The Mineralogy and Organic Composition of Bennu as
Observed by VNIR and TIR Spectroscopy
V. E. Hamilton1, A. A. Simon2, H. H. Kaplan2, P. R. Christensen3, D. C. Reuter2, D. N. DellaGiustina4, C. W. Haberle5, L. B.
Breitenfeld6, B. E. Clark7, and D. S. Lauretta4
1Southwest Research Institute, Boulder, CO, USA (hamilton@boulder.swri.edu)
2NASA Goddard Space Flight Center, Greenbelt, MD, USA
3Arizona State University, Tempe, AZ, USA
4University of Arizona, Tucson, AZ, USA
5Northern Arizona University, Flagstaff, AZ, USA
6Stony Brook University, Stony Brook, NY, USA
7Ithaca College, Ithaca, NY, USA

NASA's Origins, Spectral Interpretation, Resource Identification, and Security — Regolith Explorer (OSIRIS-REx) mission
characterized the surface composition of the carbonaceous asteroid Bennu at visible to infrared wavelengths (~0.4 - 100 µm).
Spectral features of hydrated minerals (phyllosilicates) are dominant at both visible to near infrared (VNIR) [1, 2] and thermal
infrared (TIR) wavelengths [1], with ~90 vol.% of the silicates being comprised of phyllosilicates (≤~10 vol.% olivine pluspyroxene) [3]. Features of iron oxides are observed in both the VNIR (magnetite, goethite) [4] and the TIR (magnetite) [1]. In
the 3.2-3.6 µm region, we observe spatially variable evidence for carbonate minerals (some associated with meter-long, cmwide veins) and organic compounds comparable to insoluble organic material (IOM), with classification results showing little
or no evidence of any correlation between these features and surface morphology or spectral slope/band depth [5-9]. A halfdozen isolated, meter-sized boulders exhibit pyroxene signatures consistent with those in the howardite-eucrite-diogenite(HED) meteorites from (4) Vesta [10]. There is evidence for non-uniform deposits of dust (~5-10 µm thick) superposed on alargely boulder-dominated surface [3]. The majority of VNIR features show only small band depth variations across the surface [4] and the TIR features appear to vary dominantly with particle size [3]. Nanophase magnetite produced by space weathering may account for Bennu’s visible blue slope [12]. Two populations of rocks are observed in visible imaging, distinguishedby albedo and surface texture [11], but spectral data analyzed to date have not shown any evidence of these two populations
having different compositions.
The observed mineralogy of Bennu has been identified as being most consistent with a highly aqueously altered, CI- or CMlike, carbonaceous chondrite (CC) composition. In addition, we have recently recognized that GRO 95577 (CR1) also exhibits
NIR and TIR spectra that are compatible with the observed spectroscopy and inferred mineralogy of Bennu (Figures 1 and 2).
(We note that none of the NIR analogues shown here are good matches to Bennu at visible wavelengths, where the best fit is toa sample of Ivuna (CI) heated to 700°C [14]; however, the effects of space weathering on Bennu are not represented by theanalogue spectra.) Isolated boulders containing pyroxene are interpreted as exogenic, basaltic material from Vesta, the preserved evidence of inter-asteroid mixing that occurred after the conclusion of planetesimal formation [10]. The manifestationof carbonate veins at scales much larger than has been observed in CC meteorites suggests that Bennu’s parent body experienced fluid flow and hydrothermal deposition on kilometer scales for thousands to millions of years [6]. In October 2020,
OSIRIS-REx collected a sample of the surface of Bennu for return to Earth in September 2023. We predict that the returned
sample will contain the minerals and compounds described here (phyllosilicates, iron oxides, carbonates, and organics), representing a significant degree of alteration. In addition, minerals that are difficult to detect with remote sensing data, such as sulfides, also may be present, as well as exogenous materials.
References[1] Hamilton, V. E. et al. 2019. Nature Astronomy 4:332. [2] Praet, A. et al. 2021. Icarus 363:114427. [3] Hamilton, V. E. et al.
2021. Astronomy & Astrophysics 650:A120. [4] Simon, A. A. et al. 2020. Astronomy & Astrophysics 644:A148. [5] Simon, A.
A. et al. 2020. Science 370:eabc3522. [6] Kaplan, H. H. et al. 2020. Science 370:eabc3557. [7] Kaplan, H. H. et al. 2021. Astronomy & Astrophysics 653:L1. [8] Barucci, A. et al. 2020. Astronomy & Astrophysics 637, L4. [9] Ferrone, S. et al. 2021.
Icarus 368:114579. [10] DellaGiustina, D. N. et al. 2020. Nature Astronomy 5:31. [11] DellaGiustina, D. N. et al. 2020. Science 370:eabc3660. [12] Trang, D. et al. 2021. Planetary Science Journal 2:68. [13] Takir, D. et al. 2013. Meteoritics & Planetary Science 48:1618. [14] Clark, B. E. et al. 2011. Icarus 216:462.

Figure 1. OVIRS spectrum of Bennu [1] in the “3Bennuµm region” as compared to spectra of CC

Reflectance (offset and scaled)

meteorites from [13] and a RELAB spectrum ofGRO 95577.

GRO 95577 (CR1)

Ivuna (CI1)

LAP 02277 (CM1)

MET 00639 (CM2)

Cold Bokkeveld (CM2)

2.4

8

2.6

2.8
3.0
Wavelength (µm)

3.2

3.4

Wavelength (µm)
10 12 15 20 25
Figure 2. OTES spectrum of Bennu [3]as compared to CC analogues. TheEmissivity and 1-R (offset and normalized)

Christiansen feature (~1090 cm-1, ~9.2µm) is not particularly diagnostic.
Bennu T1 (least dusty)
Orgueil (CI1)
GRO 95577 (CR1)
Almahata Sitta 91A_1 (C1)
Moapa Valley (CM1)
ALH 83100 (CM1/2)
LAP 02277 (CM2.0)
LAP 031166 (CM2.1)
ALH 85013 (CM2.2)
Cold Bokkeveld (CM2.2)
Murray (CM2.4/2.5)
Murchison (CM2.5)

1200 1000 800 600-1Wavenumber (cm )400

However, the peak at 528 cm-1 (18.9 µm)
is a diagnostic feature in the Bennu
spectrum, and it weakens and shifts withincreasing abundances of anhydrous
silicates (e.g., pyroxene and olivine). As
the abundance of anhydrous silicatesincreases, the Si-O bending mode (theminimum at ~440 cm-1 or 22.7 µm)
broadens and features of pyroxene andolivine become apparent (e.g., Cold
Bokkeveld, Murray, Murchison).