Ion irradiation of Allende meteorite probed by visible, IR, and Raman spectroscopies

• Published in: Icarus (New York, N.Y. 1962) Vol. 237; pp. 278 - 292
• Main Authors: Brunetto, R., Lantz, C., Ledu, D., Baklouti, D., Barucci, M.A., Beck, P., Delauche, L., Dionnet, Z., Dumas, P., Duprat, J., Engrand, C., Jamme, F., Oudayer, P., Quirico, E., Sandt, C., Dartois, E.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15-07-2014; Elsevier
• Subjects: Asteroids; Asteroids, surfaces; Astrophysics; Earth and Planetary Astrophysics; Experimental techniques; Infrared spectroscopy; Ion irradiation; Irradiation; Magnesium; Meteorites; Olivine; Physics; Reflectance; Sciences of the Universe; Solar wind; Spectra; Spectroscopy; Weathering; Spectroscopy; Experimental techniques; Meteorites; Asteroids, surfaces; Solar wind
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2014.04.047

•We irradiate Allende meteorite with 40keV ions to simulate asteroid space weathering.•VIS–NIR reflectance spectra show weak reddening and darkening with increasing dose.•Sputtering and amorphization of matrix olivine induce modifications of IR bands.•Carbonaceous component evolves towards a highly disordered amorphous carbon.•Spectral changes scale with elastic dose and correspond to ∼103–104yrs in main belt.
Little is known about carbonaceous asteroids weathering in space as previous studies have struggled to define a general spectral trend among dark surfaces. Here we present experiments on ion irradiation of the Allende meteorite, performed using 40keV He+ and Ar+ ions, as a simulation of solar wind irradiation of primitive bodies surfaces. We used different fluences up to 3×1016ions/cm2, corresponding to short timescales of ∼103–104yrs in the main asteroid belt. Samples were analyzed before and after irradiation using visible to far-IR (0.4–50μm) reflectance spectroscopy, and Raman micro-spectroscopy. Similarly to what observed in previous experiments, results show a reddening and darkening of VIS–NIR reflectance spectra. These spectral variations are however comparable to other spectral variations due to viewing geometry, grain size, and sample preparation, suggesting an explanation for the contradictory space weathering studies of dark asteroids. After irradiation, the infrared bands of the matrix olivine silicates change profile and shift to longer wavelength, possibly as a consequence of a more efficient sputtering effect on Mg than Fe (lighter and more volatile species are preferentially sputtered backwards) and/or preferential amorphization of Mg-rich olivine. Spectral variations are compatible with the Hapke weathering model. Raman spectroscopy shows that the carbonaceous component is substantially affected by irradiation: different degrees of de-ordering are produced as a function of dose, to finally end with a highly disordered carbon. All observed modifications seem to scale with the nuclear elastic dose.