Visible–near infrared spectral indices for mapping mineralogy and chemistry with OSIRIS‐REx

Abstract

The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission is to return to Earth a pristine sample of carbonaceous material from the primitive asteroid (101955) Bennu. To support compositional mapping of Bennu as part of sample site selection and characterization, we tested 95 spectral indices on visible to near infrared laboratory reflectance data from minerals and carbonaceous meteorites. Our aim was to determine which indices reliably identify spectral features of interest. Most spectral indices had high positive detection rates when applied to spectra of pure, single-component materials. The meteorite spectra have fewer and weaker absorption features and, as a result, fewer detections with the spectral indices. Indices targeting absorptions at 0.7 and 2.7-3 lm, which are attributable to hydrated minerals, were most successful for the meteorites. Based on these results, we identified a set of 17 indices that are most likely to be useful at Bennu. These indices detect olivines, pyroxenes, carbonates, water/OH-bearing minerals, serpentines, ferric minerals, and organics. Particle size and albedo are known to affect band depth but had a negligible impact on interpretive success with spectral indices. Preliminary analysis of the disk-integrated Bennu spectrum with these indices is consistent with expectations given the observed absorption near 3 lm. Our study prioritizes spectral indices to be used for OSIRIS-REx spectral analysis and mapping and informs the reliability of all index-derived data products, including a science value map for sample site selection.

Phase reddening on asteroid Bennu from visible and near-infrared spectroscopy

“…In fact, OVIRS probes the uppermost surface of Bennu, where scattering properties are sensitive to very small particles comparable in size to that of the incoming light, while OTES is sensitive to larger particles and sounds the first subsurface. Hamilton et al (2020) showed that a ∼ 5 µm-thick coating of dust, with dominantly particles size in the 5-50 µm range, result in a very small change in total emissivity spectral contrast. The presence of dust blanketing the surface of Bennu was also suggested from the analysis of DellaGiustina et al (2019), who first reported the spectral phase reddening evident in OCAMS data.…”

Thermal metamorphism of CM chondrites: A dehydroxylation‐based peak‐temperature thermometer and implications for sample return from asteroids Ryugu and Bennu

“…Based on early OSIRIS-REx mission data (Hamilton et al 2019), Bennu is still most similar to CM2s, now more specifically Cold Bokkeveld (CM2.2 in the classification of Rubin et al 2007), other CM2.2 and CM2.1 chondrites, and CM1/2 chondrites, although not an exact match to any one as-recovered meteorite. As new meteorite spectra are acquired and added to spectral libraries (Takir et al 2013;McAdam et al 2015;Garenne et al 2016;Kaplan and Milliken 2016;Beck et al 2018;Lantz et al 2018;Bates et al 2020;Kaplan et al 2020), additional good matches may be recognized, enabling refined interpretation.…”

“…Early OSIRIS-REx OVIRS and OTES whole-diskintegrated spectra are spatially uniform across Bennu's surface at the spatial resolutions of tens to hundreds of meters reported to date; acquisition, reduction, and interpretation of spatially resolved data are ongoing (Hamilton et al 2019;Kaplan et al 2020). A strong absorption feature at~2.7 μm in OVIRS spectra is ubiquitous on Bennu's surface (Hamilton et al 2019;Kaplan et al 2020).…”

“…Early OSIRIS-REx OVIRS and OTES whole-diskintegrated spectra are spatially uniform across Bennu's surface at the spatial resolutions of tens to hundreds of meters reported to date; acquisition, reduction, and interpretation of spatially resolved data are ongoing (Hamilton et al 2019;Kaplan et al 2020). A strong absorption feature at~2.7 μm in OVIRS spectra is ubiquitous on Bennu's surface (Hamilton et al 2019;Kaplan et al 2020). The rounded shape of the feature resembles those of intermediately altered CMs (e.g., Cold Bokkeveld, CM2.2 in the classification of Rubin et al 2007, and MET 00639;Hamilton et al 2019) but also resembles Murchison (CM2.5; Bates et al 2020).…”

“…Meteorite spectra vary with particle size, viewing geometry, illumination conditions, and terrestrial weathering (McAdam et al 2015;Campins et al 2018;Cloutis et al 2018;Takir et al 2018). Spectroscopic measurements of asteroids provide information only about the optically active outermost surface that (1) may not be representative of the composition of bulk materials collected below the surface by sample return missions and (2) may have been affected by phenomena that modify their spectra relative to (a) conditions of acquisition of laboratory spectra from meteorites (e.g., temperature, vacuum versus laboratory ambient atmosphere) and (b) material that was never exposed to processes at the parent body's surface such as space weathering (Takir et al 2018;Kaplan et al 2020). Hypotheses for the interpretation of asteroid surface composition from some spacecraftmeasured spectral properties will ultimately be tested and understood only by terrestrial laboratory analyses of the returned sample (Kaplan et al 2020).…”