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

Context. The reflectance spectra of some main-belt asteroids contain a 3-mu m 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-mu m band in goethite has a minimum around 3.10 mu m, similar to observations of 24-Themis. Its overall shape matches well Themis's 3-mu m band; goethite-like oxy-hydroxydes are a viable alternative way of explaining 24-Themis near-IR spectra