Etude nano-spectroscopique et nano-isotopique du continuum astéroïde - comète avec des échantillons de l'astéroïde Ryugu et de micrométéorites antarctique (AMMS)

Asteroids and comets are dynamically distinguished by their orbital trajectories, roughly circular between Mars and Jupiter for asteroids, and elliptical for comets. Other major differences are the remnant ices at the surface of comets and not of asteroids. However, the hypothesis of an asteroid-comet continuum, rather than a dichotomy, has gained support thanks to the analyses of samples returned from the Stardust space mission, which sampled comet 81P/Wild2. Crystalline phases formed at high temperature were in particular discovered in these samples, showing large similarities with their counterparts in primitive (asteroidal) meteorites.To investigate the asteroid-comet continuum, we analyzed samples returned from the C-type asteroid Ryugu by the JAXA space mission Hayabusa2, and compared them to meteorites and micrometeorites (MMs). MMs dominate the extraterrestrial material input on Earth nowadays, and they have been collected at Dome C (Concordia station, Antarctica) for several decades. Based on numerical simulations, MMs are believed to have a cometary origin for the most part, and an asteroidal origin for a minor fraction of them.The analyses of Ryugu show a link with CI chondrites, a rare type of meteorites showing the most similar elemental composition to that of the Solar photosphere. The Concordia MM collection also contain a small proportion of CI-like particles. CI chondrites, CI-like MMs and Ryugu samples consist of complex mixtures of minerals with a small fraction of organic matter (OM), which recorded conditions in the low-temperature reservoirs of the accretion region and the subsequent chemical and isotopic evolutions on their parent bodies.This thesis focuses on the chemical and isotopic characterization of the OM in Ryugu samples, CI-like MMs, and CI chondrites, using state-of-the-art micro- to nano- scale analytical methods. The aim is to compare and link these materials, exploring the composition of organics at different scales, to retrace their formation processes, dynamics, and evolution, ultimately placing them on the asteroid-comet continuum.The mineralogy of CI chondrites, CI-like MMs and Ryugu samples show evidence for significant aqueous alteration on their parent body. We used Fourier transmission infrared microscopy (µ-FTIR) to provide an overview of the OM distribution within this mineral matrix. Advanced techniques, such as nanoscale isotopic analysis (NanoSIMS) and IR nanospectroscopy (AFM-IR) were then employed, and a protocol for correlating AFM-IR and NanoSIMS analyses has been successfully developed, allowing to measure the isotopic and the chemical compositions of sub-micrometer organic patches at an unprecedented spatial scale within the mineral matrix. Furthermore, these techniques also prevent the need for extraction of OM from the mineral matrix, which degrades the molecular fractions of OM. The AFM-IR technique combines the spatial resolution of AFM with IR spectroscopy, and this work is the first systematic study of extraterrestrial materials by AFM-IR.In the samples studied, OM is heterogeneous and can be found at the nanoscale as sub-micrometer sized patches in the matrix or as a diffusely scattered phase. In MMs the OM is C=C-rich with minor contributions of C=O & CH in patches, while in Ryugu both patches and diffuse OM is C=O-rich. The H, C & N isotopic compositions of these patches in the MMs are comparable to that of the CI Orgueil and Ryugu samples, except for one CI-like MM, which shows exceptional 15N and D enrichments, as well as 13C depletions. No systematic correlations between the chemical and isotopic compositions were found. These results point toward diverse origins for the OM within individual CI-like MMs, from both the inner (asteroidal) and the outer (cometary) regions of the protoplanetary disk, placing them as ideal samples to study the asteroid-comet continuum. They also suggest the possible presence of hydrated minerals in cometary matter.