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

¹ Laboratoire de Planétologie de Grenoble (LPG/IPAG), Université J. Fourier, CNRS/INSU, UMR5109, OSUG, Université Joseph Fourier, 122 rue de la Piscine, 38041 Grenoble Cedex 9, France
e-mail: pierre.beck@obs.ujf-grenoble.fr
² Institut des Sciences de la Terre (IsTerre), Université J. Fourier, CNRS/INSU, 1381 rue de la piscine, 38041 Grenoble Cedex, France
³ Space Research & Planetary Sciences Division, Physikalisches Institut-Universitaet Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Received: 1 October 2010
Accepted: 12 November 2010

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