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

B Augé,J L Guerquin-Kern,G Slodzian,P Boduch,A N Agnihotri,C Engrand,E Dartois,H Vermesse,T D Wu,H Rothard,J Duprat

doi:10.1051/0004-6361/201935143

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.

Highlights

Extraterrestrial organic matter is observed in meteorites and interplanetary dust particles (IDPs), originating from asteroids, comets, and icy bodies of the solar system (Hayatsu et al 1977; Flynn et al 2003)
The irradiation simulated the effect of Galactic cosmic ray (GCR) on icy bodies surfaces in the outer solar system region, beyond the nitrogen snow line
The irradiation with swift heavy ions was monitored with Fourier-transform IR (FTIR) spectroscopy and we observed the formation of numerous molecules and radicals, some of which contain deuterium

Summary

Introduction

Extraterrestrial organic matter is observed in meteorites and interplanetary dust particles (IDPs), originating from asteroids, comets, and icy bodies of the solar system (Hayatsu et al 1977; Flynn et al 2003). The bulk D/H ratio of the insoluble organic matter (IOM) extracted from meteorites exhibits values ranging from the Vienna Standard Mean Ocean Water, i.e. 155.76 ± 0.1 × 10−6 (Lecluse & Robert 1994), hereafter referred to as VSMOW, to about twice the VSMOW value. These ratios are one order magnitude above the value of the protosolar H2 reservoir recorded in the atmosphere of giant planets (Lecluse et al 1996; Mahaffy et al 1998). The δD1 values in meteorites ranges from VSMOW to a few thousands a reference, expressed in permil

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