Sulfur Isotopic Fractionation in Vacuum UV Photodissociation of Hydrogen Sulﬁde and Its Potential Relevance to Meteorite Analysis

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Sulfur isotopic fractionation in vacuum UV photodissociation of hydrogen sulﬁde and its potential relevance to meteorite analysis

Subrata Chakrabortya,1, Teresa L. Jacksona, Musahid Ahmedb, and Mark H. Thiemensa

aDepartment of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356; and bChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Edited by James Farquhar, University of Maryland, College Park, MD, and accepted by the Editorial Board December 7, 2012 (received for review August 31, 2012) Select meteoritic classes possess mass-independent sulfur isotopic abundance of 5–6%, and in certain mineral phases, it is as high as compositions in sulfide and organic phases. Photochemistry in the 15% (12). The heavy sulfur isotope (δ34S) in sulfide phases from solar nebula has been attributed as a source of these anomalies. ordinary and enstatite chondrites (bulk analysis) is fractionated Hydrogen sulfide (H2S) is the most abundant gas-phase species in within a couple of permil from the value of Canyon Diablo the solar nebula, and hence, photodissociation of H2S by solar Troilite and indistinguishable from each other (Fig. 2A) (6, 13, 14). vacuum UV (VUV) photons (especially by Lyman-α radiation) is a [Sulfur isotope ratios are written in terms of δ33S, δ34S, and δ36S. relevant process. Because of experimental difficulties associated These δ-values refer to the per mil difference from the ratio in i i 32 i 32 with accessing VUV radiation, there is a paucity of data and a lack a standard reservoir (j) such that δ S = (( S/ S)/( S/ S)j − 1) × of theoretical basis to test the hypothesis of a photochemical origin 103‰, where i refers to 33, 34, or 36. For sulfur isotopes, the of mass-independent sulfur. Here, we present multiisotopic mea- standard reservoir j is Canyon Diablo Troilite. Δ33S and Δ36S surements of elemental sulfur produced during the VUV photolysis represent the degree of mass-independent fractionation and 33 33 34 36 36 of H2S. Mass-independent sulfur isotopic compositions are observed. are defined as Δ S = δ S − 0.515δ S and Δ S = δ S − The observed isotopic fractionation patterns are wavelength- 1.9δ34S. These quantities represent the departure from the mass- dependent. VUV photodissociation of H2S takes place through sev- dependent (e.g., Δ33S, Δ33S = 0) fractionation.] The extent of eral predissociative channels, and the measured mass-independent isotopic composition in carbonaceous chondrites is significantly fractionation is most likely a manifestation of these processes. higher (∼14‰), and aqueous alteration was suggested on the Meteorite sulfur data are discussed in light of the present experi- chondrite parent body (14, 15), leading to formation of sulfates ments, and suggestions are made to guide future experiments from sulfide grains enriching the sulfide in heavy sulfur isotopes. and models. It has been shown that the sulfur isotopic composition in in- dividual chondrules and matrix material is normal (mass-de- chondrites | achondrites | isotope pendent), except for the chondrules from the Dhajala (H3) chondrite, which posses a slight mass-independent component xygen and sulfur are the only two elements in solar system (Fig. 2B, Δ33S ∼ 0.11‰). The chondrule rim material has been Osilicates that process an isotopic abundance anomaly at the suggested as the source of this anomaly. Apart from silicates, whole-meteorite level (1–3). Both elements belong to group VI organic isolates [methyl, ethyl, isopropyl, and n-propyl sulfonic and are the 3rd and 10th most abundant elements in the solar acids] from Murchison (carbonaceous chondrite) also reveal a system (with S/O ratio of 2.1%) (4, 5), respectively. Similar to large mass-independent sulfur composition (up to ∼2‰ in Δ33S oxygen, sulfur possesses multiple stable isotopes (masses 32, 33, and Δ36S) (16). 34, and 36 amu with natural abundances of 95.02%, 0.75%, 4.21%, and 0.02%, respectively). In an oxidizing environment, Sulfur in Achondritic Meteorites oxygen condenses to high-temperature (refractory) minerals; like- The typical weight percent of FeS in achondrites is about 0.5 or wise, sulfur directly condenses in refractory phases under reducing ∼10 times lower compared to chondrites (12). The range of conditions in the solar nebula (Fig. 1) (6). Oxygen is partitioned heavy isotope fractionation (δ34S) in different types of achon- between two major oxygen-bearing molecules (e.g., CO and H2O drites is small compared to ordinary and enstatite chondrites with fractions of 1/2 and 1/3, respectively; the rest resides in solid (17). Mass-independent sulfur isotopic compositions (enrich- 33 oxides), whereas H2S is the major sulfur-carrying gas under solar ment in S) have been reported in achondritic meteorites, such nebular conditions (4, 7). There are several other minor sulfur- as Ureilites, Oldhamite (CaS) in the Norton county, Aubrite, and bearing gas-phase species that exist in the solar nebula, such as Acapulcoite-Lodranites (17, 18) (Fig. 2B). In another enigmatic 33 SiS, OCS, CS2, CS, NS, SO, and SO2, because of multiple avail- achondrite group, Ureilites, a range of S anomalies has been 2− 6+ – able chemical valence states (S to S )(5,810). Oxygen and observed that extends from 0.02‰ to 0.1‰ (Fig. 2B) (18, 19). sulfur are two chemically distinct entities and exist independently Several possible sources of mass-independent sulfur measured in the solar nebula. However, as a consequence of sulfur chem- in meteorites have been suggested (17). A photochemical origin istry, it has been shown that the abundance of H2S is severely is considered to be one of the viable mechanisms. Photochemistry affected by the abundance of oxygen in the solar nebula (9). Sulfur in Chondritic Meteorites Author contributions: S.C. and M.H.T. designed research; S.C., T.L.J., and M.A. performed Chondritic meteorites are one of the most primitive classes of research; S.C., T.L.J., and M.H.T. analyzed data; and S.C., M.A., and M.H.T. wrote meteorites containing round shaped millimeter- to centimeter- the paper. sized glassy balls (chondrules). Chondrules are one of the earliest The authors declare no conflict of interest. formed solids in the solar system, composed mainly of ferromag- This article is a PNAS Direct Submission. J.F. is a guest editor invited by the Editorial Board. nesian silicates with associated Fe–Ni metal and minor sulfide 1To whom correspondence should be addressed. E-mail: [email protected]. phases (11). Sulfur is abundant in chondrules and chondrites. The This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. weight percent of FeS varies across a wide range, with typical 1073/pnas.1213150110/-/DCSupplemental.

17650–17655 | PNAS | October 29, 2013 | vol. 110 | no. 44 www.pnas.org/cgi/doi/10.1073/pnas.1213150110 Downloaded by guest on September 30, 2021 SPECIAL FEATURE

Fig. 1. Schematic diagram of the solar nebula showing the snowline that separates the inner and outer nebula.

plays a signiﬁcant role in shaping the chemical reaction network a slope value of ∼−3 (with near-zero intercept) in Δ33S vs. Δ36S in the solar nebula and interstellar clouds (5, 10, 20–26). Labo- plot (Fig. 4). Elemental sulfur produced at 139.1-nm wavelength ratory-based photochemistry experiments have provided some also shows a mass-independent composition in both 33S and veriﬁcation that the isotopic fractionation during photolysis of 36S and is similar to measurements at 121.6 nm (Fig. 4). The two various sulfur-containing molecules by UV photons is mass- elemental sulfur samples produced at 90 and 157.6 nm are – independent (27 29). Determination of the isotope effect in the spiked (with commercial Ag2S) because of the small sample size solar vacuum UV (VUV) photodissociation of relevant triatomic and the requirements to perform isotopic measurements (SI molecules, such as H2S and H2O, is of importance to fully un- Materials and Methods) associated with somewhat larger uncer- derstand the oxygen and sulfur isotopic signatures recorded in tainties (1σ = 0.3‰ for Δ33S compared with 0.1‰ for other EARTH, ATMOSPHERIC,

the meteorites. In this paper, we report experiments that measure data) (circled in Figs. 3 and 4) and possess mass-independent AND PLANETARY SCIENCES the multisulfur isotopic fractionation during the VUV photo- sulfur isotopic compositions. For 90-nm photolysis, except for this dissociation of H2S. There is, at present, no experimental data at particular (spiked) sample, the other two samples show isotopic 33 36 relevant VUV wavelengths to understand the role of H2S pho- composition close to normal for S, whereas for S, the frac- tolysis in the early solar system. A limited set of data exists for tionation is higher (slightly positive Δ36S values). The sizes of > UV wavelengths ( 250 nm), which show mass-dependent sulfur these two samples (SF6) are more than 5 μmol, and the un- isotopic fractionations (28). The experimental results at the certainty (1σ) associated with Δ36Sislessthan0.2‰ for this short VUV wavelengths and potential relevance to the early sample size. Therefore, the measured Δ36S values are higher solar nebula are discussed. than the uncertainty limit. For 157.6-nm photolysis, except for the spiked sample, the other elemental sulfur samples yield near Results and Discussion to normal sulfur isotopic compositions in both 33S and 36S. We The sulfur isotopic composition (δ33S, δ34S, and δ36S) of product do not see a clear pressure dependency in isotopic fractionation elemental sulfur is enriched with respect to initial H2S (except in any of the experiments. for two samples at 90- and 139.1-nm photolysis products, respectively) and shown in a combined three-isotope plot (Fig. 3 VUV Photochemistry and Predissociation Dynamics of H2S. Absorp- and Table S1). Photolysis at 121.6 nm (Lyman-α line) reveals a tion spectra of H2S exhibit several Rydberg series in the VUV mass-independent fractionation with a maximum value of −1.5‰ spectral region (up to 160 nm) and converge to the ionization in Δ33S and a corresponding Δ36S value of 4.6‰, giving rise to limit beyond 90 nm (30–34). There are several experimental and

Fig. 2. Histograms showing (A) the range in sulfur isotopic fractionation (δ34S) and (B) the corresponding Δ33S (mass-independent component) values as observed in different groups of meteorites. The achondrites show mass-independent fractionation in the bulk level. Only the chondrule rim (from Dhajala) shows a mass-independent composition. All data are taken from the literature (6, 13–15, 17–19).

Chakraborty et al. PNAS | October 29, 2013 | vol. 110 | no. 44 | 17651 Downloaded by guest on September 30, 2021 34 33 34 Fig. 3. Three isotope diagrams showing the sulfur isotopic compositions measured in the present H2S photolysis experiments: (A) δ Svs.δ Sand(B) δ Svs. δ36S, respectively. The values of δ34S, δ34S, and δ34S are plotted in logarithmic scale, where δiS = ln(1 + δiS/1,000) and i refers to 33, 34, or 36. Mass-dependent slope lines are shown for reference. The elemental sulfur samples for different VUV wavelengths yield mass-independent sulfur isotopic composition. The circled data points are associated with larger uncertainties as describe in the text; otherwise, for all other data points, the 1σ uncertainty is less than the size of the symbols.

theoretical spectroscopic studies (35–41) that describe the photo- repulsive surface and predissociates (43) in several branches dissociation dynamics of H2S at the experimental VUV wave- (35, 41): lengths (i.e., 121.6, 139.1, and 157.6 nm) explored in the present + v → + ð 2Π ...... ; [1] study. Theoretical studies (37–39, 42) suggest that two strongly H2S h H SH X coupled excited electronic states are involved in the Rydberg P B 1A 2 + levels of H2S: an upper 2 state that is bound and the lower → H + SH A ...... ; [2] A 1B dissociative state. In the photodissociation process of 1 H S, the molecule is ﬁrst excited to the bound state and sub- 2 → + + 3 ...... ; [3] sequently, through coupling (of the two states), accesses the H H S P → H + H + S 1D ...... ; [4]

and 3 → H2 + S P ...:: ...... [5]

The major factor in predissociation dynamics at different Rydberg levels for varying photoexcitation energies is that the relative contributions for branching differ. For example, at 121.6 nm, pathways 2–4 are important (35), whereas at 157.6 nm, branch 1 is dominant (36, 41), and the photodissociation yields for 121.6 and 157.6 nm differ. Predissociation from various excited states of H2S is known to be a sensitive function of parent rotational level (and not dependent on the vibrational state), and a significant isotope selectivity in predissociation is expected (38). Accidental near- resonances between vibrational levels of different electronic states can significantly enhance radiationless transfer into the final dis- Fig. 4. Δ33S vs. Δ36S plot highlighting the mass-independent fractionation sociative state, and such resonances are responsible for the vibra- α measured during VUV photolysis of H2S. The data from 121.6 nm (Lyman- tional level-dependent predissociation. Isotope specificity caused line) photolysis define a slope ∼−3. The mean values of the meteorite data by accidental predissociation has been observed in diatomic mo- are plotted (6, 17), which show a spread (although small) along the same lecular systems, including N and CO (25, 44, 45). Unlike diatomic line. The mass-independent fractionation trends for other wavelengths are 2 different compared to the measurements of 121.6 nm and described in the molecules, where the potential energy surface is a function of bond text. The composition of organics from Murchison (16) is also shown and may length, for triatomic molecules, the added variable of bond angle be explained by a combination of fractionation at different wavelengths. further complicates the dissociation dynamics (36). Moreover, the

17652 | www.pnas.org/cgi/doi/10.1073/pnas.1213150110 Chakraborty et al. Downloaded by guest on September 30, 2021 SH photofragments (expressions 1 and 2) also rapidly predissociate (35), potentially generating an additional isotope effect. SPECIAL FEATURE

Probable Origin of Mass-Independent Sulfur Isotopic Composition. The mass-independent isotopic fractionation during VUV photodissociation has been measured and theoretically calculated for triatomic molecules, such as CO2,NO2,N2O, and SO2 (27, 29, 46–53). There is no common physical chemical mechanism for these anomalous processes, and several molecule-speciﬁcmecha- nisms have been proposed. A quantum mechanical basis has been proposed for the various molecules involving electronic, vibrational, and rotational energy states as the origin of mass- independent fractionation (47, 48, 54). One exception is a proposal involving SO2 photolysis through isotope selective photodissociation in a column of gas, which produces differ- ential photoabsorption among different isotopologues (isotope self-shielding) (51). Fig. 5. Solar luminosity at 1 My in the VUV spectral region as reconstructed For isotope self-shielding to occur, the absorption spectra should α ﬁ in ref. 56. The Lyman- line at 121.6 nm is the strongest in the t-tauri phase possess a ne structure, and the absorption lines for different iso- of the sun. The wavelengths used in the present experiment are marked

topologues must be separated in the frequency domain (55) and along with the integrated absorption cross-sections of H2S (relative to the is not fulﬁlled in the VUV photoabsorption zone for H2S (34). cross-section at 121.6 nm) in the parenthesis (34). Hence, the likely source of the mass-independent isotopic fractionation observed in the present experiments is from predissociative processes associated with the different dissociation measured for 121.6-nm photolysis in this work (along the slope ∼− Δ33 Δ36 pathways. As discussed earlier, available predissociative pro- 3 line in S vs. S plot). cesses (expressions 1–5) are not equivalent and contribute in different proportions after photoexcitation at different wave- Sulfur Isotopic Composition in Chondrites. In the early stages of nebular disk evolution, a fundamental difference in the chemis-

lengths. Hence, the measured isotopic compositions of elemental EARTH, ATMOSPHERIC, sulfur vary for different photolysis wavelengths and generate dif- try of the two volatile species, H2O and H2S, is that H2O con- AND PLANETARY SCIENCES denses directly to a solid at the snow line and beyond (Fig. 1), ferent mass-independent fractionation trends. As an illustration, whereas H S chemically evolves through gas–solid reactions the major difference in fragmentation pathways during photo- 2 [Fe + H S = FeS + H ] to form solids (9, 57). dissociation at 121.6, 139.1, and 157.6 nm is that the product (solid) 2 (gas) (solid) 2 (gas) + Laboratory experiments have shown that, under solar nebula SH fragment at 121.6 nm populates the A 2Σ state, whereas it conditions, sulfurization of Fe (FeS or troilite formation) pro- populates the X 2Π state at the other two wavelengths. The SH ceeds in the temperature range of 500–690 K, well within (<2a.u.) fragments subsequently predissociate to yield elemental sulfur in – the snow line (58). This sulfurization is consistent with H2Scon- all cases (35 37, 41). Because predissociation is quantum state- fi fi densation as a sul de form in the inner solar nebula that is sub- speci c (44), the isotope effect caused by predissociation of SH sequently incorporated into chondritic and achondritic parent fragment from these two states is different and may give rise to bodies at a later secondary stage. Two competitive processes may an additional effect that varies from one state to other. arise involving H2S. First, gas to solid condensation of H2SasFeS Based on the product sulfur isotopic composition measured in in the dense midplane of the inner solar nebula may occur within the present experiments, potential nebular scenarios to explain 2 a.u. of the protosun (Fig. 1) (9, 58), and second, photodissoci- the meteoritic observations may be suggested. If H2S is the major ation of H2S is in the photoactive zone of the solar nebula (17). gas-phase species and undergoes photodissociation from solar The formation of troilite is kinetically a fast process (58), and δ34 VUV or other stellar sources, the isotopic shift in Softhe because it is operative at the dense midplane, the effective fi residual H2S reservoir would not be signi cant (within a few thermochemical sulfide formation rate dominates H S photol- δ34 2 permil) and is consistent with S variations observed in dif- ysis in that zone, which is optically opaque. In this scenario, A ferent meteorite groups (Fig. 2 ) (13). The photolysis isotope the product troilite would not possess an isotope component data obtained in this experiment are wavelength-dependent. Be- resulting from H2S photolysis. cause H2S photolysis in the nebula occurs through the entire VUV Chondrites and chondrules contain overwhelmingly large region, an effective isotopic fractionation can be modeled com- amounts of sulfur as troilite and other sulfide minerals (12). It fi bining the wavelength-speci c fractionation and nebular line was recently suggested that, in the early solar nebula, the forma- strengths. Only four Rydberg bands are covered in the present tion of calcium aluminum-rich inclusion (CAI), the first product experiments using broadband (∼2 nm) VUV photons from the of condensation, was as early as 80,000 y (59), and chondrules synchrotron; the entire VUV region was not covered, and there- formed within 2 My of CAIs (60), although some evidence sug- fore, the effective fractionation in VUV photodissociation cannot gests that they are contemporary (61). Therefore, chondrules may be computed. However, Lyman-α (121.6 nm) is the strongest line have directly captured condensed troilite with no apparent mass- in the t-tauri phase of the sun (Fig. 5) (56). The integrated ab- independently fractionated sulfur. A similar proposition was sorption cross-sections at 90, 139.1, and 157.6 nm are ∼2, 0.88, suggested in the work in ref. 6. and 0.31 times larger, respectively, compared to 121.6 nm (Fig. Recent oxygen isotopic measurements (62) of CAIs document 5). Although the cross-section at 90 nm is two times larger a systematic variation of isotopic ratios (δ18O, Δ17O), which compared to 121.6 nm, the VUV flux at 121.6 nm is ∼20 times starts at the margin of CAIs, and they indicate that they were higher. The combination of VUV flux and cross-section renders circulated within the solar nebula after formation and exposed to Lyman-α as the strongest contributor in VUV photodissocia- nebular environments of different oxygen isotopic composition. tion. Although the fractionation obtained at this wavelength This scenario may also be applicable to chondrules, where chon- can be used to interpret the meteorite data, a full model drule sulfide rims are formed from different gas/dust while the treatment is clearly needed. As shown in Fig. 4, there is a broad chondrules were circulating within the nebula and archived the range in the meteorite data, and it is consistent with the spread isotopic composition of different nebular environments, especially

Chakraborty et al. PNAS | October 29, 2013 | vol. 110 | no. 44 | 17653 Downloaded by guest on September 30, 2021 on the rims. At this time, the chondrule rims might have cap- disk and subsequently synthesize organic molecules. Association tured the mass-independent isotopic signature produced by H2S of mass-independent sulfur isotopic components (as a result of photolysis, which was measured for Dhajala chondrule rims (6). VUV photodissociation of H2S) with hydrocarbons might have The stepwise leaching protocol followed in ref. 6 clearly shows occurred in the low-temperature nebular regime, become a part of that the δ34S value shifts about 2‰ between the chondrule in- the organics, and ultimately, incorporated in the carbonaceous terior and the rim of Dhajala, and the corresponding shift in chondrite parent bodies during the accretion process. The sulfur Δ33Sis∼0.12‰. The isotopic compositional shift between the isotopic anomalies measured in different organic fractions vary (Fig. chondrule interior and the rim can best be explained through the 4). The methyl group is more enriched in 33Scomparedwiththe chondrule migration mechanism after formation as described in other higher-order hydrocarbon compounds. As discussed, ref. 62 and exposure to a photochemically fractionated H2S gas. migration of the ice particles and irradiation by UV radiation may be facets of the organic synthesis process. Available VUV Mass-Independent Sulfur Isotopes in Achondrites. A distinction of light at different locations of the disk is variable, especially at the achondrites vs. chondrites is the low abundance of sulfur in surface layer of the disk and outer region of the nebula where the achondrites, generally a factor of ∼10 (12), which may provide disk is irradiated by solar as well as external stellar sources. The an opportunity to observe their primitive character. Oldamite stellar light is attenuated with depth of penetration of the light (CaS) has been identified as one of the carrier phases of mass- inside the disk by absorption of different gaseous molecules, such independent sulfur (17). As modeled, at the early evolutionary as H2 and CO. This scenario allows H2S photolysis at different time period of the solar nebula, sulfur chemistry altered with VUV wavelengths in different regions in the disk during grain mi- changing oxygen fugacity (9). The H2O condensation front (snow gration. The present experimental results show wavelength-spe- line at ∼3 a.u.) in the nebula introduces a strong zoning between cific mass-independent fractionation (Figs. 3 and 4); thus, it is the inner and outer nebula (Fig. 1). As water vapor was pro- plausible that different sulfur reservoirs of unique isotopic gressively diffusing out of the inner solar nebula with a rapid rate composition existed in the nebula, and the icy particles recor- in the first 105 y (63, 64), FeS was being replaced by CaS as the ded one of these compositions while migrating within the dominant sulfur condensate. A timescale of ∼106 y was required nebular disk. for this change in sequence, and in that time period, the majority of sulfur condensed as FeS (9) in the chondrule formation zone. Conclusions Therefore, while CaS was being formed, the proportion of pho- Multisulfur isotopic compositions have been measured in ele- tochemically processed sulfur fraction increased compared with mental sulfur produced during the VUV photolysis of H2Sat total sulfur and became the carrier phase of mass-independent four different wavelength bands at a synchrotron. The measured sulfur, archiving the signature of nebular photochemistry. sulfur isotopic fractionations are mass-independent. VUV photo- fi It is not precisely de ned how and where chondritic and dissociation of H2Sat121.6nm(Lyman-α line), one of the most achondritic parent bodies formed and produced the quantity prominent lines in the t-tauri phase of the sun, yields a slope value of sulfur contained in the planetesimals. A suggested scenario is of −3 in four-isotope Δ33S − Δ36S space. The source of mass- that the achondritic parent body underwent thermal alteration independent sulfur isotopic composition in H2S photodissocia- at a later stage and lost most of the volatile sulfur during this tion is likely a predissociative process, and it introduces isotope process (65); a highly refractory phase (e.g., CaS) might, how- selectivity in the resonance-governed curve-crossing dynamics. In ever, survive this thermal episode and be found in achondritic light of the H2S photodissociation data obtained in the present meteorites, and it would have a mass-independent sulfur com- experiments, a plausible nebular scenario to explain the sulfur position (e.g., Norton County Aubrite) (17). Ureilites are the most isotope data from meteorites is suggested. enigmatic of achondrites and show dual signature of thermal metamorphism as well as primitive texture (19). The measured Materials and Methods 33 sulfur isotopic compositions (Δ S) process a wide range (Fig. H2S photodissociation was performed at four different wavelength bands (90, 2B) but do not correlate with oxygen isotopic anomalies (18, 66). 121.6, 139.1, and 157 nm) in the VUV region using photons from the Advanced It is inferred that the oxygen and sulfur systems are decoupled Light Source synchrotron at Lawrence Berkeley National Laboratory. A 60-cm- from each other, and the variation and sulfur mass-independent long windowless reaction chamber was used for this purpose, which was at- composition reflect the extent of thermal metamorphism in ure- tached to the beamline with a three-stage differentially pumped vacuum system (Fig. S1) as described in ref. 25. In these flow experiments, high-purity ilite parent body (or bodies) and the survival of the reduced re- H2S gas was passed through the reaction chamber from a lecture bottle while fractory phases. The parent body of HEDs (howardite, eucrite, keeping the pressure constant for a particular experiment. For each wave- and diogenite, believed to be the Vesta asteroid) is a totally length, the pressure was varied between 100 and 240 mtorr. Before flowing

differentiated body, and survival of reduced refractory sulfur H2S inside the chamber in each experiment, a precleaned cylindrical aluminum phases is unlikely because of large-scale homogenization; hence, jacket was coaxially inserted into the reaction chamber. After each photolysis no mass-independent sulfur isotopic composition generated by experiment, the jacket was carefully removed and stored. During VUV pho- photochemistry is preserved. Additional studies of minor sulfide tolysis of H2S, elemental sulfur was produced (35) and deposited on the sur- components of achondrites would be definitive in defining face of the Al-jacket. Chemically extracted elemental sulfur from the Al- this process. jacket (at University of California at San Diego) was subsequently treated to convert to SF6 and measured for all four stable sulfur isotopes by isotope ratio MS (MAT 252). Full details for the experiments, process, and isotope Mass-Independent Sulfur Isotopes in Meteorite Organics. Sulfonic measurements are provided in SI Materials and Methods. acid extracts from the Murchison meteorite contain mass-independent sulfur isotopic compositions (16) as shown in Fig. 4. ACKNOWLEDGMENTS. S.C. thanks Ryan D. Davis for his help in performing These anomalies are significantly larger compared with obser- experiments at Advanced Light Source. Support from Ms. Sarah Ferrell and vations in achondrites (17). Associated with the 33S enrichment Dr. Amir Golan (Chemical Dynamics Beamline at the Advanced Light Source) in organic material are high deuterium to hydrogen (D/H) ratios, and Dr. Doug Taube (Chemistry Laboratory Manager of Advanced Light Source) is acknowledged. Two anonymous reviewers and the editor are which have been interpreted as being indicative of C-H bond acknowledged for comments that improved the manuscript. The work is formation at lower temperatures (67). Recently, the synthesis of funded through National Aeronautics and Space Administration’s Origins and organic molecules in the solar nebula has been modeled (68). It Cosmochemistry programs. M.A. and the Advanced Light Source are supported by the Director, Office of Science, Office of Basic Energy Sciences and the was shown that ice grains that originate in the outer nebula in Division of Chemical Sciences, Geosciences, and Biosciences of the US De- a high D/H region may experience UV photochemistry from the partment of Energy at Lawrence Berkeley National Laboratory under centralstarornearbystarsduringlong-rangepassagewithinthe Contract DE-AC02-05CH11231.

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