Research Article

Bright carbonate veins on asteroid (101955) Bennu: Implications for aqueous alteration history

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Science  06 Nov 2020:
Vol. 370, Issue 6517, eabc3557
DOI: 10.1126/science.abc3557

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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

Carbonaceous asteroids formed early in Solar System history and experienced varying degrees of aqueous (water-rock) and thermal alteration. Most models of the evolution of these asteroids suggest that aqueous alteration was driven by hydrothermal convection. However, it is debated whether this alteration occurred in a chemically closed or open system. The bulk chemical compositions of the carbonaceous chondrite meteorites imply that the system was closed. Models predict that large-scale fluid flow in an open system took place on at least some asteroids. In this scenario, fluids would have flowed through fractures from the interior, and minerals would have precipitated into these fractures, forming veins.

RATIONALE

Global spectral observations by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft have shown that carbon-bearing materials, including organics and/or carbonates, are widespread on the surface of near-Earth asteroid (101955) Bennu. To understand the composition of these carbon-bearing materials and their implications for Bennu’s aqueous alteration history, we examined visible–near-infrared spectra obtained at resolutions of ~4 m in the region around the mission’s primary sampling site, Nightingale. We compared these spectra with laboratory spectra of organics and carbonates. Broadband panchromatic images of the Nightingale region at pixel scales of ~1.4 cm allowed us to search for surface features that could contextualize the spectral data.

RESULTS

On the basis of the wavelength position and shape of spectral absorption features, the closest matches to a subset (~15%) of spectra collected in the Nightingale region are the carbonate minerals calcite, dolomite, and magnesite, with calcite being the most prevalent. These minerals are found in the highly aqueously altered CM- and CI-group carbonaceous chondrites, proposed meteorite analogs of Bennu.

Bright, centimeters-thick, roughly meter-long veins are apparent in images of some boulders near the Nightingale site. The veins have albedos of 10% to at least 19%, much brighter than Bennu’s average of 4.4%. These albedos are consistent with the veins being filled by carbonates with minor abundances of opaque materials such as magnetite and organics. The albedos are too high for the veins to be dominantly composed of organics or any other material that has been identified on Bennu, except carbonates.

There are only a few bright vein-filling materials found in meteorites, and carbonate is the only one of these that is plausibly present on Bennu. Linear mixture modeling suggests that only a small fraction of a spectrometer field of view (<1%) needs to contain carbonate to explain the observed spectral signatures, which is consistent with the observed extent of the veins.

CONCLUSION

The observed bright veins on Bennu are most likely dominantly composed of carbonates. The detection of these meter-sized, putative carbonate veins indicates that extensive water flow occurred in a chemically open hydrothermal system on the larger parent asteroid of Bennu, before the catastrophic disruption of the parent asteroid led to Bennu’s reaccumulation as a rubble pile. Kinetic modeling assuming a calcite vein composition shows that a hydrothermal system kilometers in size, with alteration likely taking place over thousands to millions of years, would have been needed to create veins of the dimensions preserved in the constituent boulders of Bennu.

Some CM- and CI-group meteorites have very small (micrometers to millimeters) carbonate veins or presumed vein fragments; veins on the scale that we observe on Bennu are not present in meteorites. It is possible that such veins are present on other carbonaceous asteroids that have not been observed by spacecraft. We predict that the sample of Bennu’s surface material that is planned to be returned to Earth by the OSIRIS-REx spacecraft will contain carbonates and that they will be distributed with a structure and scale distinct from those that occur in meteorites.

OSIRIS-REx panchromatic image (~1.4 cm pixel–1) showing a boulder with a bright vein on Bennu.

This linear feature is more than twice as bright as the surrounding rock. Its most plausible composition is carbonate, suggesting that a large, long-lived hydrothermal system operated on Bennu’s parent asteroid. The image was collected by the PolyCam imager on 26 October 2019 at 61.60°N, 50.57°E.

Abstract

The composition of asteroids and their connection to meteorites provide insight into geologic processes that occurred in the early Solar System. We present spectra of the Nightingale crater region on near-Earth asteroid Bennu with a distinct infrared absorption around 3.4 micrometers. Corresponding images of boulders show centimeters-thick, roughly meter-long bright veins. We interpret the veins as being composed of carbonates, similar to those found in aqueously altered carbonaceous chondrite meteorites. If the veins on Bennu are carbonates, fluid flow and hydrothermal deposition on Bennu’s parent body would have occurred on kilometer scales for thousands to millions of years. This suggests large-scale, open-system hydrothermal alteration of carbonaceous asteroids in the early Solar System.

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