Ion irradiation of the Murchison meteorite: Visible to mid-infrared spectroscopic results

¹ Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique (LESIA) – Observatoire de Paris, CNRS (UMR 8109)/UPMC (Paris 6) / Univ. Paris Diderot (Paris 7), 92195 Meudon Cedex, France
e-mail: cateline.lantz@obspm.fr
² Université Paris Diderot, Sorbonne Paris Cité, 75205 Paris Cedex 13, France
³ Institut d’Astrophysique Spatiale (IAS), CNRS (UMR 8617)/Université Paris-Sud (Paris 11), 91405 Orsay Cedex, France
⁴ Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM), IN2P3 – CNRS (UMR 8609)/Université Paris-Sud (Paris 11), 91405 Orsay Cedex, France
⁵ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université J. Fournier – Grenoble 1/CNRS-INSU (UMR 5274), 38041 Grenoble Cedex 9, France

Received: 24 November 2014
Accepted: 3 March 2015

Abstract

Aims. The goal of this study is to simulate space weathering processes on primitive bodies. We use ion implantation as a simulation of solar wind irradiation, which has been suggested by several authors to be the major component of space weathering on main belt asteroids. The laboratory analogs we irradiate and analyze are carbonaceous chondrites; we started the study with the Allende CV meteorite and in this companion paper we present results on the Murchison CM meteorite.

Methods. We performed irradiations on pressed pellets of Murchison with 40 keV He⁺ and Ar⁺ ions using fluences up to 3 × 10¹⁶ ions/cm². Reflectance spectra were acquired ex situ before and after irradiation in the visible to mid-infrared range (0.4–16 μm). A Raman analysis was also performed to investigate the modifications of the aromatic carbonaceous component.

Results. Our results indicate that spectral variations after irradiation within the visible range are smaller than spectral variations due to sample grain size or viewing geometry of the Murchison meteorite. The aqueous alteration band profile near 3 μm changes after irradiation, as adsorbed water is removed, and phyllosilicates are affected. Raman spectroscopy highlights the insoluble organic matter (IOM) modification under irradiation. We observe a shift of the silicates band at 9.9 μm, probably due to a preferential loss of Mg (compared to Fe, the lighter Mg is more easily sputtered backward) and/or amorphization of Mg-rich materials. We compare our results to previous experiments on organic-rich materials (like asphaltite or carbonaceous chondrites), and on ordinary chondrites and olivine grains. We find that the reddening/darkening trend observed on silicate-rich surfaces is not valid for all carbonaceous chondrites, and that the spectral modifications after irradiation are a function of the initial albedo.