Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving ...landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records.
Two of the instruments onboard the OSIRIS-REx spacecraft, the MapCam color imager and the OVIRS visible and 20 infrared spectrometer, observed the surface of asteroid (101955) Bennu in partially ...overlapping wavelengths. 21 Significant scientific advances have been enabled by using data from these two instruments in tandem, but a robust 22 statistical understanding of their relationship is needed for future analyses to cross-compare their data as accurately 23 and sensitively as possible. Here we present a cross-instrument comparison of data acquired by MapCam and 24 OVIRS, including methods and results for all global and site-specific observation campaigns in which both 25 instruments were active. In our analysis, we consider both the absolute radiometric offset and the relative 26 (normalized) variation between the two instruments; we find that both depend strongly on the photometric and 27 instrumental conditions during the observation. The two instruments have a large absolute offset (>15%) due to their 28 independent radiometric calibrations. However, they are very consistent (relative offset as low as 1%) when each 29 instrument’s response is normalized at a single wavelength, particularly at low phase angles where shadows on 30 Bennu’s rough surface are minimized. We recommend using the global datasets acquired at 12:30 pm local solar 31 time for cross-comparisons; data acquired at higher phase angles have larger uncertainties.
The OSIRIS-REx Camera Suite (OCAMS) onboard the OSIRIS-REx spacecraft is used to study the shape and surface of the mission’s target, asteroid (101955) Bennu, in support of the selection of a ...sampling site. We present calibration methods and results for the three OCAMS cameras—MapCam, PolyCam, and SamCam—using data from pre-flight and in-flight calibration campaigns. Pre-flight calibrations established a baseline for a variety of camera properties, including bias and dark behavior, flat fields, stray light, and radiometric calibration. In-flight activities updated these calibrations where possible, allowing us to confidently measure Bennu’s surface. Accurate calibration is critical not only for establishing a global understanding of Bennu, but also for enabling analyses of potential sampling locations and for providing scientific context for the returned sample.
Abstract Rubble pile asteroids are widely understood to be composed of reaccumulated debris following a catastrophic collision between asteroids in the main asteroid belt, where each disruption can ...make a family of new asteroids. Near-Earth asteroids Ryugu and Bennu have been linked to collisional families in the main asteroid belt, but surface age analyses of each asteroid suggest these bodies are substantially younger than their putative families. Here we show, through a coupled collisional and dynamical evolution of members of these families, that neither asteroid was likely to have been created at the same time as the original family breakups, but rather are likely remnants of later disruptions of original family members, making them second, or later, generation remnants. Our model finds about 80% and 60% of asteroids currently being delivered to near-Earth orbits from the respective families of New Polana and Eulalia are second or later generation. These asteroids delivered today in the 0.5-1 km size range have median ages since their last disruption that are substantially younger than the family age, reconciling their measured crater retention ages with membership in these families.
In early 2019, NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission surveyed asteroid (101955) Bennu with a collection of instruments, ...including the OSIRIS-REx Camera Suite (OCAMS) PolyCam imager. Using PolyCam panchromatic images, we constructed a globally controlled basemap of Bennu at an approximate ground sample distance of 5 cm with a mean spatial accuracy of ~30 cm. The basemap was photometrically normalized using a Minneart phase angle correction. New mapping methods were developed to combine images of Bennu's irregular shape and extremely rough surface into a nearly seamless mosaic. Here we present the global basemap of Bennu and discuss the image processing techniques used to construct a high-resolution mosaic of an irregular small body.
•We produced a global basemap of asteroid Bennu at a ground sample distance of 5 cm.•New methods were developed to deal with Bennu's rough surface and irregular shape.•Images were rectified to a 3D digital terrain model with a mean facet size of 80 cm.•Images were photometrically normalized to 30° phase angle using a Minneart model.•Overlaps were masked along natural feature boundaries for a nearly seamless result.
ABSTRACT
The mutual gravitational interaction of binary asteroids, which make up approximately 15 per cent of the near-Earth asteroid (NEA) population, provides a continuous tidal force, creating ...ground motion. We explore the potential of kilometre-sized binary asteroids as targets for seismological studies of their interior structure. We use a numerical model wherein each body is constructed of discrete particles interacting via gravity and contact forces. The system's orbital properties are modelled based on those of typical binary NEAs: a secondary body orbits a primary body at a distance of a few to 10 primary radii, resulting in orbital periods of a few tens of hours. We varied the elastic moduli (stiffness) of the constituent particles and measured a strain of a few micrometres caused by the orbiting satellite. Over eight orbital periods, the acceleration of the strain vector along the primary body's equatorial axis indicates that tidally induced ground motion generated by a binary asteroid system is detectable by modern seismometers, like the instruments deployed on the InSight mission to Mars. Owing to the relatively short orbital period of the satellite – a mean of 25.8 h for known binary NEAs – only a modest mission lifetime would be required for a seismometer to adequately characterize an asteroid's interior through tidally induced deformation. Future deployment of seismometers on binary asteroids will allow for a detailed characterization of the structure of these objects.
The exploration of near‐Earth asteroids has revealed dynamic surfaces characterized by mobile, unconsolidated material that responds to local geophysical gradients, resulting in distinct morphologies ...and boulder distributions. The OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission confirmed that asteroid (101955) Bennu is a rubble pile with an unconsolidated surface dominated by boulders. In this work, we documented morphologies indicative of mass movement on Bennu and assessed the relationship to slope and other geologic features on the surface. We found globally distributed morphologic evidence of mass movement on Bennu up to ~70° latitude and on spatial scales ranging from individual boulders (meter scale) to a single debris flow ~100 m long and several meters thick. The apparent direction of mass movement is consistent with the local downslope direction and dominantly moves from the midlatitudes toward the equator. Mass movement appears to have altered the surface expression of large (≥30m diameter) boulders, excavating them in the midlatitudes and burying them in the equatorial region. Up to a 10 ± 1 m depth of material may have been transported away from the midlatitudes, which would have deposited a layer ~5 ± 1 m thick in the equatorial region assuming a stagnated flow model. This mass movement could explain the observed paucity of small (<50‐m diameter) craters and may have contributed material to Bennu's equatorial ridge. Models of changes in slope suggest that the midlatitude mass movement occurred in the past several hundred thousand years in regions that became steeper by several degrees.
Plain Language Summary
Mass movement is the flow of loose material such as rock fragments across the surface of a planetary body (for instance, a landslide). We searched images of the surface of asteroid (101955) Bennu for evidence of mass movement. We found that rocks of various sizes have moved downslope, and evidence of this movement is apparent at most locations on the asteroid. By measuring the distribution of, and surface elevation around, the largest boulders on the surface of Bennu, we also found that the downslope movement of material appears to have excavated large boulders from the subsurface in the midlatitudes and buried large boulders near the equator. Our observation that material on Bennu has moved in what is currently the local downslope direction is not necessarily expected, because the downslope direction can change based on how quickly the asteroid is rotating, which varies over time. Thus, we can infer that this movement happened in the geologically recent past—probably within the past several hundred thousand years. These results can help us understand how geologic features like craters are erased, how the equatorial ridge formed, and how Bennu (and potentially other asteroids) change shape over time.
Key Points
Signatures of mass movement on Bennu are globally distributed at multiple spatial scales
Mass movement may have removed a ~10‐m‐thick layer of material from the midlatitudes and deposited a ~5‐m‐thick layer near the equator
Mass movement that left visible evidence on Bennu occurred within the past several hundred thousand years
•We present a summary of our spectral characterization of inner-belt primitive families.•Two compositional groups are found: Polana-like and Erigone-like.•We analyze visible spectra of Bennu ...(OSIRIS-REx) and Ryugu (Hayabusa2).•Expected color variations for Bennu are computed from the families’ visible spectra.
NASA's OSIRIS-REx and JAXA's Hayabusa2 sample-return missions are currently on their way to encounter primitive near-Earth asteroids (101955) Bennu and (162173) Ryugu, respectively. Spectral and dynamical evidence indicates that these near-Earth asteroids originated in the inner part of the main belt. There are several primitive collisional families in this region, and both these asteroids are most likely to have originated in the Polana–Eulalia family complex. We present the expected spectral characteristics of both targets based on our studies of four primitive collisional families in the inner belt: Polana–Eulalia, Erigone, Sulamitis, and Clarissa. Observations were obtained in the framework of our PRIMitive Asteroids Spectroscopic Survey (PRIMASS). Our results are especially relevant to the planning and interpretation of in situ images and spectra to be obtained by the two spacecraft during the encounters with their targets.
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