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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., In their study, the authors discuss the potential of thermal weathering on airless bodies. As a case study, they use boulder and fracture morphologies on asteroid Bennu.

The complex history of Bennu's surface The near-Earth asteroid (101955) Bennu is a carbon-rich body with a rubble pile structure, formed from debris ejected by an impact on a larger parent asteroid. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft is designed to collect a sample of Bennu's surface and return it to Earth. After arriving at Bennu, OSIRIS-REx performed a detailed survey of the asteroid and reconnaissance of potential sites for sample collection. Three papers present results from those mission phases. DellaGiustina et al. mapped the optical color and albedo of Bennu's surface and established how they relate to boulders and impact craters, finding complex evolution caused by space weathering processes. Simon et al. analyzed near-infrared spectra, finding evidence for organic and carbonate materials that are widely distributed across the surface but are most concentrated on individual boulders. Kaplan et al. examined more detailed data collected on the primary sample site, called Nightingale. They identified bright veins with a distinct infrared spectrum in some boulders, which they interpreted as being carbonates formed by aqueous alteration on the parent asteroid. Together, these results constrain Bennu's evolution and provide context for the sample collected in October 2020. Science , this issue p. eabc3660 , p. eabc3522 , p. eabc3557

An asteroid’s history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu’s metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 ± 0.75 million years. Analysis of the size and depth of craters on boulders on the asteroid (101955) Bennu indicates that Bennu has been in near-Earth space for 1.75 ± 0.75 million years.

Thermophysical analyses of planetary bodies such as the Moon, Mars, and numerous asteroids have allowed for remote estimates of regolith physical properties, such as particle size and packing density, as well as the relative spatial abundance of boulders. Here we define “regolith” as a particulate assemblage where most particles are comparable to or smaller than the length scale of the diurnal skin depth (the e-folding depth of the diurnal thermal wave). Until recently, regolith and boulders were believed to be thermally quite distinct; regolith on the Moon, Mars, and most asteroids was usually known or suspected to be fine (i.e., ranging from fine dust to sand), meaning that it was known or assumed to have a thermal inertia much lower than that of boulders and bedrock. Upon the arrival of NASA OSIRIS-REx at asteroid Bennu and JAXA Hayabusa2 at asteroid Ryugu and the subsequent thermophysical analyses of the respective asteroid surfaces, this preconceived notion of thermophysically distinct regolith and boulders/bedrock was found to be flawed (DellaGiustina and Emery et al., 2019; Sugita et al., 2019). Boulders cover the vast majority of both asteroids’ surfaces, yet the thermal inertia values determined for these boulder-rich surfaces fall within a range that was previously believed to exclusively represent coarse, sand-to-pebble–sized regolith. Recent work has been devoted to the analysis of the boulders’ thermal inertia and the physical interpretation thereof (Grott et al., 2019; Rozitis et al., in revision); the general conclusion so far is that the boulders have very low thermal conductivity and density owing to the presence of numerous pores and fractures, more so than most (or perhaps all) carbonaceous chondrite meteorites in Earth’s collections. As such, these boulders are likely to be structurally distinct from all known meteorite specimens. Although fine-particulate regolith is rare on the surface of Bennu, it is present in some locations. It is still of great interest to probe the physical properties of this regolith by means of thermal analysis so as to make predictions about the properties of the samples that will be returned by OSIRIS-REx and to learn about the evolution of the asteroid surface and the mechanisms by which regolith is produced and lost or destroyed. There remains the enigmatic question: although we suspect that the boulders on Bennu are distinct from the meteorite collection, will the returned samples of particulate material share these distinct properties? At what scales can we define the physical thermally relevant properties of boulders and regolith particles on Bennu? In other words, if the boulders are indeed highly fractured and porous, are the regolith particles also fractured and porous or are their dimensions below the relevant length scales? It is very challenging to estimate the physical properties of the regolith on Bennu using thermal data, even in regions where it appears to be abundant, due to the coarseness of the regolith. Commonly used planetary thermophysical models rely on the assumption that the material on the surface, be it regolith or rock, can be approximated as a continuous, non-discretized material with physical properties that are either constant with depth or are allowed to vary with depth in some well-defined way (e.g. an exponential density increase, or up to a few layers of physically distinct material, such as regolith on bedrock or dust coatings). This assumption of material continuity is valid when regolith particles are smaller, or perhaps even much smaller, than the diurnal skin depth of the thermal wave. However, high-resolution images of the Nightingale Crater on Bennu, which is the OSIRIS-REx mission’s primary sample collection site, revealed a particle size frequency distribution (SFD) that crosses this threshold; i.e., there are particles present that are smaller than, comparable to, and larger than the diurnal skin depth (~1­–5 cm) present within a single, meters-wide observation footprint. The thermophysical behavior of such a regolith configuration has never been comprehensively studied and likely cannot be properly approximated with standard 1D thermal modeling methods. We will present preliminary results using a 3D regolith model where we render hundreds of regolith particles and rocks, approximated as spheres, in a finite element mesh framework. The model is heated diurnally with a solar source to study the thermal response of such skin depth–crossing SFDs under Bennu surface-like conditions. The SFD of the particles is informed by particle size counts in Nightingale Crater from high-resolution visible images. With the SFD partially constrained, we are able to focus our efforts on exploring the material property parameter space, namely to estimate the thermal conductivity and density of individual regolith particles. Given the coarseness of the regolith on Bennu, we find that the model is more sensitive than one might expect to the thermal conductivity of the individual regolith particles owing to the effects of particle non-isothermality (Ryan et al., 2020). Although this present modeling work is focused on analyzing thermal emission data of Bennu obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES, Christensen et al., 2018), we aim to expand our efforts to study the more general thermal behavior of coarse regoliths and regoliths with wide SFDs under a range of solar heating conditions. Acknowledgements This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. References: Christensen, P.R. et al. (2018). The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument. Space Science Reviews 214, 87. DellaGiustina, D.N., Emery, J.P., et al. (2019) Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis, Nature Astronomy, DOI:10.1038/s41550-019-0731-1. Grott, M., Knollenberg, J., et al. (2019) Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu, Nature Astronomy, DOI:10.1038/s41550-019-0832-x. Rozitis, B., Ryan, A.J., Emery, J.P., et al. (in revision) Asteroid (101955) Bennu’s Weak Boulders and Thermally Anomalous Equator, Science Advances, submitted March 2020. Ryan, A., Pino Muñoz, D., Bernacki, M., Delbo, M. (2020) Full-Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates, JGR:Planets. DOI:10.1029/2019JE006100. Sugita, S., Honda, R., et al. (2019) The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes, Science. DOI:10.1126/science.aaw0422

Introduction: The OSIRIS-REx mission’s observation campaigns [1] using the PolyCam instrument, part of the OSIRIS-REx Camera Suite (OCAMS) [2 3], have returned images of the surface of near-Earth asteroid (NEA) (101955) Bennu. These unprecedented-resolution images resolved cavities on Bennu’s boulders (Fig. 1) that are near-circular in shape and have diameters ranging from 5 cm to 5 m. We made hundreds of measurements of these cavities in image and laser altimeter data and found more than 100 boulders that exhibit at least one on their surface (Fig. 1). The most likely mechanism for the creation of these cavities is impacts. However, it is unclear whether these “mini-craters” were formed during Bennu’s residence in the main asteroid belt, or if they were formed more recently, after Bennu became a near-Earth asteroid (NEA). We use our observations of mini-craters to derive the strength of solid C-type objects against impacts. Our results have implications for Bennu’s history in the main-belt and in near-Earth space. The Strength of Monolithic C-type Objects: The strength of asteroids against collisions is crucial for understanding the surface evolution of airless planetary bodes, the dynamical evolution of asteroids throughout Solar System history, and the incorporation of planetesimals into planets [4,5]. Laboratory data on centimeter-scale meteorites have been extrapolated and buttressed with numerical simulations and analytic formalisms to derive the cratering threshold at the asteroid scale [6–8]. However, thus far it has not been possible to directly assess the strength of the boulders that constitute the building blocks of a rubble-pile asteroid. Apollo lunar rocks and spacecraft missions to near-Earth asteroids indicate only two modes of impact-induced breakdown of boulders: 1) abrasion by micro-meteorites (sand-blasting), and 2) catastrophic rupture by a single large impact [9–11]. Widespread cratering on boulders has not been observed heretofore. Figure 1: Centimeter-scale impact features on the sur-face of a boulder (image 20190703T044506S720_pol, taken July 3, 2019, by the OCAMS PolyCam imager; 1 cm/pixel). We developed a new method to derive the cratering efficiency and the disruption threshold of C-type objects by combining scaling laws and observations of craters on C-type boulders and asteroids [12­–14]. We postulate that the largest crater on a boulder of a given size signifies an impact energy close to that required for disrupting that boulder. This type of analysis has been previously done for the study of the largest craters on planetary bodies larger than tens of kilometers using scaling laws [6] and laboratory experiments [15]. Here, we extend that analysis to objects of smaller size. We find that the crater to impactor size ratio on C-type objects is ~ 15, and that the collisional disruption of 1-m radius boulders on the surface of Bennu is efficient in the main belt (~ 1 Myr) but effectively ceases in near-Earth space as their collisional lifetime (~50 Myr) becomes greater than the dynamical lifetime of NEAs ( Towards a mini-crater clock for NEA surfaces: Detailed analysis of the surface density of mini-craters on Bennu’s boulders may provide a new way to determine relative ages of different regions of the surface. This can be used as a basis of comparison or calibration for alternative approaches to chronology, such as assessments of the small crater population on Bennu’s surface [12] or space-weathering on Bennu’s surface [17]. Our findings may be validated by analyzing the cosmic ray exposure ages of the returned sample. Acknowledgements This work was supported in part by NASA grant NNX15AH90G awarded by the Solar System Workings program. S.R.S. acknowledges support from NASA Grant no. 80NSSC18K0226 as part of the OSIRIS-REx Participating Scientist Program. This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. P.M. acknowledges support from the Centre National d’Études Spatiales and from the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d’EXcellence “Joint, Excellent, and Dynamic Initiative” (IDEX JEDI) of the Université Côte d’Azur. We are grateful to the entire OSIRIS-REx team for making the encounter with Bennu possible. References [1] Lauretta et al. (2017) Space Science Reviews 212, 925.[2] Rizk, B. et al. (2018) Space Science Reviews, 214, 26. [3] Golish, D.R. et al. (2020) Space Sci. Rev. 216, 12. [4] Michel, P. et al. (2001) Science, 294, 1696. [5] Bottke, W.F. et al. (2005) Icarus, 179, 63. [6] Holsapple, K.A. (1993) AREAP, 21, 333. [7] Benz, W., & Asphaug, E. (1994). Icarus, 107, 98. [8] Jutzi, M. et al. (2010) Icarus, 207, 54. [9] Hörz, F., et al. (1975) The Moon, 235. [10] Cheng, A.F. (2002) Asteroids III, University of Arizona Press, 351-366. [11] Nakamura, A.M. et al. (2008) Earth, Planets, and Space, 60, 7. [12] Walsh, K.J. et al. (2019) Nature Geoscience, 12, 242. [13] Sugita, S. et al. (2019) Science, 364, 252. [14] Veverka, J. et al. (1997) Science, 278, 2109. [15] Leliwa-Kopystyński, J. et al. (2008) Icarus, 195, 817. [16] Michel, P., & Delbo, M. (2011) Icarus, 209, 520. [17] Trang, D. et al. (2019) LPSC L, Abstract #2172.[1] Lauretta et al. (2017) Space Science Reviews 212, 925

The complex history of Bennu's surface The near-Earth asteroid (101955) Bennu is a carbon-rich body with a rubble pile structure, formed from debris ejected by an impact on a larger parent asteroid. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft is designed to collect a sample of Bennu's surface and return it to Earth. After arriving at Bennu, OSIRIS-REx performed a detailed survey of the asteroid and reconnaissance of potential sites for sample collection. Three papers present results from those mission phases. DellaGiustina et al. mapped the optical color and albedo of Bennu's surface and established how they relate to boulders and impact craters, finding complex evolution caused by space weathering processes. Simon et al. analyzed near-infrared spectra, finding evidence for organic and carbonate materials that are widely distributed across the surface but are most concentrated on individual boulders. Kaplan et al. examined more detailed data collected on the primary sample site, called Nightingale. They identified bright veins with a distinct infrared spectrum in some boulders, which they interpreted as being carbonates formed by aqueous alteration on the parent asteroid. Together, these results constrain Bennu's evolution and provide context for the sample collected in October 2020. Science , this issue p. eabc3660 , p. eabc3522 , p. eabc3557