Volume 650, June 2021
|Number of page(s)||13|
|Section||Planets and planetary systems|
|Published online||16 June 2021|
Evidence for limited compositional and particle size variation on asteroid (101955) Bennu from thermal infrared spectroscopy
Southwest Research Institute,
2 Arizona State University, Tempe, AZ, USA
3 NASA Goddard Space Flight Center, Greenbelt, MD, USA
4 Stony Brook University, Stony Brook, NY, USA
5 Lunar and Planetary Institute, Houston, TX, USA
6 Smithsonian Institution, National Museum of Natural History, Washington, DC, USA
7 Institut d’Astrophysique Spatiale, CNRS/Université Paris Sud, Orsay, France
8 University of Texas, Austin, TX, USA
9 INAF-Astrophysical Observatory of Arcetri, Firenze, Italy
10 Ithaca College, Ithaca, NY, USA
11 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
Accepted: 31 March 2021
Context. Asteroid (101955) Bennu is the target of NASA’s Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission. The spacecraft’s instruments have characterized Bennu at global and local scales to select a sampling site and provide context for the sample that will be returned to Earth. These observations include thermal infrared spectral characterization by the OSIRIS-REx Thermal Emission Spectrometer (OTES).
Aims. To understand the degree of compositional and particle size variation on Bennu, and thereby predict the nature of the returned sample, we studied OTES spectra, which are diagnostic of these properties.
Methods. We created and mapped spectral indices and compared them with the distribution of geomorphic features. Comparison to laboratory spectra of aqueously altered carbonaceous chondrites constrains the amount of compositional variability that is observable.
Results. The OTES spectra exhibit two end-member shapes (or types), and compositional variability appears limited at the spatial resolution of the observations. The global distribution of these spectral types corresponds with the locations of regions composed of (i) large, dark, relatively rough boulders and (ii) relatively smooth regions lacking large boulders.
Conclusions. The two spectral types appear to be diagnostic primarily of particle size variations, with contributions from other properties. The spectra resemble experimental data of solid substrates with very thin accumulations (a few to tens of microns) of fine particles (<~65–100 μm). The dustier surfaces commonly correspond with rougher rocks that may produce and/or act as traps for the particles. Anhydrous silicates are limited in abundance, and the bulk mineralogy is consistent with the most aqueously altered carbonaceous chondrites. We expect the returned samples to include these physical and mineralogical characteristics.
Key words: minor planets, asteroids: individual: (101955) Bennu / methods: data analysis / techniques: spectroscopic
© ESO 2021
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