Research Article

Widespread carbon-bearing materials on near-Earth asteroid (101955) Bennu

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Science  06 Nov 2020:
Vol. 370, Issue 6517, eabc3522
DOI: 10.1126/science.abc3522

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The complex history of Bennu's surface

The near-Earth asteroid (101955) Bennu is a carbon-rich body with a rubble pile structure, formed from debris ejected by an impact on a larger parent asteroid. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft is designed to collect a sample of Bennu's surface and return it to Earth. After arriving at Bennu, OSIRIS-REx performed a detailed survey of the asteroid and reconnaissance of potential sites for sample collection. Three papers present results from those mission phases. DellaGiustina et al. mapped the optical color and albedo of Bennu's surface and established how they relate to boulders and impact craters, finding complex evolution caused by space weathering processes. Simon et al. analyzed near-infrared spectra, finding evidence for organic and carbonate materials that are widely distributed across the surface but are most concentrated on individual boulders. Kaplan et al. examined more detailed data collected on the primary sample site, called Nightingale. They identified bright veins with a distinct infrared spectrum in some boulders, which they interpreted as being carbonates formed by aqueous alteration on the parent asteroid. Together, these results constrain Bennu's evolution and provide context for the sample collected in October 2020.

Science, this issue p. eabc3660, p. eabc3522, p. eabc3557

Structured Abstract

INTRODUCTION

Owing to their low reflectance and spectral similarity to primitive carbonaceous chondrite meteorites, C-complex asteroids are thought to contain carbon-bearing material. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft is designed to return a sample of carbonaceous material from the near-Earth C-complex asteroid (101955) Bennu. The selection of a suitable sample site necessitated global mapping and characterization of Bennu’s surface. Spatially resolved spectral mapping can determine the surface properties and composition of Bennu. It also provides context for both the sample that will be returned and the interpretation of unresolved observations of other dark asteroids.

RATIONALE

We used data acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS), a point spectrometer that covers the wavelength range from 0.4 to 4.3 μm, to map the physical and compositional characteristics of Bennu’s surface. These data allow us to search for spectral signatures of carbon bearing materials on Bennu. The 3.4-μm region is sensitive to carbonate or organic materials, which produce absorption bands at this wavelength because of either CO32– stretching and vibration or C-H stretching. OVIRS mapping provided global coverage of Bennu at ~600-m2 areal resolution at several local solar times. Using the data with the highest solar illumination (~9° phase, 12:30 p.m. local solar time), we mapped the depth of the 3.4-μm absorption band, peak temperature, 0.55-μm brightness, spectral slope from 0.5 to 1.5 μm, and the distribution of the 2.74-μm absorption band of hydrated minerals, which was previously detected in unresolved observations.

RESULTS

The 3.4-μm absorption band, indicative of carbon-bearing materials, is detected over all of Bennu’s surface with band depths of a few percent. The band shape varies with surface location and spans the range of 3.4-μm band shapes seen on other dark C-complex asteroids. The differing band shapes persist at higher areal resolution (60 m2) and at several phase angles. The spectra collected at 60 m2 show that the deepest bands occur over distinct boulders.

The distribution of the 3.4-μm band on Bennu’s surface does not correlate with the distributions of temperature, brightness, spectral slope, or the 2.74-μm absorption band, although some of these features correlate weakly with each other. At low phase angles, the darkest areas (~3% reflectance at 0.55 μm) are correlated with the hottest surface temperatures (~350 K), with a Spearman’s rank correlation coefficient, r, of 0.65.

The absorption feature at 2.74 μm, indicative of hydrated phyllosilicates, is globally present, with band depths of 12 to 17% that correlate with surface temperature and latitude (|r| = 0.76 and 0.58, respectively). When the temperature trend is removed, the correlation of hydrated phyllosilicates with latitude is weaker (|r| = 0.48). In OVIRS data, Bennu’s global surface has an overall blue (negative) spectral slope from 0.5 to 1.5 μm, with some boulders and craters that are redder (less negative) than average, consistent with results from multispectral imaging. Some of the darkest material is spectrally blue, whereas some is spectrally red, indicating local differences in composition, space weathering, and/or particle size.

CONCLUSION

The variation in the shape of the 3.4-μm band indicates a mix of organics and carbonates on Bennu’s surface, likely inherited from the collisional disruption of its parent asteroid. To retain a widespread 3.4-µm organic feature, most of the material on Bennu’s surface could not have been exposed to the space environment for more than a few million years. The samples returned to Earth by the OSIRIS-REx spacecraft should contain ample amounts of these materials, regardless of sampling location. Variable 3.4-μm band depths over individual boulders may be due to compositional differences or to exposure of fresh material by means of thermally driven fracturing.

Spectral variations on Bennu’s 60°E hemisphere.

(A) Visible to near-infrared (0.5 to 1.5 μm) slope. Blue denotes more steeply negative slopes (decreasing brightness with increasing wavelength); red denotes shallower slopes. (B) Band area at 3.4 μm, indicative of carbon-bearing materials. Blue indicates smaller band areas; red, larger band areas. (C) Band depth at 2.74 μm, indicative of hydrated phyllosilicates. White indicates shallower bands; blue, deeper bands.

Abstract

Asteroid (101955) Bennu is a dark asteroid on an Earth-crossing orbit that is thought to have assembled from the fragments of an ancient collision. We use spatially resolved visible and near-infrared spectra of Bennu to investigate its surface properties and composition. In addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micrometer absorption feature, which we attribute to a mix of organic and carbonate materials. The shape and depth of this absorption feature vary across Bennu’s surface, spanning the range seen among similar main-belt asteroids. The distribution of the absorption feature does not correlate with temperature, reflectance, spectral slope, or hydrated minerals, although some of those characteristics correlate with each other. The deepest 3.4-micrometer absorptions occur on individual boulders. The variations may be due to differences in abundance, recent exposure, or space weathering.

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