4 research outputs found
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Global geologic map of asteroid (101955) Bennu indicates heterogeneous resurfacing in the past 500,000 years
Global geologic maps are useful tools for efficient interpretation of a planetary body, and they provide global context for the diversity and evolution of the surface. We used data acquired by the OSIRIS-REx spacecraft to create the first global geologic map of the near-Earth asteroid (101955) Bennu. As this is the first geologic map of a small, non-spherical, rubble-pile asteroid, we discuss the distinctive mapping challenges and best practices that may be useful for future exploration of similar asteroids, such as those to be visited with the Hera and Janus missions. By mapping on two centimeter-scale global image mosaics (2D projected space) and a centimeter-scale global shape model (3D space), we generated three input maps respectively describing Bennu's shape features, geologic features, and surface texture. Based on these input maps, we defined two geologic units: the Smooth Unit and the Rugged Unit. The units are differentiated primarily on the basis of surface texture, concentrations of boulders, and the distributions of lineaments, mass movement features, and craters. They are bounded by several scarps. The Rugged Unit contains abundant boulders and signs of recent mass movement. It also has fewer small (<20âŻm), putatively fresh craters than the Smooth Unit, suggesting that such craters have been erased in the former. Based on these geologic indicators, we infer that the Rugged Unit has the younger surface of the two. Differential crater size-frequency distributions and the distribution of the freshest craters suggest that both unit surfaces formed ~10â65 million years ago, when Bennu was located in the Main Asteroid Belt, and the Smooth Unit has not been significantly resurfaced in the past 2 million years. Meanwhile, the Rugged Unit has experienced resurfacing within the past ~500,000âŻyears during Bennu's lifetime as a near-Earth asteroid. The geologic units are consistent with global diversity in slope, surface roughness, normal albedo, and thermal emission spectral characteristics. The site on Bennu where the OSIRIS-REx mission collected a regolith sample is located in the Smooth Unit, in a small crater nested within a larger one. So although the Smooth Unit is an older surface than the Rugged Unit, the impact-crater setting indicates that the material sampled was recently exposed. Several similarities are apparent between Bennu and asteroid (162173) Ryugu from a global geologic perspective, including two geologic units distinguishable by variations in the number density of boulders, as well as in other datasets such as brightness
The Role of Hydrated Minerals and Space Weathering Products in the Bluing of Carbonaceous Asteroids
International audienceThe surfaces of airless bodies such as lunar and S-type asteroids typically become spectrally redder in visible to near-infrared reflectance with longer exposures to space weathering. However, some carbonaceous asteroids instead become spectrally bluer. Space weathering experiments on carbonaceous meteorites have provided some clues as to the space weathering products that could produce spectral bluing. We applied these experimental results to our Hapke radiative transfer model, with which we modeled spectral data from the OSIRIS-REx mission in order to determine whether these space weathering productsâspecifically, nanophase and microphase metallic iron, troilite, and magnetiteâcould explain the globally blue spectrum of the carbonaceous asteroid (101955) Bennu. The model suggests that the surface of Bennu has microphase iron, nanophase magnetite, and nanophase and microphase troilite. Considering previous space weathering experiments together with our spectral modeling of Bennu, we posit that the presence of nanophase magnetite is what causes a carbonaceous asteroid to become spectrally bluer with exposure time. Nanophase magnetite can form on asteroids that have Fe-bearing hydrated minerals (phyllosilicates). On anhydrous carbonaceous asteroids, nanophase iron forms instead of magnetite, leading to spectral reddening. We therefore predict that samples returned by the OSIRIS-REx mission from Bennu will have more nanophase magnetite than nanophase iron with nanophase and microphase sulfides, whereas samples returned by the Hayabusa2 mission from the carbonaceous asteroid (162173) Ryugu, which is spectrally red, will contain nanophase and microphase sulfides as well as more nanophase iron than nanophase magnetite
Asteroid (16) Psycheâs primordial shape: A possible Jacobi ellipsoid
International audienceWe found mini-craters on Bennu's boulders. We measure their sizes. We then use scaling laws to derive the strength and collisional lifetimes of C-type objects
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OSIRIS-APEX: An OSIRIS-REx Extended Mission to Asteroid Apophis
The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft mission characterized and collected a sample from asteroid (101955) Bennu. After the OSIRIS-REx Sample Return Capsule released to Earthâs surface in 2023 September, the spacecraft diverted into a new orbit that encounters asteroid (99942) Apophis in 2029, enabling a second mission with the same unique capabilities: OSIRIS-Apophis Explorer (APEX). On 2029 April 13, the 340 m diameter Apophis will draw within âŒ32,000 km of Earthâs surface, less than 1/10 the lunar distance. Apophis will be the largest object to approach Earth this closely in recorded history. This rare planetary encounter will alter Apophisâs orbit, will subject it to tidal forces that change its spin state, and may seismically disturb its surface. APEX will distantly observe Apophis during the Earth encounter and capture its evolution in real time, revealing the consequences of an asteroid undergoing tidal disturbance by a major planet. Beginning in 2029 July, the spacecraftâs instrument suite will begin providing high-resolution data of this âstonyâ asteroidâadvancing knowledge of these objects and their connection to meteorites. Near the missionâs end, APEX will use its thrusters to excavate regolith, a technique demonstrated at Bennu. Observations before, during, and after excavation will provide insight into the subsurface and material properties of stony asteroids. Furthermore, Apophisâs material and structure have critical implications for planetary defense. © 2023. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]