WHAT WE NEED TO KNOW BEFORE ANALYZING ASTEROID BENNU: METEORITES,REMOTE SENSING, AND OSIRIS-REx
H. C. CONNOLLY Jr1,2, L. P. KELLER3, D. S. LAURETTA4, and the Carbonaceous Meteorite and SpectralAnalysis Working Groups of the OSIRIS-REx Science Team. 1Dept. of Geology, School of Earth and the Environment, Rowan University, 201 Mullica Hill Rd., Glassboro, NJ 08028 U.S.A.(connollyh@rowan.edu) 2Earthand Planetary Sciences, AMNH, Central Park West, New York, NY 11024, U.S.A., 3Code X13, ARES
NASA/JSC Houston, TX77058, U.S.A., and 4Lunary and Planetary Laboratory, University of Arizona, Tucson,
AZ U.S.A.
Introduction: The near-Earth asteroid (101955) Bennu, the target of NASA’s OSIRIS-REx asteroid sample
return mission, is one of the most heavily investigated astronomical objects to date, including multiple telescopeobservations from visible out to radar wavelengths [1]. However, our knowledge is currently limited to whole disk
spectral and photometric data and spatially resolved radar measurements data. The key to confirming any remotesensing data of Bennu, either ground-based or space-based, is to explore the asteroid with high spatial resolution
and return a carefully selected sample for laboratory-based analyses. The major goal of the OSIRIS-REx remotesensing campaign is to determine the optimal site for sample collection using data to produce Sample Site Selection
maps that incorporate concepts such as safety, sampleability, and science value. Visible to near-infrared (VNIR)
and thermal infrared (TIR) spectra obtained with the OVIRS and OTES instruments will provide key mineralogicaland chemical inputs for the science value maps.
A major question related to our ability to effectively use the spectral data collected during real-time asteroid
operations for sample site selection is: Are existing spectral libraries (e.g., RELAB database, ASU TIR SpectralLibrary, etc.) sufficient to interpret OVIRS and OTES data? The bottom-line is that the ability of the science teamto accurately interpret the spacecraft data depends on the completeness of the available spectral libraries (in termsof species, compositions, grain sizes, etc.). If these are insufficient to meet the needs of the mission, it is crucial todevelop a custom spectral library for the mission to enhance existing ones. Thus, we asked ourselves a second
question: How well is the OSIRIS-REx mission prepared for the remote-sensing data analysis during asteroid
operations and what additional research is needed? To better assess the need for a custom spectral library and thus
understand the level to which the current library and processing methods for spectral data, the OSIRIS-REx Carbonaceous Meteorite Working Group (CMWG) in collaboration with the Spectral Analysis Working Group(SAWG) undertook a two-year investigation to analyze actual meteorite and analog spectra with known constituent
proportions using the mission data analysis software, known as ‘The Blind Test”.
Methods: The CMWG prepared a series of 10 bulk mixtures of known phases, with selected size ranges foreach phase. Additionally, seven bulk chondrite samples were also prepared. Known grain sizes were prepared andwell mixed for the final bulk compositions. After preparation, samples were sent to Ed Cloutis at the Universityof Winnipeg for VNIR analysis (0.4 – 4.3 µm) and to Neil Bowles for analysis in the TIR (4.35 – 25 µm). Finaldata was sent to the SAWG for reduction, phase identification, and abundance determination. The SAWG thensent their final report on the contents of each bulk composition to the CMWG for review.
VNIR Results: Overall the VNIR data correctly resolved the analog samples and meteorites samples intohydrous and anhydrous groups where these were major constituents with only minor deviations. Detection ofminor phases at the 5% level (a requirement for the mission) was mixed. Organics in analog samples at the 5%level were correctly identified with only 2 false positives (at the trace level). Curiously, organics were not detectedin any of the meteorite samples. Transparent minerals minerals and minerals lacking strong, distinctive absorption
features in the visible/near-IR were not detected in the VNIR analyses, including plagioclase, FeNi metal, troilite,and magnetite.
TIR Results: Overall the TIR analyses correctly resolve the analog samples and meteorite samples into hydrous
and anhydrous groups where hydrated or anhydrous minerals are major constituents. No information was gleaned
from the TIR data for the detection of organic features. In analog samples, olivine and phyllosilicates were consistently identified when they were major constituents of the mixtures, but pyroxene and plagioclase present in theanalog samples at the 10 wt.% level went undetected. However, phyllosilicates were not identified in the spectraof the CM chondrites Murchison and ALH 83100. Detection of minor phases at the 5% level was unreliable.
Summary and Path Forward: The CMWG ‘Blind Study’ was informative and a worthy undertaking. TheCMWG made 7 recommendations that will be discussed during the talk to the project for consideration, including
the production a custom OSIRIS-REx spectral library. The custom library will enhance the mission’s ability toaccurately determine phases and their abundances on the surface of Bennu to better inform the ideal location forcollection of a sample. Of course, the final ground-truth for any of the remoting sensing data will come from theanalysis of the returned sample in September of 2023.
References: [1] D. S. Lauretta et al., (2015) The OSIRIS-REx target asteroid 101955 Bennu: Constraints on its
physical, geological, and dynamical nature from astronomical observations. Meteoric. Planet. Sci., 50, 834.