COMETary dust ORigin – COMETOR

This project develops an innovative protocol to characterize the association of organic matter and minerals within UCAMMs, using cutting-edge analysis techniques. Experimental simulations of the production of cometary organic matter are carried out by high energy ion irradiation of cometary ice analogs.
The handling of UCAMMs requires a clean room environment and micromanipulation devices. Each UCAMM is fragmented into several parts to allow complementary analyses on each fragment. Scanning electron microscopy equipped with X-ray spectroscopy (SEM-EDX) gives access to the overall composition of the UCAMM. The association of organic matter and mineral matter is studied by synchrotron-based infrared microscopy (µ-FTIR), µ-Raman, IR nanospectroscopy (AFM-IR), synchrotron-based transmission X-ray microscopy (STXM-XANES) coupled with transmission electron microscopy (TEM). The isotopic compositions of the light elements (H, C, N, O) are analyzed by NanoSIMS. A tentative search for soluble organic matter was carried out by high resolution mass spectrometry (Orbitrap). Irradiation experiments of cometary ice analogues with heavy ion accelerators (GANIL) are performed to simulate the formation of cometary organic matter and study the preservation of isotopic heterogeneities.

The IR spectroscopic signature of UCAMMs was acquired for the first time at the ~100 nm scale by AFM-IR, showing heterogeneities of the chemical composition of the organic matter (Mathurin et al. 2019, 2020).
The mineralogy of UCAMMs requires transport of crystalline minerals from the inner to the outer regions of the Solar System. The presence of a probably hydrated phase in a UCAMM suggests either a possible aqueous alteration on the comet, or the (late) incorporation of this mineral from a processed asteroid (Guérin et al. 2020, and 2023 in prep).
Some minerals inside the organic matter of UCAMMs recorded traces of solar irradiation, suggesting pre-accretion irradiation of these minerals. This constrains the time of residence in the inner and outer regions of the solar system (Engrand et al. 2019, 2020).
Three organic phases of different chemical compositions are present in UCAMMs. One of them is nitrogen-rich, and does not exist in insoluble organic matter from meteorites. The two other UCAMM organic phases bear similarities with the insoluble organic matter recovered by acid etching of carbonaceous chondrites. The nitrogen-rich phase contains glassy mineral phases (or none), while the other organic phases can contain crystalline minerals.
New data on the isotopic composition of light elements (H, C, N) in UCAMMs organic matter confirm the high D/H ratios of UCAMMs opening the possibility to study correlations between NanoSIMS isotopic compositions and AFM-IR data at the sub-mm scale (Rojas et al. 2020a, 2022, 2023).
High energy heavy ion irradiation of CH4- and N2-rich ices can produce organic residues with spectroscopic IR signatures similar to that of the UCAMM N-rich organic matter, suggesting a scenario for the formation of cometary organic matter by irradiation at the surface of comets.
The high energy ion irradiation of “sandwich layers” of isotopically labelled N-rich ice precursors produces an organic residue with large isotopic heterogeneities on spatial scales comparable with that observed in UCAMM organic matter (Augé et al. 2019, Rojas et al. 2020b, 2021).
The current flux of interplanetary dust accreted by the Earth measured using the CONCORDIA micrometeorite collection yielding a value of about 5,200 tons/year (Rojas et al. 2021a), allowing to estimate the amount of carbon brought to the early Earth.
Spectroscopic analogues for the aromatic infrared bands (AIB) observed in the interstellar medium can be produced in the laboratory by mechanochemical synthesis, and could be used to produce synthetic UCAMMs (Dartois et al. 2020).
We analysed samples from the Ryugu asteroid returned by the Hayabusa2 space mission. Ryugu samples show similarities with CI meteorites, which have been proposed to have a cometary origin. When comparing Ryugu data with that obtained on UCAMMs, we found no similarity between these two kinds of objects, suggesting different precursor materials, or strong post-accretional effects on Ryugu material.

The results obtained in the course of this project are opening several perspectives :
i) Continuing the search for correlation between isotopic compositions and the chemical and functional characteristics of the organic matter of UCAMMs, by combining µ-FTIR, µ-Raman, NanoSIMS, AFM-IR, and STXM-XANES.
ii) Detailed characterization of UCAMM mineralogy by TEM and study of the organic matter/mineral interface by AFM-IR.
iii) Study of the isotopic compositions of organic residues formed by swift heavy ion irradiation of isotopically labelled ices.
iv) Production of synthetic UCAMMs and simulation of their evolution in the interplanetary environment and since their accretion by the Earth.
v) Search for soluble organic compounds in UCAMMs, to better assess the role of cometary dust in the delivery of prebiotic matter on the early Earth.
These results have implications in the fields of astrophysics, planetology-cosmochemistry and astrobiology. They bring an original contribution to the understanding of the formation and evolution of solid matter in the outer regions of the protoplanetary disk, as well as important inputs for a better interpretation of data from spatial mission (Rosetta, Stardust, Hayabusa2, OSIRIS-Rex) and for the observations of protoplanetary disks by the James Webb Space Telescope (JWST).

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T. Mannel et al. Dust of comet 67P/Churyumov-Gerasimenko collected by Rosetta/MIDAS: classification and extension to the nanometer scale. A&A, 2019, 630, A26 (14 p.). (10.1051/0004-6361/201834851). (insu-02155727)
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J. Mathurin et al. Nanometre-scale infrared chemical imaging of organic matter in ultra-carbonaceous Antarctic micrometeorites (UCAMMs). A&A, 2019, 622, pp.A160. (10.1051/0004-6361/201833957). (hal-02073521)
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B. Augé et al. Hydrogen isotopic anomalies in extraterrestrial organic matter: role of cosmic ray irradiation and implications for UCAMMs. A&A, 2019, 627, pp.A122. (10.1051/0004-6361/201935143). (cea-02182692)
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Cometary dust particles rain on Earth. However, they can only be found in collections performed in the cleanest regions of the Earth (the stratosphere and Antarctica). From Antarctic snow at the vicinity of the French-Italian Concordia Antarctic base, we recovered large (> 50-100 µm) particles of very probable cometary origin, the Ultracarbonaceous Antarctic Micrometeorites (UCAMMs). UCAMMs are constituted of a dominant fraction of a solid macromolecular organic matter intimately mixed with a minor mineral component. The organic matter is structurally disorganized, shows large deuterium enrichments and exhibit an unusually large bulk nitrogen concentrations (up to 20 at%). Preliminary studies have shown that several types of organic matter co-exist in UCAMMs, with different nitrogen contents and mixed with different amounts of minerals. The minerals embedded in the organic matter have typical sizes around 50-100 nm. Both crystalline and amorphous minerals are present and exhibit a wide range of compositions. Some precursors of UCAMM organic matter (the most N-rich) could have been formed by galactic cosmic rays’ irradiation of nitrogen-rich ices at the surface of icy bodies in the outer regions of the protoplanetary disk.

UCAMMs are remarkable particles as their subcomponents preserved records of early solar system formation and evolution. The association in UCAMMs of minerals (formed at high temperatures) among large amounts of organic matter (necessarily formed at lower temperatures) opens a new window on the study of the origin and formation mechanisms of matter originating from the outer regions of the solar system. This proposal focuses on the formation mechanism and evolution of cometary dust organics and their relation with the mineral components embedded within, following 3 main questions:
1.    What is the origin of the subcomponents of cometary matter? "Inner and outer solar system…"
2.    What are the variations of the composition of organics and their embedded minerals with heliocentric distance?
3.    How did the different environments encountered (radiative interplanetary medium, terrestrial atmosphere, Antarctica) modify the cometary particles collected on Earth?

This project proposes innovative analysis protocols of UCAMMs using state-of the-art analysis techniques to characterize both the organic matter, the minerals and their association. Experimental simulations of their evolution from interplanetary space to their collection in Antarctica will be performed on cometary organic analogues and on synthetic UCAMMs produced in the laboratory.

The originality of the COMETOR proposal resides in four points: i) the availability in the laboratory of well-preserved cometary samples; ii) the analysis of these complex particles with a combination of complementary and state-of-the-art techniques – including infrared spectroscopy coupled with atomic force microscopy (AFMIR) that allows infrared analysis at the ~ 50-100 nm scale; iii) the production of analogues of cometary solids and the real-time observation of their evolution under irradiation thanks to the unique JANNuS platform, coupling a transmission electron microscope with two ion accelerators; iv) the search for soluble organic compounds (including amino acids) in UCAMMs with the very high mass resolution Orbitrap technique to probe the input of prebiotic molecules on the early Earth by cometary dust.

The expected results will have implications in the fields of astrophysics, planetology-cosmochemistry and astrobiology. They will bring an original contribution to the understanding of the formation and evolution of solid matter in the outer regions of the protoplanetary disk, as well as important inputs for the interpretation of data from Rosetta and Stardust samples, from samples returned by future space missions such as Hayabusa 2, OSIRIS-Rex, and for the observations of protoplanetary disks by the future James Webb Space Telescope (JWST).