Dathon R. Golish

IntroductionSamples returned from asteroids Ryugu and Bennu exhibit mineralogical and isotopic similarities to CI carbonaceous chondrites [1–3]. Although CIs are among the leading candidates for delivering the building blocks of life to terrestrial planets [4], their distribution in the solar system remains poorly understood. This is largely due to the featureless nature of primitive asteroids, as well as terrestrial weathering in meteorites. With the availability of pristine spectra from the returned samples, we are now better positioned to identify CI-like asteroids among the large population of primitive asteroids observed in the past decades.However, remote sensing of Ryugu and Bennu have revealed that space weathering effects may be more complex on primitive asteroids than previously assumed, and its mechanism needs to be better understood to infer the bulk asteroid composition from telescopic data. Despite their compositional and physical similarities, Ryugu and Bennu exhibit differences in their visible spectra. More intriguingly, their visible spectra likely evolved in opposite directions. On Ryugu, fresher craters are brighter and bluer [5, 6], whereas on Bennu, fresher craters are darker and redder [7]. Notably, the spectra of the freshest materials on both asteroids are similar, suggesting a similar initial state. These observations imply that compositionally similar asteroids can undergo divergent spectral evolution, leading to differences in their global spectra [8].In this study, we investigate the causes of the opposing spectral evolutions of Ryugu and Bennu by integrating spectral analyses of returned samples with remote-sensing measurements of crater spectrophotometry and thermal inertia.MethodsWe measured multi-band visible spectra of over 400 grains and aggregates from Ryugu samples and approximately 660 mg of Bennu aggregate samples at the JAXA curation facility.We analyzed the visible spectra and phase function slopes of 119 craters on Ryugu based on ONC-T data and 568 craters on Bennu based on MapCam/PolyCam data. The phase function slope was determined by fitting all available photometric data using a Hapke model [9] and calculating the ratio of radiance factors observed under incidence angle (i), emission angle (e), and phase angle (α) of i = 5°, e = 0°, α = 5°, and i = 15°, e = 0°, α = 15°. Thermal inertia values for the craters were obtained from previous studies [10–12].ResultsLimited influence from solar wind and micrometeorites: Our spectral analyses of returned samples suggest that the opposing evolutions of visible spectra may not be fully explained by classical space weathering mechanisms involving solar wind irradiation and micrometeorite bombardment. Although morphological and compositional signatures of these processes—including microcraters, amorphization, and dehydration—have been identified in Ryugu samples [13, 14], the average visible spectra of weathered grains are not statistically distinguishable from those of less weathered grains. Furthermore, although previous studies suggested that solar wind irradiation can exert opposite spectral effects when the hydration state of the material differs [15, 16], the comparable phyllosilicate abundance and bulk hydrogen content of Ryugu and Bennu samples argue against this hypothesis.Spectral evolution driven by the evolution of surface microphysical properties: We propose that changes in surface microphysical properties —grain size, porosity, and roughness— through thermal fatigue, impacts, and electrostatic levitation may be a more important mechanism. Remote-sensing data show that fresher craters on Ryugu have brighter radiance factor, bluer spectral slope, higher thermal inertia, and shallower phase function slope. All of these trends are inverted on Bennu. Such coevolution of visible spectra with phase function slope and thermal inertia, which are known indicators of microphysical properties, suggests that the opposite evolution of microphysical properties may have driven the opposite spectral evolutions on Ryugu and Bennu.These results are further validated by the significant spectral change observed after minor powder adhesion on Ryugu grains. A millimeter-sized Ryugu grain (C0179) showed a bluer spectral slope than the remote-sensing spectra after removing the powders adhered to the surface by brushing and high-pressure gas. In contrast, the powders (

Modeling Bennu’s Crater Development P.H. Smith (1), D.N. DellaGiustina (1), K.N. Burke (1), D.R. Golish (1), D. S. Lauretta (1)(1) Lunar and Planetary Laboratory, University of Arizona, USA (psmith@lpl.arizona.edu) IntroductionSeveral hundred craters have been identified and mapped on Bennu [1]. Their spectral properties have been studied using data from the MapCam instrument onboard the OSIRIS-REx spacecraft [2]. The crater frequency versus spectral slope across the wavelength range of 0.550 µm (v band) to 0.860 µm (x band) shows a skewed distribution varying from the median spectral slope of –0.17 micron–1 (x/v = 0.948) to a neutral or flat slope (x/v = 1.0). DellaGiustina et al. [2] attribute this trend to space weathering. They suggest that freshly formed craters are red, and as they age, they evolve to the bluish slope seen across the asteroidal surface. This abstract presents the results of an age model of Bennu’s craters in the size range of 1 to 10 m. Craters in this size range were selected as having likely been formed during the time that Bennu has orbited in near-Earth space. Larger craters on Bennu are thought to have formed during its long history in the main asteroid belt [3]. Elements of the Crater ModelOur model covers three stages in the evolution of craters. First, a creation function produces a small number of craters randomly selected for size according to a power law distribution. These craters appear each time step. Because the modeled crater distribution will be compared to the measured one, an uncertainty is introduced into the color slope. In other words, at each time step, craters form with an x/v of 1.0±0.01 and with sizes randomly drawn from a power law distribution (radius–3). Once a crater is created, it is tracked throughout its evolution. With each time step, the crater weathers toward a bluish slope. The number of time steps required is determined through an exponential function whose 1/e time is a variable in the model. The slope decreases rapidly at first but approaches the final Bennu slope asymptotically. This is the final destination for stages one and two. The third stage is the erasure of craters. This can occur through mass wasting that is either gravitationally forced or caused by impact-driven seismic shaking [4]. OSIRIS-REx has observed particles thrown off into orbit that return to the surface, which may also contribute to erasure [5]. Because of the limited observational data concerning these processes, they are modeled together in a linear fashion such that the smaller craters have shorter lifetimes than the larger ones. The slope of this fill-in rate is a variable in the model. The code runs time step by time step, and all craters are tracked throughout their lifetime. The variables are optimized so that the spectral slope distribution matches that observed. Model Results Figure 1 illustrates a typical output of the model. The variables are set in the following manner: The 1/e time for weathering is 50 time steps. The erasure algorithm starts eliminating the smallest craters after an age of 160 time steps and will eliminate all craters

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission recently returned a sample of rocks and dust collected from asteroid Bennu. We analyzed the highest-resolution thermal data obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES) to gain insight into the thermal and physical properties of the sampling site, including rocks that may have been sampled, and the immediately surrounding Hokioi Crater. After correcting the pointing of the OTES data sets, we find that OTES fortuitously observed two dark rocks moments before they were contacted by the spacecraft. We derived thermal inertias of 100–150 (±50) J m⁻² K⁻¹ s^−1/2 for these two rocks—exceptionally low even compared with other previously analyzed dark rocks on Bennu (180–250 J m⁻² K⁻¹ s^−1/2). Our simulations indicate that monolayer coatings of sand- to pebble-sized particles, as observed on one of these rocks, could significantly reduce the apparent thermal inertia and largely mask the properties of the substrate. However, the other low-thermal-inertia rock that was contacted is not obviously covered in particles. Moreover, this rock appears to have been partially crushed, and thus potentially sampled, by the spacecraft. We conclude that this rock may be highly fractured and that it should be sought in the returned sample to better understand its origin in Bennu’s parent body and the relationship between its thermal and physical properties.

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 Earth’s 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 ∼32,000 km of Earth’s 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 Apophis’s 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 spacecraft’s instrument suite will begin providing high-resolution data of this “stony” asteroid—advancing knowledge of these objects and their connection to meteorites. Near the mission’s 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, Apophis’s material and structure have critical implications for planetary defense.

Carbonaceous asteroids, such as (101955) Bennu, preserve material from the early Solar System, including volatile compounds and organic molecules. We report spacecraft imaging and spectral data collected during and after retrieval of a sample from Bennu’s surface. The sampling event mobilized rocks and dust into a debris plume, excavating a 9-meter-long elliptical crater. This exposed material is darker, spectrally redder, and more abundant in fine particulates than the original surface. The bulk density of the displaced subsurface material was 500 to 700 kilograms per cubic meter, which is about half that of the whole asteroid. Particulates that landed on instrument optics spectrally resemble aqueously altered carbonaceous meteorites. The spacecraft stored 250 ± 101 grams of material, which will be delivered to Earth in 2023.

When the OSIRIS-REx spacecraft pressed its sample collection mechanism into the surface of Bennu, it provided a direct test of the poorly understood near-subsurface physical properties of rubble-pile asteroids, which consist of rock fragments at rest in microgravity. Here, we find that the forces measured by the spacecraft are best modeled as a granular bed with near-zero cohesion that is half as dense as the bulk asteroid. The low gravity of a small rubble-pile asteroid such as Bennu effectively weakens its near subsurface by not compressing the upper layers, thereby minimizing the influence of interparticle cohesion on surface geology. The underdensity and weak near subsurface should be global properties of Bennu and not localized to the contact point.