Composition of organics on asteroid (101955) Bennu

H H Kaplan,D P Glavin,J Brucato,D S Lauretta,B E Clark,D C Reuter,J P Emery,H Campins,M S Thompson,A A Simon,X D Zou,V E Hamilton,M A Barucci,E A Cloutis,J P Dworkin,S A Sandford,S Fornasier

doi:10.1051/0004-6361/202141167

Abstract

Context. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission detected an infrared absorption at 3.4 μm on near-Earth asteroid (101955) Bennu. This absorption is indicative of carbon species, including organics, on the surface. Aims. We aim to describe the composition of the organic matter on Bennu by investigating the spectral features in detail. Methods. We use a curated set of spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer that have features near 3.4 μm (3.2 to 3.6 μm) attributed to organics. We assess the shapes and strengths of these absorptions in the context of laboratory spectra of extraterrestrial organics and analogs. Results. We find spectral evidence of aromatic and aliphatic CH bonds. The absorptions are broadly consistent in shape and depth with those associated with insoluble organic matter in meteorites. Given the thermal and space weathering environments on Bennu, it is likely that the organics have not been exposed for long enough to substantially decrease the H/C and destroy all aliphatic molecules.

Highlights

The Recon A data set in this study comprises 15 585 photometrically corrected OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) spectra that cover 10% of the surface; it focuses on regions of interest, including the Nightingale site (Hokioi crater), where OSIRIS-REx collected its sample
The same least-squares comparison method was previously used to identify carbonate-rich spots on Bennu by identifying 3.4 μm features with a doublet spectral shape and band positions indicative of carbonates rather than organics; we find that least-squares fits with χ2 values
We find that carbonaceous chondrite insoluble organic matter (IOM) is the closest laboratory analog to Bennu’s organics based on the available spectral data at the wavelengths covered by the OVIRS instrument

Summary

Introduction

These findings, based on the presence of an infrared absorption near 3.4 μm, indicate that the sample of Bennu’s regolith that the OSIRIS-REx spacecraft will return to Earth in 2023 (Lauretta et al 2021, 2017) is likely to contain carbonbearing material. The soluble organic matter (SOM) represents a much smaller fraction (up to 0.1%) of the total carbon, with the remainder of carbon either unaccounted for with current techniques or contained in inorganic forms (e.g., carbonates and nanodiamonds). Meteoritic SOM can be highly complex, with a variety of different compound classes, such as amino acids, carboxylic acids, hydroxy acids, amines, alcohols, aldehydes, ketones, N-heterocycles, polyols, aliphatic and aromatic hydrocarbons, and sugars (e.g., Glavin et al 2018)

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