Wavelength Dependence of the Opposition Effect for Asteroids Steins and Lutetia: Analysis of the Rosetta OSIRIS Data

Rosetta OSIRIS optical system is equipped with a total of 25 medium and narrow-band filters which allow studying spectral dependence of photometric data obtained by this instrument. In this study we analyze the OSIRIS data for asteroids Steins and Lutetia at small phase angles to see how characteristics of their opposition effect depend on the wavelength and what mechanism can explain this dependence. For asteroid Steins, the data are available for the phase angles ranging from 0.36° up to 136° acquired with 9 filters that cover the wavelength range 295.9 -- 631.6 nanometers; the images of asteroid Lutetia cover phase angles from 0.15° up to 156° in 16 filters in the range of wavelengths 269.3 -- 989.3 nanometers. The data for Lutetia have a good coverage for phase angels smaller than 5° for three filters, whereas in the case of Steins only for the WAC filter 631.6 nm the angles smaller than 5° are covered. For other filters, a special interpolation procedure was performed; it is described in detail in La Forgia et al. (Mem. S.A.It. Suppl. Vol. 20, 15, 2012).Both asteroids demonstrate significant brightness surge at small phase angles, however, its dependence on the wavelength is opposite: the sharpness of the surge increases with wavelength for asteroid Steins and decreases for asteroid Lutetia. The wavelength behavior of Steins' opposition effect is consistent with the coherent backscattering as the main mechanism that produces the opposition surge. However, the wavelength behavior of Lutetia's opposition surge cannot be explained either by coherent backscattering or by shadow hiding. We explore other opportunities to explain the specifics of the opposition effect for asteroid Lutetia. Particularly, we consider if the properties of individual particles can be responsible for the wavelength dependence of Lutetia's opposition effect.