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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.

The shape of boulders on planetary surfaces can be used to investigate the geological processes that influence them and to estimate key surface mechanical properties. Such studies are typically based on 2D images, although 2D projections of irregular particles are known to differ from their 3D geometry. To investigate how well 2D images represent 3D morphological parameters, we assess to compare particle shapes in both representations (2D and 3D). Three different granular samples were investigated (LHS-1 lunar highland simulant, Øysand sand, and glass grit) using 3D X-ray computed tomography (3DXRCT) to reconstruct 3D particle geometries within the samples. More than 2700 particles are analyzed using both their full 3D shapes and different 2D projections derived from the 3D reconstructions. Using correlations between 2D and 3D morphological analyses of the 3DXRCT scan data set, we establish statistical relationships, with associated confidence intervals, between the two representations. The outcome is a methodology for estimating 3D size and shape parameters from 2D measurements. We apply this approach to boulders observed on asteroid Bennu by the NASA OSIRIS-REx mission. The resulting 3D size and shape parameters, estimated from 2D images, closely match the true 3D parameters derived from LIDAR data. We conclude with a set of recommendations for performing reliable morphological analyses of boulders and particles in 2D planetary images, including considerations of image resolution and sample size.

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.