Light element isotopic heterogeneities in organic residues that formed by the ion irradiation of ices

Context. Ultra-carbonaceous Antarctic micrometeorites (UCAMMs) are carbon-rich micrometeorites. Their organic matter formed in a N-rich environment, possibly via the galactic cosmic ray (GCR) irradiation of ice mantles at the surfaces of small bodies. Nanoscale secondary ion mass spectrometry (NanoSIMS) revealed that UCAMMs exhibit large H, C, and N isotopic heterogeneities at the micron scale. Aims. We aim to investigate the transfer of H, C, and N isotopic heterogeneity from an ice mixture to its ion-irradiation-induced organic residue. The goal of this work is to understand the formation of H, C, and N bulk- and micron-scale isotopic heterogeneities in the organic matter of UCAMMs. Methods. We performed irradiation experiments of isotopically heterogeneous ice films at 10 K, with swift heavy ions to model the irradiation of isotopically heterogeneous icy surfaces via GCR. The irradiated ice mixtures were subsequently annealed to room temperature, which led to the formation of refractory organic residues. NanoSIMS imagery was performed on the organic residues to identify micron-scale H, C, and N isotopic heterogeneity. The ice films consisted of N2-CH4 (9:1) and NH3-CH4 (9:1) ices that contain a thin layer of D-, 13C-, and 15N-labeled species. Results. The irradiation-induced organic residues exhibit micron-scale isotopic anomalies. The transfer of isotopic anomalies from the ice to the organic residue appears to depend on the chemical composition of the ice film. Conclusions. These experiments show it is possible to transfer isotopic heterogeneity that is initially present in cometary ices to refractory organic residues that formed by heavy ion irradiation. Gaseous reservoirs of volatile species with highly fractionated isotopic compositions are predicted to exist in the early Solar System and should coexist under the form of ice mantles at the surface of small bodies at large heliocentric distances. The study of organics in UCAMMs can thus shed light on the composition of N- and C-rich ices at the surface of outer Solar System small icy bodies.