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

¹ Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM), Université Paris Sud, UMR 8609-CNRS/IN2P3, 91405 Orsay, France
e-mail: augebasile@yahoo.fr
² Institut des Sciences Moléculaires d’Orsay (ISMO), Université Paris Sud, UMR 8609-CNRS/IN2P3, 91405 Orsay, France
³ Institut Curie, PSL Research University, INSERM, U1196, 91405 Orsay, France
⁴ IFP Energies Nouvelles, direction Géosciences, 92500 Rueil-Malmaison, France
⁵ Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP) CEA/CNRS/ENSICAEN/Université de Caen Normandie, Boulevard Henri Becquerel, BP 5133 14070 Caen Cedex 05, France

Received: 28 January 2019
Accepted: 11 June 2019

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.