Anatomy of a marine-target impact structure by a “rubble-pile” asteroid in field observations, impact experiments, and numerical simulation.

The Lockne crater (7–12 km) and its smaller companion Målingen (0.7 km) formed simultaneously at 458 Ma in a shallow sea, resulting in exceptional preservation of crater fill and near-field ejecta. Their paired formation constitutes the only confirmed terrestrial impact by a binary asteroid. The event is linked to a major Middle Ordovician breakup in the Main Asteroid Belt (~470 Ma), implying that the impacting bodies were rubble-pile aggregates. The marine setting, with seawater and sedimentary strata overlying a flat crystalline basement, represents an extreme case of layering with strong property-contrasts, known to influence crater morphology and produce concentric structures. Such effects also have relevance for Mars, where concentric craters can indicate sedimentary rock and former habitable environments.

At Lockne, an inner 7.5 km wide basement crater is surrounded by a shallow ~12 km outer crater recorded in the sedimentary target rocks. It formed by a shallow excavation flow prior to deposition of basement crater ejecta, and is offset downrange due to oblique impact. At Målingen, the 0.7 km basement crater’s ejecta distribution indicates a wider but poorly preserved outer crater. Lockne subsurface geology is known from 11 shallow cores to ~335 m depth, but this is estimated to represent only a third of the crater’s true depth.

Binary asteroids are commonly rubble-piles, and although ~16% of asteroids are observed to be binary, the fraction of rubble-piles is likely much higher because original companions may have been lost. Several aspects of the Lockne morphology, notably an abnormally wide shallow outer crater surrounding the basement crater, are interpreted as consequences of a rubble-pile impact in the stratified target.

Previous 3-D simulations of the Lockne impact used a monolithic impactor. For an impact at 45° and 15 km/s, these models indicate a ~600 m projectile and target water depth slightly less than the projectile diameter, producing a ~5 km transient basement crater. Målingen was estimated at ~150 m if massive. However, rubble-piles of this size may fragment during atmospheric entry forming a “pancake-like” cluster significantly wider than the original body. Such clustered impacts distribute more energy near the surface producing shallower, wider craters. Obliquity increases breakup, enhances near-surface energy release, and intensifies downrange asymmetry. Thus, a rubble-pile could produce a wider crater than a monolithic equivalent and potentially influence basement crater depth.

To investigate crater formation mechanisms, we performed impact experiments and numerical simulations of clustered impactors. Experiments were carried out with the EPIC single stage gas gun at CAB CSIC-INTA, Spain, to launch Delrin projectiles up to ~400 m/s. Clustered projectiles were made from weakly bonded 2 mm spheres to obtain equal mass to 20 mm solid reference projectiles, and high-speed cameras recorded both half-space and quarter-space impacts. Numerical modeling in iSALE-2D is ongoing, testing several rubble-pile configurations.

Acknowledgements: This work was supported by grant PID2021-125883NB-C22 by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’, and the Spanish Research Council (CSIC) support for international cooperation I-LINK (#ILINK22061).