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Synthesis of refractory organic matter in the ionized gas

phase of the solar nebula

Maïa Kuga, Bernard Marty, Yves Marrocchi, Laurent Tissandier

To cite this version:

Maïa Kuga, Bernard Marty, Yves Marrocchi, Laurent Tissandier. Synthesis of refractory organic

matter in the ionized gas phase of the solar nebula. Proceedings of the National Academy of Sciences

of the United States of America , National Academy of Sciences, 2015, �10.1073/pnas.1502796112�.

�hal-01346065�

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Synthesis of refractory organic matter in the ionized

gas phase of the Solar Nebula

Maïa Kuga, Bernard Marty, Yves Marrocchi, Laurent Tissandier

CRPG-CNRS

Submitted to Proceedings of the National Academy of Sciences of the United States of America

Refractory organic compounds are ubiquitous in primitive chon-drites and cometary samples, though their origin is poorly under-stood. Those organic compounds are the main host of primordial noble gases, known as Q-gases, and nitrogen, which isotopic fractionations recorded physicochemical conditions of the solar system formation. Here, we report the characterization of organic compounds synthesized under ionizing conditions in a plasma setup from gas mixtures (H2(O)-CO-N2-Noble gases) reminiscent

of the protosolar nebula composition. Ionization of the gas phase was achieved at temperature up to 1000 K. Synthesized solid compounds share chemical and structural features with chondritic organics, and trapped noble gases reproduce the elemental and isotopic Q-gases patterns. These results suggest that both the formation of chondritic refractory organics and the trapping of Q-gases took place simultaneously in ionized areas of the proto-planetary disk, via photon- and/or electron-driven reactions and processing. Thus synthesis of primitive organics might not have required a cold environment as often assumed, and could have occurred anywhere it is ionized in the disk, including in its warm regions. This scenario also supports N2 photodissociation as the

cause of the large nitrogen isotopic range in the solar system.

solar nebula | organics | meteorites | noble gases | nitrogen

Introduction

The solar system formed from the gravitational collapse of a dense core within a molecular cloud that led to the ignition of a central star surrounded by a protoplanetary disk. Protoplane-tary disks are evolving and dynamic systems, in which complex chemistry occurs and results in the formation and agglomeration of solids. Among them, organic molecules are of great astrobio-logical interest because they may represent the very first building blocks of prebiotic molecules. Moreover, they are the main carri-ers of volatile elements (i.e., H, C, N, and noble gases) that were directly involved in shaping the atmospheres of rocky planets. Organic matter is ubiquitous in primitive solar system bodies (e.g., chondrites especially the carbonaceous ones, interplanetary dust particles - IDPs, cometary dust - Comet 81P/Wild 2 and Ultra-Carbonaceous Antarctic Micro-Meteorites - UCAMMs (1-3)). Most of meteoritic organics (up to 99%) are in the form of a re-fractory and insoluble macromolecular solid, commonly referred to as insoluble organic matter (IOM) (4). Notably, IOM displays bulk D and15N isotope excesses relative to solar composition,

which can reach extreme values at a microscopic scale (5, 6). These compositional and isotopic characteristics bear a unique record of the processes and environmental conditions of IOM synthesis, and, by extension, of processing of solid materials and volatile elements in the early solar nebula, but the message is still cryptic.

IOM is also the main carrier of the heavy noble gases (Ar, Kr and Xe) as well as a small fraction of He and Ne trapped in chon-drites. The exact host phase of these elements - nicknamed Phase Q (7) - is not yet characterized but appears to be a part of, or at least chemically associated with refractory organic compounds (7,

8). This Q component is ubiquitous in primitive chondrites (9, 10),

IDPs and in Antarctic micrometeorites (11). Thus, Q-gases may represent the most important noble gas reservoir outside the Sun

at the time of solid accretion in the protosolar solar nebula (PSN). Q-gases are elementally and isotopically fractionated relative to the solar composition (as inferred by the analysis of solar wind composition, (12-15)), in favor of heavy elements and isotopes, by about 1%/amu for Xe isotopes (10). So far, only plasma ex-periments involving noble gas ionization were able to fractionate them elementally and isotopically to extents comparable to those of Q (16-20). Both Q-gases and IOM are intimately related and ubiquitous in primitive solar system bodies, likely pointing to a common and pre-accretion origin.

Several processes for IOM synthesis have been proposed and/or studied experimentally, reflecting the variety of astrophys-ical regions where organosynthesis may occur. Those include: (i) cold scenarios (e.g., below 40 K) via UV-induced grain surface polymerization of organic molecules in the icy mantles of dust grains, either in the interstellar medium (ISM) or in the outer solar nebula (21, 22); (ii) warm or high temperature scenarios (e.g., 300 K or above) such as Fischer-Tropsch-like reactions onto metal grains in the inner part of the solar nebula (23,

24); or (iii) organosynthesis onto the parent body of chondrites

via polymerization of interstellar formaldehyde (25, 26). Labo-ratory analogues reproduce partially the compositional features of chondritic/cometary refractory organics, but generally fail to reproduce the large D and15N excesses observed in inner planets,

meteorites and comets (relative to the PSN composition), and do not address the elemental and isotopic fractionations of noble gases.

In this work, we present a plasma experiment designed to produce refractory solid organics from mixtures of C- and N-bearing gases representative of the composition of the solar nebula (i.e, CO, N2 with addition of noble gases). This is the

Significance

In this paper, we report an experimental investigation of the origin of refractory organics as well as of primordial noble gases, known as Q-gases, in primitive chondrites. We designed and performed a plasma experiment to examine for the first time organosynthesis and noble gas issues simultaneously. The synthetic compounds share chemical and structural fea-tures with chondritic organics, and trapped noble gases repro-duce the elemental and isotopic characteristics of Q-gases. Our results demonstrate that Q-gases formed via ionization of no-ble gases within the early solar nebula and points to the most ionized areas of the nebula as the place were organosynthesis occurred, consistently with N2photodissociation at the origin

of15N enrichments in chondritic organics.

Reserved for Publication Footnotes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136

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Fig. 1. FTIR baseline-corrected spectra of the carbonaceous solids synthesized

from CO ± Noble gas (top) or from CO + N2±Noble gas (bottom) mixtures in the Nebulotron. Shown for comparison: IOMs isolated from Orgueil, Cold Bokkeveld and PCA91008 ((34), top), tholins from the PAMPRE setup (PAMPRE-5%, produced from a mixture composed of 5% CH4in N2, (56),

bottom), and organics from the UCAMM particle DC65 ((38), bottom). The main absorption bands are highlighted. All spectra are normalized to the intensity at 1600 cm-1(C=C), in order to compare the relative intensities of

the other bands.

first integrative study that investigates organosynthesis and no-ble gas issues simultaneously. Refractory and insoluno-ble organic compounds are produced at temperatures >500 K, and share compositional and structural similarities with IOM. These solids also contain trapped noble gases with Q-like elemental/isotopic fractionation. Our results suggest that noble gas-rich complex organic compounds could have been formed in the ionized re-gions of the protoplanetary disk via gas phase reactions and grain coagulation, even in the warm regions of the disk.

Methods

Production of carbonaceous solids was undertaken in a microwave (2.45 GHz) plasma reactor, called Nebulotron ((27), Fig. S1), from gas mixtures composed of CO ± N2±noble gases (+ traces of H2/H2O). Typical experiments

were run with a total gas flow of 6-10 sccm at∼ 1 mbar for two to six

hours. Carbonaceous solid particles synthesis is initiated by the ionization and dissociation of CO/CO-N2by electronic and heavy particle processes in the gas

phase at∼ 800-1000 K, a temperature typical of microwave discharges (27, 28). The synthesized carbonaceous solids from CO ± noble gases and CO-N2

±noble gases experiments were recovered and analyzed by a combination of analytical techniques in order to address their nature and structure and their noble gas content (see SI for experimental and analytical details).

Results

Complex CHON compounds

Elemental analyses of the Nebulotron-synthesized solids re-veal their organic nature, with carbon, hydrogen, oxygen and nitrogen representing 30-70 mol%, 23-35 mol%, 6-20 mol% and 1-22 mol%, respectively (Table S1). Solids produced from CO and CO-Noble gas mixtures are N-poor samples (i.e., N/C <

2

ture contain significant amounts of nitrogen (i.e., N/C > 0.5), demonstrating the very efficient incorporation of nitrogen into the skeleton of the organic solids when N2is present in the flowing

gas mixture. H2was not used in this study but H2/H2O impurities

from the gas tanks and/or from reactor’s surfaces contributed to the gas budget during the experiments. Interestingly, hydrogen is efficiently incorporated into molecular precursors of solid com-pounds and certainly helps to build the organic skeleton of the macromolecules recovered after the experiments.

The organic and complex nature of the Nebulotron-synthesized compounds is also revealed by their FTIR spectra (Fig. 1), which display characteristic bands of aliphatic and aro-matic/oleifinic groups, and O-rich functional groups (hydroxyl, carbonyl and ether groups). The N-rich Nebulotron samples synthesized from CO-N2 experiments present additional bands

identifying N-rich functional groups (amine/amide, nitrile and imine groups). The C=O and C=C bands display the largest relative intensities, suggesting an unsaturated molecular structure for the Nebulotron-synthetized organics. The absence of the aromatic C-H stretching band (∼ 3050 cm-1) is consistent with high cross-linking and substitution of aromatics in those solids. The addition of noble gases in the gas mixture, as well as the increase of the electric discharge power, is expected to modify the electric equilibrium of the plasma and the gas temperature, thus changing the chemical reactions and/or their rates in the gas phase (28). Such effects can account for differences in the relative intensities of the bands in the FTIR spectra of samples Neb-CO28, Neb-COGR29 and Neb-COGR30 (Table S1).

A low degree of carbon structural organization: HRTEM and Raman

High resolution transmission electron microscopy (HRTEM) images (Fig. 2) illustrate a gradual increase degree of organiza-tion of the carbon structure of the Nebulotron samples with the temperature of the discharge, as well as some heterogeneities at the nanoscale. The degree of structural organization ranges from highly disordered or “fluffly” (29) with very short fringes staked by two or three, to lamellar and porous nanostructure with fringes staked by more than five and longer than 10 nm. Consistently, qualitative Raman observations show that most of the Nebulotron samples display large D and G carbon bands in 514 and 244 nm Raman spectra (Figs. S2 and S3), confirming a low degree of carbon structural organization (30, 31). Quantita-tive extraction of 244 nm Raman spectral parameters illustrate a limited heterogeneity among Nebulotron samples (Fig. 3), except for the ID/IGratio (Fig. 3a), and for the FWHM-G, much larger

for N-rich Nebulotron samples (Fig. 3b). The Nebulotron CO-9 sample falls out the trend, clearly reflecting a higher degree of structural organization consistent with HRTEM imaging (Fig. 2). This particular sample was produced with a higher electric discharge power compared to other samples, implying a higher gas temperature in the plasma setup (estimated > 800 K, (28)). This illustrates a gradual organization of the carbon structure with the plasma temperature.

Trapping, elemental and isotopic fractionation of heavy noble gases

Both Ar and Kr trapped in the synthesized solids are depleted by one order of magnitude relative to Xe and to the flowing noble gas mixture composition (Fig. 4a: the Ar depletion factor is an upper limit as the blank contribution for Ar was larger than the trapped Ar content). All the Nebulotron samples produced from CO-Noble gases ± N2 mixtures present large Kr and Xe

mass-dependent isotopic fractionations, where heavy isotopes are enriched relative to the lighter ones by 1.3±0.7%/amu and 1.0±0.4%/amu, respectively (Figs. 4b and 4c). Both the elemental and isotopic fractionation factors are also comparable with those obtained in previous plasma noble gas-trapping experiments 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204

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Fig. 2. High resolution transmission electron microscopy images of two Nebulotron solids (CO-28 and CO-9) presenting different degrees of structural organization and of chondritic IOMs (adapted from (29)). Nebulotron samples CO-28 and CO-9 were produced at discharge power of 30 and 60 W, respectively.

Fig. 3. Quantitative extraction of spectral parameters from 244 nm (a and b) and 514 nm (c) Raman spectra. (a) D band and ID/IGratio extracted from 244 nm

Raman spectra. Chondritic IOM and UCAMM data are from (57) and (35), respectively. (b) G band Raman spectral parameters extracted from 244 nm Raman spectra. Chondritic IOM data are from (57). The N-rich synthetic samples (Nebulotron/PAMPRE) present a much larger G band as pointed in Fig. S3. N-poor Nebulotron solids present G band parameters that are in the area covered by chondritic IOMs within errors. (c) G band spectral parameters extracted from the 514 nm Raman spectra for N-poor Nebulotron samples (empty circles). Data for UCAMMs (35), IDP and Stardust grains (58-60), carbonaceous chondrites (30,

31, 57, 61) and ion-irradiated organics (62) are shown for comparison. Arrows illustrate the evolution of Raman spectral parameters with thermal alteration

and irradiation with ions in the keV range. Nebulotron samples plot close to primitive extraterrestrial organics, with one sample (CO-9) showing thermal alteration.

Fig. 4. Noble gases. (a) Elemental fractionation of noble gases in the Nebulotron solids (black dots), in synthetic solids produced in plasma setups (grey area, (16-18)) and in chondritic IOM for Q-gases (red dots, (10)) relative to132Xe and to the gas reference (air composition for laboratory compounds and solar

composition for Q-gases). Arrows show upper limits. (b) Isotopic composition of Xe in the Nebulotron solids (grey range corresponding to 15 samples), in other plasma synthesized solids (16, 17, 19), and in Q (average of chondrites measured by (10)). The delta notation is used and the ratios are normalized to132Xe

and to air composition for plasma experiments and to solar composition for Q. Typical errors (1σ) are shown for Q only for visibility. Xe-Q isotopes do not follow a linear trend with the mass contrary to Xe trapped in plasma-synthesized organics. This has been interpreted as the result of addition of other noble gas components found in meteorites such as Xe-HL, carried by nanodiamonds, s-process Xe and radiogenic129Xe to account for the129Xe excess displayed

by Xe-Q (15). (c) Isotopic composition of Kr in the Nebulotron solids (grey range corresponding to 7 samples), in plasma synthesized solids by (16) and in Q (average of chondrites measured by (10)). The delta notation is used and the ratios are normalized to84Kr and to air composition for plasma experiments and

to solar composition for Q. Errors (1σ) are smaller than the symbol size for Kr-Q and are ± 5-40‰ for synthetic samples, depending on the isotopic ratio.

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Fig. 5. Schematic diagram of the solar nebula (0.1-100 AU), irradiated by stellar and interstellar UVs and X-rays. White lines represent electron fraction isolines. Both the electron fractions and the gas temperature scale are adapted from (51). Synthesis of15N- and noble gas-rich organics is

possible in the most ionized areas of the disk, via photon and/or electron-gas interactions. The source of15N-enrichment is the UV photodissociation

of N2in the PDR region only. Dispersion of organics within the disk is possible

thanks to turbulence and settlement. Organics may interact with ices in the cold and shielded middle part of the disk.

performed in ionizing conditions (16-19). The concentration of trapped Xe in the Nebulotron-synthesized solids is very high (from 2.0×10-12 to 2.4×10-9 mol/g), in agreement with results

from (19). It is worth noting that without ionization, the trapping efficiency of Xe atoms in organics is very low and does not induces detectable isotopic fractionation (32). The present data confirm that ionization of noble gases is the sine qua non condition for their efficient trapping and for the generation of elemental and isotopic fractionations.

Comparison of laboratory synthesized organics with refrac-tory organic solids from primitive chondrites

The elemental analyses and FTIR data show that the N-poor Nebulotron samples (produced from CO + traces of H2/H2O ±

noble gases) display a chemical composition very close to that of IOM from unheated chondrites (Table S1 and Fig.1a). N-poor Nebulotron samples display elemental ratios in the range of those of IOM from primitive chondrites (33) and their FTIR spectra are comparable to those of IOM. The main differences are: (i) the larger O/C ratio for N-poor synthesized samples compared to chondritic IOM, also illustrated in the FTIR spectra by the larger relative intensities of C=O and OH bands in the Nebulotron samples relative to IOM FTIR spectra, and (ii) the lower aliphatic content in the Nebulotron synthesized solids than in IOM, as shown by the overall weaker aliphatic stretching modes in FTIR spectra of the Nebulotron samples relative to IOM (Fig. 1a).

Raman and HRTEM studies also support common structural features between the Nebulotron-synthesized organics and IOM or other extraterrestrial organics (Figs. 2 and 3). Both the carbon nanotextures (Fig. 2) and the Raman spectral parameters (Fig. 3) are comparable (although not exactly identical as illustrated by the D band parameters (Fig. 3a)). Interestingly, the Nebu-lotron samples plot close to the extraterrestrial primitive organics in the Raman spectral parameters space (Fig. 3). Altogether, these observations show that organosynthesis from very simple gas molecules at temperatures > 500 K can account for the production of complex and unsaturated organic compounds with carbon structural disorder fairly similar to the one of chondritic IOMs.

and Kr-Q in meteorites relative to the solar composition (10) are well reproduced in the Nebulotron plasma experiment (Fig. 4). Ionization, which increases dramatically the trapping efficiency of noble gases in growing solids (19) may also account for the extremely high noble gas content of Phase Q (10).

FTIR and 244 nm Raman spectra show that organics pro-duced in the Nebulotron setup from a mixture of CO+N2 (+

traces of H) are richer in nitrogen (up to 22 % mol) than chon-dritic IOM ((34, 35), max. 4 %mol. in CR, (33)). This difference was already observed for laboratory analogues produced in a N2

-CH4plasma (tholins from PAMPRE setup, (36), Figs. 1 and S3).

Interestingly, N-rich refractory organics have been identified in some IDPs (37) and in UCAMMs (38), which are believed to be of cometary origin. Both N-rich Nebulotron samples and PAMPRE-tholins display fairly similar FTIR spectra to the one of UCAMM (but with either too much oxygen or aliphatic abundance, Fig. 1). The low nitrogen abundance in chondritic IOM may result from different processes. Organic precursors of IOM might have been initially N-rich, such as “cometary” organics. Processing in the disk or onto the parent-body might have resulted in selective loss of nitrogen (36). The fact that IOM still contains trapped noble gases with a common composition among different classes of meteorites may not support this possibility. However, one could argue that Phase Q is only a small and refractory fraction of IOM, so that nitrogen would have been lost from more labile sites than Phase Q. Alternatively, meteoritic IOM might have originated from synthesis of organics in a gas of non canonical composition, that is, in a gaseous environment depleted in ni-trogen relative to C-bearing species, or from precursors having trapped preferentially C and H relative to N. Both scenarios have to be investigated in details and experiments are currently under way at CRPG.

Discussion

Noble gases and nitrogen isotopes: tracers of organic synthesis in ionized areas of the solar nebula

The origin of the refractory organic compounds found in chondrites is actively debated. Cold scenarios propose UV-induced photo-polymerization of organic molecules in the icy mantles of dust grains, either in the interstellar medium (21) or in the solar nebula (22). Low temperature conditions are consistent with models calling for ion-molecule reactions as the origin of D and15N enrichments (relative to solar values) of chondritic IOMs (39, 40). However, theoretical studies suggest that ion-molecule reactions are unable to produce the extreme15N-enrichments of several thousands of permils observed in some IOM hotspots, or under specific conditions only (40, 41). Other scenarios favor an origin within the inner solar system via high temperature condensation reactions, including Fischer-Tropsch-like reactions (23, 24) and condensation reactions assisted by electric discharge, such as in the present experiment (42-44). More recently, a post-accretion scenario was proposed, via interstellar formaldehyde polymerization onto the parent body (25, 26). However, none of them satisfy all the existing constraints. More specifically, the trapping of volatile elements into meteorite organics and their associated isotopic fractionation have received little attention even though these elements are essential tracers for deciphering the origin of IOM.

The similarity between the organic solids synthesized in the Nebulotron and chondritic IOM suggests that gas phase chem-istry under ionizing conditions was able to form organic com-pounds in the disk of gas and dust surrounding the T-Tauri Sun. Plasma-synthesized organic dust also reproduces the noble gas elemental and isotopic fractionations of chondrites, contrary to the experiments simulating neutral or extremely weakly ionized cold environments via noble gas trapping into ices (e.g. ISM 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476

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dark clouds or mid-plane regions in the outer solar nebula (45)).

However, our experiments do not reproduce the15N-enrichments

observed in IOM, of∼ 500-750 ‰ in for bulk IOM (up to 1,100 ‰ for CR chondrites, (33)) and up to 5,800 ‰ in IOM hotspots (Bells CM2 chondrites, (6)), all data relative to the PSN nitrogen isotopic composition (46). We have shown in a previous study that nitrogen trapped in organics produced during plasma organosyn-thesis is moderately depleted in15N by about 15-25‰ (27). There-fore, electron-driven reactions cannot account for solar system

15N-enrichments. The only experiment done so far leading to

strong15N enrichments involves the illumination of N

2+H2 gas

mixtures by synchrotron-generated UV photons in the range 80-98 nm. Extreme15N enrichments of∼ 2,200‰ in average, up to

13,400 ‰ at 90 nm, were obtained in forming the photoproduct NH3(47). Such enrichments were interpreted as the result of

self-and mutual-shielding of N2during photodissociation, associated

with quantum perturbations that may explain the extreme15N ex-cursion at 90 nm (47, 48). None of these processes can take place in an electron dominated environment because (i) ionization by electrons is volume-correlated thus precluding self or mutual shielding, and (ii) contrary to photons, plasma electrons have a continuum spectrum of energy that prohibits quantum effects (28). The noble gases are not concerned by this limitation because their elemental and isotopic fractionations are not related to molecular dissociation as N2is.

Noble gas fractionation requires that their host phase formed in an ionizing environment. The15N enrichments of solar system

objects and reservoirs permit to identify the mode of ionization. Indeed, experimental results point to UV-irradiated regions for the generation of15N-rich compositions (47). Other modes of

ionization (e.g., electrons, radioactivity) could have contributed to noble gas fractionation, while not affecting nitrogen isotopes. Thus variations of the N isotope composition in the solar system, together with near-constant noble gas fractionation, could be the result of heterogeneities in the respective modes of ionization.

Ionization in the solar nebula and organosynthesis

In protoplanetary disks, ionization sources are multiple, re-sulting in the common occurrence of plasma environments, i.e., partially ionized regions (49). Beyond 1 AU, ionization is mainly controlled by irradiation sources, such as X-ray and UV photons from the central star and/or from the interstellar radiation field (50). In the very inner part of the disk, temperatures are high enough (> 103 K) for thermal ionization to occur. Radioactive

decay may also contribute but to a lesser extent (49). The electron fraction, depending on disk parameters, is expected to be in the range 10-12 (in dead zones) - 0.1 (in the most irradiated surface region close to the star), generating a stratified ionization structure of the disk (51). Hence, weakly to almost fully ionized plasmas constitute the main fraction of the PSN. In these ionized regions, electrons generated by direct cosmic rays and by X-ray and UV photons are likely suprathermal. The exact energy spectrum of electrons is not known but it has been suggested that the mean energy of the secondary electrons may be around 20 eV or higher (52, 53), well above the dissociation and ionization thresholds of H2, CO, N2 and noble gases. Therefore, these

electrons may drive chemical reactions such as the ones occurring in laboratory plasmas and be responsible for the synthesis of organic compounds.

In the PSN, organosynthesis and isotopic fractionation of volatiles may have occurred either via electron-driven reactions, provided an electron fraction of at least the ones of laboratory plasmas (10-6or above), or via photon-driven reactions, the latter

specifically for the generation of15N excesses by self-shielding. Such modes of ionized regions are consistent with the so-called photon-dominated region (PDR) and the warm molecular layer (WML) in disk models. The PDR, at the very surface of the disk, is necessarily the place were N2 self-shielding occurs in

disk because extreme UV photons may rapidly be absorbed by gas molecules and dust in the underlying layers. This region is almost fully ionized because of strong irradiation (51), and is expected to be a place of synthesis by photochemistry, but also of destruction of organic molecules (54). As a consequence, the organic molecules that form in the PDR may have a short lifetime but, in any case, the carrier of 15N-enrichments must survive to a certain extent. The WML, although partly shielded from photon irradiation, is largely ionized too and is likely to be a place where organosynthesis and noble gas fractionation occur. Vertical transport, predicted to occur in turbulent disk, may also help to conserve a part of the newly formed organics and the noble gas and nitrogen isotopic signatures by moving to deeper, colder and more shielded parts of the disk (22). As a matter of fact, the PDR an the WML temperatures are expected to be > 500 K and 100 - 500 K, respectively (51). Such “high” temperatures are generally not considered for organosynthesis in disk models, but our experiment demonstrates that even higher temperatures promote, with satisfactory yields, the formation of compounds that present organic complexity that is required to ac-count for IOM precursors. In numerical models of protoplanetary disk chemistry, gas phase reactions are generally efficient only in producing primary organic molecules such as HCO+, HCN, N2H+ or H2CO, but not for producing more complex organic

compounds (51). However, our experimental results challenge this view, and suggest that suprathermal electrons as a source of ionization and chemical reactivity may be underestimated. In the hot and thermally ionized part of the disk within 1 AU, organic synthesis via gas-phase reactions and fractionation of noble gases may occur as well. Simple organic molecules such as HCN, C2H2,

CO2, H2O and OH have been observed within 3 AU in the

protoplanetary disk of AA Tauri (55), supporting the hypothesis of an active organic chemistry in the hot and ionized inner part of the disk. The densities invoked for protoplanetary disks in such active regions (∼ 108cm-3) are orders of magnitude lower than those used in laboratory plasmas at∼ 1 mbar (1015-1016 cm-3), likely resulting in much less efficient synthesis of organic compounds. Nonetheless, the time during which those processes operate would be much longer in the disk than in a laboratory, likely counterbalancing the lower gas density.

The present experimental study provides a challenging but nonetheless plausible scenario predicting that precursors of chon-dritic IOM, Q-gases and the 15N enrichments originate in a

common environment from the interaction of photons and/or electrons with gas species (Fig. 5). Heterogeneities in the com-position of refractory organics and radial variations of15N/14N

ratios among solar system objects and reservoirs with heliocentric distance could reflect heterogeneities in ionization processes. UV photon ionization and processing may have dominated in the outer solar system whereas electron-dominated reactions may have been prevalent in hotter regions of the inner solar system.

ACKNOWLEDGMENTS. This study was funded by the European Research Council (FP/7 2007-2013, grant agreement 267255 to B.M.) and by the Programme National de Planétologie (PNP) through grants to Y. M. and to L. T. We warmly thank L. Zimmermann for help during noble gas analysis and G. Cernogora, E. Quirico, F.R. Orthous-Daunay for helpful comments, discussions and IR measurements..

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