A possible explanation for the blue spectral slope observed on B-type asteroids

In an effort to better understand the role dark material plays in the reflectance spectra of carbonaceous asteroids, we performed laboratory studies focusing on quantifying how the addition of relevant dark material (graphite, magnetite and troilite) can alter the ultraviolet-visible and near-infrared spectrum of a neutral silicate mineral. We find that addition of graphite, magnetite and troilite all darken the reflectance spectrum of forsterite and cause the spectral slope to decrease (become blue). These spectral changes can be caused by both nm- and μm-sized grains. In the ultraviolet-visible region, we find that graphite is most efficient at altering the spectral slope, while in the near-infrared, magnetite is the most efficient. At all wavelengths studied, graphite is the most efficient at darkening our sample spectrum. However, the observation that troilite also alters the slope and albedo of our samples suggests that the spectral changes caused by magnetite and graphite may not be unique. In addition, we find that the spectral slopes in our mixtures compare generally well to what has been observed on Bennu suggesting that a significant portion of fine-grained dark material, including sulfides, present in the regolith can cause the observed negative (blue) slope found on B-type asteroids.