Chris J. Bennett

A combined experimental and theoretical study on the formation of the amino acid glycine (NH2CH2COOH) and its isomer (CH3NHCOOH) in extraterrestrial ices

We have investigated the synthesis of the simplest aminoacid, glycine, by Galactic cosmic-ray particles in extraterrestrialices.Laboratoryexperimentscombinedwithelectronicstructurecalculationsshowedthatamethylamine molecule [CH3NH2(X 1 A 0 )] can be dissociated through interaction with energetic electrons in the track of a cosmicray particle to form atomic hydrogen and the radicals CH2NH2(X 2 A 0 )a nd CH3NH(X 2 A 0 ). Hydrogen atoms with sufficient kinetic energy could overcome the entrance barrier to add to a carbon dioxide molecule [CO2(X 1 � þ )], yielding a trans-hydroxycarbonyl radical, HOCO(X 2 A 0 ). Neighboring radicals with the correct geometric orientation then recombine to form glycine, NH2CH2COOH(X 1 A), and also its isomer, CH3NHCOOH(X 1 A). These findings expose for the first time detailed reaction mechanisms of how the simplest amino acid glycine and its isomer can be synthesized via nonequilibrium chemistry in interstellar and cometary ices. Our results offer an important alternative to aqueous and photon-induced formation of amino acids in comets and in molecular clouds. These results also predict the existence of a hitherto undetected isomer of glycine in the interstellar medium, suggest that glycine should be observable on Saturn’s moon Titan, and help to account for the synthesis of more complex amino acids in the Murchison and Orgueil meteorites.

Laboratory Studies on the Irradiation of Methane in Interstellar, Cometary, and Solar System Ices

Pure methane ices (CH4) were irradiated at 10 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The experiments were monitored via an FTIR spectrometer (solid state) and a quadrupole mass spectrometer (gas phase). Combined with electronic structure calculations, this paper focuses on the identification of CHx (x = 1-4) and C2Hx (x = 2-6) species and also investigates their formation pathways quantitatively. The primary reaction step is determined to be the cleavage of a carbon-hydrogen bond of the methane molecule to form a methyl radical (CH3) plus a hydrogen atom. Hydrogen atoms recombined to form molecular hydrogen, the sole species detected in the gas phase during the irradiation exposure. In the matrix two neighboring methyl radicals can recombine to form an internally excited ethane molecule (C2H6), which either can be stabilized by the surrounding matrix or was found to decompose unimolecularly to the ethyl radical (C2H5) plus atomic hydrogen and then to the ethylene molecule (C2H4) plus molecular hydrogen. The initially synthesized ethane, ethyl, and ethylene molecules can be radiolyzed subsequently by the impinging electrons to yield the vinyl radical (C2H3) and acetylene (C2H2) as degradation products. Upon warming the ice sample after the irradiation, the new species are released into the gas phase, simulating the sublimation processes interstellar ices undergo during the hot core phase or comets approaching perihelion. Our investigations also aid the understanding of the synthesis of hydrocarbons likely to be formed in the aerosol particles and organic haze layers of hydrocarbon-rich atmospheres of planets and their moons such as Titan.

On the Formation of Glycolaldehyde (HCOCH2OH) and Methyl Formate (HCOOCH3) in Interstellar Ice Analogs

Binary mixtures of methanol (CH3OH) and carbon monoxide (CO) ices were irradiated at 10 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The formation of glycolaldehyde (HCOCH2OH) was established through the appearance of new bands in the infrared spectrum at 1757, 1700, 1690, 1367, 1267, and 1067 cm-1. A second C2H4O2 isomer, methyl formate (HCOOCH3), was also identified by absorptions appearing at 1718, 1159, and 914 cm-1. Mass spectrometer signals during the warm-up of the ice sample showed sublimation of both the glycolaldehyde and methyl formate; these species were monitored via the C2H4O2+ molecular ion at mass-to-charge ratio, m/e, of 60 originating from both glycolaldehyde and the methyl formate isomer. The latter was distinguishable by the presence of a second signal at m/e = 45, i.e., the HCO2+ ion. Kinetic fits of the column densities of the reactants and products suggest the initial step of the formation process is the cleavage of a C–H bond in the methanol molecule to generate either the hydroxymethyl (CH2OH) or methoxy (CH3O) radical plus atomic hydrogen. The hydrogen atom holds excess kinetic energy, allowing it to overcome entrance barriers required; therefore, a hydrogen could add to a CO molecule, generating the formyl radical (HCO). This can recombine with the hydroxymethyl radical to form glycolaldehyde or with the methoxy radical to yield methyl formate. Similar processes are expected to form glycolaldehyde and methyl formate in the interstellar medium on grains and possibly on cometary ices, thus providing alternatives to gas-phase processes for the generation of complex species whose fractional abundances compared with H2 of typically a few times 10-9 cannot be accounted for solely by gas-phase chemistry.

Laboratory Studies on the Formation of Ozone (O3) on Icy Satellites and on Interstellar and Cometary Ices

The formation of ozone (O3) in neat oxygen ices was investigated experimentally in a surface-scattering machine. At 11 K, solid oxygen was irradiated with energetic electrons; the chemical modification of the target was followed on-line and in situ via Fourier transform infrared spectroscopy (FTIR; solid state) and quadrupole mass spectrometry (QMS; gas phase). The dominant product identified was the ozone molecule in the bent, C2v symmetric structure, O3(X 1A1); the cyclic D3h isomer was not observed. The associated van der Waals complexes [O3...O] and [O3...O3] could also be detected via infrared spectroscopy, verifying explicitly the existence of oxygen atoms in the matrix at 11 K. Three different formation mechanisms of ozone were revealed. Two pathways involve the reaction of suprathermal oxygen atoms with molecular oxygen [O2(X 3Σ)] at 11 K. Once the sample was warmed after the irradiation to about 38 K, a third, thermal reaction pathway involving the barrierless reaction of ground-state oxygen atoms with molecular oxygen sets in. During the warm-up phase, the inherent sublimation of oxygen and ozone was monitored by mass spectrometry and occurs in the ranges 28-43 and 58-73 K, respectively. Our data are of help to understand the mechanisms of ozone formation within apolar interstellar and cometary ices and could also be applicable to outer solar system icy bodies, such as the moons of Jupiter (Ganymede, Europa, and Callisto) and Saturn (Rhea and Dione), where ozone and/or condensed oxygen has been observed.

Untangling the formation of the cyclic carbon trioxide isomer in low temperature carbon dioxide ices

The formation of the cyclic carbon trioxide isomer, CO3(X 1A1), in carbon-dioxide-rich extraterrestrial ices and in the atmospheres of Earth and Mars were investigated experimentally and theoretically. Carbon dioxide ices were deposited at 10 K onto a silver (111) single crystal and irradiated with 5 keV electrons. Upon completion of the electron bombardment, the samples were kept at 10 K and were then annealed to 293 K to release the reactants and newly formed molecules into the gas phase. The experiment was monitored via a Fourier transform infrared spectrometer in absorption-reflection-absorption (solid state) and through a quadruple mass spectrometer (gas phase) on-line and in situ. Our investigations indicate that the interaction of an electron with a carbon dioxide molecule is dictated by a carbon–oxygen bond cleavage to form electronically excited (1D) and/or ground state (3P) oxygen atoms plus a carbon monoxide molecule. About 2% of the oxygen atoms react with carbon dioxide molecules to form the C2v symmetric, cyclic CO3 structure via addition to the carbon–oxygen double bond of the carbon dioxide species; neither the Cs nor the D3h symmetric isomers of carbon trioxide were detected. Since the addition of O(1D) involves a barrier of a 4–8 kJ mol−1 and the reaction of O(3P) with carbon dioxide to form the carbon trioxide molecule via triplet-singlet intersystem crossing is endoergic by 2 kJ mol−1, the oxygen reactant(s) must have excess kinetic energy (suprathermal oxygen atoms which are not in thermal equilibrium with the surrounding 10 K matrix). A second reaction pathway of the oxygen atoms involves the formation of ozone via molecular oxygen. After the irradiation, the carbon dioxide matrix still stores ground state oxygen atoms; these species diffuse even at 10 K and form additional ozone molecules. Summarized, our investigations show that the cyclic carbon trioxide isomer, CO3(X 1A1), can be formed in low temperature carbon dioxide matrix via addition of suprathermal oxygen atoms to carbon dioxide. In the solid state, CO3(X 1A1) is being stabilized by phonon interactions. In the gas phase, however, the initially formed C2v structure is rovibrationally excited and can ring-open to the D3h isomer which in turn rearranges back to the C2v structure and then loses an oxygen atom to ‘recycle’ carbon dioxide. This process might be of fundamental importance to account for an 18O enrichment in carbon dioxide in the atmospheres of Earth and Mars.

THE FORMATION OF ACETIC ACID (CH3COOH) IN INTERSTELLAR ICE ANALOGS

Binary ice mixtures of methane (CH4) and carbon dioxide (CO 2) ices were irradiated at 12 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The formation of trans-acetic acid (CH3COOH) was established through the appearance of new bands in the infrared spectrum at 1780, 1195, 1160, 1051, and 957 cm-1; two dimeric forms of acetic acid were assigned via absorptions at 1757 and 1723 cm-1 . During warm-up of the ice sample, the mass spectrometer recorded peaks of m/z values of 60 and 45 associated with the C2H 4O2+ and COOH+ molecular ion and fragment, respectively. The kinetic fits of the column densities of the acetic acid molecule suggest that the initial step of the formation process appears to be the cleavage of a carbon-hydrogen bond from methane to generate the methyl radical plus atomic hydrogen. The hydrogen atom holds excess kinetic energy allowing it to overcome entrance barriers required to add to a carbon dioxide molecule, generating the carboxyl radical (HOCO). This radical can recombine with the methyl radical to form acetic acid molecule. Similar processes are expected to form acetic acid in the interstellar medium, thus providing alternatives to gas-phase processes for the generation of complex chemical species whose fractional abundances compared to molecular hydrogen of typically a few × 10-9 cannot be accounted for by solely gas-phase chemistry.

MECHANISTICAL STUDIES ON THE PRODUCTION OF FORMAMIDE (H2NCHO) WITHIN INTERSTELLAR ICE ANALOGS

Formamide, H2NCHO, represents the simplest molecule containing the peptide bond. Consequently, the formamide molecule is of high interest as it is considered an important precursor in the abiotic synthesis of amino acids, and thus significant to further prebiotic chemistry, in more suitable environments. Previous experiments have demonstrated that formamide is formed under extreme conditions similar throughout the interstellar medium via photolysis and the energetic processing of ultracold interstellar and solar system ices with high-energy protons; however, no clear reaction mechanism has been identified. Utilizing a laboratory apparatus capable of simulating the effects of galactic cosmic radiation on ultralow temperature ice mixtures, we have examined the formation of formamide starting from a variety of carbon monoxide (CO) to ammonia (NH3) ices of varying composition. Our results suggest that the primary reaction step leading to the production of formamide in low-temperature ices involves the cleavage of the nitrogen-hydrogen bond of ammonia forming the amino radical (NH2) and atomic hydrogen (H), the latter of which containing excess kinetic energy. These suprathermal hydrogen atoms can then add to the carbon-oxygen triple bond of the carbon monoxide (CO) molecule, overcoming the entrance barrier, and ultimately producing the formyl radical (HCO). From here, the formyl radical may combine without an entrance barrier with the neighboring amino radical if the proper geometry for these two species exists within the matrix cage.

Laboratory Studies on the Formation of Three C2H4O Isomers—Acetaldehyde (CH3CHO), Ethylene Oxide (c-C2H4O), and Vinyl Alcohol (CH2CHOH)—in Interstellar and Cometary Ices

Laboratory experiments were conducted to unravel synthetic routes to form three C2H4O isomers—acetaldehyde (CH3CHO),ethyleneoxide(c-C2H4O),andvinylalcohol(CH2CHOH)—inextraterrestrialicesviaelectronicenergy transfer processes initiated by electrons in the track of MeV ion trajectories. Here we present the results of electron irradiation on a 2:1 mixture of carbon dioxide (CO2) and ethylene (C2H4). Our studies suggest that suprathermal oxygen atoms can add to the carbon-carbonbond of an ethylene molecule to form initially an oxirene diradical (addition to one carbon atom) and the cyclic ethylene oxide molecule (addition to two carbon atoms) at 10 K. The oxirenediradicalcanundergoa(1,2)-Hshifttotheacetaldehydemolecule.Boththeethyleneoxideandtheacetaldehyde isomerscanbestabilizedinthesurroundingicematrix.Toaminoramount,suprathermaloxygenatomscaninsertinto a carbon-hydrogen bond of the ethylene molecule, forming vinyl alcohol. Once these isomers have been synthesized inside the ice layers of the coated grains in cold molecular clouds, the newly formed molecules can sublime as the cloudreaches thehot molecular corestage. These laboratory investigations helpto explainastronomicalobservations by Nummelin et al. and Ikeda et al. toward massive star-forming regions and hot cores, where observed fractional abundances of these isomers are higher than can be accounted for by gas-phase reactions alone. Similar synthetic routes could help explain the formation of acetaldehyde and ethylene oxide in comet C/1995 O1 (Hale-Bopp) and also suggest a presence of both isomers in Titans atmosphere. Subject headingg astrobiology — astrochemistry — comets:general — ISM:molecules — methods:laboratory — molecular processes — planets and satellites: individual (Titan)

LABORATORY STUDIES ON THE IRRADIATION OF SOLID ETHANE ANALOG ICES AND IMPLICATIONS TO TITAN'S CHEMISTRY

Pure ethane ices (C2H6) were irradiated at 10, 30, and 50 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray (GCR) particles to simulate the interaction of GCRs with ethane ices in the outer solar system. The chemical processing of the samples was monitored by a Fourier transform infrared spectrometer and a quadrupole mass spectrometer during the irradiation phase and subsequent warm-up phases on line and in situ in order to extract qualitative (products) and quantitative (rate constants and yields) information on the newly synthesized molecules. Six hydrocarbons, methane (CH4), acetylene (C2H2), ethylene (C2H4), and the ethyl radical (C2H5), together with n-butane (C4H10) and butene (C4H8), were found to form at the radiation dose reaching 1.4 eV per molecule. The column densities of these species were quantified in the irradiated ices at each temperature, permitting us to elucidate the temperature and phase-dependent production rates of individual molecules. A kinetic reaction scheme was developed to fit column densities of those species produced during irradiation of amorphous/crystalline ethane held at 10, 30, or 50 K. In general, the yield of the newly formed molecules dropped consistently for all species as the temperature was raised from 10 K to 50 K. Second, the yield in the amorphous samples was found to be systematically higher than in the crystalline samples at constant temperature. A closer look at the branching ratios indicates that ethane decomposes predominantly to ethylene and molecular hydrogen, which may compete with the formation ofn-butane inside the ethane matrix. Among the higher molecular products, n-butane dominates. Of particular relevance to the atmosphere of Saturn’s moon Titan is the radiation-induced methane production from ethane—an alternative source of replenishing methane into the atmosphere. Finally, we discuss to what extent the n-butane could be the source of “higher organics” on Titan’s surface thus resembling a crucial sink of condensed ethane molecules.

Investigating the Mechanism for the Formation of Nitrous Oxide [N2O(X 1Σ+)] in Extraterrestrial Ices

The formation of nitrous oxide, N2O(X 1Σ+), in interstellar space and in ices on Pluto and Triton has been experimentally investigated. A molecular nitrogen (N2) and carbon dioxide (CO2) ice mixture was irradiated at 10 K with 5 keV electrons to simulate the electronic interaction effects of Galactic cosmic-ray bombardment of extraterrestrial ice samples over a time of 5 × 106 yr. By monitoring the experiment with a Fourier transform infrared spectrometer on line and in situ, the temporal evolution of the 2235 cm-1 absorption band of nitrous oxide was found to follow pseudo-first-order kinetics. This indicates that the mechanism of formation is most likely a reaction between ground-state molecular nitrogen, N2(X 1Σ), and an oxygen atom, either in the ground state (3P) or in the first electronically excited state (1D), within the matrix cage through an addition of the oxygen atom to a nonbonding electron pair on the nitrogen molecule. The observation of nitrous oxide together with the kinetics and dynamics studies investigated in this paper underline the role of nonequilibrium processes in low-temperature ice matrices, aid in the understanding of chemical reaction pathways that exist in extraterrestrial ices, and assist a prospective identification of nitrous oxide on the surfaces of Pluto and Triton.