Interstellar ice is a reaction site for molecular evolution. Gaseous molecules are frozen at low temperature (~10 K) to form ice mantles and the energy supplied by UV photons and other energy sources can lead to the synthesis of complex organics. Nitrogen-containing organic molecules are of special interest because of their biomolecular importance and their anomalous stable nitrogen isotopic composition (15N/14N) in the interstellar dust environment. Thus, N-containing organic molecules are the keys to understanding the evolution of organic molecules and the solar system. We focused on amino acids and amines in refractory organic residues formed from ultraviolet (UV) irradiated interstellar ice analogues. We developed analytical techniques that enable the identification of the small quantities of molecules formed from the simulated interstellar ice analogues. Organic residue analysis of the UV-irradiated H2O-CH3OH-NH3 ice showed the formation of three amines (methylamine, ethylamine and propylamine) and 11 amino acids (e.g., glycine, α-alanine, β-alanine, sarcosine, α-aminobutyric acid and β-aminoisobutyric acid). Furthermore, the compound-specific isotope analysis of nitrogen within the amino acids and the bulk organic film revealed that little isotopic fractionation occurred during formation in the simulated environment.
The organic matter in carbonaceous chondrites is of two kinds: one is called Insoluble Organic Matter, made of extremely large molecules that cannot be named with the usual nomenclature; one can be extracted by laboratory solvents and analyzed as a molecular mixture. Both are of debated origin. Retracing their natural histories requires putting strong constraints on their possible place of birth and their life time in space environments. It cannot be excluded they were formed in an interstellar medium before accretion on the chondritic parent bodies. As ultraviolet rays are the most common in the star forming regions and during the accretion phase of solar system, we propose to test the resilience of the natural organic matter of the Murchison meteorite against photolysis. The meteoritical soluble molecules were extracted by maceration and artificially exposed to a Lα photon dose commensurate to the one expected in molecular clouds and disks. The gaseous photolysis products were analyzed on the fly whereas the solid state mixture was solubilized again after irradiation for Orbitrap High Resolution Mass Spectrometry monitoring. We found that ultraviolet photons do modify the molecular mixture, removing H2 and small carbon bearing species, shifting the mass distribution toward lower masses and increasing the number of cycles and double bonds in the molecules structure. A noteworthy effect of the irradiation is its selective preservation of species with a double bond equivalent consistent with aromatic rings in their structure. This is explained by the higher stability of such compounds. As the pristine Murchison extract lacks those features, we estimate it has not undergone significant irradiation after its synthesis. The extract we used for experiment being water insoluble, we assume its reactivity in hydrothermal condition would have been limited and have had no effect on the irradiation fingerprints. As a result the soluble fraction of Murchison was whether formed where the UV photon flux was negligible or it has been accreted quickly and shielded from photolysis in a parent body.
Very complex mixtures of organic compounds occur in extraterrestrial materials such as carbonaceous meteorites. These intricate signatures of meteoritic organic matter can provide clues to elucidate chemical evolution processes in space. Previously, these complex organic molecules have not been well-resolved in primitive meteorites, so the formation mechanisms of extraterrestrial organics remain largely conjectural. In this study, the occurrence and abundance of soluble CHN organic compounds were examined in CM vs. CR meteorites to investigate possible chemical processes associated with the different parent bodies. Hydrogenated alkylpiperidines (CnH2n+1N) are more abundant in the CR chondrite, in contrast to more abundant aromatic alkylpyridines (CnH2n–5N) in the CM chondrites. Both alkylpyridines and alkylpiperidines are most likely synthesized from simple aldehydes and ammonia on meteorite parent bodies, but the differences between the distribution of N-cyclic compounds are consistent with different redox conditions of the parent bodies which influenced the organic molecular evolution processes in extraterrestrial materials.
Carbonaceous chondrites contain up to 2 wt% organic carbon, which is present as acid and solvent insoluble solid organic matter (IOM) and solvent soluble organic matter (SOM). The extraterrestrial organic matter should record chemical processes occurred in different environments in the early history of the Solar System, and the role of parent body aqueous alteration in the synthesis or subsequent modification of IOM and SOM still requires accurate constraints. We conducted hydrothermal experiments to simulate the synthesis of organic molecules during aqueous alteration on small bodies. Bulk chemical characteristics of soluble organic matter synthesized from formaldehyde in aqueous solutions were studied to compare them with that of chondritic SOM. We found that the redox state of synthesized organic molecules depends on temperature; the molecules become richer in hydrogen at higher temperatures. This can be explained by a cross-disproportionation reaction between organic molecules and formic acid, which occurs as a side reaction of the aldol condensation and works more effectively at higher temperatures. Comparison of the bulk chemical characteristics between the synthesized molecules and SOM extracted from the Murchison meteorite with methanol shows that the soluble organic molecules in Murchison are more reduced than the synthesized molecules. Considering the temperature condition for aqueous alteration on the CM parent body that is lower than or equivalent to the experimental temperatures, the reduced nature of Murchison organic molecules requires a reducing environment for them to be formed during hydrothermal alteration or imply that processes other than hydrothermal alteration were responsible for their synthesis. In case of hydrothermal synthesis, reducing conditions might be established by the interaction between water and iron-bearing silicates or metals on the parent body.
The Tanpopo mission is an astrobiology space experiment at the Japanese Experiment Module (JEM) ‘Kibo’ on the International Space Station (ISS). One of the sub-divided themes of the Tanpopo mission is for the intact capture of organic bearing micrometeoroids in low Earth orbit using ultralow density silica aerogel (0.01 g/cm3). In order to evaluate damage to organic matter in micrometeoroids during hyper velocity impacts into the aerogel, Murchison meteorite powdered samples, analogs of organic bearing micrometeoroids, were fired into flight-grade silica aerogel (0.01 g/cm3) using a two-stage light-gas gun with velocities of 4.4 and 5.9 km/s. The recovered Murchison grains were analyzed using scanning transmission X-ray microscopy/X-ray absorption near edge structure (STXM/XANES), transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). TEM observation did not show significant modifications of the recovered Murchison grains. Carbon-XANES spectra, however, showed a large depletion of the organic matter after the 5.9 km/s impact, but no such effects nor any significant hydrogen isotopic fractionation were observed after the 4.4 km/s impact.
Laboratory investigations of noble gas trapping in amorphous water ice have been used to predict the noble gas composition of comets and infer on the origin of volatile elements within planetary bodies. However, the recent measurement of the noble gas composition of ice sublimating from comet 67P/Churyumov-Gerasimenko by the Rosetta mission calls for novel experiments regarding the mechanisms of noble gas trapping and evolution in cometary ice analogues. Here, we investigated the ionization dynamics of Xe atoms interacting with water ice using the recently developed Resonant Two-Step Laser Ablation Mass Spectrometry (2S-LAI-MS). Xenon-water mixed ice was ablated with an infrared beam set on the maximum absorption wavelength for water (λ = 2948 nm) at which xenon atoms are kept neutral. Subsequent multiple ionization of xenon and oxygen resulted in periodic Coulomb explosions of Xen+ components (n ∈ [1;6]) and ionized water degradation products (OH+, H+, O+, O2+, O3+). Such explosions could only be detected when Xe and water were mixed together in ice, and not when separated in two overlaid layers. This paper discusses the potential mechanisms accounting for the generation of Coulomb explosions in these experiments and its relevance to cometary ice at closer distances to perihelion. We conclude that multiple ionization of xenon and oxygen in our experiment may be due to electron impact processes resembling cometary electron and ion bombardment, whereby energetic particles of hundreds of eV to a few keV are accelerated towards the comet’s nucleus. Electron and ion bombardment could induce significant chemical modifications to, and potentially outgassing from, the cometary surface.
Diffusivity and solubility of gas molecules in crystalline water ice are fundamental parameters to understand physicochemical processes in various environments in space, icy bodies, planets, and the Earth. For instance, diffusion and solubility of methane in ice Ih could constrain the methane flux from sub-surface Martian ice, and could be used to discuss diffusive elemental fractionation of gases trapped in polar ice cores that could disturb the reconstruction of the Earth’s paleoatmosphere. The diffusion coefficient and the solubility of methane in ice Ih, however, have not yet been directly determined. In this study, we performed diffusive degassing experiments of helium and methane from ice Ih at 251–259 K to measure both the diffusion coefficient and solubility of methane in ice Ih. We first determined the diffusion coefficient and solubility of helium in ice Ih, and could successfully reproduce those reported in previous studies. We then determined the diffusion coefficient of methane in ice Ih, and the obtained diffusion coefficient of methane at 257 K was (5.2 ± 1.4) × 10−11 m2 s−1. This is well consistent with that estimated by a molecular-dynamics simulation, where methane molecules do not diffuse through interstitial sites but by breaking hydrogen bonds between H2O molecules. The methane solubility in ice Ih was estimated to be (7.2 ± 1.1) × 10−7 mol molH2O−1 MPa−1 at 257 K, which is smaller than those of He and Ne and could be due to a larger van der Waals radius of methane than those of light noble gases.