Earth and Planetary Science Letters 385 (2014) 206–215

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Earth and Planetary Science Letters

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Highly stable meteoritic organic compounds as markers of asteroidaldelivery

George Cooper a,∗, Friedrich Horz b, Alanna Spees c, Sherwood Chang a

a Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035, USAb Astromaterials Research and Exploration Science, NASA–Johnson Space Center, Houston, TX 77058, USAc Department of Medical Microbiology and Immunology, University of California, Davis, CA 95616, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 July 2013Received in revised form 7 October 2013Accepted 9 October 2013Available online 21 November 2013Editor: B. Marty

Keywords:meteoritesimpact experimentsoxidationphosphonicsulfonicamino acids

Multiple missions to search for water-soluble organic compounds on the surfaces of Solar System bodiesare either current or planned and, if such compounds were found, it would be desirable to determinetheir origin(s). Asteroid or comet material is likely to have been components of all surface environmentsthroughout Solar System history. To simulate the survival of meteoritic compounds both during impactswith planetary surfaces and under subsequent (possibly) harsh ambient conditions, we subjected knownmeteoritic compounds to comparatively high impact–shock pressures (>30 GPa) and/or to extremelyoxidizing/corrosive acid solution. Consistent with past impact experiments, α-amino acids survived onlyat trace levels above ∼18 GPa. Polyaromatic hydrocarbons (PAHs) survived at levels of 4–8% at a shockpressure of 36 GPa. Lower molecular weight sulfonic and phosphonic acids (S&P) had the highest degreeof impact survival of all tested compounds at higher pressures. Oxidation of compounds was donewith a 3:1 mixture of HCl:HNO3, a solution that generates additional strong oxidants such as Cl2 andNOCl. Upon oxidation, keto acids and α-amino acids were the most labile compounds with prolineas a significant exception. Some fraction of the other compounds, including non-α amino acids anddicarboxylic acids, were stable during 16–18 hours of oxidation. However, S&P quantitatively survivedseveral months (at least) under the same conditions. Such results begin to build a profile of the morerobust meteoritic compounds: those that may have survived, i.e., may be found in, the more hostile SolarSystem environments. In the search for organic compounds, one current mission, NASA’s Mars ScienceLaboratory (MSL), will use analytical procedures similar to those of this study and those employedpreviously on Earth to identify many of the compounds described in this work. The current resultsmay thus prove to be directly relevant to potential findings of MSL and other missions designed forextraterrestrial organic analysis.

Published by Elsevier B.V.

1. Introduction

Understanding the survival and alteration of organic matterduring and after impacts into Earth, Mars and other Solar Sys-tem bodies is necessary to assess the importance of exogenousdelivery of organic matter in prebiotic environments. Very lit-tle is known about the general stability of organic matter underthe multitude of conditions possible in the early Solar System,such as impact generated shock waves or near surface residencein reducing or oxidizing conditions. For example, NASA’s MES-SENGER spacecraft shows the probable preservation of asteroidand/or comet organic material and water in permanently shad-owed portions of predominately high-latitude craters of the planetMercury (Neumann et al., 2013; Paige et al., 2013). In additionto Mars Surface Laboratory (MSL; Mahaffy et al., 2012), the Phi-

* Corresponding author.E-mail address: george.cooper@nasa.gov (G. Cooper).

0012-821X/$ – see front matter Published by Elsevier B.V.http://dx.doi.org/10.1016/j.epsl.2013.10.021

lae lander on the European Space Agency’s ROSETTA (Ulamecaet al., 2012) will target several water-soluble compounds, includ-ing amino acids and amides (Meierhenrich, 2008), for analysis oncomet 67P/Churyumov–Gerasimenko.

The goals of the present study were to determine the effectsof impact shock stress and extremely corrosive/oxidizing condi-tions on the survival of selected meteoritic organic compounds.The shock–induced heat and pressure that may have altered ordestroyed pre-existing organic matter in both targets and pro-jectiles depends strongly on impact velocity and the respectivedensities of the carrier materials. Utilizing the hypervelocity im-pact facility at NASA’s Johnson Space Center, we subjected or-ganic compounds, mixed within a relatively pristine matrix of theMurchison meteorite, to impacts of known velocity and shock–pressure amplitude. The amount of organic compounds (standards)added to the meteorite powder insured that any indigenous com-pounds would not interfere with subsequent analyses (Section2.1). After impact, molecular compositions of the products were

G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215 207

analyzed by ion chromatography (IC), UV-VIS spectroscopy, and/orgas chromatography–mass spectrometry (GC-MS) to determine thedegree of survival with increasing shock pressure. Our impactmethods resulted in shock pressures as high as 42.9 gigapascal(GPa, 1 GPa ∼ 9.9 × 103 atm) corresponding to ∼6 km/s impactsof common silicate projectiles and targets at vertical incidence andto somewhat higher velocities at oblique angles, as absolute stressdepends on the vertical component of impact. Our impact exper-iments are thus relevant to the survival of organic compounds ina variety of inner and outer Solar System settings including Mars,the asteroid belt and small bodies such as Titan and Callisto, whererelative encounter speeds are modest (Zahnle et al., 1992). Cur-rent impact velocities in the asteroid belt are estimated to rangefrom about 4 to 12 km/s with an average value of 5.8 ± 1.9 km/s(Farinella and Davis, 1992).

In chemical stability/oxidation tests, selected compounds wereplaced in excess “aqua regia”, i.e., a 3:1 molar mixture of hy-drochloric acid:nitric acid, for various times at either 22 ◦C or100 ◦C. Each individual acid is of course corrosive and oxidizing,however, their mixture generates additional oxidizing species in-cluding molecular chlorine (Cl2) and nitrosyl chloride (NOCl). Partof its long usefulness is the ability to decompose a variety of sub-stances – elemental (e.g., gold), mineral, and organic – at relativelylow temperatures. The oxidation stability of organic compoundsmay also be relevant in the analysis of Martian soil (Benner et al.,2000). Perchlorates (XClO4) found in this soil (Hecht et al., 2009)could have destroyed potential organics, by oxidation, during heat-ing experiments in NASA’s 1976 Viking mission (Navarro-Gonzálezet al., 2010). Although other oxidants were also possibly present(Hunten, 1979), Cl2 can be generated by the heating of a hydratedperchlorate (MgClO4·6H2O) (Devlin and Herley, 1986; Solymosi,1968). General interaction of organic compounds with chlorine alsocommonly results in chloro-substitution or chloro-addition com-pounds (e.g., Fig. 5). These are all potential outcomes during thecurrent MSL analyses. It is almost certain that in the past, organiccompounds in planetary sediments also interacted with various ox-idants in a liquid phase. Therefore, our laboratory conditions arerelevant to, and likely surpass in harshness, many of the oxidizingconditions such compounds actually encountered.

The organic compounds selected for the experiments are shownin Fig. 1. All were chosen because they are known constituentsof carbonaceous meteorites (Pizzarello et al., 2006). These include:S&P; “alpha-amino acids” (α-amino acids), i.e., compounds withthe amino group on a carbon adjacent to a carboxyl group; non-αamino acids; keto acids; PAHs; dicarboxylic acids and hydroxy-tricarboxylic acids (citric acid and isocitric acid). α-Amino acidswere included in shock experiments as a comparison to previousshock–impacts of amino acids in a meteorite matrix (Peterson etal., 1997). Also shown in some cases are the cyclic or open-chainanalogs of non-α amino acids and relevant parent-body reactions(in parentheses). Due to their distinctive compositions, the S&Pmay serve as markers of the asteroidal delivery of organic mate-rial (see Section 4). Derivatization of potential non-volatile com-pounds followed by GC-MS is now being used on MSL (Mahaffyet al., 2012): this technique includes the same derivatizing reagent(N-Methyl-N-(tert-butyldimethylsilyl) trifluoroacetamide) originallyused to identify S&P in meteorites therefore the mass spectra areknown. Our current results may be relevant to questions such as,which organic compounds from asteroids and comets survive im-pacts and what are their subsequent chemical stabilities in rela-tion to potentially harsh conditions found at the surfaces or near-surfaces on bodies such as the early Earth, Mars or outer moons?

2. Material and methods

2.1. Shock experiments and sample preparation

For the impact–shock experiments, a mixture of water-solublestandard compounds, Murchison meteorite matrix, and water wasvortexed and sonicated (for homogenization) and dried by ro-tary evaporation. The sample was then sat over sodium hydrox-ide for further drying. Likewise a mixture of PAHs, acetonitrile,and matrix was homogenized and dried. The following com-pound abbreviations are used below: MSA = methanesulfonic acid;ESA = ethanesulfonic acid; iPSA = isopropanesulfonic acid; nPSA= n-propanesulfonic acid; MPA = methylphosphonic acid; tBPA= tert-butylphosphonic acid. Samples containing the followingamounts (weight%) of each compound were shocked. ExperimentalSeries A (Fig. 1; Table 1): MSA, 1.7; ESA, 0.065; nPSA, 0.028; iPSA,0.031; MPA, 0.09; tBPA, 0.41; glycine, 1.9; alpha-amino isobutyricacid (AIB), 0.3. Series B (PAHs: Fig. 3, Table 2): coronene, 0.13;2-methyl anthracene, 0.25; acridine (deuterated, d9), 0.14. SeriesC (Supplementary Table 1): MSA, 0.17; iPSA, 0.17, butane sulfonicacid (BSA), 0.27; MPA, 0.32; tBPA, 0.31; AIB, 1.2; glutamic acid, 0.2.The given weight percent of the sulfonic and phosphonic acids arewell above (∼500–1000 times) the reported indigenous meteoriticamounts of each compound (Cooper et al., 1992); therefore any in-digenous components did not contribute to the observed survivalrates. The same is likely true for PAHs – with reported (bulk) con-centrations of 15–28 ppm in Murchison (Pizzarello et al., 2006).

The target powders were encapsulated into metal target-containers that were impacted by relatively thin, flat metal plates,accelerated by a 20 mm powder propellant gun. This apparatus,sample containers, and sample loading and recovery procedureswere described previously (Peterson et al., 1997; Horz, 1970; Skálaet al., 2002). By selecting materials for which the equations ofstate are known (Marsh, 1980) (e.g. Lexan, Al 2024; SS 304, Tung-sten) for both the target-container as well as the projectile flyerplate, one can solve for the shock stress (Duvall et al., 1958) ex-perienced by the target from only the measurement of projectilevelocity; the latter was measured via the projectile-induced occul-tation of 4 IR lasers that are trained onto photodiodes. The peakpressure depends not only on impact velocity, but also on bothtarget and projectile density which is the reason why materialsranging in density from 1.2 g/cm3 (Lexan) to 16.8 g/cm3 (W-alloy)were employed, such that a wide variety of shock stresses couldbe achieved despite the relative modest projectile velocities (up to2 km/s) typical for powder guns. By judicious selection of targetand projectile density, one can simulate silicate/silicate (3 g/cm3)as high as 6 km/s or 45 GPa with an impactor that actually onlytravels at 2 km/s. In a first set of impact experiments (Series A)all target containers were made from SS304. Series B and C em-ployed Al containers at low pressures (<20 GPa), SS304 containersat intermediate pressures (20–30 GPa) and the tungsten alloy at>30 GPa. Flyerplates (projectiles) varied from pure Lexan to Al, SSand W-alloy.

The actual target is sufficiently thin to assure minimum pres-sure decay over the total sample thickness; total target charge(sample) was 70 mg, pressed into a flat cylinder of 8 mm diam-eter. The target container was sealed to prevent the combustiongases from contaminating the target sample. The impact cham-ber was evacuated to a pressure of 10−2 Torr during all experi-ments. A stream of chilled N2 gas, fed from a LN2 Dewar, wasused to cool the impacted target assembly during all machin-ing operations that were needed to exhume the target of in-terest from the substantially deformed metal container. As withmany past impact experiments, peak temperatures could not bemeasured directly (e.g., Bunch et al., 1996; Peterson et al., 1997;Mimura and Toyama, 2005), but independent experiments and

208 G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215

Fig. 1. Compounds used in impact and oxidation studies. Compounds and schemes in parentheses illustrate open-chain/cyclic pairs and possible reactions on meteoritesparent-bodies.

G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215 209

Table 1(Series A) Percent survival of sulfonic and phosphonic acids vs. shock pressure.

Shock press(GPa)

% Survival1

MSA ESA iPSA nPSA MPA tBPA

3.7 84 84 – 69 23 164.8 103 103 66 87 22 129.4 91 87 50 61 33 19

18.1 100 96 55 76 20 1330.7 33 30 0 36 18 842.9 18 21 0 0 23 7

1 Amino acids (not shown) only survived at trace levels (<1%) above at ∼� 18 GPa.

Table 2Percent survival of PAHs vs. shock pressure (Series B).

Shock press(GPa)

% Survival

Coronene Met. anthracene Acridine

9.2 61 52 8418 69 47 2828 20 14 1236 4 6 8

thermodynamic considerations suggest some 400–450 ◦C at 42 GPa(Stoffler, 2000). The target containers are typically removed fromthe impact chamber within a few minutes after impact and allwere, at most, warm to the touch, including the 42.9 GPa experi-ment. During the impacts of PAHs embedded in mineral matrices,Mimura and Toyama (2005) estimated that peak and “residual”temperatures were quenched on the order of ∼1 μs–10 ms.

2.2. Preparation of samples for oxidation experiments

All aqua regia trials were done in Teflon-capped Pyrex tubes.Tubes were previously heated for �10 hr at 550 ◦C for clean-ing. S&P were put in two separate tubes: 2.2 μmole of eachsulfonic acid in one tube; 2.0 μmole MPA, 2.0 μmole EPA and2.8 μmole tBPA in the other. 2 ml of fresh aqua regia was addedto each tube and each sample was allowed to sit at times andtemperatures indicated in Table 1. Other compounds were testedin groups (i.e., in same tubes): group (1) = glycine, L-alanine(2-13C) and DL-glutamic acid; group (2) = glycine, DL-alanine(12C) and DL-glutamic acid; group (3) = 3-aminopropanoic acid(β-alanine), 2-methylalanine (α-AIB), 4-aminobutanoic acid, L-proline, 6-aminohexanoic acid and DL-phenylalanine; group (4) =DL-5-methyl-2 pyrollidinone, hydantoin, (S)(−)β-hydroxy−γ -bu-tyrolactone [(S)(−) 2(3H)-furanone-dihydro-4-hydroxy], (R)(+)α-hydroxybutyrolactone[(R)(+) 2(3H)-furanone-dihydro-3-hydroxy],isovaline, Succinic acid, (R)(+)methyl succinic acid, L-leucine,6-oxoheptanoic acid, 7-oxooctanoic acid, pimelic acid, DL-isocitriclactone, L-cystine (the dimer of cysteine).

Group (1): from ∼ 14 to 20 micromole (μmole) each weremixed with 3 ml aqua regia; group (2): ∼4.8 to 7 μmole each+1 ml aqua regia; group (3): ∼3.8 to 5.6 μmole each +1 ml aquaregia; group (4): ∼1.8 to 2.5 μmole (succinic acid was 5.8 μmole)+1.2 ml aqua regia. The compounds were treated at times andtemperatures indicated in Table 4.

2.3. Analysis of impact sulfonic acids and phosphonic acids (S&P)

Individual samples in containers with polar compounds (SeriesA and C) were initially removed with a spatula and placed in flasksof ultra pure (18 M�) water. Afterwards, to recover any adheringcompounds, each container was placed in the corresponding flaskof water; the entire flask was then sonicated and heated with mix-ing. The analyses of S&P in both the impact and oxidation experi-ments were performed by ion chromatography. Although multiple

chromatographic conditions were used, depending on the com-pound(s), in general these methods were similar to previous anal-yses (Cooper et al., 1992): a portion of the solutions were filteredand injected directly into an ion chromatograph (IC) with con-ductivity detection (Dionex Corp.). Compounds were eluted witha sodium carbonate (Na2CO3)/sodium bicarbonate (NaHCO3) buffer(2 ml/min) and 10 × 100 mm “Star Ion” column (PhenomenexCorp.). Buffer concentrations varied from 1.5 to 7 mM with nearlyequal concentrations of Na2CO3 and NaHCO3. All procedures werefirst performed on portions of unshocked powder (from the sameoriginal mixture as the shocked powder) and it was demonstratedthat the S&P could be recovered quantitatively from the meteoritematrix (by comparison to a solution of pure standard). The re-sponse (IC) of individual compounds in the unshocked sample isregarded as the baseline from which to evaluate the shock–induceddecomposition.

2.4. Analysis of impact amino acids

The solution remaining after S&P analysis was dried and ap-plied to a column of cation exchange resin. The column was thenrinsed with water and ammonium hydroxide to remove all organiccompounds. The combined rinses were then dried and the organicsderivatized in preparation for analysis by gas chromatography–mass spectrometry (GC-MS). The derivatization reagent was N-Methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA)with 1% tert-butyldimethylsilyl chloride, Regis Chemical Co. AFinnigan ion trap GCQ GC-MS equipped with a DB-17 (30 or 60 m× 0.25 mm) fused silica capillary column (J&W Scientific) wasused for separation and verifications of compounds. Typical con-ditions were: injector and transfer line temperature, 230 ◦C; ovenprogram, 45 ◦C to 230 ◦C at 3 ◦C/min and held for 30 min; heliumwas the carrier gas at 1 ml/min.

2.5. Analysis of impact polyaromatic hydrocarbons

PAHs were analyzed by extracting the shocked samples withacetonitrile. This solvent was found to be the most suitable for awider range of PAHs than described here. A Gilson model G121fluorometer was used for detection. Typical conditions: solventcompositions = 80% acetonitrile–20% water; flow = 0.5 ml/min;column = an Alltech C18 adsorbosphere-150 × 4.6 mm. The fluo-rometer was connected to the above ion chromatograph.

2.6. Analysis of oxidation (aqua regia) samples

After each specified oxidation treatment, samples were evap-orated to dryness (rotary evaporation), rinsed with water andmethanol (redrying after each) and sat in a sodium hydroxidedesiccator for at least 24 hrs. For IC analysis of S&P, measuredamounts of water were then added to each for injection into theIC utilizing the same general conditions as above (impact S&P) alsosee Supplementary Fig. 1. Amino acids were analyzed by GC-MS astheir isopropyl ester/trifluoroacetyl derivatives: to the above dried

210 G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215

Table 3Oxidation stability (% survival) of sulfonic and phosphonic acids1. Compounds were treated with excess aqua regia (3:1 HCL:HNO3) for the times and temperatures indicated.

Compound Time Temp(◦C)

% Survived % SO4 or PO42

MSA 24 hrs 22 100 0.07 mos 22 86 85 min 100 95 0.040 hrs 100 97 3.7

ESA 24 hrs 22 867 mos 22 885 min 100 9340 hrs 100 73

MPA 25 hrs 22 92 –7 (& 7.5) mos 22 99 –5 min 100 94 –42 hrs 100 102 –

EPA 25 hrs 22 98 –7 (& 7.5) mos 22 100 –5 min 100 88 –42 hrs 100 97 –

tBPA 42 hrs 100 52 –7.5 mos 22 100 –

1 Some values might be minimum: it is seen that with only 5 min (100 ◦C) or 24–25 hrs (22 ◦C) of aqua regia, the apparent recoveries of some compounds are decreased:from 2% in (EPA) to 14% (ESA). However, this likely indicates that recovery of the compounds are initially decreased by unknown (i.e., non-oxidation) losses, e.g., irreversiblebinding to glass tubes, etc. Consistent with this, there is no observed oxidation of the sulfonics to SO4 at these time points. Such effects would be unpredictable given thatsome samples do show complete recovery. Errors for two samples each at 7 mos: MSA = (±1); ESA = (±3); MPA = (±2); EPA = (±3); % SO4 = (±6).

2 SO42− values apply to both MSA and ESA: these compounds were in the same tube. The listed SO4 levels seem to show a better qualitative correlation to MSA oxidation,

i.e., MSA oxidizes to SO4 while ESA either did not or did so to a much lesser degree than MSA. Data with error bars are from two experiments; others are from one. No PO4

was detected (Section 3.2).

Fig. 2. Survival of sulfonic and phosphonic acids vs. shock pressure. MSA =methanesulfonic acid; ESA = ethanesulfonic acid; iPSA = isopropanesulfonic acid;nPSA = n-propanesulfonic acid; MPA = methylphosphonic acid; tBPA = tert-butylphosphonic acid.

samples were added ∼0.5–1.0 ml of a mixture of isopropyl alco-hol/acetyl chloride (Grace, Waukegan Ill, USA). Samples were thenheated at 100 ◦C for 30–45 min. After drying a 2:1 mixture of tri-fluoroacetic anhydride: ethyl acetate was added, the samples wereheated at 100 ◦C for 15 min allowed to cool and an appropri-ate volume of acetonitrile added before injection into a GC-MS.The GC-MS was an Agilent Tech. (California, USA) 6890N GC/split-less injection, interfaced to a 5975B quadrupole mass spectrometerwith electron ionization. Typical run parameters were: oven ini-tial temperature = 35 ◦C – held 1.5 min, then ramped to 200 ◦Cat 3◦/min – held 60 min. The injector and auxiliary (transfer line)temperatures were 220 ◦C in most cases. The GC columns were aDB-17 (as above) and an Agilent Chirasil-Dex CB (25 m, 0.25 mm,0.25 μm) with helium as the carrier gas at 1.0 ml/min.

3. Results

3.1. Impact stability of organic compounds

Fig. 2 and Table 1 show the results from the first impacttests (Series A) of sulfonic and phosphonic acids with the un-shocked starting mixture representing 100% in each case. Sup-

Fig. 3. The survival of selected PAHs vs shock pressure (Series B).

plementary Fig. 1 is one illustration of the recovery of shockedand unshocked compounds from the meteorite matrix. In the tar-get powders of Series A and C (Section 2.1) were the α-aminoacids glycine, 2-methylalanine (α-aminoisobutyric acid) and glu-tamic acid. These amino acids were only included as a comparisonto previous shock–impacts of amino acids in a meteorite matrix(Peterson et al., 1997). Our results show that only trace levels(<1%) of these compounds survived above ∼18 GPa (not shown).This is consistent with the low shock-survival (at �30 GPa) ofamino acids embedded in solid matrices (Peterson et al., 1997;Bertrand et al., 2009) in previous work where various aspectsof amino acid decomposition, racemization, and synthesis of newcompounds during impacts have also been described (Peterson etal., 1997; Bertrand et al., 2009; Blank et al., 2001).

Fig. 3 and Table 2 show the corresponding results of shockedPAHs (Series B): from 4 to 8% survived at the highest pressure of36 GPa. This is in rough agreement with the results of Mimuraand Toyama (2005) who observed the decomposition of approxi-mately 95% of PAHs at 30 GPa: however, in the latter experiments

G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215 211

Table 4Oxidation stability (% survival) of selected meteoritic compounds. Compounds were treated with excess aqua regia (3:1 HCL:HNO3) for the times and tempera-tures indicated. See Fig. 1 for structures.

Compound Time (hrs) Temp(◦C)

% Survived1

α-Amino acids2

Glycine 16 22 043 22 018 100 0

Alanine 16 22 043 22 018 100 0

Glutamic acid 16 22 043 22 018 100 0

2-Methylalanine (α-Aminoisobutyric acid) 16 22 0Phenylalanine 16 22 0Isovaline 17 22 0Cystine 17 22 0Leucine 17 22 0Hydantoin (cyclic N-carbamyl glycine) 17 22 n.d.Proline 16 22 97

Non-α amino acids3-Aminopropanoic acid (β-Alanine) 16 22 166-Aminohexanoic acid-cyclic (lactam) form 16 22 336-Aminohexanoic acid3 16 22 505-Aminopentanoic acid4 16 22 664-Aminobutanoic acid 16 22 775-Methyl-2-pyrrolidinone (a lactam) 17 22 100

Keto acids4-Oxopentanoic acid (Levulinic acid)4 18 22 06-Oxoheptanoic acid 17 22 07-Oxooctanoic acid 17 22 0

Tricarboxylic-hydroxy acidsCitric acid 17 22 9Isocitric acid-cyclic (lactone) form 17 22 71

Deoxy sugar acids2,4-Dihydroxybutanoic acid-cyclic (lactone) form 17 22 793,4-Dihydroxybutanoic acid-cyclic (lactone) form 17 22 92

Dicarboxylic acids5

Succinic acid 17 22 (100)2-Methylsuccinic acid 17 22 86Pimelic acid 17 22 94

1 Single ion monitoring showed that at “0%” trace amounts of some compounds are possibly present. Compounds were tested in groups – see (Section 2.2).Errors for two trials each: proline = (±3); 2-Aminopropanoic acid = (±7); 6-Aminohexanoic acid = (±20); 4-Aminobutanoic acid = (±7). Leucine = L-leucine.Most other % survival values resulted from single runs at the indicated time point.

2 Alanine used for the 43-hour experiment was the 13C L-enantiomer and 12C-DL for 16- and 18-hours experiment. No cysteine, the monomer of cystine, wasseen after aqua regia treatment. The survival of hydantoin is as yet undetermined – there is survival of an uncalculated amount of glycine resulting from itshydrolysis/oxidation. n.d. = Not determined.

3 One run (out of three) was run alone in a separate aqua regia solution.4 Run alone in separate aqua regia solution.5 Methyl succinic acid is the (R)(+) enantiomer. In one experiment the dicarboxylic acids were grouped with the keto acids and their abundances could have

been increased by decomposition of the keto acids. Subsequent runs of these acids singly showed this to be unlikely or not significant for methyl succinic acidand pimelic acid but the single sample of succinic acid was lost.

PAHs were added to mineral (olivine and serpentine) matrices asopposed to the current meteorite matrix. The literature on PAHsadded to matrices for shock impact is limited. Work on the impactsurvival of native PAH in meteorite matrices (i.e., shocked sam-ples of meteorites with no added PAHs: e.g., Tingle et al., 1992;Bunch et al., 1996) generally show (qualitative) PAH survival atlower shock pressures (∼� 20 GPa: Tingle et al., 1992) while bothof these studies find it likely that PAH’s are synthesized from mete-oritic “macromolecular” carbon at higher shock pressures or higherimpact velocities. In the current work synthesized PAHs were notspecifically searched for, however significant amounts of new com-pounds after impacts were not observed.

The sulfonic acids survive to a higher degree at lower shockpressures than other compounds while some fraction of the ma-

jority of compounds survive at the highest pressure (42.9 GPa).Series C (Supplementary Table 1) is a repeat impact test for someof the compounds, but with a different composition of metal targetcontainers and flyer plates (Section 2.1). Results are qualitativelysimilar to those of Series A with the exception of MPA; it survivesat comparable percentages to MSA. There is considerable varia-tion of survival rates at lower pressures between Series A and Cpossibly related to different combinations of target and flyer platemetals. These effects could also be due to factors such as samplehandling and recovery of post-shocked compounds. Occasionallyseen is the apparent overabundance of some compounds. In pre-vious work, such overabundance (of an amino acid) was ascribedto possible inhomogeneity of initial mixing and sampling or chro-matographic co-elution (Peterson et al., 1997). However, the most

212 G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215

Fig. 4. Ion chromatograms illustrating the oxidation and hydrolysis stability of sul-fonic and phosphonic acids in aqua regia. Each aqua regia sample is only one of twotrials. The amount of SO4

2− (from the oxidation of sulfonic acids) in this MSA/ESAsample is by far the largest seen in all trials (Table 3). More consistent with sulfonicstability, other samples show very little or no SO4

2− . There is no obvious evidencefor phosphate, a possible oxidation product of the phosphonic acids. The shownMSA, ESA, and SO4

2− were run under 1.5 mM Na2CO3/NaHCO3 (equimolar) buffer(Section 2.3). The phosphonic acid chromatograms were obtained under conditionsto quantify only tBPA: a 3.5 mM CO2−

3 buffer was the eluent. A more dilute buffer

(1.5 mM CO2−3 ) was also used for better separation and quantification of MPA and

EPA (not shown): MPA eluted at ∼16.4 min and EPA at ∼19.9 min.

significant point of the results is that both sets of data indicatethat some S&P may survive impacts of even higher shock pressuresthan used in the present experiments.

There is no previous data on the survivability of S&P in impactshocks: our results likely reflect the relatively stability of the C–Sand C–P bonds. Although, to date, there are no direct measure-ments of temperatures in impact experiments, if the organic com-pounds in the present experiments experienced temperatures nearthe estimated 400–450 ◦C (Section 2.1) at our highest shock pres-sure (42.9 GPa), the noted thermal stability of several simple alkylsulfonic acids may offer an explanation for the observed partialsurvivals. Several of these compounds have shown high survivalrates at 345 ◦C for three hours (under caustic conditions, Sec. 3.2)while our sample recovery procedures limit the high-temperatureexposure of organic compounds to minutes (e.g., Peterson et al.,1997). Likewise, alkyl phosphonates are relatively thermally stable(Freedman and Doak, 1957).

Although, in general, there is variation in the data on thestrength of atomic bonds, the C–S and C–P single bond strengthsare comparable to C–C. For example, a range of bond dissocia-tion energies (298 K) of C–C (142–145 kcal/mol) and C–S (177–182kcal/mol) have been reported (Darwent, 1970) as well as bond en-ergies (298 K) of C–C (64 kcal/mol), C–S (62 kcal/mol) and C–S(57 kcal/mol) (Huggins, 1953). However, unlike amino acids, rel-atively facile shock-decomposition pathways such as decarboxy-lation and deamination (Peterson et al., 1997) are not availableto S&P. Possible resonance stabilization, i.e., partial double bondcharacter (C=S, C=P) would further add to the stability of S&P.The addition of water to the meteorite matrix (Section 2.1) mayhave played a role in the overall survival of compounds. For ex-ample, methanesulfonic acid is calculated to form relatively stablehydrated clusters (Li et al., 2007). Clearly more laboratory and the-oretical data is desirable for C–S and C–P compounds.

Fig. 5. Results of same-tube treatment of α- and non-α amino acids with aqua re-gia (GC-MS chromatograms). 22 ◦C sample taken at 16 hours. The initial amounts ofamino acids are the same in the control and sample therefore scales are comparable(actual % survivals were calculated from standard curves). Amino acids in parenthe-ses were not detectable or possibly present in trace amounts. The compounds at∼50 minutes are chloro-substitution or – addition products – others are also seenbut definitive identifications are pending.

3.2. The oxidation stability sulfonic and phosphonic acids

Table 3 gives individual survival rates of S&P subjected to aquaregia for various times and temperatures: the data show the re-markable relative stabilities of S&P towards oxidation, e.g., compareamino acids in Table 4. Fig. 4 is a representative chromatogram ofthe recoveries of S&P subjected to aqua regia. The one and two-carbon compounds (MSA, MPA, and ESA, EPA) show survivals of atleast 86% up to ∼ seven months at 22 ◦C in aqua regia and thelower survival values might be minima – see error discussion inTable 1 footnotes. Still, considering this possibility, the phospho-nic acids appear to be more stable than the sulfonic acids at sevenmonths (∼100% survival vs. ∼87%, respectively). Consistent withthis, phosphate, an expected phosphonic acid oxidation product,was not seen. However, there is also no evidence for phosphate inthe 100 ◦C/42 hour sample where the survival of tBPA is loweredto 52%. Perhaps tBPA is decomposed to other products in this par-ticular oxidation mixture (3HCl:HNO3). The 52% survival of tBPA(100 ◦C) is the lowest survival of all S&P and may reflect the lowerrelative stability of branched compounds, however, like the otherphosphonic acids, it also survives quantitatively for seven monthsat 22 ◦C. The general stability of the C–P bond has been noted(Freedman and Doak, 1957; Kononova and Nesmeyanova, 2002;Ford et al., 2010) and is in contrast to the more common biologi-cal organic phosphate and sulfate esters. In addition to the abovestability in acid, primary sulfonic acids, up to seven carbons, haveshown survival rates of at least 80% under extremely caustic high-temperature conditions: 3.7 N NaOH, 345 ◦C for three hours (ethylsulfonic acid was an exception with ∼35% survival; Wagner, F.C. &Reid, 1931).

3.3. The oxidation stability of most α-amino acids

In the case of amino acids Table 4 shows that at 22 ◦C pro-line was the only α-amino acid to survive oxidation/hydrolysis,

G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215 213

Fig. 5. While glycine itself does not survive oxidation, some fractionof glycine is seen after oxidation of hydantoin, a cyclic derivativeof glycine, implying some protection by hydantoin. Other cyclicderivatives might also confer stability to certain tested compounds(see following sections). The issue of acidic racemization of aminoacids is not significant in the present results: Table 4 shows com-plete destruction (0% survival) of nearly all the chiral amino acids.The two that survive (proline and 4-methyl-4-aminobutanoic acid)do so quantitatively (97 and 100%, respectively), therefore the bondbreaking and re-forming necessary for racemization was likelyminimal, i.e., when there is racemization there is nearly alwaysat least some decomposition. No evidence of racemization of thesurviving chiral amino acids is seen, consistent with generally ob-served low levels of amino acid racemization in acidic solutions,e.g., racemizations conducted in concentrated HCl, 110 ◦C for 22hours (Manning, 1970). Overall, amino acids are known to un-dergo decomposition, principally by decarboxylation and deamina-tion, under a variety of conditions (Hare et al., 1978).

3.4. The oxidation stability of proline

Proline is the only common amino acid that contains analiphatic-cyclic side chain and a secondary nitrogen in the α po-sition. These features likely lend stability under certain acidic/oxi-dizing conditions: for example proline is the only protein α-aminoacid whose amino group is not lost as part of N2 during reactionwith nitrous acid (HNO2), i.e., the Van Slyke Method of determin-ing amino nitrogen. Under the current conditions proline also didnot undergo significant loss of the amino group by reaction withNOCl (generated by aqua regia) as is also common with aminesand amino acids. Proline, being an amino acid with a non-aromaticside chain, should be more stable than phenylalanine towardschloro-substitution which takes place readily in unsaturated com-pounds: several chloro-substitution (or -addition) compounds areseen in samples with phenylalanine (Fig. 5).

Proline might also have a survival advantage over other α-ami-no acids under actual neutral-alkaline pH conditions of the mete-orite parent-body. Carbonaceous meteorite water extracts are com-monly in the pH range of ∼7–10. The reported values of the pKaof proline’s amino group (∼10.5–11) is the highest of α-aminogroups (i.e., not referring to side chains) of the protein aminoacids therefore its carboxyl and amino groups will remain a rela-tively stable internal salt comparatively longer with increasing pH.In contrast, if a meteorite parent-body were to reach pH ∼ 10–11,e.g., by drying and concentrating alkaline salts, a substantial frac-tion of the amino groups of the other amino acids – and simpleamines – will be neutral (e.g., R-NH2); a state in which oxidationreactions are facile. It would be of interest to measure proline rel-ative abundances in the more altered meteorites.

3.5. The oxidation stability non-α amino acids

Our data on the oxidation stability of the non-α amino acidsrelative to alpha-amino acids may qualitatively relate to the phe-nomenon of significantly higher relative abundances of severalnon-α amino acids in some, more altered, carbonaceous meteorites(Glavin et al., 2010; Burton et al., 2012; Burton et al., 2013). Itwas suggested that the general amino acid profile in meteoritesmight be due to, among other factors, oxidizing conditions thatprevailed on their parent-bodies (Martins et al., 2007). Possiblyrelated to this, and unlike the above α-amino acids, the presentnon-α amino acids can form cyclic-anhydrous derivatives, lactams.Lactams with more than three carbons in the ring are generallythe more stable forms of the non-α amino acids (Fig. 1): an en-tire suite occurs in the Murchison (Cooper and Cronin, 1995) and

other meteorites. It is generally known that under mild conditionslactams are difficult to hydrolyze to their corresponding open chain(non-α) amino acids (Wan et al., 1980; Lánská et al., 1992; Lánská,1997) where further alteration and decomposition would be morelikely to occur. It was also suggested that, due to their lactams orto the general structure of the non-α amino acids, they might havea higher degree of protection during various modes of alterationin meteorite parent-bodies (Monroe and Pizzarello, 2011). Even inaqua regia, temperatures higher than 22 ◦C are apparently requiredfor lactam ring-opening: when two non-α amino acids were ini-tially added in their lactam forms (Table 4) their open-chain formswere not observed in the final products even though some decom-position had occurred in one case (6-aminohexanoic acid).

In general, water-soluble meteoritic organic compounds arethought to have formed under parent-body conditions of rela-tively low alteration temperatures and neutral to slightly alkalinepH (Brearley, 2006). However, due to their ability to polymerize,some knowledge of lactam stability might be gained from stud-ies in polyamide chemistry although such studies often employvery contrasting conditions, e.g., utilizing solvents, radical initia-tors, relatively high temperatures (Lánská et al., 1992; Lánská,1997). With this in mind, 2-pyrrolidone was shown to be theleast stable towards oxidation among four- to thirteen-memberedring lactams (Lánská et al., 1992): 2-pyrrolidone is the lactam of4-aminobutanoic acid (Table 4). In contrast, comparisons amonghomologous lactams show that those with alkyl substituents onthe ring generally have higher stability than the unsubstituted(Reimschuessel, 1971; Ebenda, 1976; Ravve, 2012) due to favor-able enthalpy and entropy effects of the number (and position;Ebenda, 1976) of ring substituents (Reimschuessel, 1971; Ebenda,1976). This is apparently consistent with our data as shown by the100% survival of 5-methyl-2-pyrrolidinone (Table 4) – the highestsurvival of all non-α amino acids/lactams. While our current re-sults on the oxidation of compounds may suggest an explanationfor the relative abundance of non-α amino acids in certain mete-orites, there are suggested formation mechanisms that might havelead to the presence and/or high relative ratio of these compoundsin meteorites (Glavin et al., 2010; Burton et al., 2012, 2013). Ourstudy presents data relevant to certain extreme conditions of oxi-dation that might be found in Solar System environments but doesnot yet allow definite conclusions to be drawn on non-α aminoacid/lactam stability under those meteorite parent-body conditionsthat were not as harsh.

3.6. The oxidation stability of other cyclic compounds and dicarboxylicacids

Citric acid and its isomer isocitric acid (added as the cyclic, lac-tone, form) have vastly different survival rates. The much greatersurvival of isocitric acid – including possible citric–isocitric isomer-ization – is in contrast to the low isocitric/citric abundance ratio inmeteorites, perhaps adding to the case for a specific and prolongedmeteoritic synthesis of citric acid and certain other products ofpyruvate chemistry (Cooper et al., 2011). The relative stability ofisocitric acid may be due to the ability of some hydroxy com-pounds to form lactones: the same holds for the deoxy sugar acids.Also, no evidence of racemization of the lactones (or any com-pound) was seen. Lactones, as with the above lactams, are gen-erally more stable than the corresponding open-chain compound(Reimschuessel, 1971). The dicarboxylic acids also have significantlevels of survival (at 22 ◦C; 17 hours) although the abundance ofsuccinic acid may have been influenced by the production of thisacid from the keto acids (see footnotes in Table 4).

214 G. Cooper et al. / Earth and Planetary Science Letters 385 (2014) 206–215

4. Discussion

Our data opens the possibility that relatively stable and struc-turally distinctive compounds may be the best signatures of as-teroidal (and possibly cometary) delivery to Solar System bodies.Along with high D/H and 13C/12C ratios (Cooper et al., 1997) themeteoritic sulfonic and phosphonic acids contain sulfur and phos-phorus bonded directly to carbon. Far more common on Earthare the relatively labile sulfate and phosphate esters, (SO4)Rx and(PO4)Rx, where sulfur, phosphorus and R (organic groups) arebonded to oxygen. However, these esters are due to biological ac-tivity on Earth and no simple sulfate or phosphate esters havebeen identified (to date) in meteorites indicating the possibilitythat such esters are not formed abiotically, were relatively un-stable after formation, or are present but in very low amountsin meteorites. Of the rare C–P compounds on Earth (i.e., in biol-ogy) the most common are 2-aminoethylphosphonic acid and itsderivatives (Rosenberg and La Nauze, 1967). Also, the catabolism ofmethylphosphonic acid (for a source of P) by a microbe has beenfound to play a key role in methane generation in the oceans (Met-calf et al., 2012). In the meteoritic compounds, the stable C–S andC–P bonds are thought to be descendant from interstellar precur-sors. Another distinguishing feature of the group (homologous se-ries) of meteoritic sulfonic acids is the non mass-dependent sulfurisotope fractionations (enrichment or depletions of 33S) found inmethane-, ethane- and propanesulfonic acids (Cooper et al., 1997).These are thought to be possible only by a gas-phase UV irradia-tion of symmetrical precursors: perhaps in the interstellar mediumor proto-solar nebula. These compounds or their immediate pre-cursors were then incorporated into asteroids: their discovery onany Solar System body would serve as a distinctive indicator of ex-ogenous contribution of organic compounds to planetary surfaces.

While the identification of any non-volatile organic compoundson other planets or moons would be significant, the question oftheir origin(s) might still remain. For example, in addition to as-teroidal/meteoritic sources of such compounds as carboxylic acids,aromatic compounds, etc., experiments that simulated hydrother-mal vent conditions (likely present on multiple early Solar Systembodies) also produced a variety of interesting compounds (Huberand Wachtershauser, 1997; Cody et al., 2000; Hazen and Deamer,2007) – some with properties similar to meteoritic components(Hazen and Deamer, 2007). Likewise, the presence of α-aminoacids could be the result of, for example, the in situ reaction ofammonia with carbonate or other carbon sources utilizing a varietyof energy sources. Meteoritic amino acids and other compounds dohave high D/H and 12C/13C ratios suggestive of interstellar origins(Pizzarello et al., 2006). Therefore, asteroidal delivery might be in-dicated if such compounds, as members of homologous series ofcompounds (including non-biological isomers as seen in carbona-ceous meteorites) were present on other Solar System objects. Themost abundant organic carbon phase in carbonaceous meteorites(“macromolecular carbon”) may not suffice, by itself, as a markerof asteroidal/cometary delivery although it constitutes roughly70–99% of total organic carbon in such meteorites (Pizzarello etal., 2006). For example, the origin of ”macromolecular” carbon inseveral Martian meteorites has been attributed to indigenous ig-neous processes (Wright, et al., 1989; Grady et al., 2004; Steele etal., 2012). This is in spite of the fact that this carbon is associatedwith PAHs (as in carbonaceous meteorites) in at least one Martianmeteorite (DaG 476) (Steele et al., 2012) and carbon isotope ratios(13C/12C) of the Martian reduced carbon are approximately −16 to−24� (Grady et al., 2004; Steele et al., 2012) – near that of themacromolecular carbon of carbonaceous meteorites.

Our meteoritic impact simulations cover the range to over40 GPa impact shock pressures. However, we are aware that at-tempts at laboratory re-creation of percent impact survivals may

be approximate at best. For example, at a given impact pressurethe disruptive thermal effects on organic compounds in artificialsample containers may occur at significantly lower pressures inthe natural chondrite/parent-body: see discussions in Tomeoka etal. (1999) and Mimura et al. (2005). Also, Scott et al. (1992) con-cluded that CM chondrites experienced shocks of < 5–0 GPa andTomeoka et al. (1999) estimated that Murchison had not experi-enced shocks of >20 GPa.

However, it is likely that, as occurs on Earth, there were alsomechanisms of gently delivering meteoritic carbonaceous materialonto various surfaces throughout Solar System history: this mate-rial likely has (or had) definite residence times in specific locales.This raises the question of stability: how long would organic com-pounds survive in the more harsh environments? From a studyof known (literature) oxidation reactions it was concluded thatvarious carboxylic acids may be present on Mars as oxidation prod-ucts of meteoritic insoluble carbon, meteoritic polar and non-polarorganic compounds, and also as products of possible indigenousMartian organic compounds (Benner et al., 2000). Left open wasthe question of how one would distinguish between indigenous vs.exogenous organic material. Combined with their unique molecu-lar/isotopic properties and meteoritic abundances, our impact andchemical stability results point to S&P as potentially significantmarkers of asteroidal (and possibly cometary) delivery of organicmaterial throughout the Solar System. Depending on the severityof conditions, non-α amino acids and deoxy sugar acids (Table 4),if present as homologous series of compounds, could also indicateasteroidal contribution.

Acknowledgements

We thank M. Carter for her assistance with oxidation samples.We also thank T. Tan and A. Rios for their assistance with themanuscript and D. Macon for preparation of lab equipment.

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