists at pH < 8.4 due to the formation of brucite Mg(OH)2 in solutions of higher basicity), it was predicted that mirabilite (Na2SO4·10H2O) would be the first species to freeze out, consistent with experimental observations (Vu et al., 2016). If there were an excess of sodium or sulfate ions, then hydrohalite (NaCl·2H2O) or MgSO4 hydrates would be the next species to precipitate out of solution, respectively, followed by MgCl2·12H2O. Any anions that remained after this process would eventually crystallize as acid hydrates (Fig. 1 in Johnson et al. (2019)).
This simple model hinges on the assumption that freezing is sufficiently slow to occur in thermodynamic equilibrium. Therefore, it may not always apply to surface emplacement mechanisms where the brines are frozen rapidly (such as in plume areas). In the present work, we expand on this study by exploring the impact of freezing rate (flash versus slow) on mineral formation, in order to simulate various possible resurfacing mechanisms. For simplicity, we focus on a very specific set of aqueous solutions in which the concentrations of sodium and chloride ions are exactly twice those of magnesium and sulfate. These ratios are chosen to limit the number of thermodynamic products expected upon freezing to only two (mirabilite and MgCl2·12H2O), since there is no excess of either sodium or sulfate. Any departure from the equilibrium composition would be readily identifiable by the presence of other hydrated minerals. Two concentration regimes (molar vs tenths of molar) are examined, representing near-saturation/Dead Sea-level versus near-average Earth ocean salinity, respectively. These are chosen to assess the differences, if any, in the relative abundances of the precipitated minerals, as well as testing the detection limits for these species using in-situ techniques.
The Europa Lander Science Definition Team has recommended an onboard vibrational spectrometer (Raman or infrared) for the mission concept's strawman payload (Hand et al., 2017). Such an instrument is expected to support the objectives of characterizing the mineral environment, and determining the composition of organic materials. In previous studies (Vu et al., 2016; Vu et al., 2017; Johnson et al., 2019), we have utilized cryogenic micro-Raman spectroscopy as the technique of choice for mineral identification because of its (i) high spatial resolution, owing to a small laser spot size (~4 μm) which enables detection of salt minerals even in dilute frozen solutions; (ii) ability to unambiguously characterize the hydration states of the salts of interest (Wang et al., 2006; Hamilton and Menzies, 2010); (iii) ease of implementation, as no direct sample handling and processing is necessary, and the frozen brines can be studied from a short distance (~a few cm) while being kept under controlled conditions.
While micro-Raman spectroscopy is a powerful technique for surface interrogation, its aforementioned advantages are, by design, not well-suited for investigating the bulk composition of solid materials. Furthermore, accurate quantification of mineral phases using Raman spectroscopy is notoriously challenging due to unknown scattering cross-sections and overlapping features in the broad O-H stretch region of water ice, in addition to the dependence of the measured signal on integration time, beam focusing, etc. As a result, extensive calibration efforts using reference standards with well-defined bulk compositions would be required to reliably retrieve mineral abundances.
This obstacle consequently calls for another investigative technique that can accurately quantify the composition of bulk samples, which can then provide a reference for the Raman analyses. One possible approach is X-ray diffraction (XRD) which, by virtue of its short excitation wavelength, directly probes the atomic arrangement within the unit cell of crystalline materials. Moreover, the light elements that are typically present in icy solids (e.g. hydrogen and oxygen) have low X-ray absorptivity, enabling investigation of the bulk of these samples. Mineral abundances can be determined via standard Rietveld refinement of the diffraction patterns. Although no XRD instrument has been recommended for the Europa Lander strawman payload, this technique (as the gold standard for mineralogy) appears likely to be used for the ground calibration of the vibrational spectrometer. Therefore, the accuracy in the composition of the standards determined via XRD could represent the best accuracy expected for Raman quantification, unless multiple other suitable techniques are used in complement.
This study combines Raman and XRD analyses of putative Europa frozen brines to achieve the following objectives: (i) determining the effect of emplacement mechanism on the composition of resultant frozen brines; (ii) constraining the accuracy to which the composition of icy materials can be known via XRD under Europa's surface conditions; (iii) informing analytical protocols for a potential Raman spectrometer to best provide constraints on emplacement mechanism and composition of frozen salt minerals. Ultimately, these results can help better elucidate the mineralogy of Europa as well as guide the selection of landing sites that may yield the most insights into the subsurface ocean composition from a Europa surface lander.
Section snippets
Materials and methods
Aqueous solutions were prepared at the desired ionic concentrations (samples 1–4, Table 1) using commercial NaCl (Mallinckrodt, ACS grade), MgSO4·7H2O (J. T. Baker, ACS grade), and deionized water. Dissolution of these components was observed to be endothermic, consistent with the known increase in aqueous solubilities of these salts with temperature (Rumble, 2018). Hereafter, samples 1 and 2 are referred to as “molar” brines, and samples 3 and 4 as “submolar”.
Raman experiments utilized a
Bulk (XRD) vs local (Raman) observations
Table 1 summarizes the experimental results obtained in this work. In all combinations of brine concentration and freezing rate, the XRD experiments observed only water ice and the two products predicted by the chemical divide model (mirabilite and MgCl2·12H2O). The relative proportions of these three solid phases are shown in the last column. On the contrary, the Raman results turned out to be more complex, with additional products detected in a number of scenarios.
In sample 1 where the molar
Discussion
Various hydrated minerals have been proposed to exist on the surface of Europa based on geochemical predictions of the ocean composition as well as interpretation of Galileo NIMS data (McCord et al., 1998; McCord et al., 1999; Kargel et al., 2000; Dalton, 2007). These include a mixture of sodium, magnesium, chloride, and sulfate salts, which have motivated a number of recent experimental studies (Hanley et al., 2014; Vu et al., 2016; Thomas et al., 2017). The potential presence of individual
Conclusions
We have conducted combined cryogenic Raman and X-ray diffraction studies on the freezing of a simple quaternary aqueous brine system where [Na+] = [Cl−] = 2[Mg2+] = 2[SO42−] under two freezing conditions: slow cooling at <1 K/min and flash freezing under liquid nitrogen. The bulk XRD technique observed only two salt products after annealing of the glassy phases that formed upon flash-freezing the samples, mirabilite and MgCl2·12H2O, consistent with predictions from chemical divide modeling.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA, and was supported by the NASA Astrobiology Institute (Icy Worlds node) and the JPL Research and Technology Development program. U.S. Government sponsorship is acknowledged. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement by the United States Government
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