Description du sujet de thèse

The incorporation and evolution of nitrogen from pre-stellar environments to the formation of the Solar system, its distribution within the various bodies of the Solar system, and in particular the origin of terrestrial nitrogen, are very poorly understood (Pontoppidan et al., 2014; Füri and Marty, 2015). How was nitrogen incorporated to planets by planetesimals, whose relics today are comets and asteroids, in what form(s) was this nitrogen? Recent discoveries suggest that these objects may have delivered a significant quantity of nitrogen in the form of ammonium (NH4+, in the forms of salts, phyllosilicates or organic grains) (Poch et al., 2020; Altwegg et al., 2022). Yet very few studies have looked for ammonium in carbonaceous chondrites, the most volatile-rich meteorites, having the closest composition to the Sun (Cloëz, 1864; Laize-Générat et al., 2024; Glavin et al., 2025). To improve our understanding of the origin and evolution of nitrogen during the formation of the Solar system, and of the relationship between meteorites and small bodies, it is important to study the presence, quantity and origin of the ammonium contained in these carbonaceous chondrites. Between 2021 and 2024, a PhD thesis, carried out thanks to a transdisciplinary collaboration between the Institute of Planetology and Astrophysics of Grenoble (IPAG) and the Institute of Environmental Geosciences (IGE), quantified ammonium and characterized its 14N/15N isotopic ratio in different carbonaceous chondrites CI, CR, CM, C2-ung, obtaining several promising results (Laize-Générat et al., 2024; Laize-Générat, 2024). We plan to continue this work with a new thesis, aimed at analyzing not only carbonaceous chondrites but also asteroidal samples returned to Earth, in particular those from Bennu (Glavin et al., 2025). The innovative aspect of these analyses is to establish how nitrogen is distributed within carbonaceous chondrites and returned samples, i.e. the contribution of each nitrogen-bearing phase to the total nitrogen in the rock. In addition, measurements of the 14N/15N ratio of NH4+, NO3- and soluble and insoluble organic molecules reveal the origins, evolution and possible genetic-link of these different nitrogen-bearing phases.
The aims of this thesis are (1) to constrain more precisely the abundance, origin and nitrogen isotopic composition of NH4+ and NO3- ions extracted from carbonaceous chondrites (CI, CM, CR, C2-ung), (2) determine the nitrogen distribution within these carbonaceous chondrites by establishing an experimental protocol for successively extracting the nitrogen-bearing phases (NH4+, NO3-, soluble and insoluble organic molecules) from the same sample, and then (3) reduce the mass required to enable analysis of samples from the asteroid Bennu.
Extraction of nitrogen-bearing phases by grinding, leaching and, if necessary, freeze-thawing, as well as elemental and isotopic analysis of total nitrogen and nitrogen in organic phases by isotope ratio mass spectrometry, will be carried out at IPAG. Analyses of the abundance and nitrogen isotopic composition of NH4+ and NO3- will be carried out by ion chromatography-mass spectrometry at the IGE.
References
Altwegg, K., et al. (2022) Abundant ammonium hydrosulphide embedded in cometary dust grains. Monthly Notices of the Royal Astronomical Society 516, 3900–3910. https://doi.org/10.1093/mnras/stac2440
Cloëz (1864) Comptes rendus hebdomadaires des séances de l'Académie des sciences / publiés par MM. les secrétaires perpétuels, 5. http://gallica.bnf.fr
Füri, E., Marty, B., 2015. Nitrogen isotope variations in the Solar System. Nature Geoscience 8, 515–522. https://doi.org/10.1038/ngeo2451
Glavin, D.P., et al. 2025. Abundant ammonia and nitrogen-rich soluble organic matter in samples from asteroid (101955) Bennu. Nat Astron 1–12. http://doi.org/10.1038/s41550-024-02472-9
Laize-Générat, L., et al. (2024) Nitrogen in the Orgueil meteorite: Abundant ammonium among other reservoirs of variable isotopic compositions. Geochimica et Cosmochimica Acta 387, 111–129. http://doi.org/10.1016/j.gca.2024.10.001
Laize-Generat, L., 2024. Origines et évolutions de l'azote sur les planétésimaux du système solaire : étude de la nature et de la composition isotopique des phases azotées des chondrites carbonées (These de doctorat). Université Grenoble Alpes. https://theses.fr/2024GRALU036
Poch, O., et al. (2020) Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids. Science 367. http://doi.org/10.1126/science.aaw7462
Pontoppidan, K. M., et al. (2014) Volatiles in Protoplanetary Disks. In Protostars and Planets VI University of Arizona Press. https://doi.org/10.48550/arXiv.1401.2423

Contexte de travail

The Institute of Planetology and Astrophysics of Grenoble (IPAG) is a joint research unit of the Institut des Sciences de l'Univers (INSU) of the CNRS and the Université Grenoble Alpes, located on the Saint-Martin d'Hères campus. It is one of the main laboratories of the Observatoire des Sciences de l'Univers de Grenoble (OSUG). Around 170 people work at IPAG, in several scientific teams and technical and administrative departments. Its research themes range from the solar system to the distant universe, with approaches combining laboratory measurements, intensive computing, astronomical observation and cutting-edge instrumentation. The PhD student will join the Planeto team at 122 rue de la piscine.
The doctoral student will be supervised at IPAG by Olivier Poch, CNRS researcher, and Lydie Bonal, CNAP astronomer, respectively co-supervisor and PhD thesis director. At IPAG, they will also benefit from the assistance of Laurène Flandinet, research engineer. At IGE, Joël Savarino, CNRS research director, will contribute to the scientific supervision, and engineers Nicolas Caillon and Patrick Ginot will provide assistance with chemical analyses.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

This work involves risks mainly related to experimental activities in chemistry. The doctoral student will be trained in good practices for working safely in the laboratory, by his/her supervisors and the prevention assistants.

Informations complémentaires

More details about this PhD subject are available here: https://sdrive.cnrs.fr/s/T3383AGZpzeF8ZN