Asteroid Bennu Research Articles

Analyzing the structure of periodic orbit families that exist around asteroid (101955) Bennu

Analyzing the structure of periodic orbit families that exist around asteroid (101955) Bennu

Integrated robust navigation and guidance for the kinetic impact of near-earth asteroids based on deep reinforcement learning

The defense against near-Earth asteroids (NEAs) using kinetic impact is faced with various challenges, including limited maneuverability of the impactor, inaccurate dynamic models, poor observability of relative navigation, and control execution errors. To address these challenges, an integrated robust navigation and guidance method for the kinetic impact of NEAs based on deep reinforcement learning (DRL) is proposed in this paper, which can directly map angle measurements by a monocular camera to guidance maneuvers. Firstly, a discrete partial observable Markov decision process (POMDP) is modeled for the integrated navigation and guidance problem of NEA interception. To address the situation where impactors are usually only equipped with aiming cameras and only the line of sight (LOS) angle is measured, past and current LOS measurements are concatenated into a one-dimensional state observation vector to directly introduce the memory of historical state information. A shaping reward function design based on potential energy has also been proposed to solve the problem of sparse reward as the main goal. Subsequently, proximal policy optimization (PPO) is used to solve the established POMDP model, obtaining an integrated navigation and guidance policy that directly maps from the original output of the navigation sensor to the guidance command. The potentially hazard asteroid (PHA) Bennu is used as the target and a typical kinetic impact defense scenario is designed to simulate and verify the model and method proposed in this paper, considering the influence of multiple factors. Numerical simulation results show that the proposed method achieves an interception accuracy of 203.58 m (mean value). The proposed method abandons the traditional separate design of navigation and guidance algorithms, and its robustness has been tested and verified in a wide range of uncertain environments.

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 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.

Mixed Fidelity Shape Models for Efficient Small Body Gravity Modeling

The polyhedral gravity model is the de facto standard for computing the gravity field near the surface of small bodies due to their irregular shapes. However, the computational load to compute the gravitational acceleration can increase significantly for high-resolution shape models. In this work, an algorithm for creating mixed-resolution shapes is developed with the goal of getting similar accuracy in the gravitational acceleration while reducing the computational cost. This method is tested for full integration into low-altitude trajectory simulations. The results show that blended models can provide comparable accuracy while improving the computation performance. Additionally, this approach does not require any fitting or estimation process as it only involves creating shape model connections, which is done in a deterministic manner. Test cases with asteroids Bennu and Itokawa demonstrate that the errors in the gravitational accelerations computed with the hybrid models are comparable to expected uncertainties from unknown heterogeneous mass distributions. For precomputed hybrid shape model strategies, we see savings in computational time on the order of 50% compared to using the high-resolution shape model, while getting much lower propagation errors than using only a low-resolution model alone.

Phyllosilicate decomposition on Bennu due to prolonged surface exposure

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission to carbonaceous asteroid (101955) Bennu performed detailed mapping with a suite of instruments to characterize the composition and geology of its surface. Here we use data from the OSIRIS-REx Thermal Emission Spectrometer (OTES) instrument to investigate the relationship of OTES-derived spectral indices to other derived data products from OTES, the OSIRIS-REx Camera Suite (OCAMS), and the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) at global and local scales. We quantitatively confirm that high values of the OTES silicate stretching slope (from ∼10 to ∼12 μm) in midday spectra that are indicative of thin and/or patchy dust cover are strongly associated with low thermal inertia (high porosity), low albedo boulders on Bennu. These high porosity boulders have brecciated textures with embedded clasts that likely originated on Bennu's parent body or during its disruption. The high porosity of these boulders is a key factor in the local production of the dust or its entrapment, as some large, brecciated boulders with a lower porosity have little evidence of dust. A second OTES spectral parameter, the silicate bending band depth near 22.7 μm applied to early evening spectra, is not correlated to thermal inertia, but is weakly to strongly correlated to albedo, OVIRS-derived 1.05 μm and 2.74 μm band depths, OVIRS-derived hydrogen abundance, and modeled nanophase troilite abundance. In several regions on Bennu there is a strong spatial relationship between these parameters, whereby areas with shallower silicate bending bands also have shallower 1.05 μm and 2.74 μm bands and lower albedo with higher nanophase troilite abundances. These correlations, combined with analysis of the silicate bending band in laboratory experiments of space weathered and mildly heated carbonaceous chondrites, suggests that decreased silicate bending band depths signify decomposition of phyllosilicates, likely Fe-bearing, due to space weathering or mild heating (<600 °C) via solar radiation during Bennu's time in near-Earth space. There is a strong association of larger silicate bending band depths in areas dominated by small rocks and unresolved material and in areas with small (≤ 25 m) craters identified as the spectrally reddest on Bennu, suggesting that this material has been more recently exposed due to impact and/or mass wasting processes. The shallowest silicate bending depths are associated with larger rocks and boulders that appear to have the longest surface exposure history, although there is band depth variation among them suggesting either initial composition variation that resulted in different responses to space weathering or heating, or varied exposure history of individual boulders themselves. We predict that any grains returned from Bennu with a significant surface exposure history will be characterized by shallower 22.7 μm, 1.05 μm and 2.7 μm band depths and increased sulfide (troilite) abundance, as well as textural and chemical evidence for phyllosilicate dehydration.

Stereophotoclinometry for OSIRIS-REx Spacecraft Navigation

We summarize a decade of effort by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission team to build up the unique capabilities, processes, and procedures required to accomplish the unprecedented navigation performance required during proximity operations at asteroid (101955) Bennu. Stereophotoclinometry was a key technology used for digital terrain model (DTM) generation and landmark navigation, enabling estimation of spacecraft trajectories and Bennu’s geophysical parameters. We outline the concept of operations for OSIRIS-REx landmark navigation and the wide array of testing and verification efforts leading up to OSIRIS-REx’s arrival at Bennu. We relate the outcome of these efforts to the experiences during proximity operations. We discuss navigation and DTM performance during operations, including detailed lessons learned to carry forward for future missions.

Development of preparation methods of polished sections of returned samples from asteroid Ryugu by the Hayabusa2 spacecraft

AbstractPreparation procedures of polished sections of the Ryugu samples returned by the Hayabusa2 spacecraft were established through tests using CI and CM chondrites as analog materials of the Ryugu samples and processing of the Ryugu samples. The procedures consisted of four steps: epoxy‐coating, embedding in epoxy cylinders, cutting with a wire saw, and dry polish by hand. There are three key points for successful preparation of the polished sections: (1) ethanol‐mixed epoxy with low viscosity for reinforcing the fragile samples, (2) handling under dry conditions to avoid breakup of the samples on contact with liquids due to their highly porous nature, and (3) X‐ray computed tomography data for exposing maximum surface areas of target mineral phases and clasts. These key points may also be important for processing of samples returned from asteroid Bennu and the Martian Moon Phobos, as those samples are likely to be hydrous carbonaceous chondrite‐like materials. The established procedures induce two side effects: zoning of the polished surface of the Ryugu samples in scanning electron microscope images reflecting differences in carbon contents due to permeation of low viscosity epoxy resin into the sample surface and fractures in anhydrous minerals possibly due to shear stress during dry polishing.

Global Mapping of Fragmented Rocks on the Moon with a Neural Network: Implications for the Failure Mode of Rocks on Airless Surfaces

Failure modes of lunar boulders depend both on rheology and the erosion agents acting in the lunar surface environment. Here, we address the failure modes of lunar boulders and their variations at a quasi-global scale (60°N to S). We deploy a neural network and map a total of ∼130,000 fragmented boulders (width > ∼10 m) scattered across the lunar surface and visually identify a dozen different disintegration morphologies corresponding to different failure modes. Our findings suggest that before a boulder is catastrophically shattered by an impact, there is an internal weakening period with minor morphological evidence of damage at the rock scale at the resolution of the used imagery. We find that some of the rare pre-shattering morphologies (e.g., fractures) are equivalent to morphologies observed on asteroid Bennu, suggesting that these morphologies on the Moon and on asteroids are likely not diagnostic of their formation mechanism (e.g., meteoroid impact, thermal stresses). In addition, we identify new morphologies such as breccia boulders with an advection-like erosion style. We publicly release the produced fractured boulder catalog along with this paper.

Post-flight Evaluation of Lidar-based Digital Terrain Models for OSIRIS-REx Navigation at Bennu

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft spent more than 2 yr characterizing near-Earth asteroid (101955) Bennu. The OSIRIS-REx Laser Altimeter (OLA) was responsible for producing the most accurate reconstruction of the asteroid’s surface—down to a global resolution of around 5 cm with a data precision of ±1.25 cm. However, the best-quality global OLA digital terrain model (DTM), version 21 (v21), was not available for navigation during proximity operations, nor was the utility of this model evaluated for processing images and altimeter data for navigation. The focus of this paper is the post-flight assessment of the final OLA v21 DTM, its performance for navigation-related analysis, and estimates of corrections needed for the DTM and measurement models. We created 15 cm resolution maplets for processing optical navigation (OpNav) data, and 5 cm resolution DTMs for processing altimeter data, to estimate a combined spacecraft trajectory over five phases of the mission. Our estimated corrections to the OLA instrument model produce altimeter data residuals with a precision of 7.12 cm (1σ; one standard deviation from the mean). The OpNav maplets produce image residuals at 0.2 px (1σ) and estimated landmark locations accurate to ±6 cm, outperforming DTM navigation-related performance requirements. Finally, our estimate of the global DTM scale is more precise and within 1.1σ of previously reported values. We find that a slight discrepancy persists between the image and altimeter data, with image data suggesting that the DTM is too small by 0.049%, but nevertheless is exceptional for navigation.

The low surface thermal inertia of the rapidly rotating near-Earth asteroid 2016 GE1

Context. Asteroids smaller than about 100 m in diameter are observed to rotate very fast, with periods often much shorter than the critical spin limit of 2.2 h. Some of these super-fast rotators can also achieve a very large semimajor axis drift induced by the Yarkovsky effect, which, in turn, is determined by internal and surface physical properties. Aims. We consider here a small super-fast-rotating near-Earth asteroid, designated as 2016 GE1. This object rotates in just about 34 s, and a large Yarkovsky effect has been determined from astrometry. By using these results, we aim to constrain the thermal inertia of the surface of this extreme object. Methods. We used a recently developed statistical method to determine the thermal properties of near-Earth asteroids. The method is based on the comparison between the observed and the modeled Yarkovsky effect, and the thermal conductivity (inertia) is determined via a Monte Carlo approach. Parameters of the Yarkovsky effect model are fixed if their uncertainty is negligible, modeled with a Gaussian distribution of the errors if they are measured, or deduced from general properties of the population of near-Earth asteroids when they are unknown. Results. Using a well-established orbit determination procedure, we determined the Yarkovsky effect on 2016 GE1 and confirm a significant semimajor axis drift rate. Using a statistical method, we show that this semimajor axis drift rate can only be explained by low thermal inertia values below 100 J m−2 K−1 s−1/2. We benchmarked our statistical method using the well-characterized asteroid Bennu and find that only knowing the semimajor axis drift rate and the rotation period is generally insufficient for determining the thermal inertia. However, when the statistical method is applied to super-fast rotators, we find that the measured Yarkovsky effect can be achieved only for very low values of thermal inertia: namely, 90% of the probability density function of the model outcomes is contained at values smaller than 100 J m−2 K−1 s−1/2. Conclusions. We propose two possible interpretations for the extremely low thermal inertia of 2016 GE1: a high porosity or a cracked surface, or a thin layer of fine regolith on the surface. Though both possibilities seem somewhat unexpected, this opens up the possibility of a subclass of low-inertia, super-fast-rotating asteroids.

Change of boundary: towards a mathematical foundation of global gravity models

Change of boundary is a method that iteratively downward continues data from a star-shaped boundary to a regular surface, such as the sphere or the ellipsoid of revolution, and then solves a boundary value problem using the downward continued data on the changed boundary. Although the method belongs to concepts involved in the development of recent high-degree Earth’s gravity models, it is still unclear whether the iterations converge to the true solution for boundaries as complex as, for instance, the Earth’s surface. In this paper, we revisit the method and show that convergence in terms of the Cesaro limit can be achieved under the assumption that the operator performing the iterations is non-expansive. The validity of the assumption is, however, still not proved. Therefore, we examine the hypothesis numerically using boundaries of various complexity. We start with a simple synthetic topography defined by a Legendre polynomial and move to more realistic finite-degree shapes of the asteroids Bennu and Eros. The numerical experiments indicate that the assumption is valid as long as the boundary deviates not too far from a sphere and the truncation degree of the gravity model is not too high (the experiments with the synthetic topography and Bennu). Otherwise, the hypothesis seems to be false (the Eros case). Finding an analytical condition to separate between shapes for which the change of boundary method converges/diverges remains an open issue.

Finite Element Method approach 3-dimensional thermophysical model for YORP torque computation

We present a new thermophysical model suitable for simulating the thermal conditions of small celestial bodies, such as asteroids and comet nuclei, as well as local topographic features like rocks, boulders, and craters. The model employs a Finite Element Method (FEM) approach to solve the 3-dimensional heat conduction problem, explicitly accounting for heat conduction between neighboring FEM elements. This sets it apart from other thermophysical models that typically neglect lateral heat conduction. The model takes into account scattering sunlight, self-heating due to thermal radiation, and both horizon and cast shadows. We validated our model against an analytical solution for the transient temperature distribution of a sphere under convective boundary conditions. Additionally, we compared our results with earlier thermophysical modeling works and observational data of asteroid (101955) Bennu. Using the shape models of Bennu as well as scaled versions of (162173) Ryugu, (25143) Itokawa, and (6489) Golevka to match Bennu’s surface area, we investigated the effects of shape and lateral heat conduction on the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) torque. Results indicate that contact binary and non-classified/irregular objects experience a stronger YORP torque compared to spheroidal objects. The lateral heat conduction can enhance or dampen the YORP torque. Further analysis is important to obtain more robust statistical conclusions.

Evaluating the effects of space weathering on magnetite on airless planetary bodies

Magnetite is a relevant mineral component of asteroids as it has been identified in carbonaceous chondrites, on the surface of asteroid Bennu through remote sensing observations, and in samples returned from asteroid Ryugu. However, the effects of space weathering processes on magnetite have not yet been explored. To investigate how this mineral phase responds to space weathering, here we simulate micrometeoroid bombardment and solar wind irradiation of magnetite using pulsed laser and ion irradiation experiments. We performed X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and visible to near-infrared (VNIR) reflectance spectroscopy analyses to characterize the chemical, microstructural, and spectral response of magnetite to simulated space weathering. In addition, we carried out ion impact simulations using the SDTrimSP software to evaluate the calculated response of magnetite to 1 keV H+ and 4 keV He+ ions and compared these results to our XPS and TEM results. Ion irradiation simulated ∼750 years on the surface of asteroid Bennu, with a solar-wind appropriate total H:He fluence ratio (∼24). Within this time, depletion of O was observed with H+ and He+ ion irradiation, with significantly greater change via protons due to the larger fluence, where preferential sputtering promotes the formation of a metallic iron layer at the magnetite surface. This suggests that solar wind ions act as reducing agents on Fe oxides, with a fraction remaining implanted in these phases. Indeed, we observe elongated defects contained in a crystalline rim created by He+ implanted ions in the TEM. Pulsed laser irradiation, analogous to micrometeoroid impacts, generates melts on the surface of the magnetite grains. The impact melts and H+-generated metallic iron rims both result in increased VNIR spectral reflectance, but lower fluence He+ implantation has no significant spectral effect. These results suggest that space weathered magnetite could contribute to bright regions detected in remote sensing analyses of the Ryugu and Bennu surfaces by the Hayabusa2 and OSIRIS-REx missions and will contribute to the identification and interpretation of space weathered magnetite in returned samples retrieved from both asteroids.

Temporal evolution of the dynamical environment around asteroid (101955) Bennu

In this work the changing dynamical environment about asteroid (101955) Bennu, one of the most accurately modeled asteroids to date, is analyzed. This is done through the analysis of the evolving equilibrium points and corresponding dynamical ridge lines about this body, as a function of the asteroid’s spin rate. This is motivated by the measurement of an accelerating YORP effect on Bennu, causing its spin rate to increase over time with a doubling time on the order of 2 Myr. This study looks at the changes in the dynamical environment about Bennu over times ranging from 1 Myr in the past to 500 kyr in the future. Over this time interval several changes in this environment occur due to pairs of equilibria coming into and out of existence and changing stability properties of these equilibria. As a result of these bifurcations, if Bennu’s shape is assumed to be fixed, the number of equilibria increases from a minimum of four (1 Myr in the past), to eight (at the current epoch), to a maximum of 12 (430.6 kyr in the future). After this point, the number of equilibria will decrease as equilibria annihilate with each other. Several equilibria have undergone stability changes and in the future several will cross the surface of the asteroid. Criteria for mathematically detecting all of these transitions were developed and are presented in this work. It is also found that simulated particles initially at rest on the Bennu surface will be lofted when Bennu reaches a rotation period of 3.4304 hr, which will occur within the next 400 kyr. At this point we show that Bennu may appear as an active asteroid.

Curation planning and facilities for asteroid Bennu samples returned by the OSIRIS‐REx mission

AbstractNASA's OSIRIS‐REx spacecraft collected samples from carbonaceous near‐Earth asteroid (101955) Bennu on October 20, 2020, and will deliver them to the Earth on September 24, 2023. The samples will be processed at the NASA Johnson Space Center (JSC), where most of the sample collection will be subsequently curated in a new cleanroom suite. The spacecraft collected loose regolith two ways: in a bulk sample chamber capable of holding up to 2 kg, and on industrial Velcro “contact pads” intended to collect small particles at the surface. Included in the JSC collection will be the bulk sample, the contact pads, contamination‐monitoring witness plates, and supporting hardware. Planning for the curation of the samples and hardware started at the earliest phase of proposal development and continued in parallel with project development and execution. Because a major mission goal is characterization of organic compounds in the Bennu samples, extra effort was spent in the design stage to ensure a clean curation environment. Here, we describe the preparations to receive the sample, including the design, construction, outfitting, and monitoring of the cleanrooms at JSC; the planned recovery of the sample‐containing capsule when it lands on Earth; and the approach to characterizing and cataloging the samples. These curation efforts will result in the distribution of pristine Bennu samples from JSC to the OSIRIS‐REx science team, international partners, and the global scientific community for years to come.

Stereophotoclinometry on the OSIRIS-REx Mission: Mathematics and Methods

Stereophotoclinometry (SPC) makes it possible to extract the shapes of surfaces by combining information from images, namely stereo parallax data and surface shading from slopes, with knowledge of the location of a spacecraft. This technique has been used extensively in the past few decades to describe the shapes of planets and small bodies, such as asteroids and comets. It has also been used to carefully navigate spacecraft around very small bodies, as in the case of the OSIRIS-REx mission to the ∼500 m diameter asteroid (101955) Bennu. This paper describes the mathematical foundation of SPC, with examples from the OSIRIS-REx mission.

Autonomous Reconnaissance Trajectory Guidance at Small Near-Earth Asteroids via Reinforcement Learning

Autonomous capabilities could be an essential aspect of future near-Earth asteroid exploration missions, enabling a fleet of low-cost spacecraft to be distributed to various targets for increased scientific and engineering returns. This paper studies the design of an adaptive offline policy for a surface imaging task under the influence of maneuver noise via reinforcement learning (RL). An adaptive policy that responds to the changes in the environment is obtained by including asteroid parameters as part of the feedback state and randomizing them during the training. The proximal policy optimization algorithm is used to train the policy. The robustness of the policy is tested in an environment that has unmodeled dynamical effects. Further, the overall performance of the autonomous exploration scheme is studied by combining the RL-based policy with the previously proposed autonomous navigation strategy that is built around optical and measurements. The end-to-end simulations that combine both onboard navigation and guidance are performed using asteroid Bennu as an example target, and the results show that the proposed scheme is robust.

Extraterrestrial amino acids and amines identified in asteroid Ryugu samples returned by the Hayabusa2 mission

The hot water and acid extracts of two different Ryugu samples collected by the Hayabusa2 mission were analyzed for the presence of aliphatic amines and amino acids. The abundances and relative distributions of both classes of molecules were determined, as well as the enantiomeric compositions of the chiral amino acids. The Ryugu samples studied here were recovered from sample chambers A and C, which were composed of surface material, and a combination of surface and possible subsurface material, respectively. A total of thirteen amino acids were detected and quantitated in these samples, with an additional five amino acids that were tentatively identified but not quantitated. The abundances of four aliphatic amines identified in the Ryugu samples were also determined in the current work. Amino acids were observed in the acid hydrolyzed and unhydrolyzed hot water extracts of asteroid Ryugu regolith using liquid chromatography with UV fluorescence detection and high-resolution mass spectrometry. Conversely, aliphatic amines were only analyzed in the unhydrolyzed hot water Ryugu extracts. Two- to six-carbon (C2-C6) amino acids with individual abundances ranging from 0.02 to 15.8 nmol g−1, and one- to three-carbon (C1-C3) aliphatic amines with individual abundances from 0.05 to 34.14 nmol g−1, were found in the hot water extracts. Several non-protein amino acids that are rare in biology, including β-amino-n-butyric acid (β-ABA) and β-aminoisobuytric acid (β-AIB), were racemic or very nearly racemic, thus indicating their likely abiotic origins. Trace amounts of select protein amino acids that were enriched in the l-enantiomer may indicate low levels of terrestrial amino acid contamination in the samples. However, the presence of elevated abundances of free and racemic alanine, a common protein amino acid in terrestrial biology, and elevated abundances of the predominately free and racemic non-protein amino acids, β-ABA and β-AIB, indicate that many of the amino acids detected in the Ryugu water extracts were indigenous to the samples. Although the Ryugu samples have been found to be chemically similar to CI type carbonaceous chondrites, the measured concentrations and relative distributions of amino acids and aliphatic amines in Ryugu samples were notably different from those previously observed for the CI1.1 carbonaceous chondrite, Orgueil. This discrepancy could be the result of differences in the original chemical compositions of the parent bodies and/or alteration conditions, such as space weathering. In addition to α-amino acids that could have been formed by Strecker cyanohydrin synthesis during a low temperature aqueous alteration phase, β-, γ-, and δ-amino acids, including C3 – C5 straight-chain n-ω-amino acids that are not formed by Strecker synthesis, were also observed in the Ryugu extracts. The suite of amino acids measured in the Ryugu samples indicates that multiple amino acid formation mechanisms were active on the Ryugu parent body. The analytical techniques used here are well-suited to search for similar analytes in asteroid Bennu material collected by the NASA OSIRIS-REx mission scheduled for Earth return in September 2023.

Simulating impact-induced shaking as a triggering mechanism for mass movements on Bennu

Observations of near-Earth asteroid Bennu have revealed a dynamic surface composed of unconsolidated material. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission found numerous locations exhibiting evidence of mass movements of surface material. Mass movements can be a major factor in the surface evolution of a small near-Earth asteroid, and the Small Carry-on Impactor (SCI) experiment on asteroid Ryugu has shown that seismic shaking can trigger it. We selected one mass movement site on Bennu to conduct a detailed survey of the surface boulder arrangement and geomorphology. Using these data, we created dynamical simulations of mass movement events at this site initiated by seismic shaking, and we found a mass flux comparable to estimates from the site survey. The frequency ranges of the shakings are similar to what would be produced by a 0.5-m-diameter impactor, for which the expected crater size is of a scale widely seen on Bennu (32 m). In addition, the simulation exhibited the Brazil nut effect, where finer particles percolate towards greater depth, in this case up to 1.5 m. Our results demonstrate that impact-induced seismic shaking is a viable mechanism for the initiation of mass movements, and a plausible explanation for the scarcity of fine regolith, on Bennu's surface.

Experimental and analytical methods for thermal infrared spectroscopy of complex dust coatings in a simulated asteroid environment

Abstract Airless bodies in the Solar system are commonly dominated by complex regolith mixtures consisting of coarse and fine particulates. These materials often manifest as coatings with the potential to modify or obscure the spectral signatures of underlying substrates. This can make accurate spectral analysis of surface materials challenging, especially for thermal infrared (TIR) techniques of which the spectral properties concurrently depend on grain size and albedo. Further complexity is presented when these coatings occur as discontinuous patterns in which some substrate is exposed and some is masked. Discontinuous patterns are distinguished by scale as having macroscopic or microscopic discontinuity, with the former being patches of homogeneous dust covering portions of the substrate and the latter being randomly distributed individual particles on the substrate. Investigations of asteroid (101955) Bennu’s surface by NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer have revealed contradictions between spectral and thermophysical results that are hypothesized to indicate the presence of thin and/or laterally discontinuous dust coatings. To address this, we constructed an environment chamber that enables the controlled deposition of size-regulated dust particles in coatings with varying continuity and thickness. TIR spectra of coated substrates acquired in a simulated asteroid environment are used to investigate the extent to which dust coatings of different thicknesses and arrangements contribute to orbital spectral signatures of airless body surfaces.