Panel a: map of the Clementine-derived OMAT parameter as given by Lucey et al. (2000). Small “holes” due to invalid pixels were filled by interpolation. Panel b: spectrum of the bright structure (black curve) in comparison to the surrounding surface (green curve), averaged over 1 × 1 km² areas at the locations A and B indicated in Fig. 1b. The spectra were obtained by fitting the Hapke model (Hapke 1984, 2002) to the two available M³ images. Top: absolute reflectance factor. The dotted lines correspond to the fitted linear continuum. Bottom: reflectance normalized at 2.5 μm (bottom).