Abstract
Context.The OSIRIS-REx Visible and InfraRed Spectrometer onboard the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer spacecraft obtained many spectra from the surface of the near-Earth asteroid (101955) Bennu, enabling the characterization of this primitive small body. Bennu is spectrally similar to the hydrated carbonaceous chondrites (CCs), but questions remain as to which CCs, or combinations thereof, offer the best analogy to its surface.Aims.We aim to understand in more detail the composition and particle size of Bennu’s surface by refining the relationship between this asteroid and various CC meteorites.Methods.We used published absorbance and reflectance data to identify new optical constants for various CC meteorites measured in the laboratory at different temperatures. We then used the Hapke model to randomly generate 1000 synthetic spectra in order to find the combinations of these potential meteoritic analogs that best reproduce the spectral features of the asteroid.Results.Our investigations suggest that the surface of Bennu, though visibly dominated by boulders and coarse rubble, is covered by small particles (tens to a few hundreds of μm) and that possibly dust or powder covers the larger rocks. We further find that the surface is best modeled using a mixture of heated CM, C2-ungrouped, and, to some extent, CI materials.Conclusions.Bennu is best approximated spectrally by a combination of CC materials and may not fall into an existing CC group.
Highlights
In early December 2018, NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at the near-Earth asteroid (101955) Bennu, with the objective to study the surface of this small primitive object and return a sample of pristine carbonaceous regolith to Earth (Lauretta et al 2017, 2021)
From simulations based on laboratory measurements of meteorites and 1000 synthetic spectra generated using the Hapke model, covering the [2.0–3.8] μm range, we find that the mean reflectance spectrum of Bennu is best fitted by an intimate mixture of several possible carbonaceous chondrite analogs, the following
Even if there is not a unique solution, Tagish Lake seems to be one of the samples best able to reproduce the behavior of the reflectance spectrum of Bennu in the [2–3.8] μm range, when intimately mixed with CM components, such as DOM 08003 or Niger. 2
Summary
In early December 2018, NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at the near-Earth asteroid (101955) Bennu, with the objective to study the surface of this small primitive object and return a sample of pristine carbonaceous regolith to Earth (Lauretta et al 2017, 2021). Bennu is spectrally classified as a B-type asteroid, part of the larger C-complex of asteroids, with visible to near-infrared spectral properties resembling those of carbonaceous chondrite meteorites, the CMs and CIs (Clark et al 2011; Hamilton et al 2019; Simon et al 2020). These meteorites are water- and organic-bearing and may have delivered such materials to Earth early in Solar System history. Rocks on Bennu are broken down by processes such as thermal fatigue (Molaro et al 2020) and meteoroid bombardment (Bottke et al 2020; Ballouz et al 2020)
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