Astrophysical Ices Research Articles

Ion radiation in icy space environments: Synthesis and radioresistance of complex organic molecules

Ices are omnipresent in cold regions in space on, e.g., comets, dust grains, transneptunian objects, surfaces of planets and their satellites. The dominant molecule in such ices is water, but also other small molecules or even complex organic molecules (COMs) may be present. Ionizing radiation (UV photons, electrons, ions from cosmic rays or solar wind) induces several physico-chemical processes such as radiolysis. The fragmentation of initial molecules followed by chemical reactions between radicals may lead to formation of new molecules. Furthermore, also implanted projectiles can contribute to chemistry by forming new molecular species. Other observed effects include structural changes (compaction, amorphization) and desorption (sputtering) of particles from the surface. At CIMAP (Caen, France), using the different beam lines of the GANIL facility, and at GSI (Darmstadt, Germany), the interaction of swift highly charged heavy ions with astrophysical ices has been studied in a wide projectile energy range from keV to GeV. Here, two examples of our studies on astrophysical and astrochemical applications will be discussed in detail: 1) the synthesis of COMs under irradiation of ices made of small molecules, and 2) radiosensitivity of COMs such as pyridine, glycine and adenine, both for isolated molecules in the gas phase and in condensed phase. Special emphasis is given on pyridine and pyridine in water matrix.

On the correlation of the abundances of HNCO and NH2CHO: Advantages of solid para-H2 to study astrochemical H-atom addition and abstraction reactions

AbstractIn dense interstellar clouds that are shielded from high-energy radiation (e.g., UV photons or cosmic rays), H-atom addition and abstraction reactions that take place on grain surfaces play principal roles in the synthesis or decomposition of complex organic molecules (COMs). These reactions are extensively investigated with laboratory experiments by bombarding astrophysical analogue ices with a beam of low-temperature H atoms. Here we demonstrate that, although 2-4 K solid para-H2 does not represent a typical environment of the surface of interstellar grains, para-H2 matrix isolation combined with IR spectroscopy is a complementary tool to sensitively detect astrochemical hydrogenation and dehydrogenation processes.

Experimental study of the penetration of oxygen and deuterium atoms into porous water ice

Context. Many interstellar molecules are thought to form on dust grains. In particular, hydrogenation is one of the major mechanisms of the formation of mantle ice. To date it is not clear if H atoms can penetrate the bulk of the ice mantle or if it only has chemical activity on the accessible surface of grains. Aims. We wish to study the efficiency of atoms deposited on the outer surface of the amorphous solid water to penetrate into the ice bulk. Methods. NO molecules react with O and H atoms. They are easily detected by infrared (IR) spectroscopy. These two properties make this molecule an ideal chemical tracer for the penetration of O and H atoms through water ice. In our experiments we first deposited a NO undercoat and covered this layer (at 40 K) with a variable amount of water ice. Then, we exposed this undercoat to D (10 K) or O (40 K) atoms, and we followed the NO consumption and the products that appeared via IR signatures, and we finally analyzed the desorption of all species through a temperature-programmed desorption technique. We experimentally characterize the accessible surface of the ice and provide a model to interpret quantitatively our measurements. Results. Water ice limits the destruction of tracer NO molecules. The thicker the ice, the more NO remains unreacted. H and O atoms lead to the same amount of NO consumption, pointing out that access to reactants for these two different atoms is identical. We discuss different possible scenarios of NO localization (in and/or on the ice) and determine how this affects our observables (IR data and desorption profiles). Conclusions. In our experimental conditions, it is not possible to measure any atom penetration through the bulk of the ice. The surface diffusion followed by reaction with NO or by self-reaction (i.e., H + H → H2) is faster than bulk diffusion. We propose lower limit values for penetration barriers. Therefore the building of astrophysical ice mantles should be mostly driven by surface reactivity.

Photochemistry and desorption induced by X-rays in water rich astrophysical ice analogs: implications for the moon Enceladus and other frozen space environments.

Soft X-rays are an important agent for chemical processing in the Solar System and in the interstellar medium. The photolysis and photodesorption processes of H2O-rich ices triggered by soft X-rays was, experimentally, addressed in this paper. The experiments were performed at the Brazilian synchrotron facility LNLS/CNPEN employing broadband radiation (from 6 to 2000 eV; mainly soft X-rays and a small fraction of VUV) in solid samples at temperatures of 20 and 80 K. The icy samples were monitored by infrared spectroscopy. We determined the effective destruction cross section (in the order 10−18 cm2) as well as the formation cross section for the new species produced after the irradiation. Among them, we list OCN−, CO, CO3, CH3OH, H2O2, HCOO−, NH4+, HCONH2 and CH3HCO, mostly formed in the experiment at 80 K. The chemical equilibrium stage was characterized and molecular abundances were quantified. In addition, we discuss a methodology to estimate the amount of unknown species in the ice produced by photolysis. The samples reach chemical equilibrium at fluences around 2–3 × 1018 cm−2. Timescales for reaching chemical equilibrium in space environments illuminated by X-rays were given, as well as the desorption yields induced by X-rays. The astrophysical implication on the surface chemistry and desorption processes at the moon Enceladus are provided.

Deoxyribose and deoxysugar derivatives from photoprocessed astrophysical ice analogues and comparison to meteorites

Sugars and their derivatives are essential to all terrestrial life. Their presence in meteorites, together with amino acids, nucleobases, amphiphiles, and other compounds of biological importance, may have contributed to the inventory of organics that played a role in the emergence of life on Earth. Sugars, including ribose (the sugar of RNA), and other sugar derivatives have been identified in laboratory experiments simulating photoprocessing of ices under astrophysical conditions. In this work, we report the detection of 2-deoxyribose (the sugar of DNA) and several deoxysugar derivatives in residues produced from the ultraviolet irradiation of ice mixtures consisting of H2O and CH3OH. The detection of deoxysugar derivatives adds to the inventory of compounds of biological interest that can form under astrophysical conditions and puts constraints on their abiotic formation pathway. Finally, we report that some of the deoxysugar derivatives found in our residues are also newly identified in carbonaceous meteorites.

Photochemical Processes in CO2/H2O Ice Mixtures with Trapped Pyrene, a Model Polycyclic Aromatic Hydrocarbon

Abstract Vacuum ultraviolet irradiation of a polycyclic aromatic hydrocarbon molecule, pyrene, trapped in carbon dioxide (CO2)-enriched astrophysical ice analogues has been studied and presented here. Pyrene is co-deposited with a gaseous mixture of H2O and CO2 (100% CO2; 50:50 CO2:H2O; and 100% H2O) and is subjected to Ly α photons from a hydrogen-flow microwave discharge lamp. To our surprise, we found ionization of pyrene in all three ices, with ionization yields following the H2O content in the ice, indicating that H2O ice stabilizes charged ions and electrons better than CO2 ice by a factor of 10. Caution needs to be exercised not to overinterpret our finding that pyrene is ionized in pure CO2 ice, because even trace amounts of water in the CO2 matrix could result in an increased ionization of pyrene. In addition to ionization of pyrene in CO2 ice, photochemical products of the CO2 ice itself, namely CO and CO3, were found to form efficiently, as detected using Fourier transform infrared spectroscopy, in agreement with earlier studies. UV–vis spectra showed formation of ozone (O3) with prolonged irradiation. Surprisingly, O3 yields followed CO2 concentration in the ice mixtures, with the strongest bands in photolyzed CO2 ice and the weakest in photolyzed H2O ice, indicating that CO2 ice is very protoreactive and produces more O atoms than H2O ice. Pyrene-containing photoproducts, incorporating CO2 or CO or O, such as Py-COOH, Py-OH, or Py-CHO, are not explicitly seen in the UV–vis absorption spectra, but we cannot rule out the possibility that their UV absorption may be swamped under the strong absorption of O3.

The Photochemistry of Purine in Ice Analogs Relevant to Dense Interstellar Clouds

Abstract The aromatic nitrogen heterocyclic compound purine is the core structural framework of many important biomolecules, particularly nucleobases. Purine and purine derivatives have been observed in carbonaceous chondrites, and it has been hypothesized that the exogenous delivery of these compounds, along with many other biologically relevant compounds, may have played a role in the emergence of life. Numerous experiments in our laboratory have demonstrated that the nucleobases used by life to encode genetic material could have been produced abiotically under astrophysically relevant conditions. Specifically, the UV photoprocessing of pyrimidine and purine in simple ices of astrophysical interest has resulted in the production of all five biological nucleobases, namely, uracil (RNA), cytosine (RNA and DNA), thymine (DNA), adenine (RNA and DNA), and guanine (RNA and DNA). Additionally, follow-up work has examined the photochemistry of pyrimidine in more complex astrophysical ice mixtures to better understand the formation of these compounds under realistic conditions. In this work, we examine the photochemistry of purine in more complex ices of astrophysical interest and compare our results with those from simpler ice mixtures. We also examine the effects of competing parallel synthesis of organic compounds in the ices (unrelated to purine). Finally, we discuss the astrophysical and astrobiological implications of our findings.

A Photoionization Reflectron Time-of-flight Mass Spectrometric Study on the Formation of Acetic Acid (CH3COOH) in Interstellar Analog Ices

Abstract This work investigates the synthesis of complex organic molecules with special focus on acetic acid (CH3COOH) via experiments involving the processing of astrophysical model ices of carbon dioxide–methane (CO2–CH4) by low doses of ionizing radiation, exposing the initial bond-breaking processes and successive reactions initiated by energetic electrons generated in the track of galactic cosmic-ray particles penetrating ice-coated interstellar grains, deep inside molecular clouds in their early stages of evolution. The key results were obtained through single photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) and exploiting isotopically labeled samples (C18O2–CH4; CO2–CD4). Not only acetic acid (CH3COOH), along with fragments of acetic acid dimers (CH3COOH)2, but also the hitherto elusive interstellar methyl hydroperoxide (CH3OOH) and the hydrocarbons ethane (C2H6) and butane (C4H10), along with species belonging to C2H4O, C2H6O, and C3H6O2 isomers, are swiftly formed via suprathermal reactions at doses of only 0.88 ± 0.12 eV per molecule of carbon dioxide and 0.32 ± 0.04 eV per molecule of methane, which is equivalent to doses deposited in just (2.0 ± 0.5) × 106 yr in a typical molecular cloud. The results suggest further that the search for acetic acid dimers (CH3COOH)2 toward star-forming regions has a significant potential to be successful. Finally, methyl hydroperoxide (CH3OOH) and dimethyl peroxide (CH3OOCH3), as identified previously in our laboratory, are predicted to be present in the interstellar medium, thus providing a homologous series of peroxides—HOOH, CH3OOH, and CH3OOCH3—to shed light on the interstellar oxygen chemistry.

The role of external far-ultraviolet irradiation in the survival of astrophysical ices in Elias 29

The survival of astrophysical ices in the star-forming regions depends on the suitability of temperature, density and radiation conditions. In this paper, the role of Interstellar Radiation Field (ISRF) on ices in Elias 29 is addressed. This object is the most luminous protostar in $\rho$ Oph E molecular cloud, and is surrounded by many Young Stellar Objects distant only a few arcminutes. In addition, other two brightness BV stars (S1 and HD 147889) enhance the external irradiation in Elias 29. This study was carried out by using the Monte Carlo radiative transfer code RADMC-3D assuming an internal and external irradiation. As result, we found that HD 147889 dominates the ISRF, instead of the closest protostars, and contributes to enhance the external irradiation in 44 times the standard value. Furthermore, remarkable effects are observed in FIR spectrum, as well as in near-IR image. Additionally, the snowline positions of volatile compounds, such as CO, O$_2$, N$_2$ and CH$_4$ are redefined to a toroidal-shaped morphology in the envelope, with low FUV flux (10$^{-7}$ ergs cm$^{-2}$ s$^{-1}$). In such scenario, the formation of complex molecules as result of hydrogenation or oxygenation of volatile species are expected to be severely affected.

Constraining the Molecular Complexity in the Interstellar Medium—The Formation of Ethyl Methyl Ether (CH3OCH2CH3) in Star-forming Regions

Abstract We report the first confirmed synthesis of ethyl methyl ether (EME, CH3CH2OCH3) within astrophysical model ices containing water (H2O) and methane (CH4) exposed to ionizing radiation at ultra-low temperatures of 5 K. EME (also known as methoxyethane), was recently observed toward Orion KL and currently is the largest confirmed oxygen-bearing molecule found in the interstellar medium. Exploiting isomer-selective photoionization (PI) of the subliming molecules in the temperature-programmed desorption phase at 10.49, 9.92, and 9.70 eV, coupled with reflectron time-of-flight mass spectrometry and isotopic substitution experiments (H2 18O–CH4), the detection of fragment ions of EME at m/z = 45 (C2H5O+) and m/z = 59 (C3H7O+), and probing the proton transfer in subliming ethanol–EME complexes via m/z = 61 (C3H9O+), the present study reveals that EME can be formed from suprathermal reactions initiated by cosmic rays and secondary electrons generated within astrophysical ices. The detection of EME in our experiments represents a significant advance in the understanding of formation pathways of complex organic molecules present in hot cores and helps to constrain astrochemical models on the formation of such species within molecular clouds.

Glycine formation in CO2:CH4:NH3 ices induced by 0-70 eV electrons.

Glycine (Gly), the simplest amino-acid building-block of proteins, has been identified on icy dust grains in the interstellar medium, icy comets, and ice covered meteorites. These astrophysical ices contain simple molecules (e.g., CO2, H2O, CH4, HCN, and NH3) and are exposed to complex radiation fields, e.g., UV, γ, or X-rays, stellar/solar wind particles, or cosmic rays. While much current effort is focused on understanding the radiochemistry induced in these ices by high energy radiation, the effects of the abundant secondary low energy electrons (LEEs) it produces have been mostly assumed rather than studied. Here we present the results for the exposure of multilayer CO2:CH4:NH3 ice mixtures to 0-70 eV electrons under simulated astrophysical conditions. Mass selected temperature programmed desorption (TPD) of our electron irradiated films reveals multiple products, most notably intact glycine, which is supported by control measurements of both irradiated or un-irradiated binary mixture films, and un-irradiated CO2:CH4:NH3 ices spiked with Gly. The threshold of Gly formation by LEEs is near 9 eV, while the TPD analysis of Gly film growth allows us to determine the "quantum" yield for 70 eV electrons to be about 0.004 Gly per incident electron. Our results show that simple amino acids can be formed directly from simple molecular ingredients, none of which possess preformed C-C or C-N bonds, by the copious secondary LEEs that are generated by ionizing radiation in astrophysical ices.

Study of the frequency factor in the thermal desorption of astrophysical ice analogs: CH4, C2H4, C2H6, CH3OH, CO, CO2, H2O and N2

In this work the frequency factor and the influence of the temperature on this parameter, for zeroth order desorption processes, has been experimentally determined for eight molecules of astrophysical interest. In the literature, this parameter has been estimated indirectly, obtaining values that differ by as much as three orders of magnitude from different authors. As a consequence, there are very different desorption rates reported for the same molecule and additionally its temperature dependence has been systematically neglected.The frequency factor is widely used to model the dynamics of these species under low temperature conditions present in some astrophysical environments. The method reported in this work is based on the analysis of the signal of a quartz crystal microbalance acting as a sample-holder, which is able to directly detect molecules desorbing from it.Two different types of desorption experiments were necessary for this study. In a first set of experiments, carried out at a constant rate of warming up, the desorption energy is obtained. The second set of experiments were performed at several constant temperatures to calculate the frequency factor and its relationship with temperature. The reasons for some anomalous behaviour have been analyzed. The dependence of the frequency factor on temperature should be taken into account when the Polanyi-Wigner equation is used for desorption processes. Every molecule has to be independently studied as no global tendency is found for the variation of the frequency factor with temperature.

Synthesis of complex organic molecules in simulated methane rich astrophysical ices.

It has been proposed that organic molecules required for life on earth may be formed by the radiation processing of molecular ices in space environments, e.g., within our solar system. Such processes can be studied in the laboratory with surface science analytical techniques and by using low-energy electron (LEE) irradiation to simulate the effects of the secondary electrons that are generated in great abundance whenever ionizing radiation interacts with matter. Here we present new measurements of 70 eV LEE irradiation of multilayer films of CH4, 18O2, and CH4/18O2 mixtures (3:1 ratio) at 22 K. The electron stimulated desorption (ESD) yields of cations and anions have been recorded as a function of electron fluence. At low fluence, the prompt desorption of more massive multi-carbon or C-O containing cationic fragments agrees with our earlier measurements. However, new anion ESD signals of C2-, C2H-, and C2H2- from CH4/18O2 mixtures increase with fluence, indicating the gradual synthesis (and subsequent electron-induced fragmentation) of new, more complex species containing several C and possibly O atoms. Comparisons between the temperature programed desorption (TPD) mass spectra of irradiated and unirradiated films show the electron-induced formation of new chemical species, the identities of which are confirmed by reference to the NIST database of electron impact mass spectra and by TPD measurements of films composed of the proposed products. New species observed in the TPD of irradiated mixture films include C3H6, C2H5OH, and C2H6. Furthermore, X-ray photoelectron spectroscopy of irradiated films confirms the formation of C-O, C=O, and O=C-O- bonds of newly formed molecules. Our experiments support the view that secondary LEEs produced by ionizing radiation drive the chemistry in irradiated ices in space, irrespective of the radiation type.

Energetic Processing of N2:CH4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System

Abstract We experimentally address in this paper the physicochemical effects induced by ionizing photons (energies from 6 to 2000 eV) and swift heavy ions (15.7 MeV 16O5+) in the icy mixture containing N2:CH4 (19:1) at 12 K and 19 K, respectively. The experiments simulate the effect of solar photons and X-rays, cosmic rays, and solar energetic particles (medium-mass ions) on the surface of icy bodies in the outer solar system, such as Triton, Titan, Pluto, and several other Kuiper Belt objects. The ice samples were analyzed by infrared spectroscopy (FTIR) at different fluences. From the energetic processing, the production of new molecules was observed. Among them, HCN, C2H4, C2H6, and N3 have the highest production yield. Molecular half-lives of the species of interest were calculated and extrapolated to the astrophysical environment. The effective destruction yield (in molecules/impact) of the parental species processed by the swift ions is up to six orders of magnitude higher than the value determined by employing X-rays. However, due to the differences between the fluxes of both ionizing radiation types in space, the half-lives of nitrogen and methane in the astrophysical scenarios addressed may have a huge variation. Photons dominate the chemical transformations at shorter distances from the Sun. Our results are a step toward a compilation of photochemical and radiolysis data that should allow the modeling of the abundance of astrophysical ices over long periods of time.

Solid CO2 in quiescent dense molecular clouds

Context. Carbon dioxide (CO2 ) is one of the most abundant species detected in icy grain mantles in star forming regions. Laboratory experiments have shown that CO2 molecules are efficiently formed in the solid state under interstellar conditions. Specifically, solid CO2 can be formed through energetic (e.g. UV photolysis, electron and ion bombardment) and non-energetic mechanisms (atom-addition reactions).Aims. Here we investigate the role of low-energy cosmic-ray bombardment in the formation of solid CO2 in quiescent dense molecular clouds.Methods. We performed laboratory experiments to study the formation of CO2 after ion irradiation with 200 keV H+ of astrophysical relevant ice mixtures. Laboratory spectra are used to fit the profile of the CO2 bending mode band observed at about 15.2 μ m (660 cm-1 ) by the Spitzer Space Telescope in the line of sight to background sources.Results. From a qualitative point of view, good fits to observations are obtained by considering either three or four laboratory components. From a quantitative point of view, a better result is obtained with four components, i.e. when a spectrum of CO2 formed after ion irradiation of CH3 OH ice is added to the fitting procedure.Conclusions. Our results support the hypothesis that energetic processing of icy grain mantles is an efficient formation mechanism of CO2 ice also in quiescent dark cloud regions, and indirectly suggest the presence of CH3 OH in icy grain mantles in interstellar cold regions.

The Gaseous Phase as a Probe of the Astrophysical Solid Phase Chemistry

Abstract In support of space missions and spectroscopic observations, laboratory experiments on ice analogs enable a better understanding of organic matter formation and evolution in astrophysical environments. Herein, we report the monitoring of the gaseous phase of processed astrophysical ice analogs to determine if the gaseous phase can elucidate the chemical mechanisms and dominant reaction pathways occurring in the solid ice subjected to vacuum ultra-violet (VUV) irradiation at low temperature and subsequently warmed. Simple (CH3OH), binary (H2O:CH3OH, CH3OH:NH3), and ternary ice analogs (H2O:CH3OH:NH3) were VUV-processed and warmed. The evolution of volatile organic compounds in the gaseous phase shows a direct link between their relative abundances in the gaseous phase, and the radical and thermal chemistries modifying the initial ice composition. The correlation between the gaseous and solid phases may play a crucial role in deciphering the organic composition of astrophysical objects. As an example, possible solid compositions of the comet Lovejoy are suggested using the abundances of organics in its comae.

Methanol Formation via Oxygen Insertion Chemistry in Ices

Abstract We present experimental constraints on the insertion of oxygen atoms into methane to form methanol in astrophysical ice analogs. In gas-phase and theoretical studies this process has previously been demonstrated to have a very low or nonexistent energy barrier, but the energetics and mechanisms have not yet been characterized in the solid state. We use a deuterium UV lamp filtered by a sapphire window to selectively dissociate O2 within a mixture of O2:CH4 and observe efficient production of CH3OH via O(1D) insertion. CH3OH growth curves are fit with a kinetic model, and we observe no temperature dependence of the reaction rate constant at temperatures below the oxygen desorption temperature of 25 K. Through an analysis of side products we determine the branching ratio of ice-phase oxygen insertion into CH4: ∼65% of insertions lead to CH3OH, with the remainder leading instead to H2CO formation. There is no evidence for CH3 or OH radical formation, indicating that the fragmentation is not an important channel and that insertions typically lead to increased chemical complexity. CH3OH formation from O2 and CH4 diluted in a CO-dominated ice similarly shows no temperature dependence, consistent with expectations that insertion proceeds with a small or nonexistent barrier. Oxygen insertion chemistry in ices should therefore be efficient under low-temperature ISM-like conditions and could provide an important channel to complex organic molecule formation on grain surfaces in cold interstellar regions such as cloud cores and protoplanetary disk midplanes.

Thermal and energetic processing of astrophysical ice analogues rich in SO2

Sulfur is an abundant element in the cosmos and it is thus an important contributor to astrochemistry in the interstellar medium and in the Solar System. Astronomical observations of the gas and of the solid phases in the dense interstellar/circumstellar regions have evidenced that sulfur is underabundant. The hypothesis to explain such a circumstance is that it is incorporated in some species in the solid phase (i.e. as frozen gases and/or refractory solids) and/or in the gas phase, which for different reasons have not been observed so far. Here we wish to give a contribution to the field by studying the chemistry induced by thermal and energetic processing of frozen mixtures of sulfur dioxide (one of the most abundant sulfur-bearing molecules observed so far) and water. We present the results of a series of laboratory experiments concerning thermal processing of different H2O:SO2 mixtures and ion bombardment 30 keV He$^+$ of the same mixtures. We used in situ FTIR spectroscopy to investigate the induced effects. The results indicate that ionic species such as HSO$_{3}^{-}$, HSO$_{4}^{-}$, and S$_2$O$_{5}^{2-}$ are easily produced. Energetic processing also produces SO$_3$ polymers and a sulfurous refractory residue. The produced ionic species exhibit spectral features in a region that, in astronomical spectra of dense molecular clouds, is dominated by strong silicate absorption. However, such a dominant feature is associated with some spectral features, some of which have not yet been identified. We suggest adding the sulfur-bearing ionic species to the list of candidates to help explain some of those features.

The Formation of Nucleobases from the Ultraviolet Photoirradiation of Purine in Simple Astrophysical Ice Analogues.

Nucleobases are the informational subunits of RNA and DNA and are essential to all known forms of life. The nucleobases can be divided into two groups of molecules: the pyrimidine-based compounds that include uracil, cytosine, and thymine, and the purine-based compounds that include adenine and guanine. Previous work in our laboratory has demonstrated that uracil, cytosine, thymine, and other nonbiological, less common nucleobases can form abiotically from the UV photoirradiation of pyrimidine in simple astrophysical ice analogues containing combinations of H2O, NH3, and CH4. In this work, we focused on the UV photoirradiation of purine mixed with combinations of H2O and NH3 ices to determine whether or not the full complement of biological nucleobases can be formed abiotically under astrophysical conditions. Room-temperature analyses of the resulting photoproducts resulted in the detection of adenine, guanine, and numerous other functionalized purine derivatives. Key Words: Pyrimidine-Nucleobases-Interstellar; Ices-Cometary; Ices-Molecular processes-Prebiotic chemistry. Astrobiology 17, 761-770.

Mechanisms of the Formation of Adenine, Guanine, and Their Analogues in UV-Irradiated Mixed NH3:H2O Molecular Ices Containing Purine.

We investigated the formation mechanisms of the nucleobases adenine and guanine and the nucleobase analogues hypoxanthine, xanthine, isoguanine, and 2,6-diaminopurine in a UV-irradiated mixed 10:1 H2O:NH3 ice seeded with precursor purine by using ab initio and density functional theory computations. Our quantum chemical investigations suggest that a multistep reaction mechanism involving purine cation, hydroxyl and amino radicals, together with water and ammonia, explains the experimentally obtained products in an independent study. The relative abundances of these products appear to largely follow from relative thermodynamic stabilities. The key role of the purine cation is likely to be the reason why purine is not functionalized in pure ammonia ice, where cations are promptly neutralized by free electrons from NH3 ionization. Amine group addition to purine is slightly favored over hydroxyl group attachment based on energetics, but hydroxyl is much more abundant due to higher abundance of H2O. The amino group is preferentially attached to the 6 position, giving 6-aminopurine, that is, adenine, while the hydroxyl group is preferentially attached to the 2 position, leading to 2-hydroxypurine. A second substitution by hydroxyl or amino group occurs at either the 6 or the 2 position depending on the first substitution. Given that H2O is far more abundant than NH3 in the experimentally studied ices (as well as based on interstellar abundances), xanthine and isoguanine are expected to be the most abundant bi-substituted photoproducts. Key Words: Astrophysical ice-Abiotic organic synthesis-Nucleic acids-Origin of life-RNA world. Astrobiology 17, 771-785.