From top to bottom, the 3-μm band in goethite, water ice, and as observed on Themis. The Themis spectrum has been digitized from Rivkin & Emery (2010). The green spectrum is the ratio between the two mixtures with different amounts of goethite, from Fig. 1. The black curve corresponds to the transmission spectrum of synthetic goethite from Ruan et al. (2002), obtained at 110 °C. The red curve shows the imaginary index of the optical constant of water ice (Schmitt et al. 1998) for comparison. Data have been offset and rescaled for clarity.