^1,2D.N.DellaGiustina et al. (>10)
Science 370, eabc3660 Link to Article [DOI: 10.1126/science.abc3660]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
²Department of Geosciences, University of Arizona, Tucson, AZ, USA.
Reprinted with permission from AAAS

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.

^1,2H.H.Kaplan et al. (>10)
Science 370, eabc3557 Link to Article [DOI: 10.1126/science.abc3557]
¹NASA Goddard Space Flight Center, Greenbelt, MD, USA.
²Southwest Research Institute, Boulder, CO, USA.
Reprinted with permission from AAAS

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.

¹Amy A. Simon et al. (>10)
Science 370, eabc3522 Link to Article [DOI: 10.1126/science.abc3522]
¹Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
Reprinted with Permission from AAAS

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.

¹Tamilarasan Subramani,¹Kristina Lilova,¹Mykola Abramchuk,¹Kurt D. Leinenweber,¹Alexandra Navrotsky
Proceedings of the National Academy of Sciences of the United States of America Link to Article [DOI:
https://doi.org/10.1073/pnas.2017312117]
¹School of Molecular Sciences and Center for Materials of the Universe, Arizona State University, Tempe, AZ 85281

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show that greigite is a stable phase in the Fe–S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe3S4 in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field.