Composition and Evolution of Frozen Chloride Brines under the

Jan 17, 2017 - *E-mail: [email protected]. .... Raman spectra were recorded over a temperature range from 80 to 233 K, as were the NIR refle...
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Composition and Evolution of Frozen Chloride Brines under the Surface Conditions of Europa Elena C. Thomas,†,‡ Robert Hodyss,†,‡ Tuan H. Vu,†,‡ Paul V. Johnson,*,†,‡ and Mathieu Choukroun†,‡ †

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States NASA Astrobiology Institute

‡

ABSTRACT: Chloride salts have been proposed to exist on the surface of Europa based on some geochemical predictions of the ocean composition as well as some interpretations of data from the Galileo spacecraft. To help elucidate our understanding of Europa’s surface composition, we have conducted a study of frozen chloride salt brines prepared under simulated Europa surface conditions (vacuum and temperature) using both near-infrared (NIR) and Raman spectroscopies. The latter was used to determine the hydration states of various chloride salts as a function of the temperature, while NIR spectroscopy of identically prepared samples was used to provide reference reflectance spectra of the identified hydrated salts. We further investigated the stability of the formed hydrated salts under vacuum and ultraviolet (UV) irradiation. Our results indicate that, at temperatures ranging from 80 to 233 K, (i) NaCl·2H2O is formed from the freezing of NaCl brines, (ii) freezing of KCl solutions does not form KCl hydrates, (iii) freezing of MgCl2 solutions forms a stable hexahydrate, and (iv) freezing of CaCl2 solutions forms a hexahydrate. Dehydration of the salts was observed as temperatures were increased, leading to a succession of hydration states in the case of CaCl2 (hexahydrate → tetrahydrate → dihydrate). Similarly, UV irradiation is also found to induce progressive loss of water from the hydration shell of NaCl, CaCl2, and MgCl2 hydrates. KEYWORDS: Raman spectroscopy, infrared spectroscopy, surface composition, irradiation, Europa

1. INTRODUCTION Europa is one of the primary targets of exploration in our Solar System because of its astrobiological potential. Compelling evidence exists for the presence of a salty global ocean between Europa’s icy surface and its rocky interior. The Galileo spacecraft’s magnetometer measurements have indicated a distortion of Jupiter’s magnetic field around Europa that is consistent with a subsurface salt water layer.1,2 This subsurface liquid ocean is hypothesized to exist as a result of internal tidal heating, resulting from the strong gravitational forces experienced by Europa as it orbits Jupiter.3,4 The surface of Europa is relatively young and is covered with features that have been interpreted as being indicative of resurfacing from the ocean below.5,6 Hydrated salt minerals were reported to exist on the surface associated with these younger regions, possibly from the ocean.7,8 Furthermore, Hubble Space Telescope observations of excess hydrogen Lyman-α and OI (130.4 nm) emissions above the southern hemisphere9 as well as far-ultraviolet (UV) absorptions at similar and more equatorial latitudes10 are consistent with transient ∼200 km high water vapor plumes. Currently, the National Aeronautics and Space Administration (NASA) is planning a mission to Europa to further assess its habitability with either a multiple-flyby spacecraft and/or lander.11,12 However, it is unlikely that the subsurface ocean will be sampled directly in the foreseeable future. Thus, our best opportunity for understanding the composition of Europa’s subsurface ocean is by inference from the composition of © XXXX American Chemical Society

materials on the surface of Europa’s icy crust. Determination of the composition of Europa’s proposed subsurface ocean will be critical to assess its potential habitability. The suggested composition of the Europa surface hydrated salt minerals includes chloride salts, which are of interest because chlorine is abundant throughout the Solar System and has been suggested to be present on Europa.13−15 In addition, the surface of Europa is marked with brown−yellow lines, which have been suggested to indicate the presence of sodium chloride.16 This sodium chloride could originate from a chloride-rich silicate seafloor, which enriches the liquid ocean with sodium, chloride, and other ions. Furthermore, in addition to the internal tidal heating from Jupiter, chloride salts may contribute to the stability of liquid water on Europa, because they, like other salts, depress the freezing point. If Europan ocean fluids are expressed on the surface, one might expect hydrated salts to form as the ocean brine freezes. Unfortunately, much of the information we have about Europa’s surface composition comes from near-infrared (NIR) reflectance spectra, which suffer from two major drawbacks: (i) NIR features of salt hydration states can be broad and weak, although these spectral features gain higher definition at low temperatures, and (ii) there is a paucity of Received: Revised: Accepted: Published: A

October 26, 2016 January 12, 2017 January 17, 2017 January 17, 2017 DOI: 10.1021/acsearthspacechem.6b00003 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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2.3. Raman Spectra. Raman spectra were acquired with a Horiba Jobin-Yvon LabRam HR dispersive confocal Raman microscope, using a 50 mW frequency doubled Nd:YAG laser for excitation (532 nm). Spectra were obtained at a resolution of 1 cm−1 with a 600 groove/mm grating. The sharp Si feature at 520.7 cm−1 was used for frequency calibration. In the Raman experiments, samples were held at the desired temperature under vacuum using an Oxford Instruments MicrostatN2 cryostage. This system has a base pressure of ∼3 × 10−5 Torr. Raman spectra were recorded over a temperature range from 80 to 233 K, as were the NIR reflectance spectra. The samples were cycled thermally 2−3 times each, in the same manner as the IR spectra to maintain consistency. However, during the first cycle, data were only recorded at 80, 100, and 233 K rather than in 20 K increments as was performed in the NIR measurements. Raman spectra were obtained over a range from 50 to 4000 cm−1. 2.4. Vacuum UV Irradiation. Sodium chloride, calcium chloride, and magnesium chloride samples were irradiated with an Opthos electrodeless krypton resonance lamp while in the sample vacuum chambers of the NIR reflectance and Raman apparatuses described above. The lamps were typically operated with an input microwave power of 100 W. In both cases, MgF2 windows were installed in the sample chambers to ensure that UV radiation reached the samples. In the case of the NIR measurements, the volume through which the UV photons traveled prior to the MgF2 window of the sample chamber was purged with N2 to minimize absorption. In the case of the Raman experiments, the lamp was placed directly against the MgF2 window with a plastic “skirt” fitted around the lamp− window interface to facilitate an Ar purge of the volume between the lamp and window. The krypton lamp emits primarily at 116.5 and 123.6 nm.19 However, the sharp cutoff in transmittance of the MgF2 window at ∼120 nm will block essentially all of the 116.5 nm photons from reaching the sample. The choice of purge gas was an experimental convenience. Ar is often used for this purpose. However, the Fourier transform infrared (FTIR) apparatus uses the same gas source to purge both the FTIR and the diffuse reflectance accessory volumes. Ar is a good insulator and, if used to purge the FTIR, can cause the IR source to overheat and also prevents the laser from cooling properly. Therefore, N2 was used as per the recommendation of the manufacturer in the NIR experiments.

spectral data of some candidate materials under relevant conditions.17,18 The purpose of this research is dual: (1) to bridge the information gap between accessible observational data, available both now and in the future, with spectra of chloride salts in experimentally verified hydration states that may be present on the surface of Europa and (2) to investigate the evolution of these salts once exposed to the surface conditions of Europa (high vacuum and particle or UV irradiation) as a function of time. We have obtained diffuse NIR reflectance spectra of chloride salts as well as their associated Raman spectra. The NIR and Raman spectra of identically prepared samples were measured under conditions simulating those experienced on Europa as putative ocean fluids are expressed onto the surface. With this approach, the Raman data can be used to definitively identify the hydration state of a given salt and its corresponding NIR reflectance spectrum. Furthermore, spectra were measured after samples were thermally cycled (80−233 K) or irradiated to simulate the loss of water that these salts may experience on the surface of Europa. Although the surface temperature of Europa is not expected to rise above 113 K, the samples in these experiments were warmed to 233 K to expedite the loss of hydration in salt minerals expected to occur on Europa’s surface over a longer time scale. As a consequence, the data presented in this paper can be compared to observational data to determine the hydration states of chloride salts on the surface of Europa.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Saturated solutions of sodium chloride, potassium chloride, magnesium chloride, and calcium chloride were made at room temperature (295 K) from NaCl (Mallinckrodt, ACS grade), KCl (J.T.Baker, ACS grade), MgCl2·6H2O (J.T. Baker, USP), and CaCl2·6H2O (Fischer, ACS grade) powders. Each solution was flash-frozen by pipetting the supernatant, dripping into liquid nitrogen, followed by grinding of the resultant ice with a mortar and pestle (precooled under liquid nitrogen) until a fine powder was obtained. Each powdered sample was quickly transferred to a precooled gold or copper sample cup held at ∼100 K in a vacuum chamber integrated within either an infrared (IR) reflectance accessory or a microscopy stage placed under the Raman spectrometer. Once the sample was deposited, the chamber was evacuated and the temperature was lowered to 80 K. The temperature was allowed to stabilize for a few minutes before NIR reflectance or Raman spectra were recorded. 2.2. Diffuse NIR Reflectance Spectra. Diffuse NIR reflectance spectra were acquired with a Thermo Nicolet 6700 FTIR equipped with a PikeTech DiffusIR reflectance accessory (30° nominal angle of incidence) and a lowtemperature, evacuable sample chamber. The base pressure in the sample chamber typically ranged from 2 × 10−6 to 4 × 10−4 Torr depending upon the temperature. Spectra were referenced to a 600 grit gold diffuse reflectance standard (ThorLabs) and taken with a resolution of 2 cm−1. Each spectrum is the result of 1000 co-added scans. Each sample was cycled thermally at least twice between 80 and 233 K. For the first cycle, data were taken in 20 K intervals from 80 to 233 K. For the second and third cycles (where applicable), spectra were only recorded at the extreme temperatures, i.e., 80 and 233 K. Third thermal cycles were executed if the sample spectra continued to change dramatically throughout the first two temperature cycles and if it appeared that the samples continued to lose water.

3. RESULTS 3.1. Freezing and Thermal Cycling of Powdered Frozen Brines. 3.1.1. Sodium Chloride Thermal Cycling. Figure 1 shows Raman spectra of frozen powdered saturated NaCl solutions. The first spectrum taken at 80 K is very similar to that of pure water ice,20 which has diagnostic peaks at 3112, 3231, and 3336 cm−1. Upon heating the sample to 233 K and recooling to 80 K, distinct peaks are observed at 3403, 3420, 3434, and 3535 cm−1, which are diagnostic of NaCl·2H2O.21 In addition, the water ice features are significantly reduced. Figure 2 shows NIR spectra of frozen powdered saturated NaCl solution at 80 K before (blue trace) and after (red trace) temperature cycling. Additionally, an intermediate spectrum at 153 K (taken upon warming) is included, where three sharp peaks characteristic of NaCl·2H2O are visible at 5167, 5123, and 5040 cm−1. Hanley et al.14 see similar features in their NaCl spectrum at 243 K. In repeated experiments, the exact temperature where these peaks emerged varied slightly. We B

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Because the sample solution is saturated in NaCl, freezing of the brine below the eutectic results in the formation of water ice, probably nucleated on top of/around the NaCl·2H2O crystallites. Upon warming under vacuum, this water ice sublimates and exposes NaCl·2H2O that formed first. Further heating results in the dehydration of NaCl·2H2O, leading to the formation of anhydrous NaCl. Because anhydrous NaCl does not exhibit any spectral features in the NIR, the spectrum becomes flat and indistinct. Sodium chloride is not known to form stable hydrates other than NaCl·2H2O under these conditions. 3.1.2. Potassium Chloride Thermal Cycling. The Raman spectra of powdered frozen saturated KCl solution (Figure 3)

Figure 1. Raman spectra of powdered saturated NaCl brine at 80 K. The blue spectrum was taken prior to thermal cycling, while the red spectrum was taken after warming the sample to 233 K and cooling back to 80 K. The spectra show the loss of the diagnostic water bands at 3112, 3231, and 3336 cm−1 (blue) and the emergence of NaCl· 2H2O features between 3400 and 3600 cm−1 upon thermal cycling (red).

Figure 3. Raman spectra of powdered frozen saturated KCl brine at 80 K (black), after warming to 233 K (red), and after cooling back to 80 K (blue).

appeared effectively identical to that of pure water ice, with peaks at 3108, 3225, and 3334 cm−1 initially observed when the sample was flash frozen at ∼80 K (black trace). Although small peak shifts are observed upon thermal cycling, these only show the influence of the temperature on peak positions and are not indicative of new hydration states. In addition, hydrated salt peaks tend to be sharp. Thus, we conclude that no hydration state of potassium chloride was observed upon freezing or temperature cycling. Additionally, the subsequent spectra at 233 K (red) and 80 K (blue) only show residual water peaks at 3127 and 3086 cm−1, respectively. In Figure 4, the NIR spectra of KCl at 80 K has been compared to the NIR spectra of pure water at 80 K, to show the similarities between these spectra. Because anhydrous KCl is IR-inactive and hydrated minerals are expected to be active, this further supports the conclusion that KCl solutions do not form any hydrate upon freezing and temperature cycling. Hanley et al.14 see a similar water-like spectrum of KCl at 243 K. 3.1.3. Calcium Chloride Thermal Cycling. The Raman spectra of powdered frozen saturated CaCl2 solution (Figure 5) exhibit changes in the hydration state upon temperature cycling. The initial spectrum at 80 K shows peaks at 3407 and 3430 cm−1, indicating the presence of the hexahydrate, CaCl2· 6H2O.21 When heated to 233 K, it appears that α-CaCl2·4H2O forms, with peaks at 3452 and 3486 cm−1. Upon cooling back

Figure 2. NIR reflectance spectra of powdered saturated NaCl brine at 80 K. The blue spectrum was taken prior to thermal cycling, while the red spectrum was taken after warming the sample to 233 K and cooling back to 80 K. The black spectrum was taken at 153 K during the warming phase of the temperature cycle. Note the loss of water around 6500 cm−1 (1.5 μm) as the temperature cycle proceeded.

hypothesize that this could be dependent upon variables that are not easily characterized, such as the freezing rate and/or grain size. However, regardless of the exact point of emergence, these peaks are likely to be observed at Europa-relevant conditions. After the sample was heated to 233 K and cooled back to 80 K (shown in the red spectrum), the NaCl·2H2O spectral features became less distinct and the spectrum became flatter, consistent with the loss of water and the formation of anhydrous NaCl. C

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Figure 4. NIR spectra of powdered frozen saturated KCl brine. The red spectrum was taken prior to thermal cycling, while the black spectrum was taken after warming the sample to 233 K and cooling back to 80 K. A spectrum of powdered water ice at 80 K is included as a reference (blue).

Figure 6. NIR spectra of powdered frozen saturated CaCl2 brine. Spectra are shown before temperature cycling at 80 K (black), warmed to 233 K (purple), cooled to 80 K (blue), warmed again to 233 K (pink), and cooled back to 80 K (red). Spectra are offset for clarity.

solution at 80 K is shown in Figure 7. Prior to thermal cycling, prominent peaks were observed at 3344, 3394, and 3504 cm−1, which are indicative of the hexahydrate (MgCl2·6H2O).23 These peaks became more prominent and shifted on the order of a few wavenumbers as the sample warmed to 233 K and cooled back to 80 K. In the NIR spectra (Figure 8), the peaks near 6831, 5083, and 4034 cm−1 became more distinct after 1 thermal cycle. Additionally, a new peak appeared at 6402 cm−1. However, there was little change after the second thermal cycle, suggesting that the initial hydration state was very stable. A comparison to NIR spectra obtained by ref 14 of MgCl2·6H2O, MgCl2·4H2O, and MgCl2·2H2O shows that our spectrum corresponds precisely to the hexahydrate and is distinct from the other two states. This is supported by computational modeling by Callahan et al.,24 which shows that the first solvation shell of the magnesium cation coordinates with six water molecules at octahedral sites. The water molecules are held closest to the magnesium cation, a configuration so stable that even the chloride ion is held to the exterior of the hydration shell, making it very difficult to remove these water molecules from the solvation shell. This is consistent with the observations described here. 3.2. Irradiation of Powdered Frozen Brine Samples. In a series of separate experiments, each frozen chloride brine sample was also irradiated with UV to determine the stability of the hydrated chloride salts and their associated hydration states under these conditions. Because KCl was only observed in the anhydrous form, KCl samples were not irradiated. 3.2.1. Sodium Chloride Irradiation. A powder of frozen saturated sodium chloride brine was irradiated with the krypton UV lamp overnight for a total of ∼26 h at a constant temperature of 173 K (Figure 9). Despite this long period of irradiation, the peaks in the NIR spectra did not appear to change. Instead, the water region around 6500 cm−1 became less intense, suggesting a continual loss of water as the water ice sublimed from the sample. A similar experiment was conducted within the Raman microscopy stage, in which the sodium chloride brine sample

Figure 5. Raman spectrum of powdered frozen saturated CaCl2 brine. Spectra are shown before temperature cycling at 80 K (black), warmed to 233 K (purple), recooled to 80 K (blue), warmed again to 233 K (pink), and cooled back to 80 K (red).

down to 80 K, an additional peak at 3478 cm−1 emerges, indicating a further change of the hydration state to CaCl2· 2H2O,22 along with features indicative of the hexahydrate. When heated back to 233 K, the features broaden and become less intense but the peak positions remain essentially the same. These peaks persist in the spectra taken after cooling again to 80 K. This indicates that, after sublimation of water, CaCl2 is capable of forming two additional stable hydration states, the tetrahydrate and the dihydrate, which occur alongside the hexahydrate. The companion IR spectra (Figure 6) also demonstrate these changes in hydration state with each thermal cycle. The detailed peak positions are tabulated in Table 1. 3.1.4. Magnesium Chloride Thermal Cycling. The Raman signature of the powdered frozen saturated magnesium chloride D

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Table 1. Positions of Diagnostic Raman Spectral Features Used for Mineral Identification and Their Corresponding NIR Peaksa aqueous sample NaCl

hydrohalite (NaCl·2H2O)

KCl

sylvite (KCl)

CaCl2

antarcticite (CaCl2·6H2O)

MgCl2

a

observed mineral

Raman features (cm−1)

thermal history 80 K heat to 153 K cool to 80 K

not measured 3403, 3420, 3434, 3535

80 K heat to 233 K cool to 80 K 80 K

NIR features (cm−1) 5121, 5040, 3950 5167, 5123, 5041, 3948 5166, 5128, 5040, 3947 none

3407, 3430

α-tetrahydrate (α-CaCl2·4H2O) antarcticite (CaCl2·6H2O) sinjarite (CaCl2·2H2O) antarcticite (CaCl2·6H2O) sinjarite (CaCl2·2H2O) antarcticite (CaCl2·6H2O) sinjarite (CaCl2·2H2O)

heat to 233 K cool to 80 K

bischofite (MgCl2·6H2O)

80 K heat to 233 K and cool back to 80 K heat to 233 K and cool back to 80 K

3452, 3407, 3478 3407, 3483 3407, 3478

heat to 233 K cool to 80 K

3486 3430 3430 3430

6851, 6778, 6556, 6111, 5733, 5589, 5490, 5048, 4824 4783 6556, 5490, 5090, 5049 6945, 6726, 6637, 6490, 5607, 5220, 4779 broad, indistinct peaks

3344, 3394, 3504 3349, 3392, 3512

6851, 6778, 6556, 8447, 8130, 7301, 5478, 4819 6824, 5482, 5083, 6831, 6400, 6228,

5048 7156, 6490, 6044, 5746, 5563,

not measured

6831, 6400, 6228, 6110, 5482, 5083, 4514, 4034

4532, 4034 6110, 5482, 5083, 4514, 4034

The lack of Raman data indicates that the spectrum resembles that of water ice.

Figure 7. Raman spectrum of powdered frozen saturated MgCl2 brine at 80 K. Spectra are shown prior to thermal cycling (blue) and after warming to 233 K and cooling back to 80 K (red). The blue spectrum is scaled by a factor of 4 to enhance the spectral features, demonstrating similarity to the red spectrum.

Figure 8. NIR spectrum of powdered frozen saturated MgCl2 brine at 80 K. Spectra are shown prior to thermal cycling (red), after warming to 233 K and cooling back to 80 K (blue), and after a second thermal cycle to 233 K and back to 80 K (black). Note that the latter two spectra are identical.

was irradiated for 5 h. A spectrum was taken of the sample at 153 K before irradiation (Figure 10), which clearly showed the presence of NaCl·2H2O, with distinctive peaks at 3421 and 3538 cm−1.21 After 5 h of irradiation, a portion of the originally white sample had become a discolored yellow (inset of Figure 10), which is consistent with the formation of color centers in NaCl.25 Note that some regions were clearly discolored, while others were not, indicating non-uniform irradiation of the sample. Raman spectra of the non-discolored white regions revealed that NaCl·2H2O was still present in these locations (purple trace). However, the NaCl·2H2O features were more

sharply defined than in the original pre-irradiation spectra. This is likely due to water ice sublimating from the sample under vacuum during irradiation, muting the water features of the spectrum. In contrast, the Raman spectrum of the discolored− yellow area (red trace) reveals that NaCl·2H2O lost its waters of hydration to form anhydrous NaCl, consistent with the featureless NIR spectrum. The irradiation results additionally confirm that NaCl·2H2O is the only stable hydration state available to sodium chloride under these experimental conditions. E

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the FTIR diffuse reflectance stage at 173 and 153 K. At 173 K, the CaCl2 hydrate changed from CaCl2·6H2O to α-CaCl2· 4H2O after 1 h of irradiation (Figure 11). At 153 K, the

Figure 9. NIR spectra of powdered frozen NaCl brine at 173 K. The blue spectrum was taken prior to UV irradiation, while the red spectrum was taken after 26 h of UV irradiation. Note the loss of water around 6500 cm−1. Figure 11. NIR spectrum of irradiated powdered frozen saturated CaCl2 brine at 173 K. Pre-irradiation spectra are shown at 80 K (black) and after warming to 173 K (blue). Spectra taken at 173 K after 1 h (purple) and 5.5 h (red) of irradiation are also shown. Spectra are offset for clarity.

hydration state of the sample changed under irradiation (Figure 12) but at a much slower rate than at 173 K, as expected. Here, we focus on the evolution of the peaks at 5051 and 5090 cm−1, which are diagnostic of CaCl2·6H2O and α-CaCl2·4H2O, respectively. Before irradiation, the predominant hydration state of the sample was CaCl2·6H2O, shown by the peak at 5051 cm−1. After 2 h of irradiation, a second peak emerged at 5090 cm−1, indicating that some of the sample changed

Figure 10. Raman spectra of powdered frozen saturated NaCl brine at 153 K. The inset shows a photograph of the sample after 5 h of irradiation with a Kr lamp. The blue spectrum was taken prior to irradiation, while the purple and red spectra were taken postirradiation of the regions, having unmodified color (whitish) and newly formed color (yellow) centers seen in the inset.

The NIR experiments do not show significant dehydration of NaCl·2H2O to anhydrous NaCl upon irradiation, unlike the Raman experiments. This may be due to the different sampling depths of the two techniques. NIR spectra typically sample the top few hundred micrometers of a powdered, mostly nonabsorbing sample. The confocal Raman microscope used in these experiments will only probe the top few micrometers of the sample. Irradiation of NaCl·2H2O will naturally dehydrate the top of the sample first. This is observable with the shallow sampling depth of the Raman experiment, while in the NIR experiments, the thin IR-inactive NaCl layer is invisible and the spectra are representative of NaCl·2H2O underneath. 3.2.2. Calcium Chloride Irradiation. Frozen calcium chloride brines were irradiated with the krypton UV lamp in

Figure 12. NIR spectrum of powdered frozen saturated CaCl2 brine at 153 K. Spectra are shown prior to irradiation (black) and after 2 h (blue), 4 h (purple), and 5 h (red) of UV irradiation. Spectra are offset for clarity. F

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ACS Earth and Space Chemistry hydration state to α-CaCl2·4H2O. After 5 h, the peak at 5090 cm−1 was the dominant peak, indicating that most of the sample had changed hydration state to the tetrahydrate. This experiment was repeated in the Raman setup (Figure 13), where the sample was irradiated for 4 h at 153 K. The

Figure 14. Raman spectra of powdered frozen saturated MgCl2 brines. The black and red traces show the spectra of the powder at 165 K before and after 5 h of irradiation with a Kr lamp, respectively. The blue and green traces show control spectra of MgCl2·4H2O taken at 373 K and MgCl2·2H2O taken at 396 K, respectively. Figure 13. Raman spectra of powdered frozen saturated CaCl2 brine at 153 K. Spectra are shown prior to irradiation (black) and after 4 h of irradiation (blue).

form. NIR spectra of the corresponding phases can be found in the study by Hanley et al.14

4. DISCUSSION Figure 15 presents a schematic flowchart of the experiments reported here and their results. Thermal cycling experiments examined the stability of the hydration states of various chloride salts. The sodium chloride spectra in Figures 1 and 2 indicate that NaCl·2H2O is the only hydrate formed by the freezing of NaCl brines under Europa-like conditions. The potassium chloride spectra (Figures 3 and 4) indicate that no hydrate of the salt forms by freezing of the saturated KCl brine under any of the conditions found on Europa. The calcium and magnesium chloride results (Figures 5−8), however, did indicate the presence of hydration states with up to six water molecules. The calcium chloride experiments showed stable hydration states of CaCl2·6H2O, α-CaCl2·4H2O, and CaCl2· 2H2O. Magnesium chloride, on the other hand, only exhibited stable MgCl2·6H2O. Both potassium and sodium are group 1 cations that only have a formal 1+ charge, whereas magnesium and calcium have a 2+ charge. The group 2 salts are capable of holding a larger solvation shell than the group 1 salts as a result of their smaller size and higher charge densities. McCord et al.27 performed a similar investigation of frozen sulfate and carbonate solutions. Here, frozen brine samples were prepared by depositing prepared solutions of Na2SO4, MgSO4, and Na2CO3 on a cooled sample manipulator (∼150− 220 K) in a vacuum chamber under dry nitrogen. Once deposited, the chamber was evacuated and the sample temperature was quickly brought down to ∼110 K. Samples were then thermally cycled up to ∼200−235 K to eliminate water not bound as hydrates and then to simulate loss of waters of hydration that may occur as a result of radiolytic processing on Europa. After each thermal cycle, NIR reflectance spectra were recorded with the samples returned to Europa-like temperatures (110−130 K). They observed NIR spectra similar to those of the Galileo near-infrared mapping spectrometer (NIMS) from non-ice regions of Europa, which are distinct

spectrum of the initial sample shows peaks at 3427 and 3410 cm−1, indicating the predominance of CaCl2·6H2O. After 4 h of irradiation, the spectrum exhibits peaks at 3445 and 3478 cm−1, indicating that the majority of the sample had changed its hydration state to the tetrahydrate.21 The NIR and Raman spectra are thus consistent and confirm a change in hydration state as a result of irradiation at 153 K. Of note, an additional NIR experiment was carried out at 133 K, where no evolution of the hydration state was observed after a total of 21 h of irradiation. In a control experiment, the calcium chloride sample was held under vacuum at 173 K for 4 h without irradiating. The spectra grew flatter over time, but the spectral peaks did not shift, confirming that any changes in the calcium chloride spectra under irradiation were predominantly as a result of irradiation and not slow dehydration. The observed flattening may be due to recrystallization or sintering of the sample over time. 3.2.3. Magnesium Chloride Irradiation. Frozen saturated MgCl2 brines were irradiated with the Kr lamp for 5 h at 165 K. Figure 14 shows Raman spectra of the powder before and after irradiation. Prior to irradiation, three prominent features are observed, similar to those seen in Figure 7, that indicate the presence of the hexahydrate form. After irradiation, the spectrum exhibits a drastic change, showing a dominant feature at 3437 cm−1. Control spectra of lower hydration states, namely, the tetra- and dihydrates, were acquired for comparison. Following the procedure of Gurevich et al.,26 the tetrahydrate sample was prepared by heating the commercial hexahydrate powder at 373 K for 2 h, while the dihydrate was obtained upon further heating at 396 K for an additional 4 h. A comparison of these control spectra indicates that the irradiation converted the frozen powder to the tetrahydrate G

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Figure 15. Schematic flowchart summarizing the thermal cycling (T cycle) and UV irradiation (UV) experiments and their results.

Additionally, if MgCl2·6H2O is detected in addition to MgSO4 on the trailing hemisphere of Europa, our findings potentially support Brown and Hand’s hypothesis that MgSO4·7H2O is an irradiation product present on the trailing hemisphere of Europa. However, it has also been suggested that MgSO4 hydrates and other minerals could be hidden from spectral detections on the (older) leading surface by sputtered and redeposited ice frost.28 In other work related to hydrated salts on Europa, Brown and Hand29 suggested that the 2.07 μm absorption seen in ground-based observations of Europa’s trailing hemisphere obtained at the Keck observatory is indicative of the presence of MgSO4·7H2O on the surface. Given that this feature was not seen on the less irradiated leading hemisphere, they concluded that it was an irradiation product. They proposed a radiolytic mechanism for the formation of MgSO4·7H2O on the surface, whereby MgCl 2 is transformed into MgSO 4 ·7H 2O by implantation of sulfur ions originating from Io. Vu et al.30 showed that a predicted Europa ocean brine would preferentially form MgCl2 over MgSO4 hydrates upon freezing. The waters of hydration could serve as a source of oxygen in the conversion of MgCl2 into MgSO4 via sulfur implantation. This provides several species in addition to MgCl2 that could be involved in the proposed radiolytic mechanism. However, further studies of this mechanism are necessary to determine the effect of ion concentrations and irradiation dose on the rate of this process and could be used to date the age of an observed surface.

from crystalline materials. This led to the conclusion that the non-ice material on Europa’s surface contains disordered and heavily hydrated MgSO4 and Na2SO4 that is endogenic in origin. McCord et al.27 also found that sublimation (and radiolysis) removes water from hydrated salts, leading to distinctive spectra. Our observations are consistent with this result. Recently, Hand and Carlson16 have suggested that radiationinduced color centers in NaCl expressed on the surface of Europa from the ocean below can provide an explanation for the color of the non-ice material on Europa. Irradiation of alkali halides is known to result in trapping of electrons in halogen vacancies, leading to the formation of characteristic color centers.25 In their experiments, NaCl was cooled to Europa surface temperatures (100 K) under vacuum. Experiments were conducted with NaCl introduced under three conditions: as 300 μm grains, as 300 μm grains with water ice deposited on top, and as a frozen saturated NaCl solution. In all cases, upon irradiation with 10 keV electrons, color centers formed, changing the samples from white to a yellow−brown color, which closely resembles the color seen in the non-ice material of Europa’s surface. However, our experimental findings suggest that an aqueous NaCl brine kinetically frozen will form NaCl· 2H2O and not anhydrous NaCl. The current work does find, though, that irradiation will transform NaCl·2H2O into anhydrous NaCl, which upon further irradiation will form the color centers observed by Hand and Carlson. Our results suggest that the distribution of the yellow−brown diffuse component on Europa’s surface, which may likely be irradiated anhydrous NaCl, in comparison to detected NaCl·2H2O, should coincide with radiation dose mapping of Europa’s surface. The presence of sodium chloride may be directly and unambiguously identified on Europa if the characteristic NIR NaCl·2H2O features (see Table 1) are observed. If such features are found, they would indicate a relatively young terrain, because irradiation will rapidly dehydrate NaCl·2H2O to anhydrous NaCl. Further research is needed to determine the rate at which NaCl forms color centers upon irradiation and the rate at which they are destroyed by visible light. These rates may then be used to estimate the age of the discolored surface.

5. CONCLUSION We surveyed the hydration states of frozen aqueous solutions of NaCl, KCl, CaCl2, and MgCl2 over a range of temperatures simulating the exposure of these brines on Europa’s surface, using a combination of Raman and NIR spectroscopies. Our findings indicate that (i) NaCl·2H2O is formed from the freezing of NaCl brines, (ii) KCl brines do not form any observable hydrate, (iii) MgCl2 solutions form a stable hexahydrate, and (iv) CaCl2 solutions first form a hexahydrate, which dehydrates when exposed to vacuum and forms a tetrahydrate and a subsequent stable dihydrate. H

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D.; Hibbitts, C. A.; Granahan, J. C.; Ocampo, A. Hydrated salt minerals on Europa’s surface from the Galileo near-infrared mapping spectrometer (NIMS) investigation. J. Geophys. Res.: Planets 1999, 104 (E5), 11827−11851. (9) Roth, L.; Saur, J.; Retherford, K. D.; Strobel, D. F.; Feldman, P. D.; McGrath, M. A.; Nimmo, F. Transient Water Vapor at Europa’s South Pole. Science 2014, 343 (6167), 171−174. (10) Sparks, W. B.; Hand, K. P.; McGrath, M. A.; Bergeron, E.; Cracraft, M.; Deustua, S. E. Probing for Evidence of Plumes on Europa with HST/STIS. Astrophys. J. 2016, 829 (2), 121. (11) Pappalardo, R. T.; Vance, S.; Bagenal, F.; Bills, B. G.; Blaney, D. L.; Blankenship, D. D.; Brinckerhoff, W. B.; Connerney, J. E. P.; Hand, K. P.; Hoehler, T. M.; Leisner, J. S.; Kurth, W. S.; McGrath, M. A.; Mellon, M. T.; Moore, J. M.; Patterson, G. W.; Prockter, L. M.; Senske, D. A.; Schmidt, B. E.; Shock, E. L.; Smith, D. E.; Soderlund, K. M. Science Potential from a Europa Lander. Astrobiology 2013, 13 (8), 740−773. (12) Phillips, C. B.; Pappalardo, R. T. Europa Clipper Mission Concept: Exploring Jupiter’s Ocean Moon. Eos Trans. AGU 2014, 95 (20), 165−167. (13) Kargel, J. S.; Kaye, J. Z.; Head, J. W.; Marion, G. M.; Sassen, R.; Crowley, J. K.; Prieto Ballesteros, O.; Grant, S. A.; Hogenboom, D. L. Europa’s crust and ocean: Origin, composition, and the prospects for life. Icarus 2000, 148 (1), 226−265. (14) Hanley, J.; Dalton, J. B.; Chevrier, V. F.; Jamieson, C. S.; Barrows, R. S. Reflectance spectra of hydrated chlorine salts: The effect of temperature with implications for Europa. J. Geophys Res-Planet 2014, 119 (11), 2370−2377. (15) Ligier, N.; Poulet, F.; Carter, J.; Brunetto, R.; Gourgeot, F. VLT/SINFONI Observations of Europa: New Insights into the Surface Composition. Astron. J. 2016, 151 (6), 163. (16) Hand, K. P.; Carlson, R. W. Europa’s surface color suggests an ocean rich with sodium chloride. Geophys. Res. Lett. 2015, 42 (9), 3174−3178. (17) Dalton, J. B. Linear mixture modeling of Europa’s non-ice material based on cryogenic laboratory spectroscopy. Geophys. Res. Lett. 2007, 34 (21), L21205. (18) Shirley, J. H.; Dalton, J. B.; Prockter, L. M.; Kamp, L. W. Europa’s ridged plains and smooth low albedo plains: Distinctive compositions and compositional gradients at the leading side-trailing side boundary. Icarus 2010, 210 (1), 358−384. (19) Okabe, H. Intense Resonance Line Sources for Photochemical Work in Vacuum Ultraviolet Region. J. Opt. Soc. Am. 1964, 54 (4), 478−481. (20) Clark, R. N. Water Frost and Ice: The near-Infrared Spectral Reflectance 0.65−2.5 μm. J. Geophys Res. 1981, 86 (Nb4), 3087−3096. (21) Baumgartner, M.; Bakker, R. J. Raman spectra of ice and salt hydrates in synthetic fluid inclusions. Chem. Geol. 2010, 275 (1−2), 58−66. (22) Uriarte, L. M.; Dubessy, J.; Boulet, P.; Baonza, V. G.; Bihannic, I.; Robert, P. Reference Raman spectra of synthesized CaCl2·nH2O solids (n = 0, 2, 4, 6). J. Raman Spectrosc. 2015, 46 (10), 822−828. (23) Hibben, J. H. The Raman spectra of water, aqueous solutions and ice. J. Chem. Phys. 1937, 5 (3), 166−172. (24) Callahan, K. M.; Casillas-Ituarte, N. N.; Roeselova, M.; Allen, H. C.; Tobias, D. J. Solvation of Magnesium Dication: Molecular Dynamics Simulation and Vibrational Spectroscopic Study of Magnesium Chloride in Aqueous Solutions. J. Phys. Chem. A 2010, 114 (15), 5141−5148. (25) Seitz, F. Color Centers in Alkali Halide Crystals. Rev. Mod. Phys. 1946, 18 (3), 384−408. (26) Gurevich, L. M.; Rommel, I. F.; Polyakov, Y. A. Infrared Spectra and Characteristic Structural Features of Hydrates of Magnesium Chloride. J. Struct. Chem. 1978, 18 (5), 683−687. (27) McCord, T. B.; Teeter, G.; Hansen, G. B.; Sieger, M. T.; Orlando, T. M. Brines exposed to Europa surface conditions. J. Geophys. Res.: Planets 2002, 107 (E1), E15004.

Additionally, vacuum UV irradiation resulted in changes in the hydration states of NaCl, CaCl2, and MgCl2. In particular, NaCl·2H2O is converted into anhydrous NaCl upon irradiation, which is then observed to form color centers, as previously observed by Hand and Carlson.16 Both CaCl2 and MgCl2 are shown to change hydration states from the hexahydrate to the tetrahydrate upon irradiation. Generally, our results can be used to determine the hydration state of salts on the surface of Europa using IR spectroscopic data. These data could be especially useful in observations of a salt such as calcium chloride, which changed most dramatically with each thermal cycle. For example, the thermal history of a surface could be determined by observing that calcium chloride is present as the hexahydrate. This could indicate recent plume activity near the surface, if emplaced CaCl2·6H2O has not been on Europa’s surface long enough to change its hydration state as a result of warmer surface temperatures or irradiation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paul V. Johnson: 0000-0002-0186-8456 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank T. B. McCord and two anonymous reviewers for their comments, which substantially improved this paper. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). The authors gratefully acknowledge funding from the NASA Astrobiology Institute (Icy Worlds). Government sponsorship is acknowledged.

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REFERENCES

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DOI: 10.1021/acsearthspacechem.6b00003 ACS Earth Space Chem. XXXX, XXX, XXX−XXX