Chondritic Organic Matter Research Articles

Effect of Nitrogen on the Structure and Composition of Primordial Organic Matter Analogs.

Organic molecules are ubiquitous in primitive solar system bodies such as comets and asteroids. These primordial organic compounds may have formed in the interstellar medium and in protoplanetary disks (PPDs) before being accreted and further transformed in the parent bodies of meteorites, icy moons, and dwarf planets. The present study describes the composition of primordial organics analogs produced in a laboratory simulator of the PPD (the Nebulotron experiment at the CRPG laboratory) with nitrogen contents varying from N/C < 0.01 to N/C = 0.63. We present the first Fourier transform ion cyclotron resonance mass spectrometry analysis of these analogs. Several thousands of molecules with masses between m/z 100 and 500 are characterized. The mass spectra show a Gaussian shape with maxima around m/z 250. Highly condensed polyaromatic hydrocarbons (PAH) are the most common compounds identified in the samples with lower nitrogen contents. As the amount of nitrogen increases, a dramatic increase of the chemical diversity is observed. Nitrogen-bearing compounds are also dominated by polyaromatic hydrocarbons (PANH) made of 5- and 6-membered rings containing up to four nitrogen atoms, including triazine and pyrazole rings. Such N-rich aromatic species are expected to decompose easily in the presence of water at higher temperatures. Pure carbon molecules are also observed for samples with relatively small fractions of nitrogen. MS peaks compatible with the presence of amino acids and nucleobases, or their isomers, are detected. When comparing these Nebulotron samples with the insoluble fraction of the Paris meteorite organic matter, we observe that the samples with intermediate N/C ratios bracketing that of the Paris insoluble organic matter (IOM) display relative proportions of the CH, CHO, CHN, and CHNO chemical families also bracketing those of the Paris IOM. Our results support that Nebulotron samples are relevant laboratory analogs of primitive chondritic organic matter.

Diversity of chondritic organic matter probed by ultra-high resolution mass spectrometry

Diversity of chondritic organic matter probed by ultra-high resolution mass spectrometry

Exploring the link between molecular cloud ices and chondritic organic matter in laboratory

Carbonaceous meteorites are fragments of asteroids rich in organic material. In the forming solar nebula, parent bodies may have accreted organic materials resulting from the evolution of icy grains observed in dense molecular clouds. The major issues of this scenario are the secondary processes having occurred on asteroids, which may have modified the accreted matter. Here, we explore the evolution of organic analogs of protostellar/protoplanetary disk material once accreted and submitted to aqueous alteration at 150 °C. The evolution of molecular compounds during up to 100 days is monitored by high resolution mass spectrometry. We report significant evolution of the molecular families, with the decreases of H/C and N/C ratios. We find that the post-aqueous products share compositional similarities with the soluble organic matter of the Murchison meteorite. These results give a comprehensive scenario of the possible link between carbonaceous meteorites and ices of dense molecular clouds.

Kinetics in thermal evolution of Raman spectra of chondritic organic matter to evaluate thermal history of their parent bodies

AbstractIn order to establish kinetic expressions for Raman spectroscopic parameters of organic matter in chondritic meteorites with heating, a series of heating experiments (at 600–900 °C for 3–48 h) of the Murchison (CM2) meteorite was conducted. For comparison, several carbonaceous chondrites with various metamorphic degrees—Allende (CV3.2), Moss (CO3.6), Yamato (Y‐) 793321 (heated CM2), and Tagish Lake (ungrouped C2)—were also analyzed by the Raman spectrometer. Changes in the full width at half maximum of the D1 band (ΓD) of heated Murchison correlated well with temperature and time, and showed similar trends of chondrites with various metamorphic degrees. We obtained the kinetic expressions for the changes in ΓD by heating to estimate the time–temperature history of thermally metamorphosed type 3 chondrites and heated CM chondrites. Our results may also be useful for asteroids which are the targets of Hayabusa2 and OSIRIS‐REx missions.

New insights into the structure and formation of coals, terrestrial and extraterrestrial kerogens from resonant UV Raman spectroscopy

Resonant UV (244 nm-excitation) Raman spectroscopy was used to characterize a series of coals, type II kerogens, Insoluble Organic Matter (IOM) extracted from primitive chondrites and two stratospheric Interplanetary Dust Particles. UV-Raman spectra of immature terrestrial kerogens and coals are distinct from those of extraterrestrial kerogens extracted from type 1 and 2 primitive chondrites, allowing unambiguous identification. The degree of disorder in the refractory carbonaceous matter in the considered IDPs is found to be higher than in primitive chondrites, confirming former interpretation of visible (514 nm-excitation) Raman spectra of IDPs. Spectral and structural features of laboratory analogues of chondritic organic matter formed in cold plasma reactors at 500–700 °C differ from those of natural samples, challenging the hypothesis that chondritic organic matter within the protosolar disk may form through plasma reactions. Finally, UV Raman data shed new light in the structural evolution of coals during maturation and support the presence of an sp2 structural transition at vitrinite reflectance Ro ∼ 0.4–0.5%. Comparison with former visible Raman and infrared spectroscopic data confirms the existence of this structural transition, which is related to a dramatic drop of oxygenated species (carboxyl, hydroxyl, ketone and ether groups). Oxygen thus plays an indirect but significant role in the control of the Raman spectra of polyaromatic materials. A potential application of this technique is the determination of the biotic versus abiotic origin of kerogens in ancient rocks on Earth, and of soils and rocks on Mars that is otherwise difficult with classical visible-excitation Raman spectroscopy.

Organics preserved in anhydrous interplanetary dust particles: Pristine or not?

AbstractThe chondritic‐porous subset of interplanetary dust particles (CP‐IDPs) are thought to have a cometary origin. Since the CP‐IDPs are anhydrous and unaltered by aqueous processes that are common to chondritic organic matter (OM), they represent the most pristine material of the solar system. However, the study of IDP OM might be hindered by their further alteration by flash heating during atmospheric entry, and we have limited understanding on how short‐term heating influences their organic content. In order to investigate this problem, five CP‐IDPs were studied for their OM contents, distributions, and isotopic compositions at the submicro‐ to nanoscale levels. The OM contained in the IDPs in this study spans the spectrum from primitive OM to that which has been significantly processed by heat. Similarities in the Raman D bands of the meteoritic and IDP OMs indicate that the overall gain in the sizes of crystalline domains in response to heating is similar. However, the Raman ΓG values of the OM in all of the five IDPs clearly deviate from those of chondritic OM that had been processed during a prolonged episode of parent body heating. Such disparity suggests that the nonaromatic contents of the OM are different. Short duration heating further increases the H/C ratio and reduces the δ13C and δD values of the IDP OM. Our findings suggest that IDP OM contains a significant proportion of disordered C with low H content, such as sp2 olefinic C=C, sp3 C–C, and/or carbonyl contents as bridging material.

Processes of noble gas elemental and isotopic fractionations in plasma-produced organic solids: Cosmochemical implications

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn-type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk.

The deuterium/hydrogen distribution in chondritic organic matter attests to early ionizing irradiation.

Primitive carbonaceous chondrites contain a large array of organic compounds dominated by insoluble organic matter (IOM). A striking feature of this IOM is the systematic enrichment in deuterium compared with the solar hydrogen reservoir. This enrichment has been taken as a sign of low-temperature ion-molecule or gas-grain reactions. However, the extent to which Solar System processes, especially ionizing radiation, can affect D/H ratios is largely unknown. Here, we report the effects of electron irradiation on the hydrogen isotopic composition of organic precursors containing different functional groups. From an initial terrestrial composition, overall D-enrichments and differential intramolecular fractionations comparable with those measured in the Orgueil meteorite were induced. Therefore, ionizing radiation can quantitatively explain the deuteration of organics in some carbonaceous chondrites. For these meteorites, the precursors of the IOM may have had the same isotopic composition as the main water reservoirs of the inner Solar System.

Amorphization and D/H fractionation of kerogens during experimental electron irradiation: Comparison with chondritic organic matter

Irradiation is common in the interstellar medium and the protosolar nebula. We have investigated the effects of electron irradiation on kerogens of type I and III in a 200kV transmission electron microscope (TEM), at 293K and 92K, using various fluences. Using synchrotron-based scanning transmission X-ray microscopy (STXM) and NanoSIMS, we have demonstrated a progressive amorphization coupled with hydrogen loss and a significant deuterium to hydrogen ratio (D/H) fractionation, with δD increasing by up to 1000‰. Hydrogen loss is non-linearly related to the fluence. Irradiation under cryogenic conditions (92K) hinders amorphization and D/H elevation. We suggest that these effects are controlled by radiolysis (carbonhydrogen bonds are broken and hydrogen is lost), coupled with recombination. The amorphization and hydrogen loss are rate-limited by defect diffusion which controls the recombination probability. The D/H increase appears to follow a Rayleigh distillation law with an apparent fractionation factor similar to the equilibrium fractionation factor of the isotopic exchange reaction CH4+HD↔CH3D+H2.This study represents a first step to estimate the kinetics and timescales of D/H fractionation under ionizing radiation. Extrapolatation of this fractionation behavior to natural environments remains difficult at this point because simultaneous irradiation by protons and other cosmic rays at various energies also occurs. However, the present results show that isotopic fractionation by electron irradiation at 200kV alone might have been kinetically inhibited at the low temperatures of the interstellar medium and the outer region of the protosolar nebula. In addition, we show that STXM or NanoSIMS experiments should not be performed on organic samples that have already been investigated using TEM, even under low flux electron irradiation conditions.

The preservation state of organic matter in meteorites from Antarctica

Abstract— The recovery of large numbers of meteorites from Antarctica has dramatically increased the amount of extraterrestrial material available for laboratory studies of solar system origin and evolution. Yet, the great age of Antarctic meteorites raises the concern that significant amounts of terrestrial weathering has corrupted their pre‐terrestrial record. Organic matter found in carbonaceous chondrites is one of the components most susceptible to alteration by terrestrial processes. To assess the effects of Antarctic weathering on both non‐Antarctic and Antarctic chondritic organic matter, a number of CM chondrites have been analyzed. Mössbauer spectroscopy has been used to ascertain pre‐terrestrial and terrestrial oxidation levels, while pyrolysis‐gas chromatography‐mass spectrometry was used to determine the constitution of any organic matter present. Increased oxidation levels for iron bearing minerals within the non‐Antarctic chondrites are likely to be a response to increased amounts of parent body aqueous alteration. Parent body processing also appears to remove ether bonds from organic material and alkyl side chains from its constituent units. The iron in Antarctic chondrites is generally more oxidized than that in their non‐Antarctic counterparts, reflecting terrestrial weathering. Antarctic weathering of chondritic organic matter appears to proceed in a similar way to parent body aqueous alteration and simply enhances the organic responses observed in the non‐Antarctic data set. Degradation of the record of preterrestrial processes in Antarctic chondrites should be taken into account when interpreting data from these meteorites.

Clay mineral‐organic matter relationships in the early solar system

Abstract— As the solar system formed, it inherited and perpetuated a rich organic chemistry, the molecular products of which are preserved in ancient extraterrestrial objects such as carbonaceous chondrites. These organic‐rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early solar system. Chondritic organic matter is present in the inorganic meteorite matrix which, in the CM and CI chondrites, contains evidence of alteration by liquid water on the parent asteroid. An unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence? We have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals. This association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early solar system prior to the origin of life on the early Earth.

Comments on D/H ratios in chondritic organic matter

Primitive meteorites contain significant amounts of carbon, the majority of which is present as organic matter. Low molecular weight compound classes include aliphatic and aromatic hydrocarbons, amino acids, nucleic acid bases, carboxylic acids, phosphonic acids, sulfonic acids, sugar-related compounds, and organohalogens. Yet, the low molecular weight compounds are minor components relative to a high molecular weight, predominantly aromatic network. Some of the organic matter exists in morphologically distinct microvesicles or nanoglobules and appears to associate with clay minerals in the matrix. As bona fide prebiotic organic compounds, the meteoritic organic matter provides valuable insights into the chemical evolution in the early solar system and the reactions that preceded the origin of life on Earth.