Abstract
Abstract The surfaces of airless bodies such as lunar and S-type asteroids typically become spectrally redder in visible to near-infrared reflectance with longer exposures to space weathering. However, some carbonaceous asteroids instead become spectrally bluer. Space weathering experiments on carbonaceous meteorites have provided some clues as to the space weathering products that could produce spectral bluing. We applied these experimental results to our Hapke radiative transfer model, with which we modeled spectral data from the OSIRIS-REx mission in order to determine whether these space weathering products—specifically, nanophase and microphase metallic iron, troilite, and magnetite—could explain the globally blue spectrum of the carbonaceous asteroid (101955) Bennu. The model suggests that the surface of Bennu has microphase iron, nanophase magnetite, and nanophase and microphase troilite. Considering previous space weathering experiments together with our spectral modeling of Bennu, we posit that the presence of nanophase magnetite is what causes a carbonaceous asteroid to become spectrally bluer with exposure time. Nanophase magnetite can form on asteroids that have Fe-bearing hydrated minerals (phyllosilicates). On anhydrous carbonaceous asteroids, nanophase iron forms instead of magnetite, leading to spectral reddening. We therefore predict that samples returned by the OSIRIS-REx mission from Bennu will have more nanophase magnetite than nanophase iron with nanophase and microphase sulfides, whereas samples returned by the Hayabusa2 mission from the carbonaceous asteroid (162173) Ryugu, which is spectrally red, will contain nanophase and microphase sulfides as well as more nanophase iron than nanophase magnetite.
Highlights
Studies of airless bodies such as the Moon and S-type asteroids have shown that with long durations of exposure to micrometeoroid bombardment and solar wind—processes collectively known as space weathering—the visible to nearinfrared reflectance spectra of an exposed surface becomes spectrally redder (e.g., Keller & McKay 1993, 1997; Hapke 2001; Noguchi et al 2011)
In our initial phase of modeling, we focused on nanophase and microphase iron, troilite, and magnetite, because these have been observed to form as products in space weathering experiments on carbonaceous chondrites (e.g., Thompson et al 2020), suggesting that they may form on the surfaces of carbonaceous asteroids
For a carbonaceous asteroid to become spectrally bluer, hydrated iron-bearing phyllosilicates, which enable the formation of nanophase magnetite, need to be present on the surface
Summary
Studies of airless bodies such as the Moon and S-type asteroids have shown that with long durations of exposure to micrometeoroid bombardment and solar wind—processes collectively known as space weathering—the visible to nearinfrared reflectance spectra of an exposed surface becomes spectrally redder (increasing reflectance with increasing wavelength) (e.g., Keller & McKay 1993, 1997; Hapke 2001; Noguchi et al 2011). Our current understanding and models of space weathering on airless bodies (e.g., Cassidy & Hapke 1975; Hapke 2001; Noble et al 2007; Lucey & Riner 2011) have largely been shaped by knowledge gained from investigations of lunar regolith These lunar space weathering studies showed that the presence of submicroscopic particles affects the overall reflectance slope in the visible to near-infrared (Cassidy & Hapke 1975; Keller & McKay 1993, 1997). Because the main minerals on the lunar surface are plagioclase, pyroxene, and olivine, the submicroscopic particles predominantly consist of metallic iron (Fe)
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