Nanoscale infrared imaging analysis of carbonaceous chondrites to understand organic- interactions during aqueous alteration

Yoko Kebukawa (癸生川 陽子)a,1, Hanae Kobayashib, Norio Urayamab, Naoki Badenb, Masashi Kondoc, Michael E. Zolenskyd, and Kensei Kobayashia

aFaculty of Engineering, Yokohama National University, 240-8501 Yokohama, Japan; bNihon Thermal Consulting Co., Ltd., 160-0023 Tokyo, Japan; cInstrumental Analysis Center, Yokohama National University, 240-8501 Yokohama, Japan; and dAstromaterials Research and Exploration Science, National Aeronautics and Space Administration Johnson Space Center, Houston, TX 77058

Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved December 6, 2018 (received for review September 19, 2018)

Organic matter in carbonaceous chondrites is distributed in fine- have applied near-field IR microspectroscopy to carbonaceous grained matrix. To understand pre- and postaccretion history of chondrites and demonstrated organic-phyllosilicates associations (28, organic matter and its association with surrounding , micro- 29). However, the optics and probe settings were not completely scopic techniques are mandatory. Infrared (IR) spectroscopy is a useful sufficient to detect pure near-field signals, and thus the spatial res- technique, but the spatial resolution of IR is limited to a few olution was limited to ∼1 μm. Dominguez et al. (34) reported on micrometers, due to the diffraction limit. In this study, we applied submicron (∼20 nm) mineral identification of the Murchison mete- the high spatial resolution IR imaging method to CM2 carbonaceous orite and a cometary dust particle with near-field IR imaging using chondrites Murchison and Bells, which is based on an atomic force suitable background (i.e., nonnear-field scatterings) suppression, but – −1 microscopy (AFM) with its tip detecting thermal expansion of a the wavelength (wavenumber) range was limited to 800 1,100 cm , sample resulting from absorption of infrared radiation. We confirmed which did not cover characteristic organic absorptions. that this technique permits ∼30 nm spatial resolution organic analysis Alternatively, a high spatial resolution IR imaging technique that for the meteorite samples. The IR imaging results are consistent with detects photothermal-induced resonance using a tunable laser combined with an atomic force microscope was recently de- the previously reported association of organic matter and phyllosili- EARTH, ATMOSPHERIC,

veloped and applied to various research fields (35–37). When AND PLANETARY SCIENCES cates, but our results are at much higher spatial resolution. This ob- light is absorbed by the sample, the temperature increase causes servation of heterogeneous distributions of the functional groups of ∼ a characteristic thermal expansion. This thermal expansion drives organic matter revealed its association with minerals at 30 nm spa- the cantilever into oscillation. The oscillation amplitude of the tial resolution in meteorite samples by IR spectroscopy. cantilever is directly proportional to the amount of light absor- bed. The technique covers full mid-IR including ranges for or- meteorites | IR spectroscopy | AFM-IR | organic matter ganic absorption features. Here we report IR imaging analyses of carbonaceous chondrites by atomic force microscopy-based infrared rimitive extraterrestrial materials such as carbonaceous chon- spectroscopy (AFM-IR) that provides IR imaging with spatial Pdrites contain diverse organic matter (OM). Although distinct resolution far below conventional optical diffraction limits. organic grains called “nanoglobules” 100 nm to 1 μminsizeare often found in primitive carbonaceous chondrites (1, 2), they consist Results of roughly 10% or less of the total organics in chondrites (3). The Murchison Meteorite. IR absorption spectra (Fig. 1 A and B)of remaining organics are smaller than nanoglobules and distributed in the Murchison meteorite were obtained using the nano-IR at a fine-grained matrix. To understand pre- and postaccretion history of OM and its interaction with associated minerals, microscopic Significance techniques are mandatory. Scanning electron microscope obser- vation with osmium labeling has demonstrated that OM in car- Spatial relationships between organic matter and minerals are bonaceous chondrites is associated with clay minerals, which may necessary for understanding the formation and evolution of or- have had important trapping and possibly catalytic roles in the ganic matter during aqueous and thermal alteration in their par- evolution of organics in the early solar system (4). ent bodies, as well as preaccretional history. Infrared spectroscopy To obtain molecular structure information at the submicron is a powerful tool to analyze the molecular structures of organic scale, scanning transmission X-ray microscopy (STXM) com- matter and identification of minerals. However, its spatial reso- bined with X-ray absorption near-edge structure spec- lution is limited due to the diffraction limit. Recently, the atomic troscopy (C-XANES) is so far the most suitable method. The force microscopy (AFM) based IR nanospectroscopy was de- STXM with C-XANES provides molecular structure information veloped and applied in various scientific fields to overcome the at the ∼40 nm spatial resolution and is often applied to various extraterrestrial materials including carbonaceous chondrites (5– diffraction limit of IR. We applied the AFM-based IR nano- spectroscopy to carbonaceous chondrites and studied organic- 12), Antarctic micrometeorites (13, 14), interplanetary dust ∼ particles (IDPs) (15, 16), and cometary particles recovered from mineral associations at the 30 nm spatial resolution. Comet 81P/Wild 2 by the Stardust mission (17–20). Fourier transform infrared (FTIR) microspectroscopy is a Author contributions: Y.K. designed research; Y.K., H.K., and M.K. performed research; – H.K., N.U., and N.B. contributed new reagents/analytic tools; Y.K. analyzed data; and Y.K., well-established technique for extraterrestrial materials (15 17, M.E.Z., and K.K. wrote the paper. 21–27), as well as imaging analyses (28–32). IR spectroscopy The authors declare no conflict of interest. provides information about not only molecular structures of OM similar to C-XANES, but also mineral identification, and thus This article is a PNAS Direct Submission. could be a complementary method to STXM/C-XANES. How- Published under the PNAS license. ever, the spatial resolution of IR is limited to a few micrometers 1To whom correspondence should be addressed. Email: [email protected]. (roughly equal to wavelength) due to the diffraction limit. The This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. scanning near-field optical microscopy technique allows one to 1073/pnas.1816265116/-/DCSupplemental. obtain IR spectra at submicrometer spatial resolution (33). We

www.pnas.org/cgi/doi/10.1073/pnas.1816265116 PNAS Latest Articles | 1of6 Downloaded by guest on September 25, 2021 locations indicated in Fig. 1C. The IR spectra indicated in red − showed absorption bands at ∼3,400 and 1,640 cm 1, and are assigned to structural and/or absorbed H Oofhydroussilicatesat − 2 2,960, 2,930, and 2,860 cm 1; assigned to the aliphatic C–H − − stretching modes at around 1,435 cm 1; assigned to CO 2 of − − 3 at around 1,150 cm 1;assignedtoSO2 of sulfates; − 4 and at around 1,000 cm 1, assigned to the Si-O stretching modes of silicates. The spot in green (Fig. 1C) was dominated by sulfates with lesser silicates, and spots in blue are mainly silicates with less OH and sulfates. These features were mostly consistent with the IR spectra of the Murchison meteorite obtained with conventional micro-FTIR (Fig. 2) (38). However, the nano-IR spectra reflected more local characteristics, e.g., a spot with abundant aliphatic CH and sulfates that are less significant in the conventional micro- FTIR spectra obtained for larger areas (100 × 100 μm2). Fig. 3 shows 3 × 3 μm2 IR absorption maps with the IR source − − tunedto1,000cm1 corresponding to SiO, 1,150 cm 1 correspond- − − ing to sulfates, 1,730 cm 1 corresponding to C = O, 2,920 cm 1 −1 corresponding to aliphatic CH2, 2,960 cm corresponding to −1 aliphatic CH3, and 3,400 cm corresponding to OH. Some part of the silicates (Fig. 3A, Lower Right) overwrapped with the OH-rich area that indicated phyllosilicates. Fig. 4 shows the CH2/ −1 CH3 peak intensity ratio map generated from 2,920 cm (CH2) −1 map and 2,960 cm (CH3) map after image shift correction using the AFM image.

Bells Meteorite. IR absorption spectra (Fig. 5 A and B) of the Bells meteorite were obtained using the nano-IR at locations indicated in Fig. 2. IR spectra of Murchison and Bells meteorites obtained using a con- Fig. 5C. None of the five points showed the OH absorption band in ventional micro-FTIR (38). this area. One of these points showed dominant absorptions at − around 2,950 cm 1 assigned to aliphatic C–H, as well as at − − ∼1,730 cm 1 assigned to C = O, ∼1,600 cm 1 assigned to aromatic Discussion − − C = C, at around 1,450 cm 1 assigned to carbonates, at ∼1,200 cm 1 Spatial Resolution. The diffraction limit is roughly equivalent to the − − − assigned to sulfates and/or C–O, and at ∼950 cm 1 assigned to wavelength, e.g., ∼3.3 μmat3,000cm 1 and ∼10 μmat1,000cm 1. silicates. The other spots showed dominant silicate peaks at around Point spectra a few micrometer-sized areas from our meteorite −1 −1 1,000 cm , and some peaks at 1,450, 1,200, and 1,150 cm .The samples (Figs. 1 and 5) clearly showed that the spatial resolution in nano-IR spectra did not resemble the IR spectra of the Bells me- our nano-IR analysis was beyond the diffraction limit. The IR maps teorite obtained by the conventional technique (Fig. 2), probably (Figs. 3 and 6) also confirmed that the actual spatial resolution was due to heterogeneities of the meteorite since the analyzed area was at least 30 nm, e.g., based on the C = O hot spot in the Murchison × μ 2 only 2 1 m . (Fig. 3C,seetheDiscussion below).Thisismuchhigherspatial Fig. 6 shows 2 × 1 μm2 IR absorption maps with the IR source − − resolution compared with previous IR imaging measurements using tuned to 1,010 cm 1 corresponding to SiO, 1,124 cm 1 corre- − near-field IR (28, 29) and synchrotron-based IR (30–32). sponding to sulfates, 1,450 cm 1 corresponding to carbonates, −1 −1 1,724 cm corresponding to C = O, 2,920 cm corresponding to CH /CH Ratios. −1 2 3 The CH2/CH3 peak intensity ratio is a good in- aliphatic CH2, 2,960 cm corresponding to aliphatic CH3, and −1 dicator of aliphatic chain length and branching. Although the 3,400 cm corresponding to OH. Fig. 7 shows the CH2/CH3 −1 CH2/CH3 peak intensity ratio is not equal to the molar abun- peak intensity ratio map generated from the 2,920 cm (CH2) −1 dance ratio of CH2/CH3 due to the difference of molar absorp- map and the 2,960 cm (CH3) map after image shift correction. tion coefficients between CH2 and CH3, the CH2/CH3 peak intensity ratio shows a linear correlation with the actual number of CH2/CH3 in molecules (39). The CH2/CH3 peak intensity ratio of the Murchison meteorite

C 3 50 is 1.0 ± 0.1 and that of the Bells meteorite is 1.4 ± 0.2 (29). The (c) average CH2/CH3 peak intensity ratio (average of the points that −1 AB0 have 2,920 cm intensity over light blue in the color scale where 200 1.5 (nm) (μm) 800 CH2 and CH3 peaks were observed in the spectra) of the Murchison meteorite obtained from the CH2/CH3 map (Fig. 4) 0 150 01.53 -50 was 1.2 ± 0.4. This is consistent with the reported ratio for 600 (μm) Murchison (1.0 ± 0.1) (29), within analytical error. The average CH2/CH3 peak intensity ratio of Bells (Fig. 7) was 0.7 ± 0.4 100 − 400 (average of the points that have 2,960 cm 1 intensity over red- purple in the color scale where CH2 and CH3 peaks were ob- IR-Amplitude/V IR-Amplitude/V 200 50 served in the spectra). This is lower than the reported ratio of Bells meteorite (1.4 ± 0.2) (29). Considering the small analyzed 2 0 0 area (2 × 1 μm ), this difference could be due to heterogeneity of 3600 3200 2800 2000 1600 1200 OM in the Bells meteorite. Further presumption could be made Wavenumber/cm-1 Wavenumber/cm-1 that the OM associated with anhydrous matrix phases has lower Fig. 1. IR spectra of Murchison meteorite for the region of (A) 2600– CH2/CH3 ratio compared with bulk OM that is primarily asso- − − 3850 cm 1 and (B) 900–2000 cm 1. The color of the spectra corresponds to ciated with hydrated phases (phyllosilicates) (28, 29). Further the spots indicated in C, an AFM image that shows the sample height. discussion regarding the distribution of the CH2/CH3 peak ratios

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1816265116 Kebukawa et al. Downloaded by guest on September 25, 2021 -1 -1 A Silicates Si-O 1000 cm B(b) Sulfates SO4 1150 cm 0 50

IR map AFM image 60 150 0 0 (V) (V) (nm) (nm) 59 -50 -150 -1 -1 C C=O 1730 cm D(d) Aliphatic CH2 2920 cm 0 150 150 120 5 0 0 (V) (V) (nm) (nm) 54 20 -150 -150 -1 -1 E Aliphatic CH3 2960 cm (f)F O-H 3400 cm 50 150 110 120 0 0 (V) (V) (nm) (nm) 20 20 -150 -50

Fig. 3. IR map of Murchison meteorite for each frequency coupled with corresponding AFM image which was recorded during the mapping measurement to 2 −1 −1 keep track of the sample drift. Areas are 3 × 3 μm with 200 × 100 points (15 × 30 nm steps). (A) Silicates Si-O at 1,000 cm ,(B) sulfates SO4 at 1,150 cm ,(C) −1 −1 −1 −1 C = O at 1,730 cm ,(D) aliphatic CH2 at 2,920 cm ,(E) aliphatic CH3 at 2,960 cm , and (F) O-H at 3,400 cm .

and their association with minerals are in the following sections have originated from hydrolysis of sulfides such as pyrrhotite and EARTH, ATMOSPHERIC, of this paper. pentlandite, and/or by oxidation during preservation or during AND PLANETARY SCIENCES ultramicrotomy due to use of water. Murchison Meteorite. In the IR maps of the Murchison meteorite There are small spots rich in C = O. One was ∼100 nm at the (Fig. 3), ∼1.5 μm of the sulfate region is surrounded by silicates. upper middle of the map, and the other was ∼30 nm at Fig. 3C, The OH-rich region is in Fig. 3F, Lower Right, and thus the sil- Middle Left. Both consist of multiple pixels, and thus they should icates in this region are likely hydrated, i.e., phyllosilicates. It not be artifacts. One of these C = O rich spots (the smaller one) should be noted that there are some contributions from OH also shows a high abundance of aliphatic CH2 (Fig. 3D), and thus functional groups in the OM to the OH absorption. Considering appears as a hot spot in the CH2/CH3 ratio map (Fig. 4). These that the IR spectra of insoluble organic matter show much less spots (∼30–100 nm) could be nanoglobules. Typical nano- OH intensity compared with aliphatic CH intensity (roughly 1/5) globules in Murchison are aromatic rich or have insoluble or- (40), the OH from phyllosilicates should have significant con- ganic matter (IOM) like compositions (5). Although they have tributions to the OH absorption. However, we could not exclude some C = O, predominantly C = O rich nanoglobules have not the possibility that the OM in the analyzed area is dominated by OH functional groups to an unusual degree, precluding a major contribution from phyllosilicate OH in this localized area. The silicate-rich regions without OH are likely anhydrous silicates, most likely olivine and pyroxene. Note that adsorbed water from the atmosphere would contribute to the OH band, but anhydrous silicates typically do not show a significant OH band compared with phyllosilicates because structural OH in phyllosilicates en- hances atmospheric water adsorption. IR absorptions from or- ganics (Fig. 3 C–E) overlap with phyllosilicates. These results are consistent with the previous IR imaging measurements of car- bonaceous chondrites that showed an association of OM with phyllosilicates (28, 29, 31, 32), but our results demonstrate this association at much higher spatial resolution. This indicates that the OM is finely mixed with phyllosilicates, and that perhaps OM exists in the interlayer spacings. The high CH2/CH3 rich spots (<∼100 nm) are observed in aliphatic rich regions (Fig. 4, Lower Right). The heterogeneity of CH2/CH3 ratio was observed at the scale of a few tens of nanometers, suggesting a heterogeneous formation process of OM or mixing of OM of different origins. The matrix of Murchison is composed largely of tochilinite and cronstedtite (serpentine), along with minor amounts of other phases such as olivine, enstatite, pyrrhotite, pentlandite, mag- netite, and calcite (41). This mineralogy is consistent with our IR map showing hydrous and anhydrous silicates. However, sulfates 2 are not so common in CM chondrites, and it is possible that Fig. 4. The CH2/CH3 peak intensity ratio map (3 × 3 μm ) of the Murchison − − these sulfates could be terrestrial weathering products (42), as meteorite for 2,920 cm 1/2,960 cm 1 generated from Fig. 3 D and E after pointed out by Gounelle and Zolensky (43) for sulfate veins in image shift correction. The areas with intensity over 0.005 in 2,920 cm−1 map

CI chondrites. Thus, the sulfates in our Murchison sample may were calculated. The uncertainties of the CH2/CH3 intensity ratios were 16%.

Kebukawa et al. PNAS Latest Articles | 3of6 Downloaded by guest on September 25, 2021 sulfates. The C = O and aliphatic CHs are heterogeneously C 1 (c) 250 distributed in the OM-rich area. The upper region (2E to 1G) is A relatively C = O rich and the lower region (4E) is relatively al- m)

1000 1000 (nm)

(μ iphatic rich. The area around 1D shows strong absorptions of both C = O and aliphatics, due probably to enhancement by Au. 0 800 800 -250 = 0 0.67(μm) 1.33 2 The C O rich region is also rich in sulfates and carbonates, but V/edutilpmA-RI

V/edutilpmA-RI contains few silicates. The aliphatic-rich region (4E) is rich in 600 600 B sulfates, carbonates, and some silicates. C = O rich OM areas (1G and 2E) do not overlap with sili- 400 400 cates, but an aliphatic-rich OM area (4E) has some silicates. A 200 200 possible hypothesis is that this could be due to the hydrophobic nature of aliphatic moieties compared with C = O, and thus 0 0 aliphatic-rich OM is preferentially associated with anhydrous pha- ses. There is also a possibility that some of aliphatic-rich OM could 3600 3200 2800 2000 1600 1200 Wavenumber/cm-1 Wavenumber/cm-1 have originated from anhydrous processes, such as interstellar ice chemistry (46, 47). This explanation is consistent with the observa- − Fig. 5. IR spectra of Bells meteorite for the region of (A) 2,600–3,850 cm 1 tion that anhydrous chondritic IDPs tend to be rich in aliphatic − and (B) 900–2,000 cm 1. The color of the spectra corresponding to the spots indicated in C, an AFM image that shows the sample height.

A(a) Silicates Si-O 1010 cm-1 been found so far. However, the C = O rich spots in Murchison ABCDEFGH ABCDEFGH 1 1 91 were rich in C = O compared with OM in other areas which 480 mostly represent IOM. The details for these compounds are not 2 2 (V) known since we do not have full IR spectra of them, although IR 3 3 (nm) spectroscopy of bulk meteorites is not ideal for aromatic features 4 4

– 5

since the aromatic C H stretch absorption is intrinsically weaker -116 (b) Sulfates SO 1124 cm-1 than the aliphatic C–H stretch absorptions. To see the hetero- B 4 89 geneity of nanoglobule-like OM, further measurements are re- 140 quired with increasing analytical areas as well as a full spectrum (V)

for each compound. Considering that these features are not (nm) found in the control sample (heated antigorite; see SI Appendix), 5

they are not likely to be laboratory contamination. -169 (c) Carbonates CO 1450 cm-1 C 3 120

Bells Meteorite. In the IR maps of the Bells meteorite, OH ab- 117 sorption is not observed in the analytical area (2 × 1 μm2). There – (V)

are several areas, around 200 300 nm each, with strong ab- (nm) sorptions of silicates, 1D, 4F, 1H, and 2F (Fig. 6A). The corre- 5 sponding AFM images show that the heights of these regions -193 (except 2F) are significantly low. Hence, it is probably due to (d) C=O 1724 cm-1

enhancement of IR absorption by Au on the surface of the D 93 130 substrate (44). The enhancement of signals by Au coating does not affect peak intensity ratios, e.g., the CH2/CH3 peak intensity (V) ratios. Note that the Murchison sample is mounted on ZnS, thus (nm) such enhancement is not observed. Silicates and sulfates inter- 5

– -1 -106 digitately distribute at 200 300 nm scale in the left region (Fig. 6 (e) Aliphatic CH2 2920 cm A and B). Sulfates are more finely mixing or overlapping with E 110 silicates in the right region. Carbonates are largely overlapped 105 with sulfates, except some areas in 1G and 3E (Fig. 6 B and C). (V) The Bells meteorite is a highly brecciated, unusual CM2 (nm) chondrite. The fine-grained mineralogy of matrix in Bells differs 5 -121 considerably from other CM chondrites and has closer affinities (f) Aliphatic CH 2960 cm-1 to matrix in CI chondrites (45). The dominant phases are fine- F 3 83 grained saponite interlayered with serpentine. Tochilinite and 140 cronstedtite, which are typical of CM chondrite matrix, are en- (V) tirely absent (45). Fragmental olivines and pyroxenes are com- (nm) mon, pentlandite, pyrrhotite, magnetite, anhydrite, calcite, and 5 -99 rare Ti-oxides also occur as accessory phases (45). The magnetite -1 is more abundant than in other CMs. Considering this mineral- G(g) O-H 3400 cm 82 ogy, the nano-IR analytical area (2 × 1 μm2) apparently does not 110 represent the average matrix mineralogy of Bells. We do not see (V) any hydrous phases in the analytical area. The silicates in the IR (nm) map are likely olivines and/or pyroxenes. Sulfates in the IR map 5 could be anhydrite or oxidation products from sulfides, e.g., -97 pentlandite and pyrrhotite. Fig. 6. IR maps (Left) of Bells meteorite for each frequency coupled with IR absorptions of OM are observed in the green-lined area – corresponding AFM image and (Right), which was obtained at the same time (Fig. 6 D F). The OM in Bells appears as a series of beads with an IR map to confirm sample location. Areas are 2 × 1 μm2 with 400 × 50 present at grain boundaries. Anhydrous silicates in Bells do not points (5 × 20 nm steps). Grids are eye guides to correct the sample shifts −1 show significant association with OM. This is consistent with between each scan. (A) Silicates Si-O at 1,010 cm ,(B) sulfates SO4 at 1,124 −1 −1 −1 Murchison, where no association of OM and anhydrous silicates cm ,(C) carbonates CO3 at 1,450 cm ,(D)C= O at 1,724 cm ,(E) aliphatic −1 −1 −1 was observed. Rather, OM is associated with carbonates and CH2 at 2,920 cm ,(F) aliphatic CH3 at 2,960 cm , and (G) O-H at 3,400 cm .

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1816265116 Kebukawa et al. Downloaded by guest on September 25, 2021 ABCDEFG at the scale of a few tens of nanometers. We found two isolated OM spots (∼30–100 nm) very rich in C = O. 1 OM in the Bells meteorite showed beadlike structures within mineral grain boundaries with some heterogeneities in C = O and aliphatics, as well as CH2/CH3 ratios. Phyllosilicates were apparently not likely present in the analytical area. Unlike 2 phyllosilicates in Murchison, anhydrous silicates in Bells do not show a significant association with OM. Rather, OM is associ- ated with carbonates and sulfates, which could be weathering 3 products (including terrestrial) from sulfides. The CH2/CH3 peak intensity ratios of OM associated with phyllosilicates in Murchison were consistent with the same ratio 4 of bulk Murchison. However, the ratios in Bells that are not associated with phyllosilicates were lower than these of the bulk Bells and Murchison. This result indicates that the OM associ- 2 Fig. 7. The CH2/CH3 peak intensity ratio map (2 × 1 μm ) of the Bells ated with anhydrous phases has a lower CH2/CH3 ratio com- meteorite for 2,920 cm−1/2,960 cm−1 generated from Fig. 3 D and E after pared with the OM associated with phyllosilicates. image shift correction. The areas with intensity over 0.02 in 2,960 cm−1 Methods map were calculated. The uncertainties of the CH2/CH3 intensity ratios were 14%. Sample Preparation. The Murchison meteorite (CM2) and the Bells meteorite (anomalous CM2) were selected for this study, because Murchison is a typical CM2 and is the most studied meteorite for organic matter, and Bells is known moieties (15, 16). C = OrichOMcouldhaveoriginatedfrom to have abundant organic nanoglobules (51) and significantly D- and 15N-rich aqueous activities. Cody et al. (48) reported that IOM-like organic IOM (52). Antigorite (Mg-rich serpentine—an abundant phase in CM chon- solids could have been produced from formaldehyde. Such organic drites) powder which was heated in atmosphere at 500 °C for 3 h was used solids tend to be rich in C = O, particularly synthesized in lower for contamination control. We prepared ultramicrotomed thin sections us- temperatures in their IR spectra (49). ing a sulfur-embedding method based on Nakamura-Messenger et al. (2). The OM in the observed area in the Bells meteorite has a lower Grains from each sample were embedded in a molten (115 °C) sulfur droplet ± with a glass needle. The sulfur droplet subsequently solidifies and is then CH2/CH3 peak intensity ratio (0.7 0.4) compared with the OM in attached onto an epoxy stub using glue. The Murchison sample was sliced Murchison associated with phyllosilicates (1.2 ± 0.4), as well as ∼ – EARTH, ATMOSPHERIC, into 70 90 nm thick sections with a Leica ultramicrotome using a DIATOME AND PLANETARY SCIENCES reported values of bulk Bells (1.4 ± 0.2) and Murchison (1.0 ± 0.1) knife. The sections were floated onto deionized water and trans- (29). The OM shows a large heterogeneity in the CH2/CH3 ratio. ferred to a ZnS substrate. The Bells sample and heated antigorite were sliced Some spots (∼100–200 nm) in D1, D2, and E4 have higher into ∼100 nm thick sections, and transferred to Au-coated glass substrates. < < CH2/CH3 ratios, and some spots (∼200–300 nm) in G1 and E3 have Before analysis, the sections were mildly heated ( 100 °C, 15 min) until the sulfur sublimated off, leaving the ultramicrotomed samples essentially in- lower CH2/CH3 ratios. The OM associated with sulfates (likely terrestrial weathering products of sulfides) (4E in Figs. 5B and 6) tact. Such a low degree of heating should not significantly affect the or- ganics in the samples (53). has higher CH2/CH3 ratio compared with the other areas. The CH2/CH3 ratio in meteoritic OM generally increases Nano-IR. with increasing alteration and/or metamorphism (40). Thus, The IR absorption spectra were obtained using a nanoIR2 (Anasys Instruments), which consists of a tunable infrared laser (optical parametric the OM associated with anhydrous silicates could be more oscillator) that is focused onto a sample in the proximity of a probe tip from primitive compared with the OM associated with phyllosilicates. an AFM. The AFM cantilever oscillation amplitude is linearly dependent on It is also known that phyllosilicates catalyze various organic the IR absorption (54, 55). reactions(50).Furtheranalysesaswellasexperimentalin- The single spectrum data were obtained at selected points with a wave- vestigations are required to understand the roles of various number spacing of 4 cm−1. To avoid laser-induced sample damage and op- minerals in the formation and evolution of organics in the timize the signal-to-noise ratios of the peaks from OM, the incident laser powers [neutral density (ND) filter values] were set to 1.31% in the range early solar system. − − from 900 to 1,090 cm 1, and 2.19% in the range from 1,090 to 2,000 cm 1 − and 2,600 to 3,850 cm 1 for the Murchison meteorite. For the Bells meteorite Conclusions and antigorite, the incident laser powers (ND filter values) were set to 1.44% − We conducted high-resolution imaging IR analyses of two CM in the range from 900 to 1,060 cm 1, 5.64% in the range from 1,060 to − − chondrites (Murchison and Bells meteorites) using AFM-IR 1,210 cm 1, 6% in the range from 1,210 to 1,630 cm 1, 2.77% in the range ∼ from 1,630 to 1,850 cm−1, 5.64% in the range from 1,850 to 2,000 cm−1,and (nano-IR). We successfully achieved 30 nm spatial resolution, −1 which is beyond the diffraction limit. We successfully demon- 7.76% in the range from 2,600 to 3,850 cm . The IR spectra were normal- strated that the abilities of AFM-IR (nano-IR) for organic- ized to a constant laser power level at each wavenumber by calibrating with the laser power detected by an IR sensitive photodetector, and ND filter mineral associations in ultramicrotomed thin sections of chon- values that affected linearly to the IR absorption intensities. As a result, the dritic meteorites, although we would point out that caution must higher laser power enhances the signal-to-noise ratios of the spectra. be taken when (i) analyzing heterogeneous samples, since ana- For the IR maps, the laser power (ND filter values) was set to 2.77% for all lyzed areas may not represent bulk compositions since typical images of the Murchison meteorite. For the Bells meteorite, the laser power − − scanning areas are < ∼5 μmand(ii) using Au-coated substrates (ND filter values) was 1.44% for the map at 1,010 cm 1, 4.35% at 1,124 cm 1, − − − since enhancements of IR signals by Au may not be homogeneous if 5.64% at 1,450 cm 1, 2.19% at 1,724 cm 1, 11.06% at 2,920 and 2,960 cm 1, − the sample thickness is uneven although the peak ratios are and 14.96% at 3,400 cm 1. For antigorite, the laser power was 7.76% for the −1 not affected. map at 2,920 cm . The IR absorption intensities of the all IR maps were The overlapping of OH and aliphatics indicated that OM in normalized to a constant laser power level by the observed laser power at the Murchison meteorite is associated with phyllosilicates as each wavenumber and ND filter values. The scan rate was 0.1 Hz for all IR maps. AFM images were recorded during the mapping measurements reported previously, but our result reveals this association at to keep track of the sample drift. All AFM scans in the AFM-IR were done much higher spatial resolution, although we could not com- in contact mode using a PR-EX-nIR2 gold-coated cantilever (resonance pletely exclude the possibility that the OM in the analyzed area is frequency: 13 ± 4 kHz, spring constant: 0.07–0.4 N/m) with 0.3 V of the dominated by OH functional groups to an unusual degree, pre- probe voltage. −1 cluding a major contribution from phyllosilicate OH in this lo- To generate the CH2/CH3 intensity ratio maps, the CH2 (2,920 cm )and −1 calized area. The heterogeneity of CH2/CH3 ratio was observed CH3 (2,960 cm ) maps were aligned based on their AFM height images that

Kebukawa et al. PNAS Latest Articles | 5of6 Downloaded by guest on September 25, 2021 were obtained during IR imaging measurements to correct image shifts. The ACKNOWLEDGMENTS. We appreciate anonymous reviewers for helpful

uncertainties of the CH2/CH3 intensity ratios were calculated based on the comments. This work was supported by the Japan Society for the Promotion −1 −1 SDs of the CH2 (2,920 cm ) and CH3 (2,960 cm ) intensities in the organic- of Science Kakenhi (Grants JP17H02991, JP17H06458, and JP18K03722), and free areas (upper one-third of the Murchison map and left one-quarter of the Astrobiology Center of National Institutes of Natural Sciences (Grants the Bells map). AB281004, AB291005, and AB301020).

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