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Goethite as an alternative origin of the 3.1 μm band on dark asteroids

Abstract : Context. The reflectance spectra of some main-belt asteroids contain a 3-μm band, which is explained by the presence of water or hydroxyl groups in minerals. Recent observations of 24-Themis have been found to display evidence of water ice. Aims. We synthesize iron oxy-hydroxide materials and in the laboratory measure their near-infrared spectra under dry conditions to compare with asteroid observations. Methods. The syntheses are performed using either the well-established titration method or a non-conventional hydrothermal method. Bi-directional near-infrared reflectance spectra are obtained using the spectrogonio radiometer available at LPG/IPAG. Spectra are measured in a vacuum to avoid contamination by adsorbed water. Results. The reflectance spectra that we measure for synthesized goethite are consistent with published transmission spectra. The 3-μm band in goethite has a minimum around 3.10 μm, similar to observations of 24-Themis. Its overall shape matches well Themis's 3-μm band; goethite-like oxy-hydroxydes are a viable alternative way of explaining 24-Themis near-IR spectra.
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Pierre Beck, Eric Quirico, D. Sevestre, German Montes-Hernandez, A. Pommerol, et al.. Goethite as an alternative origin of the 3.1 μm band on dark asteroids. Astronomy and Astrophysics - A&A, EDP Sciences, 2011, 526, A85 (4p.). ⟨10.1051/0004-6361/201015851⟩. ⟨insu-00679298⟩



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