Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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Ultraviolet Spectroscopy and Photochemistry of SO2/H2O Ices Robert Hodyss,†,‡ Paul V. Johnson,*,†,‡ Stephen M. Meckler,†,§ and Edith C. Fayolle† †
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Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109-8099, United States ‡ NASA Astrobiology Institute ABSTRACT: The presence of sulfur dioxide on the trailing hemisphere of Europa’s surface is well-established, and both its presence and its chemistry are influenced by sulfur ion implantation. Particle irradiation is known to be a significant radiolysis source, particularly on the trailing surface, which is subject to particles in Jupiter’s plasma torus. Photochemistry driven by solar ultraviolet (UV) photons is also significant. To date, most studies have investigated the effects of vacuum UV radiation, while photochemistry at longer wavelengths is less wellstudied. This work investigates chemical changes in thin, cryogenic films of SO2/H2O at temperatures and pressures relevant to Europa’s surface when subjected to temperature changes and UV radiation at 147, 206, 254, and 284 nm. Spectra were collected in both the mid-infrared range and the UV range, which elucidates electronic transitions that are less diagnostic but perhaps more applicable to reflectance spectra of solar system bodies. These experiments show irreversible red shifting of the B̃ ← X̃ absorption peak upon heating likely as a result of crystallization and/or thermal chemical reactions. Further, photons with wavelengths up to 284 nm are shown to induce significant chemistry in SO2/H2O mixtures. This in conjunction with the increased solar flux compared to more energetic wavelengths and the strong C̃ ← X̃ and B̃ ← X̃ absorption bands in SO2 suggests that far-UV radiation plays a significant role in the sulfur cycle on Europa. KEYWORDS: far ultraviolet, photochemistry, spectroscopy, sulfur dioxide, Europa
1. INTRODUCTION Sulfur dioxide was first detected on Europa in 1981 by the presence of a weak absorption centered at ∼280 nm in the ratio of trailing to leading hemisphere reflectance spectra taken by the International Ultraviolet Explorer.1 This absorption feature was identified as the à ← X̃ transition (since attributed mainly to the B̃ ← X̃ electronic transition, although other states are involved2,3), and SO2 was hypothesized to result from implantation of energetic sulfur ions into surface water ice. Subsequent detection4 and mapping5,6 of this feature supported an exogenic source of SO2, but local variations in band strength correlated with surface terrains also suggest the possibility of local sources. From the depth of the 280 nm absorption, Hand et al.7 estimated the concentration of SO2 to be ∼0.3%. The surface of Europa is bombarded by energetic charge particles, including sulfur ions (Sn+). The global average flux of sulfur ion incident on the surface of Europa is 9.0 × 106 cm−2 s−1 (global average energy flux of 3.0 × 109 keV cm−2 s−1).8 This has led to the hypothesis that sulfur dioxide on the surface of Europa results from implantation of high-energy sulfur ions accelerated by the Jovian magnetic field into water ice, which has been investigated experimentally. Strazzula et al.9−11 have demonstrated that implantation of 200 keV S+ ions into water ice produces H2SO4 with a high yield, but no SO2 production © XXXX American Chemical Society
was observed. However, sulfur on Europa may participate in a chemical cycle driven by radiolysis, cycling between H2SO4, SO2, H2S, and polymerized sulfur.12,13 Other experiments with 33 keV S+ ions implanted into water ice14 generated an absorption feature at ∼270 nm, assigned to a “S−O bond”, lending support for the role of sulfur ion implantation in the formation of SO2. Alternately, SO2 may result from radiolysis of frozen sulfatebearing brines from Europa’s subsurface ocean. In this hypothesis, frozen ocean fluids are emplaced on the surface via spreading, rifting, subduction, and cryovolcanism.15 Salts, such as various hydrated forms of MgSO4 and Na2SO4, have been shown to yield good spectral matches to Europa’s non-ice material.16−19 Sulfate may then be converted into sulfur dioxide through particle irradiation, as part of Europa’s sulfur cycle. This idea is supported by correlations between electron energy flux, sulfur ion flux, and sulfuric acid hydrate abundance.20,21 Previous experimental work on the irradiation of sulfur dioxide and sulfuric acid bearing ices has focused mainly on Received: Revised: Accepted: Published: A
February 12, 2019 March 6, 2019 March 7, 2019 March 7, 2019 DOI: 10.1021/acsearthspacechem.9b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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ACS Earth and Space Chemistry
The deposition rate was ∼0.3 μm/h. Where appropriate, the samples were left to equilibrate for 5 min at the temperature of interest before spectra were taken. The vacuum chamber is pumped by a 700 L/s turbomolecular pump (Varian 700 HT) and routinely reaches a base pressure less than 5 × 10−10 Torr. Gas samples are introduced into the vacuum chamber through a separate turbopumped line with a base pressure of at least 1 × 10−6 Torr. The flow of gas into the chamber is regulated with a precision leak valve. The gas enters the chamber through a 1/16 in. diameter stainless-steel tube, whose tip is approximately 5 cm from the deposition window. This results in the gas impinging on the window in the form of a somewhat collimated jet. H2O and SO2 were premixed manometrically into 1 L glass bulbs prior to deposition. IR spectra were recorded in transmission with a Nicolet 6700 FTIR spectrometer, at a resolution of 2 cm−1 full width at half maximum (FWHM). Each spectrum is the result of 100− 2000 co-added single scan spectra. UV spectra were recorded with an Ocean Optics (USB-2000) fiber spectrometer, with scan times of 0.4−1.2 s and a resolution of 2.2 nm FWHM. Each UV spectrum is the result of 25−35 co-added single scan spectra. UV irradiation of the ice samples was performed with Xe, I2, and Hg resonance lamps at 147 nm (8.43 eV), 206 nm (6.02 eV), and 254 nm (4.88 eV), respectively (Opthos Instruments). Emission spectra of these lamps are given in Figure 2
proton irradiation.22−24 In these experiments, proton energies are high enough (0.8 MeV) to easily break the S−O bond, yielding a variety of sulfur oxyanions. Interestingly, lowtemperature thermal reactions between SO 2 and H2 O 2 (another molecule detected on Europa’s surface25) are also possible,26,27 yielding similar sulfur oxyanions at temperatures as low as 50 K. Most recently, Kanuchova et al.28 reported experiments investigating thermal processing and 30 keV He+ irradiation of H2O/SO2 water mixtures. Mixtures deposited at 16 K and warmed to 120 K showed the formation of HSO3− and S2O52− ions, while He+ irradiation produced SO2−, HSO4−, H3O+, and SO3 polymeric chains. Here, we present data on the temperature dependence of the far-ultraviolet (UV) absorption spectroscopy and photochemistry of sulfur dioxide/water ice films, focusing on the B̃ ← X̃ electronic transition feature, which peaks at ∼280 nm and stretches roughly from 240 to 338 nm.2,3 Although previous work on thermal processing of SO2/H2O ices has been reported, we are unaware of other work focusing on the UV region of the spectrum; other works having focused on the infrared (IR) region. The B̃ ← X̃ absorption band is shown to red shift irreversibly upon heating, suggesting that it may be possible to use the position of this band as a spectral thermometer. We also show that irradiation of sulfur dioxide/ water ice mixtures at 147, 206, 254, and 284 nm can lead to significant chemistry. Photochemistry of these solid-state ice films has not been previously reported in the literature at these long wavelengths. Given the greater brightness of the Sun at longer wavelengths compared to the vacuum UV, combined with the C̃ ← X̃ and B̃ ← X̃ absorption bands centered at ∼220 and ∼280 nm, respectively, UV radiation with wavelength values larger than 200 nm is likely the dominant driver of SO2 photochemistry on Europa.
2. EXPERIMENTAL SECTION The experimental apparatus has been described elsewhere,29,30 but the specific configuration used in these experiments is shown schematically in Figure 1. Ice films were deposited on a CaF2 window cooled by a closed cycle He refrigerator (ARS 202B). The temperature of the window was monitored with a Si diode thermometer affixed to the copper window frame. The thickness of the ice films was monitored during growth by laser interferometry and ranged from approximately 0.3 to 1 μm.
Figure 2. UV spectra of a 1:26 SO2/H2O ice film (∼1 μm thick) at various temperatures. Spectra were acquired at the temperatures indicated consecutively from the bottom up. Spectra have been shifted vertically for clarity.
by Johnson et al.,31 who measured their flux to be 1.09 × 1019, 8.68 × 1018, and 3.41 × 1018 photons m−2 s−1 using a calibrated Si photodiode (International Radiation Detectors, Inc.), respectively. Irradiation at 284 nm (4.37 eV) was performed using an Oriel Instruments high-pressure Xe arc lamp filtered with a 280 nm (nominal) bandpass filter (10 nm FWHM, Edmunds Optics). The incident flux was measured to be 9.9 × 1018 photons m−2 s−1 using the calibrated Si photodiode employed previously. A spectrum taken of the filtered Xe lamp output showed a Gaussian emission profile centered at approximately 284 nm with 10 nm FWHM. Of
Figure 1. Schematic of the experimental apparatus. B
DOI: 10.1021/acsearthspacechem.9b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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ACS Earth and Space Chemistry note, all of the UV sources emitted photons well below the ionization thresholds of H2O and SO2 (∼11.8 eV for solidphase H2O32 and ∼12.6 eV for gas-phase SO233).
3. RESULTS AND DISCUSSION 3.1. Thermal Chemistry. Figure 2 shows UV absorption spectra of a ∼ 1 μm thick 1:26 SO2/H2O ice film as a function of the temperature. Two absorption features are observed in all spectra: a broad band centered at approximately 280 nm and the edge of a feature rising to the lower wavelength limit of the spectrometer at 200 nm. As discussed earlier, the 280 nm band corresponds to the B̃ ← X̃ electronic transition, while the shorter wavelength feature is assigned mainly to the C̃ ←X̃ transition. The B̃ ← X̃ absorption is observed on Europa and is the basis for the detection of SO2 on its surface.4,5,34,35 The peak position of the B̃ ← X̃ absorption shows a strong dependence upon the temperature, as shown in Figures 2 and 3. Figure 3 includes data from two experiments. In the first, the Figure 4. IR spectra of a 1:26 SO2/H2O ice film at various temperatures (the same ice sample as in Figure 2 at the same temperatures). Red dotted vertical lines indicate the positions of new features seen after thermal cycling. These are attributed to HSO3− and S2O52−, which were observed at 1034 and 1013 cm−1 and at 953 cm−1, respectively. Spectra were acquired at the temperatures indicated consecutively from the bottom up. Spectra have been shifted vertically for clarity.
the growth of these same new bands upon heating and are consistent with the work of Loeffler and Hudson. The shift in peak position of the B̃ ← X̃ absorption band that we observe may be due to the formation of HSO3− and S2O52−, which also possess absorptions in this region of the UV.36 Alternatively or perhaps in addition, the shifting of the B̃ ← ̃X absorption peak in the UV spectrum may be related to a transformation from amorphous to crystalline ice, which is an irreversible physical change that occurs upon warming. Corresponding IR measurements of the same ice film show sharpening of the prominent SO2 and H2O peaks, indicative of crystallization. The temperature dependence of the peak position of the B̃ ← X̃ absorption band in SO2/H2O ices suggests that it could be used as a spectral thermometer, although the irreversibility of the process means that the derived temperature will be the maximum temperature that the surface has experienced. Further, the peak position could be a tracer of the thermal chemistry, as suggested by Loeffler and Hudson.26 To date, UV spectra of Europa have not had the required signal-to-noise or spatial resolution to be used in this fashion. However, Europa’s surface temperature ranges from ∼50 to 125 K from the poles to the equator, which corresponds with the observed irreversible spectral changes in SO2/H2O mixtures. Having said that, it is possible that, in the presence of energetic ions or UV photons, the profile of the band could be further modified in analogy to the transition from the crystalline to amorphous structure of water ice at low temperatures caused by energetic processing (see for example refs 37 and 38). 3.2. Photochemistry. Upon irradiation with photons at 147, 206, 254, and 284 nm, sulfur dioxide and water mixtures displayed chemistry consistent with the sulfur cycle thought to occur on Europa’s surface. Qualitatively, the differing wavelengths all caused similar chemistry with small variations. This chemistry was apparent in the IR spectra. IR spectra taken before and after irradiation with 284 nm photons are shown in
Figure 3. Peak wavelength for the SO2 B̃ ← X̃ absorption band. Two separate experiments are shown. The arrows indicate the heating and cooling paths taken in each experiment.
1:26 SO2/H2O mixture was deposited at 12 K, warmed stepwise to 100 K, and then cooled back to 12 K (the UV absorption spectra are shown in Figure 2). The peak position of the B̃ ← X̃ absorption shifts from 272.9 nm at 12 K to 280.3 nm at 100 K. The shift in peak position is not significantly reversible upon cooling, dropping to only 279.9 nm at 12 K. Data from the second experiment is shown in the upper right trace of Figure 3. This ice was deposited from a 1:20 SO2/H2O gas mixture at 100 K and then heated stepwise as shown. The position of the B̃ ← X̃ absorption peak shifts from 282.2 nm at 100 K to 286.6 nm at 140 K. The trend is roughly the same as in the heating curve from the first experiment. The irreversible change in peak position suggests a chemical and/or physical change occurring in the ice. This is consistent with previous work by Loeffler and Hudson,26 who observed thermal reaction in SO2/H2O ices upon heating to 100 K and those of Kanuchova et al.28 upon heating to 120 K. These reactions produced species, such as HSO3− and S2O52−, which were observed by the growth of new absorption bands in the IR at approximately 1034 and 1013 cm−1 and at 958 cm−1, respectively. IR spectra of the current ice films (Figure 4) show C
DOI: 10.1021/acsearthspacechem.9b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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observed at 983, 1101, and 1218 cm−1 indicating the formation of SO42− can be seen, while growth of the feature at 1056 cm−1 is evidence of the HSO4− ion. Finally, features seen at 1020 and 1003 cm−1 likely correspond to HSO3−. It should be noted that, in some spectra, sulfuric acid was recorded at 1247 cm−1 during deposition,39 possibly from a gas-phase reaction during mixing. Photons with a wavelength longer than ∼219 nm (∼5.66 eV) are not energetic enough to dissociate SO2,40,41 yet this chemistry was observed at wavelengths up to 284 nm. Therefore, the observed products must result from electronically excited SO2 reacting with ground-state H2O or SO2. In the gas phase, excitation into the 3B1 state followed by reaction with ground-state SO2 has been shown to yield SO3.40
Figure 5 as a representative example. Included in Figure 5 is a difference spectrum (found by subtracting the pre-irradiation
SO2 (3B1) + SO2 (X , 1 A1) → SO3 + SO
Sodeau et al.41 have shown that SO2 is photochemically inactive in Ar matrices, unless it is present as the dimer, supporting the above reaction pathway. SO3 or SO could subsequently react with H2O to form the various sulfur oxyanions observed. Direct reaction of excited SO2 with H2O is also possible. 3.3. SO2 Photolysis on Europa. Figure 6 shows the compilation of gas-phase (293 K) photoabsorption cross
Figure 5. IR spectra of a 1:20 SO2/H2O ice film (∼0.3 μm thick) at 100 K before irradiation (black) and after 24 h of irradiation with a Xe arc lamp at 284 nm (blue) and the difference spectrum found by subtracting the pre-irradiation spectrum from the irradiated spectrum and multiplying the result by 2 (red). Spectra are offset for clarity.
spectrum from the post-irradiation spectrum) to emphasize the changes. UV spectra were also taken during these experiments. However, very little change was evident in these spectra, and therefore, they have not been included. Given that the IR spectra show limited quantities of reaction products being formed, changes in the UV may be difficult to see where features are expected to be less prominent. Another possibility is that the result of chemical changes observed in the IR have minimal effect on the UV spectrum. This would then suggest that the changes in the UV after thermal annealing could be ascribed to physical changes. The locations of IR features observed in the irradiation experiments are listed in Table 1, along with the species and transitions assigned. Destruction of SO2 was evident in the spectra via the reduction in intensity of the features associated with the symmetric (ν1) and asymmetric (ν3) stretches at 1150 and 1339 cm−1, respectively (negative going features in difference spectra). In terms of product formation, a number of ionic species were observed in the post-irradiation spectra. Features
Figure 6. Blue line (blue axis on the left): Gas-phase (293 K) photoabsorption cross section for SO2. Black line (black axis on the right): Solar flux at Jupiter determined by scaling the solar spectral irradiance measured at 1 AU outside Earth’s atmosphere on March 29, 1992 by SUSIM and SOLSTICE to 5.2 AU (i.e., the average of Jupiter’s aphelion and perihelion). Red line (red axis on the far right): Differential photoabsorption rate per nanometer given by the product of the photoabsorption cross section and the solar flux.
Table 1. Peak Positions and Assignments in Irradiated SO2/ H2O Ices observed position (cm−1) 1339 1218 1150 1128 1101 1056 1028 1003 983
assignment SO2 (ν3) SO42− SO2 (ν1) HSO4− SO42− asymmetric stretch HSO4− symmetric stretch HSO3− HSO3− SO42−
sections for SO2 given by Manatt and Lane42 for UV photons ranging from just slightly more energetic than the hydrogen Lyman-alpha emission line up to 400 nm. Included on the plot is the solar flux incident on the Jovian satellites. The flux was calculated from the solar average of the irradiance measured by the two Upper Atmosphere Research Satellite (UARS) solar instruments, the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), and the SOLare Stellar Irradiance Comparison Experiment (SOLSTICE), on March 29, 1992 and April 15, 1993 given by Woods et al.43 The SUSIM and SOLSTICE measurements were taken above the Earth’s atmosphere at 1 AU from the Sun. Therefore, the irradiance was scaled to 5.2 AU (i.e., the average of Jupiter’s aphelion and perihelion) and multiplied by λ(hc)−1 to give the photon flux.
reference position (cm−1) 134124 121622 115124 114522 109227 105224 103527 101124 98224 D
DOI: 10.1021/acsearthspacechem.9b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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The photoabsorption cross sections exhibit a significant structure superimposed on a number of broad absorption peaks while showing an overall trend that sees the cross section decreasing by ∼7 orders of magnitude with an increasing wavelength. In contrast, the solar flux increases by ∼5 orders of magnitude over the same wavelength range. The vertical lines on the graph indicate the wavelengths of the four UV lamps used in the photochemical experiments, emphasizing their correlation with the strong absorption features. To demonstrate the net response to solar irradiation of SO2 on the Galilean satellites, a differential (in wavelength) photoabsorption rate was calculated by multiplying the photoabsorption cross sections with the solar flux and included in Figure 6. The large features in the differential photoabsorption rate centered at ∼210 and ∼290 nm demonstrate the importance of higher wavelength photons. To further emphasize this point, the differential absorption rate was integrated over three regions, which included the contributions of H Ly-α (121.025−122.175 nm), the C̃ ← X̃ feature centered at ∼220 nm (180.025−230.025 nm), and the B̃ ← X̃ feature centered at ∼280 nm (250.025−330.025 nm). This resulted in photoabsorption rates of 5.00 × 10−7, 9.73 × 10−6, and 6.53 × 10−5 s−1 for the H Ly-α, C̃ ← X̃ , and B̃ ← X̃ features, respectively. The total rate integrated over the wavelength region shown in the figures (i.e., 119.025−400.025 nm) is 7.60 × 10−5 s−1. Despite the intensity of the H Ly-α feature, it only contributes 0.7% of the total photoabsorption rate. This is in stark contrast the C̃ ← X̃ feature, which contributes 12.8%, and the B̃ ← X̃ feature, which accounts for 85.9%.
§
Stephen M. Meckler: Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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 acknowledge support from the NASA Outer Planets Research Program and the NASA Astrobiology Institute (Icy Worlds). Stephen Meckler participated in this work as an intern sponsored by the NASA Undergraduate Student Research Program. The authors are grateful to two anonymous reviewers for comments that helped improve this article. Government sponsorship is acknowledged.
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4. CONCLUSION Our work has shown several interesting aspects of cryogenic sulfur dioxide/water ice mixtures under changing temperatures and UV radiation. UV absorption spectra show measurable irreversible changes with an increasing temperature that may be caused by thermal-induced chemical production of HSO3− and S2O52− and/or crystallization of the ice. Regardless, observation of the B̃ ← X̃ absorption peak position could provide the means of constraining the thermal history of the surface. UV-induced photochemistry of sulfur dioxide/water ices produced many of the same compounds formed from charged particle irradiation studies, suggesting that Europa’s sulfur cycle may be partially solar-driven. The current experiments demonstrated that chemistry is induced by radiation not only at the energetic 147 nm wavelength but also at 206, 254, and 284 nm. Given the combination of increased solar intensity at longer wavelengths compared to the vacuum UV and the strong B̃ ← X̃ absorption, photons between 250 and 330 nm are expected to be particularly significant for the photochemistry of Europa’s surface. While particle irradiation is much heavier on the trailing side of Europa, the entire surface is exposed to solar irradiation. Therefore, destruction of SO2 is expected across Europa’s surface and not restricted to the trailing side.
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REFERENCES
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Paul V. Johnson: 0000-0002-0186-8456 E
DOI: 10.1021/acsearthspacechem.9b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsearthspacechem.9b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
