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

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 × 10 16 ions/cm 2 . 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.

Simulated space weathering of Fe- and Mg-rich aqueously altered minerals using pulsed laser irradiation

“…Thus, the spectral bluing trends of this sample may be representative of asteroids that have experienced small degrees of alteration. Additionally, Lantz et al (2015b) found a broad correlation between the spectral evolution of meteorites and their initial albedos and possibly carbon content. While a much larger set of experiments are still needed to constrain the spectral trends of volatile-rich CCs, the data presented here along with the Lantz et al (2015b) study appear to indicate that the compositional dependency of space weathering trends may be a useful tool to constrain the compositions and aqueous histories of volatile-rich asteroids.…”

Global Spectral Properties and Lithology of Mercury: The Example of the Shakespeare (H‐03) Quadrangle

“…Most commonly, space weathering produces a reddening and a darkening of the surface (Brunetto et al, ). However, it has been shown that these effects are not systematic for every asteroid class but depends also on the initial albedo (Lantz et al, ). Thus, the darkest C‐type asteroids (the richest in carbon) can have the opposite spectral behavior with respect to space weathering, that is, bluing and brightening.…”

“…Besides, the Tagish Lake meteorite, which is darker (albedo between ≃2.0 and 3.5% at 550 nm); (Izawa et al, 2015) and contains even more carbon than LRM (5.5% wt. (Lantz et al, 2015)), becomes brighter and bluer after irradiation. Therefore, the LRM of Mercury is an intermediate case between the cases of Murchison meteorite and Tagish Lake meteorite and thus can darken and redden when exposed to the space weathering.…”

“…with respect to space weathering, that is, bluing and brightening. Yet, these asteroids and their associated meteorites have an albedo (e.g., ≃5% at 550 nm for the Murchison meteorite; Lantz et al, 2015) lower than the albedo of LRM on Mercury (≃13% at 550 nm for the Degas crater, ≃8% at 550 nm for the Basho crater, which is surrounded by LRM darker than around Degas, like are Akutagawa and Sholem Aleichem craters; Denevi & Robinson, 2008). The albedo of LRM is closer to the one of the Allende meteorite (≃11% at 550 nm; Brunetto et al, 2014) for which the space weathering weakly reddens and darkens the surface.…”

“…for Murchison and ≃0.3% wt. for Allende; Lantz et al, 2015). Besides, the Tagish Lake meteorite, which is darker (albedo between ≃2.0 and 3.5% at 550 nm); (Izawa et al, 2015) and contains even more carbon than LRM (5.5% wt.…”