Supporting InformationSpencer et al. 10.1073/pnas.0801860105SI Text

Microprobe Laser-Desorption Laser-Ionization MassSpectrometryIn two-step laser mass spectrometry, the desorption laser istuned to bring surface components of a sample into the gas phasein a neutral state. No ionization occurs at this step owing to thelow laser power density. Subsequently, an ionization laser inter-cepts the gaseous plume whose wavelength is chosen to reso-nantly ionize only compounds of interest in this desorptionplume, taking advantage of the resonance enhanced multipho-ton ionization (REMPI) process (1–3).

Methods. See refs. 3 and 4 for in-depth descriptions of micro-probe laser-desorption laser-ionization mass spectrometry(�L2MS). In brief, constituent neutral molecules of the sampleare first desorbed with a pulsed infrared (IR) laser beam focusedto a spot, presently adjustable at 10 or 40 �m using either aEr:YAG (2.94 �m) or CO2 (10.6 �m) laser, respectively (Fig.S4). In the second step, a selected class of molecules in thedesorbed plume is preferentially ionized by a single-frequencypulsed UV (UV) laser beam that passes through the plume.Resultant ions are then extracted, injected into a reflectrontime-of-f light mass spectrometer, and analyzed according totheir mass-to-charge ratios. A complete mass spectrum is ob-tained for each shot. Soft, species-selective ionization, that is,ionization of molecules containing a characteristic functionalgroup with minimal accompanying fragmentation, is achievedthrough 1 � 1 REMPI (1, 2). Our instrument is designed toprobe the � 3 �* transition of systems with extended �-con-jugation, efficiently ionizing molecules with benzene-ring moi-eties and providing a selective ionization window for polycyclicaromatic hydrocarbons (PAHs). Tuned ionization of PAHsgreatly simplifies mass spectra by providing a soft ionizationroute, yielding spectra that consist of primarily parent ion masseswith a �1 charge state. PAHs are relatively stable, owing toextended �-conjugation that results in the delocalization of�-electrons over the entire cyclic structure.

Normal operating parameters, for experiments presented inthe main text unless otherwise noted, were using CO2 laserdesorption (� � 10.6 �m; 120-ns FWHM; 22 �J per pulse;�2.5 � 106 W/cm2 power density) and Nd:YAG laser fourthharmonic ionization (� � 266 nm; 4 mJ per pulse). The UVionization laser pulse typically intersected the desorption plume25 �s after IR laser desorption pulse.

Carbonaceous Chondrite Meteorite StudiesThe relative abundance and distribution of alkyl substitutedaromatic compounds in carbonaceous chondrites included�L2MS analyses of CV, CO, CI, CM, CR, CK, Tagish Lake,Coolidge, MAC88107, and ALH meteorites (4–6).

The interior fine-grained matrix for Murchison shows a con-sistent abundance of PAHs, as shown in Fig. S1. For chondrulestudies presented in ref. 7 and summarized in the main text, it isconceivable that detected PAHs are the source of terrestrialcontamination. This possibility was addressed and ruled out withseveral concurrent control studies as discussed by Plows et al. (7).Spatial maps of m/z 128 and 178 (naphthalene and phenan-threne, respectively) over a Murchison meteorite chondrule areshown in Fig. S2.

Interplanetary Dust Studies: Preliminary AnalysesTo distinguish between IDPs and other collected particles,elemental abundances for each of the analyzed particles weredetermined using scanning electron microscopy with an energy-dispersive x-ray detector (8). This enabled identification ofchondritic particles based on their high relative abundance of Si,Mg, or Fe and absence of nonmeteoritic elements (e.g., K and Ti)(9, 10).

Laboratory Analog StudiesBackground. Interstellar ice mantles likely condense on silicatedust grains and are comprised mainly of water and otherchemical species (e.g., CH3OH, NH3, CH4, etc.) (11–13). The3.28-�m absorption band has been attributed to aromatic com-pounds at low temperatures in molecular clouds (11, 13). The6.2-�m interstellar emission band could be accounted for byPAHs that have one or more nitrogen incorporated within thecyclic aromatic structure, otherwise known as polycyclic aro-matic nitrogen heterocycles (PANHs) (14).

Methods. All samples were prepared in a nearly identical manner.Sample preparation, photolysis, and IR analyses were carried outby the NASA Ames Research Center (ARC) Astrochemistrygroup (15–18) and by the University of Leiden (19), in twoseparate projects. Known quantities of PAHs and/or otherlow-mass compounds were simultaneously vapor-deposited withwater onto substrates held at �10–20 K (15). Ices were typically0.1 �m thick and had, for example, H2O:PAH ratios of 800–3,200for experiments reported by Bernstein et al. (15). Photolysis wascarried out with a microwave-powered, f lowing-hydrogen dis-charge lamp, emitting UV radiation flux equally at the Lyman� line (121.5 nm) and at 160 � 10 nm (20). With a photon yieldof �2 � 1015 photons/cm2�s, experiments were performed withinthe one-photon process regime relevant for interstellar pro-cesses. Photolysis times used were equivalent to modest densemolecular cloud exposures. The IR spectrum was monitoredbefore UV photolysis as well as throughout the irradiationperiod, typically lasting several hundred minutes, yielding adynamic picture of PAH reaction processes under interstellarice-like conditions.

For experiments performed in collaboration with the Univer-sity of Leiden (19), samples were exposed to both UV and solarradiation. After UV photolysis, as described above, samples weresealed and installed on the Exobiology Radiation Assembly(ERA) of the EURECA satellite. Here, they were exposed to thefull solar spectrum for 4 months.

Proton irradiations were done in collaboration with NASAARC and performed using a Van de Graaff generator [0.8 MeVand 1 � 1011 protons/cm2�s (21)]. Proton exposures corre-sponded to proton bombardment for �1 � 106 yr in a densemolecular cloud (17, 22) and were performed in a manner similarto other ice analog experiments discussed in the main text.

Interstellar ice analog photolysis experiments were carried outin collaboration with NASA ARC using perdeuterated PAHsand H2O, and subsequent detection with �L2MS (15). In par-ticular, coronene (C24H12)-D2O and d12-coronene (C24D12)-H2Osystems were studied.

UV plus Solar Radiation Result Details. Ice analogs only exposed toUV radiation revealed some PAHs; however, they were neitheras abundant nor as complex as in UV plus solar irradiatedsamples.

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UV and Proton Irradiation Result Details. For UV photolysis exper-iments of interstellar ice analogs, performed in collaborationwith NASA ARC, a number of coronene derivatives weredetected with �L2MS, as shown in Fig. S3. Intramolecularbridging ethers, which were observed for benzo[ghi]perylene,were corroborated by IR absorption in the 1,095 cm�1 (C-O)stretching region.

In a distinct set of experiments, coronene was exposed toproton radiation in pure solid H2O, CO2, and CO containingices, which are known to be of astrochemical significance (23–25). With �L2MS, oxygen substituted coronene photoproductswere observed in these pure solid ices, most likely a consequenceof trace H2O in the CO2 and CO samples. Hydrogen additionsimilar to that observed for UV irradiation is also seen in theseices, most notably in pure H2O ice. Results were consistent withour initial UV and solar irradiation experiments (15, 19).

Isotopic Substitution Experimental Result Details. Unexpectedly,PAHs were found to easily exchange hydrogen with availabledeuterium (D) on water in the surrounding ice (15). Focusedisotopic substitution experiments, using coronene (C24H12)-D2Oand d12-coronene (C24D12)-H2O systems, showed H/D exchangeto occur via D atom addition as well as by exchange at oxidizedand aromatic edge sites. Enrichment was found to depend on UVflux and ice deuterium concentration (18). H/D exchange was8–15% more facile on oxygenated PAHs, indicating the increas-ing lability of hydrogen on these substituted aromatic com-pounds (15). Interstellar ice mantles have shown increasedamounts of deuterium relative to cosmic abundances (26),potentially providing a source of deuterium for PAH enrich-ment. D-rich aromatic compounds have been found in a numberof extraterrestrial materials, including meteorites and IDPs (27,28). Using �L2MS as a sensitive tool for the detection of PAHsand their D-isomers, research presented by Bernstein et al. (15)and Sandford et al. (18) shows that D-enrichment can occur viaUV photolysis of PAHs in D-rich interstellar ices. It is apparentfrom extensive control studies performed in these experiments,that an icy substrate, small molecules and/or PAHs, and radia-tion are necessary in order for complex photoproducts to beobserved.

Hypervelocity Particle Capture in Aerogel

Materials. Stardust f light spare aerogel used in experimentspresented in the main text were representative of aerogel thatwas installed on the Stardust spacecraft. Flight spare aerogelsamples were obtained from Scott Sandford (NASA ARC) andare representative of the actual aerogel material installed on theStardust spacecraft. Witness coupon aerogel, consisting of smallaerogel blocks on the Stardust Sample Tray Assembly (STA),was located adjacent to particle collection aerogel tiles but wasshielded from actual impacts. This aerogel serves as an idealcontamination control standard for the entire Stardust Missionperiod. Witness coupon aerogel samples (WCARMI1CPN,0,6and WCARMI1CPN,0,7) were also obtained from Sandford andwere prepared by the NASA Stardust Curation Facility (NASAJohnson Space Center). Combined, these witness coupon sam-ples constituted a 10-mm depth profile of the witness aerogel tile.

For hypervelocity particle capture experiments describedhere, Stardust aerogel material was used to mimic the Wild 2particle collection environment. Glass beads (100% borosilicate/soda lime; 20 �m; Duke Scientific) were used as ‘‘blank’’ (i.e.,nonorganic) impactors in this study. Hypervelocity test shots ofthese beads were performed at 6 km/s into Stardust f light spareaerogel by Andrew Westphal at the NASA ARC two-stagelight-gas gun facility (29). Bead impact tracks were removedfrom aerogel cells and dissected for �L2MS surface analysis byWestphal, Christopher Snead, and Anna Butterworth [Univer-

sity of California–Berkeley, Space Sciences Laboratory (UCB-SSL)] using their keystone technique (30). Spatially resolvedanalysis of aromatic compounds on the surface of preparedkeystones was performed by rastering the laser desorption pulseover the area of interest and monitoring PAH integrated peakareas. Both Er:YAG and CO2 lasers were used during the�L2MS analysis of these samples to test the efficacy of these twodesorption wavelengths for future cometary sample analyses. Ascan of the surface was first performed with Er:YAG laserdesorption using 50-�m step sizes. Subsequently, a scan of thesurface using CO2 laser desorption was performed, also using50-�m step sizes. Er:YAG desorption was used first as this laserhas a much smaller spot size at the sample surface, hopefullyleaving detectable amounts of PAHs at the surface, if they werepresent.

Gun shot residue from the NASA ARC two-stage light gas gunwas received from Gerardo Dominguez (UCB-SSL) via MaxBernstein (NASA ARC). This material was produced during thetest shots of 50-�m-diameter borosilicate glass beads at 6 km/s.These samples consisted of three aluminum foils, which werebetween the gun and the aerogel substrate, and one witnessplate, which was behind the aerogel substrate. Fine-grainedparticulate dust covering the side that was exposed to the gun wasdirectly transferred to �L2MS sample platters. Samples wereanalyzed using �L2MS with no further preparation. �L2MS massspectra were obtained by averaging 50 mass spectra takenrandomly across the sample surface. CO2 (10.6 �m; 40-�m spotsize at sample surface) and Er:YAG (2.94 �m; 10-�m spot size)lasers were used for spatially resolved surface desorption. ANd:YAG laser (266 nm; 4 mJ; 1.25 W/cm2) was used for selectiveionization of PAHs.

Silica Aerogel Synthesis. The aerogel synthesis process begins withhydrolysis of a silicon alkoxide precursor, tetraethyl orthosilicate[TEOS, Si(OCH2CH3)4], to form silicon hydroxide. Reactionsbetween these resulting silicon hydroxide compounds ultimatelyform an extended three-dimensional network consisting of Si-O-Si bonds. For TEOS, the balanced chemical equation for theformation of this sol-gel is

Si�OCH2CH3�4 �1� � 2H2O�1�3 SiO2 �s� � 4CH3CH2OH �1�.

[1]

Ultimately an extended three-dimensional network consisting ofSi-O-Si bonds is formed. Following sol-gel formation, super-critical f luid extraction is used to remove most accessory mol-ecules; however, inevitably a small fraction of unreacted alkox-ide groups remains on the silica backbone. The unusually largeaerogel specific free surface area also makes it difficult toremove other sources of carbon [e.g., the organic solventsethanol (C2H5OH) and acetonitrile (CH3CN)] used during itsmanufacture (31–33). Carbon in Stardust aerogel is known toexist in the form of trapped organic volatiles and short (methyl,ethyl, and propyl) aliphatic groups bonded to silicon in theaerogel macrostructure. This aerogel was baked at 300°C for 72 hto remove a large fraction of these volatile organics, although upto 2% carbon by mass still remains (31).

Fullerene Detection

Methods and Materials. A surface-enhanced laser desorptionionization (SELDI) mass spectrometer (ProteinChip SELDISystem PCS4000 Enterprise Model; Bio-Rad) was used for thesestudies, which is similar to instruments used in reported mete-orite studies (ref. 34, for example). Physical barriers includeextraction thimbles and PTFE filters (0.2 �m).

High-mass fullerene envelopes were characterized by mass

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peaks with an even spacing of 24 amu. This corresponded toeven-numbered carbon clusters and was attributed to the loss orgain of C2 fragments in the fullerene structure.

LDMS Advantages. Laser desorption mass spectrometry (LDMS)is a versatile analytical technique for sensitively detectingfullerenes and is capable of reporting characteristic mass-to-charge ratio (m/z) signatures for fullerene identification. Thistechnique can also simultaneously detect a series of fullerenesfrom C60 to beyond C400, making it extremely versatile (35).

�L2MS for Fullerene Detection. Low-power IR laser desorption inthe �L2MS technique would greatly reduce the risk of creatingfullerenes, while UV ionization at 212 nm can effectivelyionize desorbed fullerenes, providing a technique with sensi-tivity comparable to that of LDMS. We discovered, however,that the low-power desorption laser used in our system (CO2laser; 10.6 �m) is unable to desorb aggregated fullerenes, acommon occurrence in evaporated fullerene samples. �L2MSis therefore not suitable for studying extractable fullerenes inmeteorites (36).

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Fig. S1. �L2MS mass spectrum taken on the surface of homogenized (crushed into a rough powder) Murchison meteorite, a CM2 carbonaceous chondrite.Tentative mass assignments are given.

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Fig. S2. Spatially resolved �L2MS analysis of a chondrule in the Murchison meteorite [original data from work performed by Plows et al. (7)]. (A) Opticalmicroscope image of the analyzed chondrule, with a box indicating the sampled area. (B) Integrated peak area map for m/z 128, which is tentatively assignedas naphthalene. (C) Integrated peak area map for m/z 178 (e.g., phenanthrene).

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Fig. S3. Photolysis products observed during UV irradiation of PAHs in water-containing interstellar ice analogs (16). (A) Coronene (C24H12; 300.3588 g/mol),an astrochemically relevant polycyclic aromatic hydrocarbon. (B) Observed photolysis-induced substitutions for different interstellar ice analog compositions.

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Fig. S4. Diagram of microprobe laser-desorption laser-ionization mass spectrometry.

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