Global spectrophotometric properties of Asteroid (21) Lutetia using Rosetta-OSIRIS images

Abstract We present a spectrophotometric analysis of the surface areas of asteroid (21) Lutetia that were imaged during the Rosetta flyby on July 2010 by the OSIRIS camera in the wavelength range 269 to 989 nm. A spectral slope change with respect to phase angle is found and interpreted as being due to areal diversity of Lutetia's surface. We applied Minnaert and Hapke models to disk-integrated and disk-resolved data of Lutetia. Except for the albedo in both models, the Minnaert (limb darkening k) and Hapke (roughness) parameter display no dependence on wavelength. The single scattering albedo, ω , shows a relative flat spectrum across the wavelength range with a slope of (1.65±0.29)% per 100 nm. The Hapke roughness parameter of Lutetia is constrained to be 28°± 1° in average for all wavelengths. We generated three different color ratio maps to investigate the overall color variation across the surface of Lutetia. Apart from differences on local scale no significant color variations are found across the regions imaged on Lutetia's surface (Achaia, Noricum, Narbonensis, Baetica and Etruria). Based on (931.9-nm/269.3-nm) color ratio map, the typical color contrast is estimated to be on the order of 10%. We note that, compared to other asteroids visited by spacecraft, the subtle color variation in order of 10% or less is observed for E-type asteroid (2867) Steins and the C-type asteroid (253) Mathilde.