C and B class asteroids (including 101955 Bennu, the destination of the OSIRIS REx mission) display spectral bluing in the visible (Lantz et al. 2013). This spectral bluing effect has been found to be temporally variable on Bennu (Binzel et al. 2015). Binzel et al. suggested this is due to a fining of the ~45 micron grain size fraction, which causes spectral reddening. This finer grain size of the ~45 micron fraction may be associated with regolith migration during formation of Bennu's equatorial ridge. de Leon et al. (2016) recently established that the Polana-Eulalia family from the inner part of the asteroid belt (and likely source region for the NEO Bennu) show a continuum of of spectral slopes from blue to moderately red, typical of primitive B and C asteroids.
In Brown (2014) the effect of grain size and optical index on the albedo of small conservative and absorbing particles as a function of wavelength was examined. The conditions necessary for maximization of spectral bluing effects in real-world situations were identified. This paper will discuss the effects of the particle morphology, or shape, affect the spectral bluing slope. We will discuss the case of axisymmetric particles such as spheroids and cylindrical puck shapes.
The spectral bluing to be discussed in this presentation was present in the Lantz et al. spectra, but not the Binzel et al. spectra, suggesting that in addition to finer grain ~45 micron material, a decrease in the sub micron grain sized fraction has taken place as Bennu's sub-Earth latitude changed between these observations. Observations of this effect may provide the strongest test yet for cohesive regolith models (e.g. Rositis et al. 2014).
In this presentation, we will discuss: 1.) the evidence for spectral bluing on 101955 Bennu (in particular) and other bodies in our solar system, 2.) the effects of particle shape on the spectral bluing slope, 3.) implications of how the OVIRS instrument on OSIRIS-REx may be used to determine the spatial variability of this spectral feature on Bennu and 4.) the potential for OVIRS to augment our understanding of the dynamics of sub micron material on asteroids.
Binzel, R. P. et al. "Spectral slope variations for OSIRIS-REx target Asteroid (101955) Bennu: Possible evidence for a fine-grained regolith equatorial ridge" Icarus 256 (2015), 22-29
Brown, Adrian J. “Spectral Bluing Induced by Small Particles under the Mie and Rayleigh Regimes.” Icarus 239 (2014): 85–95.
de León, et al. “Visible Spectroscopy of the Polana–Eulalia Family Complex: Spectral Homogeneity.” Icarus 266 (2016): 57–75.
Lantz, C., et al. “Evidence for the Effects of Space Weathering Spectral Signatures on Low Albedo Asteroids.” A&A 554 (2013): 138.
Rositis, B. et al. "Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA" Nature (2014): 174-176