Abstract
The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) onboard the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft detected ~3.4-μm absorption features indicative of carbonates and organics on near-Earth asteroid (101955) Bennu. We apply a Kolmogorov-Smirnov similarity test to OVIRS spectra of Bennu and laboratory spectra of minerals to categorize 3.4-μm features observed on Bennu as representing either carbonates or organics. Among the 15,585 spectra acquired by OVIRS during high-resolution (4 to 9 m/spectrum footprint) reconnaissance observations of select locations on Bennu's surface, we find 544 spectral matches with carbonates and 245 spectral matches with organics (total of 789 high-confidence spectral matches). We map the locations of these matches and characterize features of Bennu's surface using corresponding image data. Image data are used to quantitatively characterize the albedo within each spectrometer footprint. We find no apparent relationships between spectral classification and surface morphological expression, and we find no correlation between carbon species classification and other spectral properties such as slope or band depth. This suggests either that carbonates and organics are ubiquitous across the surface of Bennu, independent of surface features (consistent with findings from laboratory studies of carbonaceous chondrites), or that the observations do not have the spatial resolution required to resolve differences. However, we find more organic spectral matches at certain locations, including the site from which the OSIRIS-REx mission collected a sample, than at others. Higher concentrations of organics may be explained if these materials have been more recently exposed to surface alteration processes, perhaps by recent crater formation.
Highlights
NASA's OSIRIS-REx mission (Lauretta et al, 2017, 2021) is returning a sample from near-Earth asteroid (101955) Bennu, a low-albedo 500m-diameter body whose closest known analogs are the hydrated carbonaceous chondrite meteorites (Clark et al, 2011; Hamilton et al., 2019)
Correlating spectral observations to imaging data can be difficult due to projection effects (e.g. Ferrone et al, 2021a), spacecraft motion during integration, differences in viewing geometry (Zou et al, 2021), and differences in spatial resolution, we present our findings with respect to quantitative measures of surface properties independently observed within each OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) footprint by the MapCam medium-field imager of the OSIRIS-REx Camera Suite (OCAMS) (Rizk et al, 2018; Golish et al, 2020)
Organics and carbonates are found at each of the locations studied on asteroid Bennu during the Recon A phase of the OSIRIS-REx mission, which offered sufficient resolution to distinguish these carbon-bearing species
Summary
NASA's OSIRIS-REx mission (Lauretta et al, 2017, 2021) is returning a sample from near-Earth asteroid (101955) Bennu, a low-albedo 500m-diameter body whose closest known analogs are the hydrated carbonaceous chondrite meteorites (Clark et al, 2011; Hamilton et al., 2019). Kaplan et al (2020a) analyzed the higher-resolution Recon A data for Nightingale, the site that was sampled by OSIRISREx, and found evidence of specific carbonate minerals (calcite, dolo mite, magnesite) distributed throughout the area and probably associ ated with bright veins visible in boulders seen in corresponding imaging data. These observations suggest that the parent body of asteroid Bennu experienced large-scale hydrothermal aqueous alteration in a chemi cally open system
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