Research Article

Variations in color and reflectance on the surface of asteroid (101955) Bennu

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Science  06 Nov 2020:
Vol. 370, Issue 6517, eabc3660
DOI: 10.1126/science.abc3660

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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

The color and reflectance of asteroids can be used to infer their compositions and histories. Variations in these spectrophotometric properties are driven by differences in lithology and/or exposure to processes collectively known as space weathering (bombardment by meteoroids and solar wind ions). On anhydrous bodies, such as the Moon and S-type asteroids, space weathering darkens and reddens spectral slopes (where “redder” indicates a more positive slope relative to the solar spectrum) in the visible wavelengths. However, on primitive C-complex asteroids—bodies that may have delivered water and organics to early Earth—the spectral changes that result from space weathering are not well understood. Evidence from meteorites thought to be analogous to C-complex asteroids suggests that either reddening or bluing is possible. Deciphering such changes is necessary to understand the origin and relative exposure age of surface units on primitive Solar System objects.

RATIONALE

The MapCam imager on the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft acquired global multispectral images of the C-complex asteroid (101955) Bennu in four bands: b′ (0.44 to 0.50 μm), v (0.52 to 0.58 μm), w (0.67 to 0.73 μm), and x (0.82 to 0.89 μm). Bennu is a rubble pile, made up of the reaccumulated fragments of a larger C-complex progenitor that was blown apart by a catastrophic impact.

Using band ratios and principal components analysis, we mapped Bennu’s color and reflectance at a pixel scale of ~25 cm. In combination with higher-resolution (~5 cm per pixel) panchromatic PolyCam images, we assessed relationships between MapCam spectra and morphologic features on Bennu’s surface, aiming to understand sources of variation from the average and determine the relative timing associated with color differences.

RESULTS

The surface of Bennu has unexpectedly heterogeneous colors distributed on a moderately blue (gently negatively sloped) global surface. Boulders are the dominant source of heterogeneity and fall into distinct populations on the basis of reflectance. Dark boulders (reflectance of 0.034 to 0.049, encompassing Bennu’s average reflectance of 0.044) tend to be rougher and rounder, whereas bright boulders (0.049 to 0.074) are smoother and more angular. Variation in color within individual boulders is also apparent; for example, boulder faces that appear to be more recently exposed owing to fracturing are bluer than putatively older faces. Conversely, small reddish craters are observed to overlie blue craters, indicating that the former are younger (more recently exposed material). Bennu’s smallest craters have a size distribution that indicates that they are also the youngest, and they are redder than the average surface. Many of the larger (older) craters have colors indistinguishable from Bennu’s average. Crater spectral slopes indicate that terrains with intermediate ages have the bluest near-ultraviolet spectral slopes.

CONCLUSION

The differences in reflectance and texture among boulders indicate that Bennu may have inherited distinct lithologies formed at different depths in its larger progenitor asteroid, as well as debris from impactors. The color variations within boulders and among craters suggest that space weathering on Bennu does not drive a unidirectional progression from red to blue (or vice versa). Rather, freshly exposed redder surfaces, as exemplified by the small reddish craters, initially brighten in the near-ultraviolet region (i.e., become blue at shorter wavelengths), as exemplified by blue crater rims and fractured boulder faces. Brightening in the visible to near-infrared wavelengths follows, ultimately leading to more moderately blue spectral slopes, consistent with Bennu’s average. The time scale associated with space weathering–induced color changes (~105 years) is compatible with previous findings only if Bennu’s small reddish craters formed under conditions in which gravity, rather than the strength of the impacted surface, is the dominant influence. This finding offers an indication of cratering physics on small rubble-pile asteroids.

False-color image mosaic of rubble on asteroid Bennu, as observed with the MapCam multispectral imager.

Color and reflectance vary between and within boulders, resulting from different exposure ages and innate compositions. Red is the x/v band ratio (indicating redder spectral slopes), green represents relative change in the w band (an indication of composition), and blue is the b′/v band ratio (indicating bluer spectral slopes in the near-ultraviolet region).

Abstract

Visible-wavelength color and reflectance provide information about the geologic history of planetary surfaces. Here we present multispectral images (0.44 to 0.89 micrometers) of near-Earth asteroid (101955) Bennu. The surface has variable colors overlain on a moderately blue global terrain. Two primary boulder types are distinguishable by their reflectance and texture. Space weathering of Bennu surface materials does not simply progress from red to blue (or vice versa). Instead, freshly exposed, redder surfaces initially brighten in the near-ultraviolet region (i.e., become bluer at shorter wavelengths), then brighten in the visible to near-infrared region, leading to Bennu’s moderately blue average color. Craters indicate that the time scale of these color changes is ~105 years. We attribute the reflectance and color variation to a combination of primordial heterogeneity and varying exposure ages.

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