The complex history of the Bennu surface
The asteroid near Earth (101955) Bennu is a carbon-rich object with a debris pile structure, formed from debris thrown out by collisions with the larger parent asteroid. The Origin spacecraft, Spectral Interpretation, Resource Identification, Privacy, Regolith Explorer (OSIRIS-REx) is designed to collect a surface sample of Bennu and bring it back to Earth. After arriving in Bennu, OSIRIS-REx conducted a detailed survey of the asteroid and scouted potential sites to collect samples. Three papers present results from those mission phases. DellaGiustina et al. Optical and albedo color mapping of the Bennu surface and establishes how they relate to boulders and craters, finding the complex evolution caused by space weathering processes. Simon et al. near infrared spectroscopy was analyzed, finding evidence that organic materials and carbonates are widely distributed on the surface but are most concentrated on individual rocks. Kaplan et al. examined more detailed data collected on the main sample website, called Nightingale. They identified bright patterns with separate infrared spectra in a number of rocks, which they understood were carbonate salts formed by changing water in the parent asteroid. Together, these results limit Bennu̵7;s evolution and provide context for the sample collected in October 2020.
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Due to their low reflectivity and spectra similar to the primary carbon-containing chondrite meteorites, C-complex asteroids are thought to contain carbon-bearing material. The OSIRIS-REx spacecraft (Origin, Spectral Interpretation, Resource Identification and Security – Regolith Explorer) is designed to return a sample of carbon material from a complex asteroid near Earth C (101955) Bennu. Choosing an appropriate sampling site requires global mapping and surface characterization of Bennu. A spatially resolved spectral map can determine Bennu’s surface properties and composition. It also provides context for both the sample to be returned and the interpretation of unanswered observations of other dark asteroids.
We used the data obtained by the OSIRIS-REx Infrared and Visible Spectrometer (OVIRS), a point spectrometer covering the wavelength range of 0.4 to 4.3 μm, to map the Physical characteristics and composition of the Bennu surface. These data allow us to look for spectral markers of carbon-bearing materials on Bennu. The 3.4 μm region is sensitive to carbonates or organic materials, giving rise to absorption bands at this wavelength because of CO32- stretch and vibrate or CH stretch. OVIRS map provides Bennu’s global coverage at ~ 600-m2 resolution areal at some local solar moment. Using data with highest sunlight (phase ~ 9 °, 12:30 pm local solar time), we mapped the depth of absorption band 3.4 μm, peak temperature, degree light 0.55 μm, a spectral slope of 0.5 to 1.5 μm, and the distribution of the absorption band of 2.74 μm of aqueous minerals, have been previously detected in no observations. answered.
An absorption band of 3.4 μm, a sign of carbon-containing material, was detected over the entire surface of Bennu with a band depth of several percent. The band shape changes with surface position and spans the range of the 3.4 μm band shape found on other dark C complex asteroids. Different strip shapes persist at a higher resolution (60 m2) and at some phase angles. Spectra were obtained at 60 m2 shows that the deepest bands occur on different rocks.
The distribution of the 3.4 μm band on the surface of Bennu is not correlated with that of temperature, brightness, spectral slope, or absorption band of 2.74 μm, although some of these characteristics are correlated. weak together. At low phase angles, the darkest regions (reflectance ~ 3% at 0.55 μm) correlate with the hottest surface temperatures (~ 350 K), with the Spearman’s degree correlation coefficient, r, of the 0.65.
Absorbance at 2.74 μm, an indication of aqueous phyllosilicate, is globally present, with a range of 12 to 17% relative depth with surface temperature and latitude (|r| = 0.76 and 0.58 respectively). When temperature trends are removed, the correlation of hydrated phyllosilicates with latitude becomes weaker (|r| = 0.48). In the OVIRS data, the global surface of Bennu has an overall blue (negative) spectral slope of 0.5 to 1.5 μm, with some rocks and craters being redder (less negative). ) relative to the average, consistent with results from the multi-lens image. Some of the darkest matter have a spectrum of blue, while some have a spectrum of red, which shows local differences in composition, space weather and / or particle size.
The variation in the shape of the 3.4 μm band indicates a blend of organic matter and carbonates on the surface of Bennu, which can be inherited from the disruption caused by the impact of its parent asteroid. In order to retain the organic properties of 3.4 m, most of the materials on Bennu’s surface have not been exposed to the space environment for more than several million years. Samples returned to Earth by the OSIRIS-REx spacecraft must contain a large amount of these materials, regardless of the sampling location. Variable 3.4 μm band depths on individual rocks can be attributed to differences in composition or from exposure to new materials by thermal fracture.
Asteroid (101955) Bennu is a dark asteroid in trans-Earth orbit that is believed to have been assembled from debris from an ancient impact. We use Bennu’s space-resolution visible and near-infrared spectrum to examine its surface properties and composition. In addition to the hydrated phyllosilicate strip we found a universal 3.4 micrometer absorption, which we consider to be a mixture of organic materials and carbonates. The shape and depth of this absorption feature varies across the Bennu surface, spanning the range seen between similar main-belt asteroids. The distribution of an absorption characteristic is not correlated with temperature, reflectivity, spectral slope, or hydrated minerals, although some of these characteristics are correlated. The deepest absorption 3.4 micrometres occurs on individual rocks. Variations can be due to differences in abundance, recent exposure, or space weather.