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International audience; We manually mapped particles ranging in longest axis from 0.3 cm to 95 m on (101955) Bennu for the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission. This enabled the mission to identify candidate sample collection sites and shed light on the processes that have shaped the surface of this rubble-pile asteroid. Building on a global survey of particles, we used higher-resolution data from regional observations to calculate particle size-frequency distributions (PSFDs) and assess the viability of four candidate sites for sample collection (presence of unobstructed particles ≤ 2 cm). The four candidate sites have common characteristics: each is situated within a crater with a relative abundance of sampleable material. Their PSFDs, however, indicate that each site has experienced different geologic processing. The PSFD power-law slopes range from −3.0 ± 0.2 to −2.3 ± 0.1 across the four sites, based on images with a 0.01-m pixel scale. These values are consistent with, or shallower than, the global survey measurements. At one site, Osprey, the particle packing density appears to reach geometric saturation. We evaluate the uncertainty in these measurements and discuss their implications for other remotely sensed and mapped particles, and their importance to OSIRIS-REx sampling operations.

Itokawa has two distinct terrain types, rough highlands, and smooth lowlands. The lowlands formed by the movement of fine-grained materials from the highlands into topographic lows, covering up large boulders and producing a smooth surface. The topography of asteroids is a function of the shape, interior density, and spin rate. Itokawa, like many near-earth objects, may have experienced changes in its spin period due to YORP. Changes in spin period compared with the current 12.13 h period, may result in changes in the location of topographic lows and thus the concentration of fines in the lows. Under faster spin periods, ∼ 8 h or less, the northern topographic low, currently Sagamihara, changes location, but the southern lowland, Muses-Sea, stays in the same location. Above ∼ 8 h the topographic lows match the current geographic extent of the fine-grain lowlands. Current estimates of the timescale of regolith migration based on seismic shaking span several orders of magnitude. However, if these can be further refined, the location of the northern lowlands could be used as a constraint on the past spin rates of Itokawa The methods used in this study could be applied to other asteroids and may place an independent constraint on past spin periods.

International audience; Bennu is an~500-m-diameter rubble-pile asteroid that is the target of detailed study by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission. Here we use data from the OSIRIS-REx Laser Altimeter to assess depth-to-diameter ratios (d/D) of 108 impact craters larger than 10 m in diameter. The d/D of craters on Bennu ranges from 0.02 to 0.19. The mean is 0.10 ± 0.03. The smallest craters show the broadest range in d/D, consistent with d/D measurements on other asteroids. A few craters have central mounds, which is interpreted as evidence that a more competent substrate lies a few meters beneath them. The range of d/D narrows as crater size increases, with craters larger than 80 m tending toward smaller d/D. At large scales, increases in target strength with depth, combined with target curvature, may affect crater morphometry.

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Mercury’s polar regions host deposits of radar-bright material in regions of permanent shadow, commonly the interior of impact craters, and the deposits are hypothesised to be water ice (i.e., Chabot et al., 2018). Thermal modelling, prior to the arrival of the MESSENGER mission, found that water ice is not thermally stable in craters smaller than 10 km, assuming the craters had a depth-to-diameter ratio of 0.2 (Vasavada et al., 1999). Studies of the distribution of radar-bright deposits have identified deposits in craters under 10 km (Deutsch, et al., 2016). In this study, we used the high-resolution north polar topography from the MESSENGER mission to evaluate the morphometry and temperatures of craters with diameters of 5-10 km to explore if these craters could host stable water ice on geologic timescales. We measured the depth and diameter of 201 5-10 km in diameter craters between 75-85° N. MLA tracks that bisected the crater were used to measure the depth and diameter of 99 craters, spanning all longitudes of this north polar region. Thermal models for the north polar region of Mercury use the gridded MLA topography sampled at 1 km resolution (Paige et al., 2013; Chabot et al., 2018), so it was important to ensure the gridded topography accurately captured the craters’ shapes before using the results of these thermal models for these small craters, Comparisons between the MLA track profiles and the profiles taken through the gridded MLA product showed consistent depth to diameter profiles in both datasets, substantiating the use of the gridded MLA product to be used to determine depth and diameter values for these craters and the thermal models for these craters to be used to explore the stability of water ice in these craters. The average depth-to-diameter ratio of the 201 craters is 0.15, 25% lower than the estimate used in pre-MESSENGER thermal study (Vasavada et al., 1999). Thermal measurements of the 156 craters show that many of them have average temperatures below 110 K, meaning that they have thermal conditions that would allow water ice to be stable on geologic timescales under a thin layer of insulating material. Only three craters had small, single-pixel regions with maximum temperatures under 110 K, suggesting that water ice is not stable on the surface in the majority of small craters, except for isolated regions or below the 1-km scale of the thermal model. These results show that water ice would be stable in simple, sub-10 km diameter craters on Mercury and that the presence of radar-bright deposits in these craters is not a constraint on the age of radar-bright deposits. However, our mapping results do show a clear correlation with radar-bright signatures and longitude. In particular, around 60°E longitude, we observe a higher percentage of radar-bright craters. One of Mercury’s two cold poles is nearby at 90°E, but a large complex crater, Prokofiev-112 km in diameter, is also located at 64°E and many of the craters that are radar-bright appear to be secondaries of Prokofiev. Possible explanations for this longitude distribution are being actively investigated, including association with Prokofiev, cold-pole thermal conditions, effects of radar visibility, and the potential for uneven water ice distribution in the small craters near Mercury’s north pole.

Observations of impact crater morphology can be used to gain insight into the geological history and evolution of a planet's surface. Image data from the Mariner 10 mission revealed the diversity of impact crater morphologies and degradational states on Mercury, leading to early studies that sought to establish a stratigraphic column for the planet, despite only acquiring image data for ~45% of the surface. In 2011, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft entered orbit around Mercury, returning a high-resolution global image dataset that enables a robust analysis of crater morphology and degradation to be completed for the entirety of Mercury's surface. In this study, we conducted a visual classification of crater degradation according to initial crater morphology, and assigned a degradation state to all craters on Mercury ≥40 km in diameter. In our scheme, Class 1 craters are those that are heavily degraded, and Class 5 craters are very fresh with bright ray systems. We discuss the processes involved in crater degradation and erasure, and the challenges associated with applying crater degradation to derive the timing of geological events. We found that, based on the global spatial density of craters in each class, there appears to be a dearth of Class 1 craters within the intercrater plains, likely due to several ancient basin-sized impacts effectively obliterating a considerable portion of craters ≥40 km in diameter in this region. The crater degradation database we present here will serve as a useful tool for future analyses of Mercury's geological evolution.

We performed the first global surface roughness assessment of the asteroid 433 Eros at baselines (horizontal distances) of 4–200 m. We measured surface roughness using the root-mean-square (RMS) deviation over a variety of baselines after first detrending the height to remove long-wavelength slope effects. The global surface roughness of Eros is found to be self-affine at all baselines investigated. The surface roughness is statistically correlated with crater density at baselines of 100–200 m and boulders at a baseline of 5 m. No global spatial statistical correlation was found for baselines of 4–200 and mapped tectonic lineaments, ponds, slope, or geopotential elevation. The surface roughness of the crater Shoemaker (Charlois Regio) is controlled by the interplay of a high boulder density producing higher surface roughness values at small baselines and low crater density lowering surface roughness values at long baselines. We estimated the mobile regolith thickness (regolith that moves around and infills topography) to be 0.2–6.2 m from the difference in the surface roughness values at the baseline of 4 m. Furthermore, we find that the change in RMS deviation as a function of baseline compares favorably with the moon, and differs significantly from existing results for rubble-pile asteroid Itokawa.

Surface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad scale slope has been removed, and can be used to understand how geologic processes produce and modify a planet's topographic character at different scales. The statistical measure of surface roughness employed in this study of Mercury was the root-mean-square (RMS) deviation, and was calculated from 45–90°N at horizontal baselines of 0.5-250 km with detrended topographic data from individual Mercury Laser Altimeter tracks. As seen in previous studies, the surface roughness of Mercury has a bimodal spatial distribution, with the cratered terrain (dominated by the intercrater plains) possessing higher surface roughness than the smooth plains. The measured surface roughness for both geologic units is controlled by a trade off between impact craters generating higher surface roughness values and flood-mode volcanism decreasing surface roughness. The topography of the two terrain types has self-affine-like behavior at baselines from 0.5–1.5 km; the smooth plains collectively have a Hurst exponent of 0.88 +/- 0.01, whereas the cratered terrains have a Hurst exponent of 0.95 +/- 0.01. Subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms. The northern rise, a 1,000-km-wide region of elevated topography centered at 65° N, 40° E, is not distinguishable in surface roughness measurements over baselines of 0.5–250 km.

A lidar model of asteroid (101955) Bennu has a north-south asymmetry in shape and evidence of an early partial disruption., We investigate the shape of near-Earth asteroid (101955) Bennu by constructing a high-resolution (20 cm) global digital terrain model from laser altimeter data. By modeling the northern and southern hemispheres separately, we find that longitudinal ridges previously identified in the north extend into the south but are obscured there by surface material. In the south, more numerous large boulders effectively retain surface materials and imply a higher average strength at depth to support them. The north has fewer large boulders and more evidence of boulder dynamics (toppling and downslope movement) and surface flow. These factors result in Bennu’s southern hemisphere being rounder and smoother, whereas its northern hemisphere has higher slopes and a less regular shape. We infer an originally asymmetric distribution of large boulders followed by a partial disruption, leading to wedge formation in Bennu’s history.

The finding by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission that its target (101955) Bennu is an active asteroid has raised questions as to whether the observed particle ejection events are driven by temperature. To investigate sublimation of water ice and rock thermal fracture as possible temperature-driven causes, we modeled the global temperatures of Bennu and searched for correlations with the identified ejection points on the asteroid surface. We computed temperatures with the Advanced Thermophysical Model and the 75-cm-resolution global shape model of Bennu derived by the OSIRIS-REx mission. We find that ~1856 m^2 of Bennu's polar regions have orbit-averaged temperatures that are sufficiently cold to enable water ice, if buried within the top few meters of the surface, to remain stable over geological timescales. Millimeter-thick layers of surface water ice are also stable over ~10^3-year timescales within polar centimeter-scale cold traps. However, we do not find evidence of conditions enabling ice stability in the warmer equatorial regions, where ejection events have been observed, implying that sublimation of water ice is not the cause of particle ejection. Conversely, rock thermal fracture remains a possible mechanism of particle ejection. We find high amplitudes of diurnal temperature variation, a proxy for the efficacy of thermal fracturing, at all latitudes on Bennu due to its extreme ruggedness. Therefore, if rock thermal fracture is the mechanism, particles could be ejected from any latitude, which is consistent with the continued observations of particle ejection by OSIRIS-REx.

Multiple lines of evidence support the hypothesis that Mercury’s polar regions host deposits of water ice in permanently shadowed regions, often within the interiors of craters. Pre-MErcury, Surface, Space, ENvironment, GEochemisty, and Ranging (MESSENGER) thermal modeling of the temperature of idealized simple craters found that the interiors of these craters were too hot to host near-surface ice on geologic timescales unless within 2° of the poles. However, results from the Arecibo Observatory and the MESSENGER mission identified many small

Target heterogeneities, such as cracks, faults, joints, and blocks, are known to influence impact crater morphology on planetary surfaces. We perform a preliminary investigation into how the relationship of target heterogeneity size to projectile size affects the cratering process and final crater morphology for a fixed impact velocity. We use the CTH hydrocode to numerically simulate these impacts into a strong target with idealized heterogeneities where the ratio of the projectile size and heterogeneity size is varied. When the projectile is significantly smaller than the size of the heterogeneities, the pressure field decay is similar to that for a half-space impact into a homogenous target. In contrast, when the projectile size is comparable to or larger than the heterogeneities, we observe more efficient attenuation of the shockwave, resulting in decreased cratering efficiency. The attenuation of the shockwave is caused by rarefaction waves reflecting off of the free surfaces of the heterogeneities and internal energy losses resulting from void space collapse when the target strength is overcome by the impact energy.

High resolution images from MESSENGER provide morphological information on the nature and origin of Mercury's hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low-reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales

Surface roughness is an important metric in understanding how the geologic history of an asteroid affects its small-scale topography and it provides an additional means to quantitatively compare one asteroid with another. In this study, we report the first detailed global surface roughness maps of 25143 Itokawa at horizontal scales from 8–32 m. Comparison of the spatial distribution of the surface roughness of Itokawa with 433 Eros, the other asteroid for which this kind of analysis has been possible, indicates that the two asteroids are dominated by different geologic processes. On Itokawa, the surface roughness reflects the results of down-slope activity that moves fine grained material into geopotential lows and leaves large blocks in geopotential highs. On 433 Eros, the surface roughness is controlled by geologically-recent large impact craters. In addition, large longitudinal spatial variations of surface roughness could impact the role of YORP on Itokawa.

The Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer mission will return a sample to Earth from asteroid (101955) Bennu. Digital terrain models (DTMs) of the asteroid, and products enabled by them, are key to understanding the origin and evolution of the asteroid, providing geological and geophysical context for the sample, maximizing the amount of sample returned, navigating the spacecraft, and ensuring the safety of the spacecraft during sampling. The mission has two approaches for producing these DTMs: a camera-based approach and a lidar-based approach. We provide an overview of the methods used for these two approaches and how they fit into the originally planned mission. We also discuss a summary of tests using these plans to evaluate the expected performance of the DTMs and describe the data products derived from them.

In the version of this Article originally published, in the sentence “There are three identified boulders with long axes exceeding 40 m and more than 200 boulders larger than 10 m”, the term “10 m” should have been “10 m (ref.)”. This has now been corrected.

Data acquired by the Mercury Laser Altimeter and the Mercury Dual Imaging System on the MESSENGER spacecraft in orbit about Mercury provide a means to measure the geometry of many of the impact craters in Mercury's northern hemisphere in detail for the first time. The combination of topographic and imaging data permit a systematic evaluation of impact crater morphometry on Mercury, a new calculation of the diameter Dt at which craters transition with increasing diameter from simple to complex forms, and an exploration of the role of target properties and impact velocity on final crater size and shape. Measurements of impact crater depth on Mercury confirm results from previous studies, with the exception that the depths of large complex craters are typically shallower at a given diameter than reported from Mariner 10 data. Secondary craters on Mercury are generally shallower than primary craters of the same diameter. No significant differences are observed between the depths of craters within heavily cratered terrain and those of craters within smooth plains. The morphological attributes of craters that reflect the transition from simple to complex craters do not appear at the same diameter; instead flat floors first appear with increasing diameter in craters at the smallest diameters, followed with increasing diameter by reduced crater depth and rim height, and then collapse and terracing of crater walls. Differences reported by others in Dt between Mercury and Mars (despite the similar surface gravitational acceleration on the two bodies) are confirmed in this study. The variations in Dt between Mercury and Mars cannot be adequately attributed to differences in either surface properties or mean projectile velocity.