Hydrogen isotopic anomalies in extraterrestrial organic matter: role of cosmic ray irradiation and implications for UCAMMs

Abstract

Context. Micrometeorites represent, at timescales shorter than a few million years, the dominant source of extraterrestrial matter at the surface of the Earth. Analyses of ultracarbonaceous micrometeorites recovered from Antarctica, known as UCAMMs reveal an exceptionally N-rich organic matter associated with spatially extended high D enrichments. Experiments show that this specific organic matter might have been formed in the outer solar system by energetic irradiation of N-rich icy surfaces. Aims. We experimentally investigate the hydrogen isotopic fractionation resulting from irradiation of normal and D-rich N2-CH4 ices by high energy ions, simulating the exposition to Galactic cosmic rays of icy bodies surfaces orbiting at large heliocentric distances. Methods. Films of N2-CH4 ices and a N2-CH4/CD4/N2-CH4 “sandwich” ice were exposed to 129Xe13+ ion beams at 92 and 88 MeV. The chemical evolution of the samples was monitored using in situ Fourier transform infrared spectroscopy. After irradiation, targets were annealed to room temperature. The solid residues of the whole process left after ice sublimation were characterized in situ by infrared spectroscopy, and the hydrogen isotopic composition measured ex situ by imaging secondary ion mass spectrometry at the sub-micron scale (NanoSIMS). Results. Irradiation leads to the formation of new molecules and radicals. After annealing, the resulting poly-HCN-like macro-molecular residue exhibits an infrared spectrum close to that of UCAMMs. The residue resulting from irradiation of N2-CH4 ices does not exhibit a significant deuterium enrichment comparable to that found in extraterrestrial organic matter. The residue formed by irradiation of D-rich ices shows the formation of isotopic heterogeneities with localised hotspots and an extended contribution likely due to the diffusion of the radiolytic products from the D-rich layer. Conclusions. These results show that high-energy cosmic ray irradiation does not induce the large hydrogen isotopic fractionation observed at small spatial scale in interplanetary organics. By contrast, large D/H ratio heterogeneities at the sub-micron spatial scale in extraterrestrial organic matter can result from isotopically heterogeneous ices mixtures (i.e. condensed with different D/H ratios), which were transformed into refractory organic matter upon irradiation.

Organic Matter in the Solar System—Implications for Future on-Site and Sample Return Missions

“…Organic compounds may also be formed by solid-phase reactions on interstellar ice grains by energetic processing (e.g. cosmic rays and UV irradiation) (Augé et al 2016(Augé et al , 2019. Laboratory analogues of the physical and chemical conditions of these interstellar ices (or pre-cometary icy grains) results in the formation of a residue that may contain glycerol, urea, glycolic acid (Nuevo et al 2010), nucleobases (Nuevo et al 2009(Nuevo et al , 2017, hydantoin (de Marcellus et al 2011a), glyceraldehyde (de Marcellus et al 2015), aldopentoses and sugar-related compounds (Meinert et al 2016), and amino acids (Briggs et al 1992;Kasamatsu et al 1997;Bernstein et al 2002;Muñoz Caro et al 2002;Meinert and Meierhenrich 2012;de Marcellus et al 2011b;Modica et al 2014Modica et al , 2018.…”