William F. Bottke

Deimos, the smaller and outer Martian moon, is a dark, irregularly shaped body whose origin and evolution are still poorly constrained. Its surface is smoother and less cratered than the other moon, Phobos, a difference still unexplained due to limited data coverage and resolution. Here, we reveal the global distribution of craters and boulders on Deimos, including areas previously poorly observed, using high‐resolution images from the Emirates Exploration Imager onboard the Emirates Mars Mission. Craters on Deimos have not reached saturation equilibrium, suggesting a recent resurfacing event. The leading side shows a slightly higher spatial crater density than the trailing side, while the Mars‐facing side has more craters than the anti‐Mars side, contrary to theoretical predictions and observations from Phobos. These asymmetries suggest recent reorientation of Deimos and/or the reaccumulation of dust across the Martian system, providing new insights into the dynamical and geological evolution of Deimos.

The study of S-complex near-Earth objects (NEOs), the parent bodies of ordinary chondrites, has shown that they are dominated by asteroids with LL chondrite-like compositions. This is surprising because among the three subtypes of ordinary chondrites (H, L, and LL), LL chondrites are the least common, representing only 10% of all ordinary chondrite falls. This discrepancy has been attributed to the size of the NEOs studied, which are likely too large to be the immediate precursors of the meteorites that fall on Earth. To test this hypothesis, we obtained near-infrared spectra (0.7–2.5 μ m) of a group of objects with absolute magnitudes 20.0 ≤ H ≤ 29.2 (sizes ∼4–343 m). The sample was divided into subgroups based on their H value, and the composition of the asteroids was determined. We found that the dominance of LL chondrite-like objects disappears at sizes of ∼31–49 m. At this size range, asteroids with L chondrite-like compositions become dominant, matching the fraction of L chondrite meteorite falls. In contrast, the fraction of H chondrite-like NEOs was found to be much lower than the proportion of H chondrite falls, even among the smallest objects. We determined an upper size limit of ∼18 m for the parent bodies of these meteorites. The same upper limit was established for the pre-atmospheric parent bodies of LL chondrites. These results constitute the first observational evidence for a size dependence in the composition of S-complex NEOs.

Multiple studies have proposed a substantial surge in large lunar impacts approximately 800 Myr ago (Ma). Some are based on analyses of the ages of large lunar craters, such as the 93 km Copernicus crater. Others focus on the age distributions of impact glasses returned by lunar missions. A key challenge has been identifying and testing a plausible source for this putative impact spike. Here we use collisional and dynamical models to link this event to the formation of the Eulalia asteroid family, whose primitive carbonaceous chondrite-like parent body was disrupted ∼800 Ma near the 3:1 mean motion resonance with Jupiter (J3:1). Our simulations indicate that approximately three-quarters of the family’s fragments eventually entered the J3:1 over a ∼150 Myr interval. While some fragments were injected into the resonance immediately after the disruption, others migrated more gradually via nongravitational (Yarkovsky) thermal forces. Once in the J3:1, the fragments were dynamically transported into the planet-crossing region, leading to an elevated rate of bombardment on the Moon and terrestrial planets. Our results demonstrate that the Eulalia breakup can plausibly account for the observed lunar craters formed near 800 Ma. Intriguingly, this event may also have had widespread repercussions across the inner solar system. On Earth, its timing coincides with significant shifts in the biosphere, possibly linked to large impacts. On Mars, these impacts might have triggered a pulse of volcanic activity. Together, they showcase how certain catastrophic collisions in the main belt can have far-reaching consequences for the history of the terrestrial planets.

The main belt asteroid (52246) Donaldjohanson (DJ) is a likely member of the Erigone asteroid family. This implies that DJ is a fragment of a larger parent body that was destroyed in a collision about 155 million years ago. We report observations taken during a flyby of DJ by the Lucy spacecraft. We found that DJ is composed of two heavily cratered lobes, connected by a smoother neck, with overall dimensions 8.8 kilometers (km) by 4.4 km by 3.1 km. The crater density is consistent with the Erigone family’s age, except for craters <0.4 km, which have been preferentially erased. DJ rotates slowly in a tumbling state, likely owing to spin-down by radiative forces. Surface spectra show iron-bearing phyllosilicates, indicating moderate aqueous evolution on the parent body.

The South Pole–Aitken (SPA) basin-forming impact was a critical event in the Moon’s history. Despite being the oldest and largest acknowledged basin, critical details including the impactor’s size, nature, direction, and fate of the ejecta remain uncertain. Here, we simulate SPA’s formation and resultant crustal structure. We find that SPA’s observed shape of an ellipse tapered toward the south is best reproduced by a 260-km-diameter differentiated impactor striking with a north-to-south trajectory. In this scenario, the impact disperses lunar mantle materials in the cross-range and the downrange directions away from the lunar farside. Much of the mantle ejecta collapses into the basin interior, consistent with the distribution inferred from gravity. These results suggest that the Artemis landing sites near the lunar south pole should contain abundant SPA ejecta including mantle materials. Samples returned from these regions should therefore reveal the age of SPA and the composition of the lunar mantle.

The South Pole‐Aitken basin (SPA) is the largest and oldest impact basin on the Moon and was excavated through the crust and into the mantle early in lunar history. Here, we use gravity data to identify heterogeneities in crustal density that could indicate deeply sourced material in the crust despite a lack of surface spectral signatures. We show the presence of a ∼400‐km‐wide annulus of large‐amplitude, small‐scale gravity anomalies associated with the basin's rim region, which we interpret as the SPA ejecta blanket. We perform a pair of gravity inversions, varying either the thickness of a dense layer or the concentration of dense material in a mixed layer to match the gravity observations, and determine that the SPA ejecta blanket likely contains ≳5 million km ³ of mantle‐derived material. Three craters excavate the sources of these anomalies, revealing varied compositions that imply that the lunar mantle may have had orthopyroxene‐ and clinopyroxene‐rich components at similar depths at the time of the SPA impact, with important implications for the timing of mantle overturn. Gravity anomalies also reveal a previously unnoticed 1,440‐km‐diameter peak‐ring within SPA, delineating the transition between the collapsed ejecta and the central melt sheet. The annulus of strong anomalies associated with the ejecta encompasses the south polar region, suggesting a strongly heterogeneous subsurface near several future missions' landing targets, including the upcoming Artemis missions. These results place an important constraint on basin formation, reveal the composition of the mantle, and will guide future studies in targeting this material.

We use our home catalog of the asteroid proper elements to study the Karin family. The hierarchical clustering method provides formal identification with 3863 members, but this set also includes objects from the neighboring Koronis2 and Kuitaisi families, as well as interlopers originating from the much older Koronis family. By tracking the trajectories of cluster objects backward in time, we identified 2161 asteroids whose orbits converged with that of their parent body (832) Karin at 5.72 ± 0.09 Mya (95% C.L.). This method of calculating the family’s age is based on a novel convergence metric that is directly related to the velocities at which fragments were ejected from (832) Karin. We analyze the extent to which members ≤1.5 km in diameter had drifted in semimajor axis due to Yarkovsky thermal forces and find it reflects the tilt of their rotation poles away from the ecliptic, recording the influence of the YORP torque. Karin’s size frequency distribution in the ≃(0.8–3) km range follows a power law with a cumulative slope index −3.20 ± 0.01. Removing members of the Karin family from the original group, we examine the Koronis2 family, whose members are associated with (158) Koronis. We find it difficult for large members of the Koronis2 family to converge with the orbit of (158) Koronis within its previously estimated age of 7.6 Myr. Achieving such convergence would require the Koronis2 family to be older than 10 Myr, but our result must be verified with a direct numerical approach in the future.

The South Pole-Aitken basin (SPA) is the largest and oldest impact basin on the Moon and excavated through the crust and into the mantle early in lunar history. Here we use gravity data to identify heterogeneities in crustal density that could indicate deeply sourced material in the crust despite a lack of surface spectral signatures. We show the presence of a ~400-km-wide annulus of large-amplitude, small-scale gravity anomalies associated with the basin’s rim region, which we interpret as the SPA ejecta blanket. We perform a pair of gravity inversions, varying either the thickness of a dense layer or the concentration of dense material in a mixed layer to match the gravity observations, and determine that the SPA ejecta blanket likely contains ≳5 million km3 of mantle-derived material. Three craters excavate the sources of these anomalies, revealing varied compositions that imply that the lunar mantle may have had orthopyroxene- and clinopyroxene-rich components at similar depths at the time of the SPA impact, with important implications for the timing of mantle overturn. Gravity anomalies also reveal a previously unnoticed 1200-km-diameter peak-ring within SPA, delineating the transition between the collapsed ejecta and the central melt sheet. The annulus of strong anomalies associated with the ejecta encompasses the south polar region, suggesting a strongly heterogeneous subsurface near several future missions’ landing targets, including the upcoming Artemis missions. These results place an important constraint on basin formation, reveal the composition of the mantle, and will guide future studies in targeting this material.