Seeding the Pregenetic Earth: Meteoritic Abundances Of

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arXiv:1505.01465v1 [astro-ph.EP] 6 May 2015 i oeue nncetds hycm ntovari- two in come They nucleotides. in molecules tic to life. led of of that steps is appearance the and planets the understanding young formation in for the interest biomolecules particular Thus, pregenetic 2015). of mate- origin al. macromolecular et Comet (Capaccioni organic of rials re- nonvolatile surface the for the some from evidence revealed on comes has biomolecules which support i.e., 67P/Churyumov-Gerasimenko, of Additional analysis planetesimals, fallen have cent Earth. of that the organ- fragments meteorites on of chondrite the abundances carbonaceous in high the- the diame- the This measured in by km ics disk.) substantiated 100 circumstellar is are to the ory context from km this 1999; originating 1 in ter, approximately Chyba (Planetesimals & bodies Pierazzo 2012). solid 1992; al. interplane- et Sagan and Burton & Oró planetesimals, Chyba 1908; comets Chamberlin and 1961a; & (in- meteorites, (Chamberlin on particles comets dust biomolecules by nucleobases) tary in and delivered of synthesized acids and reservoir was fatty prebiotic acids, Earth amino the cluding of tion iy B 4,18 anS,Hmlo,O,LS41 Canada 4M1, L8S ON, Hamilton, St, Main 1280 241, ABB sity, Canada 4M1, L8S ON, Hamilton, BC, St, Vancouver, Rd, Agricultural Canada 6224 1Z1, V6T 325, HENN Columbia, uloae r h irgncnann,characteris- nitrogen-containing, the are Nucleobases por- significant a that proposed been long has It rpittpstuigL using ApJ typeset Preprint in publication for Accepted 5 4 3 2 1 mi:[email protected] Email: Univer- [email protected] McMaster E-mail: Astronomy, and Physics of Department Main 1280 241, ABB University, McMaster Institute, Origins British of University Astronomy, and Physics of Department EDN H RGNTCERH EERTCAUDNE FNUC OF ABUNDANCES METEORITIC EARTH: PREGENETIC THE SEEDING mn cd,btaetelataudn nncebss h abunda als The Nunataks) nucleobases. 330 in Graves are abundant least classes nucleo the (e.g. two and are meteorites acids but acids, amino CR2 amino both ppb of meteorite). 1–500 abundances Murchison of pare meteorite the in amounts the dominate within 15 the synthesis meteorites their of in nucleoba for chondrites list responsible The is a carbonaceous reaction bodies. to in parent reduced found meteorite then are within r are occur chemical reactions potentially abiotic pu could of These of b review variety the extensive performed. a i.e., An from then nucleobases, collated classes. the are meteorite chondrites of various carbonaceous abundances in study, found this In organics. o odtosta hrceieerysae fplanetesimals. of states headings: Subject early characterize that conditions for fterato ehnssfrncebs omto ugssth suggests formation ab nucleobase othe most for H (the the mechanisms glycine reaction to than the meteorites of respect CM2 in with abundant less chondrites magnitude carbonaceous in abundances 2 abncoscodie r ls fmtoiekonfrhavin for known meteorite of class a are chondrites Carbonaceous n NH and 1. 3 A ae ecigi h rsneo aaytsc sauiao silica) or alumina as such catalyst a of presence the in reacting gases INTRODUCTION T E tl mltajv 5/2/11 v. emulateapj style X srbooy—atohmsr eerts eer,meteoroid meteors, meteorites, — astrochemistry — astrobiology ehd:dt analysis data methods: — ± 5 p n 16 and ppb 250 e .D Pearce D. K. Ben OETA ECINPATHWAYS REACTION POTENTIAL cetdfrpbiaini ApJ in publication for Accepted ± 3pbrsetvl.Gaiems fe a h greatest the has often most Guanine respectively. ppb 13 ABSTRACT 1,4 n ap .Pudritz E. Ralph and a e edn&Shat 97 aas ta.1975, al. discov- et analogs 1979; nucleobase Hayatsu type some been with 1981, 2008; 1977; along CR3 1964), Schwartz have Hayatsu al. Schwartz and & 1968; et & Stoks Velden CR1 U) Martins der 1990; van CM1, 2011; and al. CI1, al. et A Shimoyama et CR2, (Callahan (G, CM2, meteorites in RNA ered or DNA oeo exrbs)ada es n phosphate (ri- one sugar one least U), at or and T (PO4 deoxyribose) C, group A, or (G, bose nucleobase one nucle- this on meteorites. nu- data within In to available abundances the approach obase presenting 2014). this first extend al. by we et cleobases (Cobb papers, subsequent record compared and and meteoritic out the carried me- then in with was occurring bodies reactions parent Strecker teorite acids aqueous carbonaceous amino of of in origin means the by acids on work amino Theoretical dis- of chondrites. and collated abundances first the (2014) Pudritz played & form- to Cobb delivery planets, their ing and biomolecules of origin physical code. to genetic essential the is un- of nucleobases or origin proteins, the the DNA building of understanding since for origin and the instructions replicate, derstanding the to stores pairs viruses RNA and base RNA organisms and/or the the known RNA DNA all DNA in Since use In and A-U. A-T, and G-C and genome. are G-C the are pairs constitutes RNA base of (or viruses) organisms ordering some in DNA particular in of (U). the chain(s) the uracil because in nucleotides and important the (T) are and thymine (A), They adenine (C), and cytosine (G) pyrimidines, guanine purines, the eties: hr r he opnnst h nucleotide: the to components three are There astro- the understand to project term long a of part As tFshrToshsnhss(..CO, (i.e. synthesis Fischer-Tropsch at tbde.Oecaso carbonaceous of class One bodies. nt uloae,bti – resof orders 1–2 is but nucleobases, r 3 − lse aaadcmae across compared and data blished idn lcso N n DNA, and RNA of blocks uilding atospouigncebssis nucleobases producing eactions cso oa uloae nthese in nucleobases total of nces which unknown currently is It . .Treo h v uloae in nucleobases five the of Three ). iiulrato ahasthat pathways reaction dividual nataioai) u survey Our acid). amino undant oiaeteaudne of abundances the dominate o e unn,aeieaduracil and adenine guanine, ses ae—h ocle M (e.g. CM2 so-called bases—the ihcneto ae and water of content high a g 2,3,5 oeua data molecular — s stems likely most the is EBSSAND LEOBASES 2 Pearce & Pudritz

(purine, 2,6-diaminopurine, 6,8-diaminopurine) and nu- abundances ranging from 5000–20000 ppb. Analyti- cleobase catabolic intermediates (hypoxanthine and xan- cal techniques then moved towards Ion-exclusion Chro- thine). Ribose and deoxyribose have not been identified matography (IEC, IEC-MS) (Stoks & Schwartz 1981, in meteorites (although the sugar dihydroxyacetone has 1979; van der Velden & Schwartz 1977) and High Per- (Cooper et al. 2001)). Phosphates have been found as formance Liquid Chromatography (HPLC, HPLC-MS) minerals in many meteorites, typically as Ca-phosphates (Callahan et al. 2011; Shimoyama et al. 1990). These or Fe-phosphate (Ebihara & Honda 1987). For infor- studies yielded nucleobase abundances in the 1–500 ppb mative reviews about carbonaceous chondrite mete- range. IEC uses an aqueous mobile phase and sepa- orites and their classification, please see Cobb & Pudritz rates the ionized compounds from the non-ionized com- (2014), Weisberg et al. (2006), Botta & Bada (2002) and pounds of a mixture by excluding the former. After the Hayes (1967). mixture is separated by the chromatograph, it is some- We begin in this paper by compiling a comprehen- times sent into mass spectrometer for further separation. sive list of the observed abundances of nucleobases in HPLC has a liquid mobile phase and improves chromato- carbonaceous chondrites as reported in the scientific lit- graphic resolution by applying pressure to the dissolved erature. We then collate and display the data from mixture, allowing the mixture to flow at a much higher these studies, emphasizing the trends in abundances and speed through the column. The use of a higher pres- frequencies of these molecules by meteoritic type—and sure allows for smaller particles to be used for the col- hence formation conditions. The data is first displayed umn packing material, which creates a greater surface by individual nucleobase abundance for each meteorite area for interaction between the stationary phase and sample, then by total nucleobase abundance for each me- the flowing molecules. This allows for a better separa- teorite sample, and finally by average relative nucleobase tion of molecules. MS is also sometimes used in tandem abundance to G for CM2 meteorites. The potential for with HPLC for further separation. nucleobase contamination in the samples of each study TLC has been found to be less sensitive and gives worse are also reviewed. separation than HPLC (Choma 2005). Stoks & Schwartz We then perform an extensive survey of all of the pub- (1981) also note that the analytical techniques applied lished chemical methods that have been employed or sug- in the earliest studies (PC, MS) are not very suitable gested as pathways for the abiotic formation of nucle- for quantitative purposes. Therefore due to their lower obases. This survey is presented as a starting point in accuracy and sensitivity, we won’t consider the studies order to understand the reaction pathway(s) that could that used PC, TLC or MS in the quantitative analysis in occur within planetesimals. The comprehensive list is this paper (i.e. Hayatsu et al. (1975, 1968) and Hayatsu reduced by disregarding reactions that are unlikely to (1964)). occur in such environments. A final list of candidate nu- Table 1 lists the data sources for the nucleobases, nucleobase reaction pathways within planetesimals is then cleobase intemediates and nucleobase analogs in each proposed. carbonaceous chondrite in this analysis. Carbonaceous In a subsequent paper, we will use this candidate list to chondrites are classified into types based primarily on investigate the synthesis of nucleobases in planetesimal composition and secondarily on amounts of aqueous and interiors. thermal processing (see Cobb & Pudritz (2014) for an 2. overview). For instance, the character ’R’ in CR1 mete- METEORITIC DATA orites signifies the meteorite is of Renazzo-like composi- The two fundamental techniques for measuring me- tion. The Renazzo meteorite is a meteorite that fell in teoritic nucleobases in the laboratory are chromatogra- Renazzo, Italy in 1824. The number ’1’ in CR1 mete- phy and mass spectrometry. Chromatography separates orites signifies the meteorite is of petrologic type 1 and a mixture by dissolving it in a fluid, a.k.a. the mo- therefore underwent the highest amount of aqueous alter- bile phase, and sending it through a structure, e.g., a ation. The level of aqueous alteration is determined by column, holding a stationary phase, e.g., a silica layer. the amount the chondrules (round grains) in the mete- The molecules in the mixture travel at different speeds orite have been changed by the presence of water. Petro- through the structure, causing them to separate for de- logic type 1 signifies the complete obliteration of chon- tection. Mass spectrometry separates a mixture by ion- drules in the meteorite, whereas petrologic type 3 sig- izing its constituents (e.g. by bombarding them with nifies the chondrules in the meteorite have not been al- electrons) and then accelerating them through an elec- tered. The majority of meteorite samples with reported tric or magnetic field. The ions are deflected as they nucleobase abundances are of the CM2 type (Mighei-like pass through the field, and separated according to their composition with an average level of aqueous alteration). mass-to-charge ratio. 2.1. The specific analytical techniques used to detect nu- Contamination Possibility cleobases in carbonaceous chondrites have improved sig- There is always a concern about terrestrial con- nificantly over time. First analyses of nucleobases ap- tamination when analyzing meteorites for organics. plied Paper and Thin-layer Chromatography (PC, TLC) Callahan et al. (2011) addressed this by analyzing the (Hayatsu et al. 1968; Hayatsu 1964) and Mass Spec- content of the terrestrial environment around which the trometry (MS) (Hayatsu et al. 1975). (PC and TLC meteorites were retrieved. In the case of Antarctic me- are very similar, with the only difference being that teorites they analyzed dried residue from an ice sample PC has its stationary phase within a tube and TLC collected in 2006 from the Graves Nunataks region, and has its stationary phase on a plane.) These stud- in the case of the Murchison meteorite they analyzed ies published extraordinarily high concentrations of G a soil sample collected in 1999 from near the original and A in the Murchison and Orgueil meteorites, with Murchison meteorite fall site. In the Antarctic residue Meteoritic Abundances of Nucleobases and Potential Reaction Pathways 3

Table 1 Meteoritic nucleobase data sources.

Type Meteorite Sample Number Analytical Technique Reference(s) CI1 Orgueil Ehrenfreund HPLC-MS Callahan et al. (2011) CI1 Orgueil MNHN IEC-MS Stoks & Schwartz (1981); Stoks & Schwartz (1979) CM1 Scott Glacier SCO 06043 HPLC-MS Callahan et al. (2011) CM1 Meteorite Hills MET 01070 HPLC-MS Callahan et al. (2011) CM2 Allan Hills ALH 83100 HPLC-MS Callahan et al. (2011) CM2 Lewis Cliff LEW 90500 HPLC-MS Callahan et al. (2011) CM2 Lonewolf Nunataks LON 94102 HPLC-MS Callahan et al. (2011) CM2 Murchison Smithsonian HPLC-MS Callahan et al. (2011) CM2 Yamato Yamato 74662 HPLC Shimoyama et al. (1990) CM2 Yamato Yamato 791198 HPLC Shimoyama et al. (1990) CM2 Murchison ASU 1 IEC-MS Stoks & Schwartz (1981); Stoks & Schwartz (1979) CM2 Murchison ASU 2 IEC van der Velden & Schwartz (1977) CM2 Murray N.A. IEC-MS Stoks & Schwartz (1981); Stoks & Schwartz (1979) CR1 Grosvenor Mountains GRO 95577 HPLC-MS Callahan et al. (2011) CR2 Elephant Moraine EET 92042 HPLC-MS Callahan et al. (2011) CR2 Graves Nunataks GRA 95229 HPLC-MS Callahan et al. (2011) CR3 Queen Alexandra Range QUE 99177 HPLC-MS Callahan et al. (2011) sample, less than 5 pptr (parts-per-trillion) of G and A from Cobb & Pudritz (2014) and (in the case of Yamato were measured, strongly suggesting the meteorite nucle- 74662) Shimoyama et al. (1979). G is typically the most obases to be of extraterrestrial origin. Interestingly 1380 abundant nucleobase in these samples, ranging from 2– ppb of C was measured in the soil sample from near the 515ppb. These G abundances are approximately 1–2 or- Murchison fall site, but no abundance of C was mea- ders of magnitude lower than the glycine measured in sured in the Murchison meteorite sample. Also, several the same meteorites. The range of G abundances in CM nucleobase analogs were measured in the Murchison me- meteorites is closer to the intermediate and less abun- teorite that are either rare or absent on Earth (purine, dant amino acids in carbonaceous chondrites: valine and 2,6-diaminopurine, and 6,8-diaminopurine). The con- aspartic acid. G is approximately 1 order of magnitude junction of these findings strongly suggests an extrater- lower to 1 order of magnitude higher than both the va- restrial origin of these meteorite nucleobases. line and aspartic acid measured in the same meteorites. Shimoyama et al. (1990) argued that because the G and aspartic acid remain in relatively close abundance amino acid mixtures measured in the analyzed mete- with each other. A and U are generally less abundant orite were racemic, i.e., composed of equal amounts than G by a factor of 2–10. of D and L enantiomers, it is unlikely that the sam- The G abundance curve in Figure 1 has a localized ple (also containing nucleobases) was subjected to much peak in the middle, but still trends downwards over- contamination. Other studies took the high uracil-to- all, generally following the glycine curve. The A abun- thymine ratios in the sample (e.g. 21 in a sample of dance curve in Figure 1 trends downward as well, and the Murchison meteorite) as evidence that the nucle- appears to have a local maximum in the same location obases are extraterrestrial in origin (Stoks & Schwartz the G curve did. There is not enough U data to say any- 1981, 1979; van der Velden & Schwartz 1977). A high thing about its curve. The correspondence between the uracil-to-thymine ratio is suggestive of an extraterres- amino acid and nucleobase curves suggests that condi- trial origin of nucleobases because studies on biogenetic tions that favour the synthesis of glycine are related to nucleobases in various soils and recent sediments have those that optimize the production of nucleobases. The measured little uracil with respect to the other nucle- 1–2 orders of magnitude difference in abundance between obases (Stoks & Schwartz 1979). nucleobases and glycine could have several causes: different decay rates, difference of synthesis or competing 3. RESULTS: NUCLEOBASE ABUNDANCES AND chemical reactions for the same basic materials. These RELATIVE FREQUENCIES will be future considerations of our theoretical work. The 3.1. Nucleobase Abundances in Meteorites similarity in the bumps created by the G and A curves in Figure 1 could suggest a similar mechanism is used to We display the individual nucleobase abundance data synthesize G and A within the meteorite parent bodies. grouped by carbonaceous chondrite type. The CM me- The nucleobase abundances in CI and CR mete- teorites are displayed in one plot, and the CI and CR orites are displayed in Figure 2; again, along with the meteorites are grouped together in another plot—due to abundances of glycine, valine and aspartic acid from limited nucleobase data of the latter group. The or- Cobb & Pudritz (2014) as a guide. These abundances dering of meteorite samples for both plots is the same are much more reduced than in the CM meteorites, most as the ordering defined by Cobb & Pudritz (2014) for laying in the 1-20 ppb range. The Orgueil meteorite ob- amino acid abundances. This order was chosen by tained from the Muséum National d’Histoire Naturelle Cobb & Pudritz (2014) to give a smooth monotonic se- (MNHN) is the only exception with slightly higher abun- quence for the most abundant amino acid: glycine. In dances of nucleobases than the other samples (∼100 Figure 1, we illustrate the ppb (parts-per-billion) abun- ppb). Abundances of nucleobases in CI and CR me- dances of the purines G and A and the pyrimidine U in teorites range from 1–4 orders of magnitude lower than CM meteorites compared with the abundances of glycine, glycine abundances in the same meteorites. Nucleobases valine and aspartic acid found in the same meteorites 4 Pearce & Pudritz

CM2 CM1 104

103 )

102

101 Abundance (ppb Abundance

guanine adenine 100 uracil glycine valine aspartic acid 10-1 Murray Yamato 791198Murchison SmithMurchison ASUMurchison 1 ASU 2LON 94102 Yamato 74662 LEW 90500 ALH 83100 MET 01070 SCO 06043

Figure 1. Abundances of three nucleobases (G: blue, A: green, U: yellow) and three amino acids (glycine: red, valine: magenta, aspartic acid: cyan) in CM meteorites. The meteorite order is monotonically decreasing with respect to glycine. Amino acid data is obtained from Cobb & Pudritz (2014) and Shimoyama et al. (1979). sured in lesser abundances (0.2–9 ppb). 106 CR2 CR3 CI1 CR1 In Figure 3, we sum up G, A and U in all the meteorite guanine sources and plot the total nucleobase abundances by me- 105 adenine uracil teorite type. The total amino acid abundances measured glycine 104 in the same meteorites in this study are added to this

) valine plot for comparison. The total amino acid abundances aspartic acid 103 include glycine, alanine, serine, aspartic acid, glutamic acid and valine, and are taken from Cobb & Pudritz 102 (2014) and Shimoyama et al. (1979). The total amino

Abundance (ppb Abundance acid abundances from Cobb & Pudritz (2014) are veri- 101 ﬁed from the original sources, and in some cases reﬁne-

0 ments are made. 10 The ordering of meteorites is in accordance with the to-

10-1 tal amino acid abundance plot in Cobb & Pudritz (2014). GRA 95229EET 92042 QUE 99177Orgueil MNHMOrgueil Ehren.GRO 95577 It is clear from this plot that CM2 meteorites dominate in total nucleobase abundance, averaging at 330 ± 250 ppb. Figure 2. Abundances of three nucleobases (G: blue, A: green, This is interesting because CM2 meteorites also appear U: yellow) and three amino acids (glycine: red, valine: magenta, to be one of the most abundant in amino acids, with to- aspartic acid: cyan) in CR and CI meteorites. The meteorite order tal abundances averaging ∼104 ppb. This could suggest is monotonically decreasing with respect to glycine. Amino acid a similarity in the synthesis mechanisms of amino acids data is obtained from Cobb & Pudritz (2014). and nucleobases in the parent bodies of CM2 meteorites, range up to 4 orders of magnitude lower than valine, and e.g., both sharing HCN as a reactant. Conversely CR2 up to 3 orders of magnitude lower than aspartic acid in meteorites, which appear to be the most abundant in to- CI and CR meteorites. tal amino acids (averaging ∼105 ppb), are one of the least Due to the relatively low abundances of nucleobases abundant in nucleobases, with a total average abundance in CM1, CR1, CR3 and CI1 meteorites in comparison of 16 ± 13 ppb. These diﬀerences could arise from diﬀer- to CM2 meteorites, and from the relatively low abun- ent reaction rates, decay rates or chemical composition dances of amino acids in the same meteorite types in within the CR2 parent bodies. Cobb & Pudritz (2014), it is likely that the parent bod- The total abundances of nucleobases in Figure 3 range ies of CM1, CR1, CR3 and CI1 meteorites have a less from ∼1–4 orders of magnitude smaller than the total suitable environment for organic synthesis than the par- abundances of amino acids in the same meteorites. A ent bodies of CM2 meteorites. general conformity is seen between the pattern given by Other purines were also measured in CM, CI and CR the total nucleobase abundances and the pattern given by meteorites. Xanthine, a catabolic intermediate of G, the total amino acid abundances when considering solely was found in abundances of 4–2300 ppb and hypoxan- the CM meteorites. Total abundances of nucleobases in thine, a catabolic intermediate of A, was found in abun- the CM2 meteorites generally maintain both an increas- dances of 3–243 ppb. Nucleobase analogs purine, 2,6- ing order, and a separation of approximately 2 orders of diaminopurine, and 6,8-diaminopurine were also mea- magnitude from the amino acids (with the main excep- Meteoritic Abundances of Nucleobases and Potential Reaction Pathways 5

CM2 CR2 CR3 CI1 CR1 CM1

105

104 (ppb) 103

Abundance Abundance 10

101 Amino Acids Nucleobases 100 ALH 83100* LEW 90500 Yamato 74662 LON 94102 Murchison ASU 2 Murchison ASU 1 Yamato 791198 Murray Murchison Smith GRA 95229 EET 92042 QUE 99177 Orgueil Ehren. Orgueil MNHM GRO 95577 MET 01070 SCO 06043

†

Figure 3. Total nucleobase abundances from 17 meteorite samples plotted with the total amino acid abundances found in the same meteorites from Cobb & Pudritz (2014) and Shimoyama et al. (1979). The amino acid data taken from Cobb & Pudritz (2014) is slightly refined. Nucleobase sum includes G, A and U. Amino acid sum includes glycine, alanine, serine, aspartic acid, glutamic acid and valine. The meteorite order conforms to the ordering of the total amino acid abundance plot in Cobb & Pudritz (2014). *ALH 83100 is a CM1/2 type meteorite. †The total amino acid abundance is the ”Murray 1” abundance from Cobb & Pudritz (2014). tion being the Murchison Smith. meteorite). Total abundances of nucleobases in the CM1 meteorites maintain a 100 decreasing order and a separation of approximately 1–2 orders of magnitude from the amino acids. The patterns given by the total nucleobase and total amino acid abundances disagree in the sections containing the CI and CR meteorites. Though, more meteoritic nucleobase assays need to be performed on these meteorite subclasses to 10-1 verify that the disagreement in patterns is not the result of a poor sample size. This general conformity between total nucleobases and total amino acids in only the CM meteorites agrees with the above suggestion: that there may be similar reaction mechanisms for nucleobase and guanine) [X]/mol (mol Abundance Relative amino acid synthesis within the CM meteorite parent 10-2 bodies, but perhaps there are different reaction and de- Guanine Adenine Uracil cay rates (e.g. due to different temperatures, hydration) or chemical compositions within the CR and CI mete- Figure 4. Average relative nucleobase abundances to G in CM2 orite parent bodies. meteorites for each remaining mechanism, and filter these reactions 3.2. Relative Nucleobase Frequencies down to a candidate list based on assumed reactant and Since CM2 meteorites were the most common to host catalyst availability within planetesimals. nucleobases and had the highest abundances, we aver- aged the relative abundances of A and U with respect 4.1. Reaction Mechanisms to G from CM2 meteorites in Figure 4. Here we can Here we categorize the known types of reactions that see G is the most common nucleobase in carbonaceous synthesize nucleobases abiotically into seven groups. The chondrites, followed by A at 0.36 ± 0.48 and then U at following list of reaction types is in rough order of de- 0.23 ± 0.19. creasing likelihood of occurrence within planetesimals. 4. Fischer-Tropsch (FT) synthesis uses carbon monoxide, ABIOTIC NUCLEOBASE SYNTHESIS hydrogen and ammonia gases as reactants in the pres- We now turn to the question of what chemical reac- ence of a catalyst (such as nickel-iron alloy or aluminum tions might be responsible for nucleobase formation in oxide) to create all five nucleobases (Hayatsu et al. 1968, planetesimals. To pursue this we first consider each well 1972; Yang & Oró1971; Anders et al. 1974). Fischer- known abiotic nucleobase-synthesizing mechanism, and Tropsch synthesis is already thought by some to be the we filter out the mechanisms that require environments mechanism for producing meteoritic organics such as dissimilar to the interior of a planetesimal. We then con- purines, pyrimidines, amino acids and aromatic hydro- sider the most frequently discussed reaction pathways carbons (Anders et al. 1974; Hayatsu et al. 1972). 6 Pearce & Pudritz

Several non-catalytic (NC) chemical reactions have hydrothermal environments, and have been proven to be been theorized or proven to synthesize all five nucle- favourable from a theoretical standpoint. If a nucleobase obases which only involve a solution being heated, cooled synthesis study suggests a chemical equation for the syn- or maintained at room temperature. The reactants in thesis of their nucleobase(s), the chemical equation from these reactions vary, although HCN is found to be the the study is copied into the reaction column of Table 3. most common (LaRowe & Regnier 2008; Hill & Orgel For studies that do not suggest a chemical equation, a 2002; Schwartz & Bakker 1989; Voet & Schwartz simple 1982; Yamada et al. 1969, 1968; Ferris et al. 1978; reactants → nucleobase (1) Wakamatsu et al. 1966; Oró& Kimball 1962, 1961). HCN is also one of the main reactants in Strecker scheme is used. In the case of FT reactions, liquid wa- synthesis, which is proposed to be the reaction that ter is also added as a potential product due to the fact produces amino acids within meteorite parent bodies that it generally forms along with nucleobases in the lab- (Cobb & Pudritz 2014; Peltzer et al. 1984). oratory experiments (Hayatsu & Anders 1981). For the Various other catalytic (CA) reactions have synthe- deamination of C into U (reaction 32), ammonia is added sized A, T, C and U (Kumar et al. 2014; Saladino et al. as a potential product in order to perfectly balance the 2001; Subbaraman et al. 1980; Oró1961b; Bendich et al. reaction. 1948). The most common of these reactions uses neat For catalytic reactions, the chemical equation includes formamide as the sole reactant, which successfully forms one or more catalyst(s) written above the reaction arrow. nucleobases in the presence of several different catalysts. In the case of multiple catalysts, a ’+’ is used to signify Spark discharge experiments first introduced by Stan- ’and,’ a ’k’ is used to signify ’or’ and a ’+k’ is used to ley L. Miller have produced G and A (Yuasa et al. 1984; signify ’and/or.’ Miyakawa et al. 2000, 1999). These experiments send a The reactions in Table 3 are firstly grouped by product high voltage electrical discharge through gaseous mix- nucleobase; in order of decreasing number of reactions. tures of reactants, e.g., methane, ethane and ammonia Then each nucleobase group is sub-grouped by reaction or water, carbon monoxide and nitrogen, to produce or- type (starting with Fischer-Tropsch synthesis). Finally ganics. the reaction type sub-groups are ordered by increasing Ultraviolet irradiation in various solutions, e.g., water, total molecular mass of reactants (ignoring coefficients). ice and urea, or water, ammonia and pyrimidine, has produced G, C and U (Menor-salván & Mar´ın-Yaseli 5. RESULTS: CANDIDATE REACTION PATHWAYS WITHIN 2013; Nuevo et al. 2012; Lemmon 1970). Photo- PLANETESIMALS dehydrogenation, i.e., using ultraviolet light to re- In order for a reaction pathway from Table 3 to be con- move hydrogen from molecules, of 5,6-dihydrouracil sidered as a candidate reaction within planetesimals, the and 5,6-dihydrothymine has yielded U and T reactants and (if applicable) catalyst(s) must be present (Chittenden & Schwartz 1976; Schwartz & Chittenden within the planetesimal environment. Since comets are 1977). And finally, electron irradiation in a gaseous thought to contain material that would have also been in- mixture of methane, ammonia and water has successfully corporated into planetesimals during the latter’s forma- synthesized A (Ponnamperuma et al. 1963). tion (Schulte & Shock 2004; Alexander 2011), we con- This research is on the potential synthesis of nucle- sider all reaction schemes with no reactant cometary obases within planetesimals, thus only the mechanisms abundances unlikely to occur in planetesimals. In the listed above that could possibly occur in such environ- case of catalytic reactions, the catalyst(s) must also be ments are of interest. Since the rocky, solid and liquid found in meteorites in order for the reaction scheme to water-filled interior of a planetesimal (Travis & Schubert be considered as a candidate. 2005) is likely not an environment where electron beams, Many spectroscopic molecular surveys and one in situ ultraviolet light or electric discharges would occur, we molecular analysis (done with mass spectrometry), have only consider FT, NC and CA reactions as likely produc- been carried out on various comets (Mumma & Charnley ers of the nucleobases measured in carbonaceous chon- 2011; Liu et al. 2007; Bockelée-Morvan et al. 2004; drites. Crovisier et al. 2004; Ehrenfreund & Charnley 2000; 4.2. Bockelée-Morvan et al. 2000). From these surveys only Reaction Pathways eight of the reactants in Table 3 have been measured: To compile a thorough list of FT, NC and CA reac- H2, H2O, CH2O, NH3, CO, HCN, Cyanoacetylene and tions that have been employed or suggested to produce Formamide. All reaction pathways with reactants not nucleobases, we use a combination of Internet browsing from the list of eight above are disregarded. Deuter- with search criteria relevant to nucleobase synthesis (e.g. ated hydrogen has only been found in comets in the “abiotic adenine synthesis”, “Fischer-Tropsch synthesis form of HDO, DCN, HDCO, NH2D, HDS, CH3OD and of nucleobases”, etc.), and reading through citations from CH2DOH (Crovisier et al. 2004), though since D2 and nucleobase synthesis studies. ND3 are not substantially different from H2 and NH3, The 60 most discussed FT, NC and CA nucleobase- we do not immediately disregard the deuterated FT re- synthesizing reactions in scientific literature are formu- actions from Hayatsu et al. (1972) as candidates. lated in Table 3, along with their experimental nu- The temperature limit for the interior of carbonaceous cleobase yields. All of the reactions in Table 3 have chondrite parent bodies based on the theoretical tem- successfully synthesized a nucleobase in the laboratory perature for which amino acid synthesis in planetesi- with the exception of the 5 reaction pathways from mals generally shuts off is ∼300◦C (Cobb et al. 2014). LaRowe & Regnier (2008). These reaction pathways In thermal evolution simulations of the interiors of car- have been studied for their thermodynamic potential in bonaceous chondrite parent bodies, Travis & Schubert Meteoritic Abundances of Nucleobases and Potential Reaction Pathways 7

(2005) found temperatures reached a maximum of 180◦C. McSween et al. (2002) had similar results, with their Table 2 Candidate reaction pathways for the smallest-radius carbonaceous chondrite parent bodies ◦ production of nucleobases within reaching a maximum interior temperature of 227 C. meteorite parent bodies. The reaction From a study that classifies temperature ranges within numbers pertain to those in Table 3. the parent bodies of the various carbonaceous chondrite subclasses and petrologic types, Sephton (2002) assigns Nucleobase Reaction Number Type ◦ a temperature limit of 400 C for the parent bodies of Adenine 1 FT the carbonaceous chondrites in which nucleobases have 3 NC been found. Furthermore, based on nucleobase decom- 4 NC 6 NC position experiments from Levy & Miller (1998), all five 7 NC nucleobases decompose in seconds at temperatures above 8 NC ◦ 400 C in an aqueous environment. 24 CA Since the FT synthesis of A, G, U and T in Uracil 29 NC Hayatsu et al. (1972) were only successful when temper- 32 NC atures reached as high as 700◦C, we disregard these reactions as candidates. This temperature is much higher Cytosine 43 FT than those within CM, CR and CI carbonaceous chon- 44 NC 49 CA drite parent bodies, and much too high to sustain nucleobases in aqueous environments (Travis & Schubert Guanine 51 FT 2005). 54 NC Jarosewich (1990) and Wiik (1956) have done com- Thymine 58 NC plete chemical analyses of several carbonaceous chondrites. From these two surveys, only Al O , SiO , Ni NC: Non-catalytic; CA: Catalytic; FT: 2 3 2 Fischer-Tropsch. and Fe (satisfying NiFe) of the above catalysts has been found. This allows us to disregard schemes 23, 25, 50 and 1972; Ferris et al. 1968), and meteorite parent bodies are 60, as the catalysts for these reactions were unlikely to thought to remain aqueous on the order of millions of have been available within carbonaceous chondrite par- years (Travis & Schubert 2005). ent bodies. Perhaps the most likely explanation of the deficit The remaining reaction schemes complete our list of in C and T is a purely chemical one. C and T are candidate reaction pathways for the synthesis of nucle- known to readily oxidize into 5-hydroxyhydantoin obases within planetesimals. This list is presented in and 5-methyl-5-hydroxyhydantoin respectively Table 2 and the corresponding reactions are indicated (Redrejo-Rodr´ıguez et al. 2011), which could ex- by asterisks in Table 3. In total there are 7 candidate plain the lack of both C and T in meteorites. Although reactions for A, 2 for U, 3 for C, 2 for G and 1 for T. 5-hydroxyhydantoin and 5-methyl-5-hydroxyhydantoin Although some of the reactions in Table 3 that did not haven’t been measured in carbonaceous chondrites, make the candidate list have experimental yields that are a few amino acid hydantoins have, e.g., 5-(2- indeed high, these pathways require reactants that may Carboxyethyl)hydantoin (Cooper & Cronin 1995). not be present in planetesimals. For this study, we limit A final possibility for the lack of C and T in meteorites our candidates to the reactions for which there is some could be that the reactions synthesizing A, G and U evidence of the reactants being present within meteorite in meteorite parent bodies are more thermodynamically parent bodies. favourable than those synthesizing C and T. This would 6. result in a reduced production of C and T with respect to DISCUSSION the other nucleobases within planetesimals, which could Because nucleobases, nucleobase catabolic intermedi- then be further diminished to negligible amounts from ates and nucleobases analogs are present in carbonaceous photodissociation, deamination or oxidation. An impor- chondrites, these molecules would have likely also been tant target of our future investigations is to address this present on the pregenetic, meteorite-accreting Earth. C question, i.e., how much C or T could be synthesized and T have yet to be found in meteorites. A plausible within meteorite parent bodies. astrophysical source of C and T could arise from ultravi- Since the abundances of xanthine and hypoxanthine olet radiation-driven chemical reactions on icy particles measured in carbonaceous chondrites are comparable to in space (Nuevo et al. 2014). This would supply the C those of G and A, the former would have been just as needed for the onset of the RNA world that likely pre- available as the latter during the evolution of the genome ceded DNA-based life (Neveu et al. 2013), as well as the and the genetic code. Xanthine and hypoxanthine were T needed for the later appearance of the DNA, RNA and therefore not selected to have a place in the genome for protein world. a reason other than availability. It is currently unknown why C or T have not been What is particularly interesting, is the low abundance measured in carbonaceous chondrites. Photodissociation of nucleobase analogs that have been measured in car- is unlikely to be the main cause as Pilling et al. (2008) bonaceous chondrites. Nucleobase analogs are struc- have found that G, A, C, T and U have similar survival turally similar to nucleobases, and 2,6-diaminopurine, rates when exposed to vacuum UV photons at energies like A, can actually base pair with T or U (Kirnos et al. ranging ∼4–30 eV. Deamination could explain the lack 1977). If nucleobase analogs can replace some of the of C in meteorites, since C deaminates to U in aque- genome nucleobases, but were perhaps only available ous solutions (Robertson & Miller 1995; Garrett & Tsau in very small (<10 ppb) abundances on the pregenetic 8 Pearce & Pudritz

Earth, then it is possible to consider meteoritic abun- sured in low abundances around 0.2–9 ppb. dance as a driving factor in A’s incorporation into the CM2 meteorites appear to be the preferred type for genome over these nucleobase analogs. nucleobases with an average total nucleobase abundance Total nucleobase abundances in carbonaceous chon- of 330 ± 250 ppb. Conversely, CR2 meteorites are the drites are consistently lower than amino acids by ∼1–4 least abundant in nucleobases, with an average total orders of magnitude. The decay rates of molecules due abundance of 16 ± 13 ppb. These results are dissimilar to hydrolysis have been measured and a useful tool in to amino acid abundances in carbonaceous chondrites, describing them is Arrhenius plots. These results show where CR2 and CM2 meteorites both contain the great- that nucleobases decay more rapidly than glycine, ala- est abundances. This may suggest that amino acids and nine, serine, glutamic acid, aspartic acid and phenylala- nucleobases could share a reaction mechanism within the nine at temperatures above 200◦C (Levy & Miller 1998; parent bodies of CM2 meteorites, but may have differing Sato et al. 2004; Qian et al. 1993). Though the amino reaction mechanisms within the parent bodies of CR2 acids studies were not performed at temperatures lower meteorites. than 200◦C, by extrapolating the Arrhenius curves from Nucleobases are typically 1–3 orders of magnitude Sato et al. (2004) and Qian et al. (1993), it can be seen less abundant in carbonaceous chondrites than glycine that amino acids should be more stable than nucleobases (which is the most abundant amino acid in carbonaceous at all aqueous temperatures. Since carbonaceous chon- chondrites). Nucleobases can be 2 orders of magnitude drite parent bodies are thought to have had aqueous inte- less abundant to even 1 order of magnitude more abun- riors for millions of years (Travis & Schubert 2005), the dant in carbonaceous chondrites than aspartic acid (one ∼1–4 orders of magnitude difference in abundance be- of the less abundant amino acids in carbonaceous chon- tween amino acids and nucleobases within carbonaceous drites). The most abundant nucleobase in carbonaceous chondrites could simply be a result of the more rapid chondrites is guanine, ranging from 2–515 ppb. Adenine hydrolysis decay rate of nucleobases. is the 2nd most abundant nucleobase with an average It is clear from the number of individual nucleobase relative abundance to G of 0.36 ± 0.48 within CM2 me- reactions in Tables 3 that A is the most commonly pro- teorites. Uracil is the least abundant nucleobase with an duced nucleobase in the lab. Five out of the seven can- average relative abundance to G of 0.23 ± 0.19 within didate reactions producing A are NC reactions (all in- CM2 meteorites. The total abundance of nucleobases in volving HCN), producing a maximum 15 % yield. G carbonaceous chondrites is 1–4 orders of magnitude less on the other hand has only one theoretical NC reaction than the total abundance of amino acids. candidate, with no laboratory yield. Since G happens to Of the 60 most discussed FT, NC and CA nucleobase- be the most abundant nucleobase within carbonaceous synthesizing reactions in the literature, 3 FT, 10 NC chondrites (Figure 4), and A is the second most abun- and 2 CA reactions are proposed as the candidate dant, NC reactions are perhaps less likely to be the main reactions producing nucleobases within meteorite parent reaction mechanism to produce nucleobases within me- bodies. It appears as if the FT synthesis reaction teorite parent bodies. CA reactions are also not an ob- mechanism best complies with the nucleobases found in vious choice for the main reaction mechanism since they carbonaceous chondrites thus far. are missing a reaction pathway for the G and U found in meteorites. We are indebted to Alyssa Cobb, whose work on me- The last candidate reaction mechanism to consider is teoritic amino acids provided an important template for FT synthesis, which produces G, A and C. In experi- our study here. We would also like to thank Dr. Allyson ments by Hayatsu et al. (1968), A had the highest yield Brady for sharing her superb knowledge on molecular (0.16 %) followed by G (0.09 %) and C (0.05 %). This analysis techniques, and Dr. Paul Ayers for his advise- mechanism may at first seem to be an unlikely candidate ment on the molecular stability of C and T. The research since C and T are not found in carbonaceous chondrites. of B.K.D.P. was supported by a NSERC CREATE Cana- However, it is possible that reaction 32 accounts for the dian Astrobiology Training Program Undergraduate Fel- missing C in carbonaceous chondrites, as in this reac- lowship. R.E.P. is supported by an NSERC Discovery tion C deaminates into U. Though G is on average the Grant. most abundant nucleobase in CM2 meteorites, there are also a few instances where A is more abundant than G in REFERENCES CR2, CR3, CM1 and CM2 meteorites. This increases the plausibility of the FT reaction pathways in Hayatsu et al. Alexander, C. M. 2011, in IAU Symp. 280: The Molecular (1968) occurring within meteorite parent bodies. Universe, ed. J. Cernicharo, & R. Bachiller (Cambridge: Cambridge University Press), 288 7. CONCLUSIONS Anders, E., Hayatsu, R., & Studier, M. H. 1974, OrLi, 5, 57 Unlike amino acids, hydroxy acids, carboxylic acids, Bendich, A., Tinker, J. F., & Brown, G. 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R. 2013, CEJ, 19, 6488 (Amsterdam: North-Holland Publishing Co.), 155 Miyakawa, S., Murasawa, K., Kobayashi, K., & Sawaoka, A. B. Yuasa, S., Flory, D., Basile, B., & Oró, J. 1984, JMolE, 21, 76 1999, JAChS, 121, 8144 Weisberg, M. K., McCoy, T. J., & Krot, A. N. 2006, in Meteorites Miyakawa, S., Murasaw, K., Kobayashi, K., & Sawaoka, A. B. and the Early Solar System II, ed. D. S. Lauretta & H. Y. 2000, OLEB, 30, 557 McSween Jr. (Tucson: Univ. Arizona Press), 19 Miyakawa, S., Cleaves, H. J., & Miller, S. L. 2002, OLEB, 32, 209 Wiik, H. B. 1956, GeCoA, 9, 279 10 Pearce & Pudritz 1 Oró(1963) 1 Grossenbacher & Knight (1965) 0.09 Saladino et al. (2001) 9.3 Fox & Harada (1961) 55 Davidson & Baudisch (1926) 20 Takamoto & Yamamoto (1971) 5 Takamoto & Yamamoto (1971) 59 Harada & Suzuki (1976) 0.62 Kumar et al. (2014) < detected Bendich et al. (1954) 0.005 Voet & Schwartz (1982) < 21.25 Bendich et al. (1948) 36 Ferris et al. (1974) theoretical LaRowe & Regnier (2008) 0.08 Nelson et al. (2001) Max Yield (%) Source(s) detected0.16 Yang & Oró(1971); Hayatsu et al. (1968) 0.0017 Miyakawa et al. (2002) detecteddetected95 Robertson & Miller Garrett (1995); & Tsau (1972); Ferris et al. (1968) detecteddetected Oró& Kimball (1962); Oró(1961b) 24 Sanchez et al. (1968) + Hayatsu et al. (1972) 18 Hill & Orgel (2002) ++ Hayatsu et al. (1972) theoretical LaRowe & Regnier (2008) detected0.06 Oró(1960) Schwartz & Bakker (1989) 0.0290.038 Miyakawa et al. (2002); Levy et al. (1999) detected15 Yamada et al. (1969); Wakamatsu et al. (1966) 0.04 Ferris et al. (1978) 0.05 Oró& Kimball (1961) detected Hayatsu et al. (1968) 4.7 Hudson et al. (2012) cleobase synthesis. A 21 Yamada et al. (1968) U 6.3 Subbaraman et al. (1980) → 3 4 PO 3 H || A 80 Shaw (1950) Cl → 4 A 25 Yamada et al. (1968) NH → || A 3 Ferris & Orgel (1966) 4 3 A 11 Sanchez et al. (1968) → U 75 Harada & Suzuki (1976) → A 1 Sanchez et al. (1967) HPO 0 A 7 Sanchez et al. (1968) icarbonate A detected Yamada et al. (1968) 2 2 e ) → Table 3 4 → → → O 3 3 2 NH U ( O U U || 2 4 A U → U → → A PO → O → 2 2 → ) U A → A A+H H O O aq 4 2 2 → → A 2( 2 U NH O −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→ 2 3 → SiO U + H A || Zeolite ) U+H A+H A U + || → Nickel 3 A U aq O 3 3 ( − 2 O → → O 2 O O U → H → 2 2 2 2 O Al · 0 0 A + 4NH ] A 2 Kaolin Al Al || 2 → 2 8 || ) A Raney U A + + + ) −−−−−−−−−−−→ 2 → 3 aq OH → CN → → ( ( 4 SiO -toluenesulfonate + NaCN + Formamidine acetate + NH 0 O NiF e NiF e NiF e || ) p −−−−−−−−−−→ −−−−−−−−−−→ −−−−−−−−−−−−−−−−→ 2 2 A + 5H 3 Mo A NH [ 3 3 3 aq A Reaction schemes and experimental yields for FT, NC and CA nu −−−−−−→ O ( 2 2 → 3 A → + H U + NH → Zn Al 3 −−−−−−−−−−−−−−−→ −−−−−−−−−−−−−−−−−−−−−→ 3 3 0 + 2CH → + ND + ND → + NH 2 ) ) 2 2 2 O aq aq 2 ( ( NC C+H FT CO+H NC 2HCN CA Formamide NC 5HCN NC HCN+NH NC 5CO+5NH NC HCN+NH NC HCN+H * * * * * * * * * 42 CA Acetylene dicarboxylic acid + Urea 41 NC Malic acid + Urea + Polyphosphoric acid 40 NC Malic acid + Urea + Sulfuric acid 39 NC Maleic acid + Urea + Polyphosphoric acid 38 NC Fumaric acid + Urea + Polyphosphoric acid 37 NC Propiolic acid + Urea + Polyphosphoric acid + H 36 NC Propiolicacid+Urea+Sulfuricacid+H 35 NC Acrylonitrile + Urea + Ammonium chloride 22 NC Aminomalonitrile 2021 NC NC 4-Formamido-5-imidazolecarboxamidine + Potassium b 6-chloropurine + Ammoniacal butanol 31 NC 2,4-Diaminopyrimidine + H 23 CA 5HCN+4NH 32 19 NC 4-amino-5-cyanoimidazole + Formamidine acetate + H 30 NC HCN + Ammonium hydroxide + H 34 NC Acrylonitrile+Urea+H No.Adenine 1 Type Reaction 27 FT CO+D 25 CA Formamide 2 FT CO+D 29 33 NC Cyanoacetaldehyde + Urea 24 13 NC 4-aminoimidazole-5-carbonnitrile + Ammonium cyanid 26 CA Thioisoguanine sulfate 28 NC Ammonium cyanide 12 NC 4-aminoimidazole-5-carbonnitrile + HCN 3 18 NC Diaminomaleonitrile + Formamidine acetate + NH 4 1011 NC NC HCN + Glycolonitrile HCN + Ammonium formate 6 5 NC Ammonium cyanide 17 NC NaCN + Ammonium chloride + Formamidine acetate + NH 8 9 NC Ammonium cyanide + H 7 16 NC NaCN + HCN + Acetamidine hydrocloride + NH Uracil 15 NC KCN + Ammonium formate + Formamide 14 NC 4-aminoimidazole-5-carbonnitrile + Sodium cyanide Meteoritic Abundances of Nucleobases and Potential Reaction Pathways 11 0.36 Kumar et al. (2014) 0.44 Saladino et al. (2001) 0.03 Kumar et al. (2014) 195 Ferris et al. (1968); Sanchez et al. (1966) 5 Ferris et al. (1968) trace Ferris et al. (1974) Max Yield (%) Source(s) 43 Sanchez et al. (1968) + Hayatsu et al. (1972) 0.00670.0035 Miyakawa et al. (2002); Levy et al. (1999) theoretical LaRowe & Regnier (2008) +++ Hayatsu et al. (1972) detected0.09 Yang & Oró(1971); Hayatsu et al. (1968) 0.2353 Nelson et al. (2001); Robertson & Miller (1995) theoretical0.1 LaRowe & Regnier (2008) Stephen-Sherwood et al. (1971) detected0.05 Yang & Oró(1971); Hayatsu et al. (1968) theoretical LaRowe & Regnier (2008) G 20 Sanchez et al. (1968) → Table 3 Continued G C O O 2 2 → → O 2 C C+H G+H O O T O 2 2 2 2 2 → ) C SiO SiO C + H aq C T || || Zeolite T+H G+H ) 2( → + + || ) → 3 3 C aq O O 3 3 ( O O O 2 2 aq O ( 2 2 2 O O T H + H H → 2 2 2 C 2 2 Al Al · · ) ] ] Kaolin Al Al || || → 8 8 aq || → ) ) G + + + + ) ( 2 ) G aq CN CN → ( aq ( ( SiO ( O NiF e NiF e NiF e NiF e → || −−−−−−−−−−→ −−−−−−−−−−→ −−−−−−−−−−−−−−−−→ −−−−−−−−−−−−−−−−→ 2 O 3 Mo Mo 2 [ 3 3 [ 3 3 O O 2 2 2 2 Zn Al Zn −−−−−−−−−−−−−−−→ −−−−−−−−−−−−−−−−−−−−−→ −−−−−−−−−−−−−−−→ + 3CH + H + CH + ND + ND + NH + NH ) ) ) 2 2 2 2 aq aq aq ( ( ( CA Formamide FT CO+H FT CO+H NC 2HCN NC 5HCN NC 3HCN * * * * * * Candidate reactions referenced in Table 2. Guanine 50 CA Formamide 49 48 NC Cyanoacetylene + Potassium cyanate + H Thymine 47 NC Cyanoacetylene + Urea + H No.Cytosine 43 Type Reaction 51 57 FT CO+D 56 NC 4-aminoimidazole-5-carboxamide + Potassium cyanate 58 54 55 NC 4-aminoimidazole-5-carboxamide + Cyanogen 53 NC Ammonium cyanide 52 FT CO+D 46 NC 2,4-Diaminopyrimidine + H 5960 NC CA U + Paraformaldehyde + Hydrazine Formamide 44 45 NC Cyanoacetaldehyde + Urea NC: Non-catalytic; CA:+++ Catalytic; very FT: abundant; Fischer-Tropsch. ++ abundant; + less abundant. *