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Isotope Fractionation of Light Elements Upon Ionization: Cosmochemical and Geochemical Implications

Periodic Reporting for period 4 - Photonis (Isotope Fractionation of Light Elements Upon Ionization: Cosmochemical and Geochemical Implications)

Reporting period: 2021-07-01 to 2022-06-30

Volatile elements such as hydrogen, nitrogen and noble gases present large isotope variations among solar system objects and reservoirs (including planetary atmospheres) that remain unexplained at present. Works based on theoretical approaches are model-dependent and do not reach a consensus. Laboratory experiments are required in order to develop the underlying physical mechanisms.

The aim of the project is to investigate the origins of and processes responsible for isotope variations of the light elements and noble gases in the Solar System through an experimental approach involving ionization of gaseous species. We investigate the significant isotope variations observed for noble gases and stable isotopes among solar system objects and reservoirs, which are presently debated between a pre-solar origin (ion-molecule interactions, or nucleosynthetic anomalies) and protosolar nebula processing. We target the second possibility by carrying out new experiments. There were three tasks:
Task 1: Synthesis of solid organic compounds by microwave plasma ionization of gas in the laboratory.
Task 2: Synthesis of gaseous and solid organics by photon ionization/dissociation of gas mixtures in a VUV cell coupled to the DESIRS line, Soleil synchrotron.
Task 3: Gas-ice interactions: fractionation of noble gases and stable isotopes. This task was designed to investigate the origin and processing of volatile elements in cometary ice, for which new constraints arose from the analysis of the coma of Comet 67P/Churyumov-Gerasimenko.

The project is to investigate the sources of volatile elements in the solar system and processes that fractionated their isotopic compositions. Doing so, we aim at understanding processes that led to the genesis of organic matter in the solar system, as well as to the the development and processing of planetary atmospheres, a long debated subject which is relevant to the origin of environmental conditions necessary for the development of life on Earth.
Task 1: Synthesis of solid organic compounds by microwave plasma ionization of gas.

We have developed a new system, called "Nebulotron" consisting of a high vacuum glass line in which adjustable gas mixtures can be flowed through a microwave (2.45 GHz) plasma discharge. The flown gases were mixtures of H2, CO, N2 and noble gases. We succeeded in producing solid organics. We carried out a series of analyses dedicated to investigate the molecular structure (gas chromatography-mass spectrometry, 13C-nuclear magnetic resonance spectroscopy, Fourier transformed infrared spectroscopy) and isotopic composition (NanoSIMS) of H and N in Nebulotron products (collaboration with the laboratory of Sylvie Derenne, Univ. Paris 6). We have coinjointly carried out a modeling approach in which the results were integrated in a global model of organic matter generation and transport in the nascent solar system, in collaboration with Dr. Sébastien Charnoz (IPGP) (Bekaert et al, 2018).

Task 2: Synthesis of gaseous and solid organics by photon ionization/dissociation of gas mixtures in a VUV cell coupled to the DESIRS line, Soleil synchrotron.

We are investigating the effect of photo-irradiation on the isotope composition of organic products formed in the context of the protoplanetary disk. Using the high flux of the DESIRS UV-photon line at Soleil syncrhotron, we aim at forming and trapping the organic products of the irradiation of simple gas mixtures (N2:CH4 and H2:N2:CO) to measure their chemical and isotopic compositions and compare them to extraterrestrial organic materials. Contrary to our expectation, we have been awarded only 5 days of beam time on DESIRS but have applied for additional time. We observed the formation of organic products (notably HCN and C2H2). The condensable gas products were collected on cold traps during or after the experiments for further isotope analysis. We also carried out original experiments of photo-ionization using the APSIS setup at LATMOS laboratory to provide experimental insights into pathways of photochemistry-driven molecular growth within outer Circum-Stellar Envelopes (CSE). EUV photons mimicking the interstellar UV field were generated by using a neon gas-discharge type VUV lamp coupled windowless to the photochemical APSIS reactor (Tigrine et al. 2016).

Task 3: Gas-ice interactions: fractionation of noble gases and stable isotopes.

We study the elemental and isotopic fractionation of noble gas and stable isotopes upon trapping in ice and irradiation by UV phortons. The EXCITING (Exploring Xenon in Cometary Ice by Trapping and Irradiating Noble Gases) experimental set-up built in our laboratory permits water-noble gas mixtures to be flown and adjusted using a quadrupole mass spectrometer, before being condensed onto a cold plate at around 25 K. The so-formed ice containing water and noble gases can then be heated up to any temperature and/or irradiated using a hydrogen lamp simulating interstellar radiations. Noble gases can then be released for the ice lattice upon warming, before being analyzed in static mass spectrometry. The experimental set up is connected directly to a noble gas mass spectrometer that has been upgraded as part of this project with the latest state of the art electronics. The system is operational since the beginning of 2019 and the experiments are currently under way. We developed a new stable isotope analytical system, as well as an on-line noble gas mass spectrometer to analyze directly the run products. This experiment is the only one worldwide to permit trapping and temperature-controlled desorption of volatile elements (all noble gases, nitrogen) under irradiation by UV photons. Run products can be analyzed for their abundances and isotopic ratios during these processes.
We develop the production of organic matter upon irradiation of gas mixtures. In particular, we aim at using photons to do so, which is a world premiere.
We develop two original systems: a new high vacuum ionization line, and a new ice irradiation experiment. With the latter, we are able to analyse directly noble gas and stable isotope compositions with online analyzers.
We use the latest analytical techniques to characterize the elemental, molecular, and isotopic compositions of synthesized organics using an large array of methods including ion probe, noble gas, stable isotope and state of the art organic analyses.
At the end of the project, we expect to be able to decipher the origin of organic matter in the solar system. We also develop a modeling approach for transport of organics in the solar system, which is fed with our experimental results. Such an approach has never be attempted before.
Summary of processes that can fractionate the isotopes of volatile elements in the solar system