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Planetary defense efforts rely on estimates of the mechanical properties of asteroids, which are difficult to constrain accurately from Earth. The mechanical properties of asteroid material are also important in the interpretation of the Double Asteroid Redirection Test (DART) impact. Here we perform a detailed morphological analysis of the surface boulders on Dimorphos using images, the primary data set available from the DART mission. We estimate the bulk angle of internal friction of the boulders to be 32.7 ± 2. 5° from our measurements of the roundness of the 34 best-resolved boulders ranging in size from 1.67–6.64 m. The elongated nature of the boulders around the DART impact site implies that they were likely formed through impact processing. Finally, we find striking similarities in the morphology of the boulders on Dimorphos with those on other rubble pile asteroids (Itokawa, Ryugu and Bennu). This leads to very similar internal friction angles across the four bodies and suggests that a common formation mechanism has shaped the boulders. Our results provide key inputs for understanding the DART impact and for improving our knowledge about the physical properties, the formation and the evolution of both near-Earth rubble-pile and binary asteroids. Planetary Defense efforts rely on estimates of asteroids' mechanical properties, which are difficult to obtain accurately from Earth. Here, authors compare images from space missions to the rubble-pile asteroids Dimorphos, Itokawa, Ryugu, and Bennu and study such properties through boulders on their surface. [ABSTRACT FROM AUTHOR]

Images collected during NASA's Double Asteroid Redirection Test (DART) mission provide the first resolved views of the Didymos binary asteroid system. These images reveal that the primary asteroid, Didymos, is flattened and has plausible undulations along its equatorial perimeter. At high elevations, its surface is rough and contains large boulders and craters; at low elevations its surface is smooth and possesses fewer large boulders and craters. Didymos' moon, Dimorphos, possesses an intimate mixture of boulders, several asteroid-wide lineaments, and a handful of craters. The surfaces of both asteroids include boulders that are large relative to their host body, suggesting that both asteroids are rubble piles. Based on these observations, our models indicate that Didymos has a surface cohesion ≤ 1 Pa and an interior cohesion of ∼10 Pa, while Dimorphos has a surface cohesion of <0.9 Pa. Crater size-frequency analyzes indicate the surface age of Didymos is 40–130 times older than Dimorphos, with likely absolute ages of ~ 12.5 Myr and <0.3 Myr, respectively. Solar radiation could have increased Didymos' spin rate leading to internal deformation and surface mass shedding, which likely created Dimorphos. Images collected during NASA's DART mission of the asteroid Didymos and its moon, Dimorphos, are used to explore the origin and evolution of the binary system. Authors analysis indicate that both asteroids are weak rubble piles and that Didymos' surface should be about 40 to 130 times older than Dimorphos. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission revealed that the asteroid Bennu has ~1500 impact craters (Bierhaus et al., 2022). Here we use data from the OSIRIS-REx laser altimeter (OLA) to measure the depths, d, and diameters, D, of a sample of small (D ≤ 10 m) impact craters that span a broad range of latitudes and longitudes. d/D in this sample ranges from 0.04 to 0.27, with a mean of 0.13 ± 0.04. Compared to larger (D ≥ 10 m) craters on Bennu, those studied here have a greater average d/D. The mean d/D of craters on Bennu's younger, Rugged Unit is statistically larger than the mean d/D of those on the older, Smooth Unit. One of the studied craters contains an interior mound that may indicate the presence of stronger material at depth. A large fraction of craters studied here are located near large boulders whose lengths are >50% the diameter of the crater. Some of these large boulders cross the crater rim crest, protruding into and perched above the crater interior. In the most extreme cases, the boulder length is ~2.5 times the crater diameter. The d/D of craters near large boulders is bimodal, which could be a consequence of how boulders affect crater formation. Laboratory experiments and results from Hayabusa2's Small Carry-On Impactor experiment indicate that preexisting boulders and heterogeneities affect crater formation and d/D. The abundance of small craters on Bennu makes the asteroid a rich resource for understanding impact processes. • Craters on Bennu smaller than ~10 m have depth-diameter (d/D) ratios between 0.04 and 0.27, with a mean of 0.13 0.04.Craters on the Rugged Unit have a larger average d/D than those on the Smooth Unit. • The number of craters with a central mound is consistent with predictions based larger craters. • The distribution of d/D suggests that crater formation is affected by how far large boulders protrude above the crater. [ABSTRACT FROM AUTHOR]

Abstract: There are ∼300 features on the Asteroid 433 Eros that morphologically resemble ponds (flat-floored and sharply embaying the bounding depression in which they sit). Because boulders on Eros are apparently eroding in place and because ponds with associated boulders tend to be larger than ponds without blocks, we propose that ponds form from thermally disaggregated and seismically flattened boulder material, under the assumption that repeated day/night cycling causes material fatigue that leads to erosion of the boulders. Results from a simple boulder emplacement/thermal erosion model with boulders emplaced in a few discrete events (i.e., large impacts) match well the observed size distribution. Under this scenario, the subtle color differences of ponds (somewhat bluer than the rest of the surface) might be due to some combination of less space-weathered material and density stratification of silicate-rich chondrules and more metal-rich matrix from a disaggregated boulder. Volume estimates of ponds derived from NEAR Laser Rangefinder profiles are consistent with what can be supplied by boulders. Ponds are also observed to be concentrated in regions of low slope and high elevation, which suggests the presence of a less mobile regolith and thus a contrast in the resistance to seismic shaking between the pond material and the material that makes up the bounding depression. Future tests include shake-table experiments and temperature cycling (fatigue) of ordinary chondrites to test the thermal erosion mechanism. [ABSTRACT FROM AUTHOR]

Recent asteroid missions have revealed that many sub-kilometer asteroids are rubble piles. Large parts of their surfaces are covered with boulders larger than tens of centimeters. An evaluation of the abundance and size distribution of boulders provides clues to understand surface processes on boulder-covered asteroids. Here we report a new method that automatically measures the abundance of small boulders (sub-pixel to a few pixels), whose boundaries cannot be recognized with visual inspection, by quantifying the surface radiance variation that occurs during the spinning of the asteroid. After validating our approach with previous boulder counting data, we apply this method to images of the asteroids Ryugu and Itokawa, which were visited by JAXA's Hayabusa and Hayabusa2, and obtain a global distribution of the boulders larger than 0.75–3 ​m, which corresponds to 1.5–6 pixels. We find that the boulder number density of this size range is smaller (1) on the western bulge than on the eastern hemisphere and (2) on the equatorial ridge than on the higher latitudes, both of which exceed the number density of boulders ​> ​5 ​m by an order of magnitude. The boulder size distribution at 1.25–20 ​m shows that the boulders smaller than 1 ​m are more abundant at the equator than at mid-latitudes, while those larger than 1 ​m in diameter are more abundant at mid-latitudes than at the equator. This contrast suggests size-dependent migration of boulders in the latitudinal direction. We also find that the typical boulder size (the size reaching the cumulative areal coverage of 50%) is 1.9 ​m at the equatorial region (10°S-10°N) while it is 2.6 ​m at mid-latitudes (40°S-50°S, 40°N-50°N). The typical boulder size is also smaller in the western bulge (2.0–2.2 ​m). We construct global maps of the power-law index of the size frequency distribution of boulders and find minor variations over the entire surface of Ryugu (−2.53 ​± ​0.03) for boulders larger than 1.25 ​m. This small variation suggests homogeneous size sorting processes on Ryugu. Surface roughness does not show a significant correlation with the v-band albedo but shows a high anti-correlation (R ​= ​−0.73) with the current geological slope on the eastern hemisphere. Our method is useful enhancement of smooth area detection and boulder distribution characterization that will be applicable to other planetary explorations in the future, including those of Phobos and other asteroids. • Automatic analysis of pixel-scale boulders based on pixel radiance variation as asteroid spins. • Distribution of sub-meter boulders on Ryugu indistinguishable from visual counting. • Method detects the east-west dichotomy in the distribution of small boulders (several meters in diameter). • Uniform power-law index of boulder size frequency distribution on Ryugu suggests homogeneous size sorting. • Boulder distribution is correlated with current geologic slope. [ABSTRACT FROM AUTHOR]