Vesta Phase Reddening from Dawn Observations

Now that Dawn has entered into orbit around the asteroid 4 Vesta, the onboard Framing Camera (FC) and the VIR spectrometer have acquired data over a wide range of phase angles. The FC observes Vesta through several narrow band filters in the 400-1000 nm wavelength range, while VIR covers the 0.25-5.0 μm range. The availability of this data set allows us to study the phenomenon of phase reddening. The reflectance spectrum of atmosphereless solar system bodies is known to vary with phase angle. This is known as phase reddening, because the spectrum is usually (but not always) observed to become redder with increasing phase. It has been found for a variety of bodies like the Moon and asteroids [1,2]. Phase reddening has been demonstrated in laboratory studies using regolith simulants [3]. It is not fully understood, but some aspects can be interpreted in terms of geometric optics simulations [4]. We determine the degree of phase reddening for various terrains on Vesta and attempt to correlate our findings with various physical properties of the regolith, like grain size, mineralogical composition, aggregation state (glasses, agglutinates), and degree of space weathering. [1] Gehrels, T. et al. (1964) Wavelength dependence of polarization. III. The lunar surface, AJ 69, 826-852 [2] Taylor, R.C. et al. (1971) Minor Planets and Related Objects. VI. Asteroid (110) Lydia, AJ 76, 141-146 [3] Adams, J.B. and A.L. Filice, (1967) Spectral Reflectance 0.4 to 2.0 Microns of Silicate Rock Powders, JGR 72, 5705-5715 [4] Grynko, Y. and Y. Shkuratov (2008) Light scattering from particulate surfaces in geometrical optics approximation, in Light Scattering Reviews 3, ed. A. Kokhanovsky, 329-382