Irradiation of nitrogen-rich ices by swift heavy ions

Clues for the formation of ultracarbonaceous micrometeorites

¹ 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
e-mail: auge@ganil.fr
² Institut d’Astrophysique Spatiale, UMR 8617 CNRS-Univ. Paris-Sud, Université Paris-Saclay, Bâtiment 121, Univ. Paris-Sud, 91405 Orsay Cedex, France
³ Centre de Sciences Nucléaires et de Sciences de la Matière, UMR 8609 CNRS/IN2P3-Univ. Paris-Sud, Université Paris-Saclay, Bâtiment 104, 91405 Orsay Campus, France
⁴ Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, 22453-900 Rio de Janeiro, RJ, Brazil
⁵ Departamento de Física, Universidade Federal do Amapá, Rod. JK Km. 02, Jardim Marco Zero, 22453-900 Macapá, Brazil

Received: 27 October 2015
Accepted: 27 May 2016

Abstract

Context. Extraterrestrial materials, such as meteorites and interplanetary dust particles, provide constraints on the formation and evolution of organic matter in the young solar system. Micrometeorites represent the dominant source of extraterrestrial matter at the Earth’s surface, some of them originating from large heliocentric distances. Recent analyses of ultracarbonaceous micrometeorites recovered from Antarctica (UCAMMs) reveal an unusually nitrogen-rich organic matter. Such nitrogen-rich carbonaceous material could be formed in a N₂-rich environment, at very low temperature, triggered by energetic processes.

Aims. Several formation scenarios have been proposed for the formation of the N-rich organic matter observed in UCAMMs. We experimentally evaluate the scenario involving high energy irradiation of icy bodies subsurface orbiting at large heliocentric distances.

Methods. The effect of Galactic cosmic ray (GCR) irradiation of ices containing N₂ and CH₄ was studied in the laboratory. The N₂-CH₄ (90:10 and 98:2) ice mixtures were irradiated at 14 K by 44 MeV Ni¹¹⁺ and 160 MeV Ar¹⁵⁺ swift heavy ion beams. The evolution of the samples was monitored using in-situ Fourier transform infrared spectroscopy. The evolution of the initial ice molecules and new species formed were followed as a function of projectile fluence. After irradiation, the target was annealed to room temperature. The solid residue of the whole process left after ice sublimation was characterized in-situ by infrared spectroscopy, and the elemental composition was measured ex-situ.

Results. The infrared bands that appear during irradiation allow us to identify molecules and radicals (HCN, CN⁻, NH₃, ...). The infrared spectra of the solid residues measured at room temperature show similarities with that of UCAMMs. The results point towards the efficient production of a poly-HCN-like residue from the irradiation of N₂-CH₄ rich surfaces of icy bodies. The room temperature residue provides a viable precursor for the N-rich organic matter found in UCAMMs.