Lockne: A layered-target impact structure from a rubble-pile impactor analysed in field observations, impact experiments, and numerical simulation.

Rubble-piles may constitute nearly all of the asteroids in the size range 0.2-10 km [1]. The reason for the high abundance has been suggested to be their high durability against disruption due to their shock-absorbent nature compared with monolithic bodies [2]. The Lockne crater (7–12 km) and its smaller companion Målingen formed simultaneously at 458 Ma in a shallow sea, resulting in exceptional preservation of crater fill and near-field ejecta [3]. The impact event is linked to the Middle Ordovician breakup in the Main Asteroid Belt (~470 Ma), implying that the impacting bodies were rubble-pile aggregates [4].

The marine setting at the Lockne impact event, 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 [5, 6]. It formed by a shallow excavation flow prior to deposition of basement crater ejecta, and is offset downrange due to oblique impact [5, 6]. At Målingen, the 0.7 km basement crater’s ejecta distribution also indicates a wider but poorly preserved outer crater.

Previous 3-D simulations of the Lockne impact used a monolithic impactor [5]. 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 [4]. However, rubble-piles of this size may deform during atmospheric entry forming a “pancake-like” cluster significantly wider than the original body [7]. Such clustered impacts distribute more energy near the surface producing shallower, wider craters. Obliquity increases the flattening, enhances near-surface energy release, and intensifies downrange asymmetry. This seems indicated in the morphology of the concentric Lockne impact structure where the shallow outer crater appears wider than what can be obtained with numerical simulation using a massive impactor [5, 6].

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 3 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 [8, 9] is ongoing, testing several rubble-pile configurations. Preliminary results indicate a peak energy release at higher position in the target for oblate rubble-pile projectiles ("pancake") than for massive spherical projectiles leading to wider, shallower impact structures consistent with observation in nature.

 

Acknowledgements: This work was supported by grants PID2021-125883NB-C22 and PID2024-160976NB-I00 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).

 

References: [1] Walsh, K. (2018), Rubble pile asteroids. Annu. Rev. Astron. Astrophys. [2] Jourdan, F. et al. (2023), Rubble pile asteroids are forever. Proc. Natl. Acad. Sci. U.S.A. 120(5). [3] Ormö, J et al. (2014), The geology of the Målingen structure: A probable doublet to the Lockne marine-target impact crater, central Sweden. Meteorit. Planet. Sci. [4] Ormö et al. (2014), First known Terrestrial Impact of a Binary Asteroid from a Main Belt Breakup Event. Sci. Rep.  [5] Lindström et al. (2005), Lockne crater as a result of marine-target oblique impact. Planet. Space Sci. [6] Ormö, J., et al. (2013), A new method to determine the direction of impact: Asymmetry of concentric impact craters as observed in the field (Lockne), on Mars, in experiments, and simulations. Meteorit. Planet. Sci. [7] Shuvalov, V. V. (2002), In Impacts in Precambrian Shields (eds Plado, J. & Pesonen, L. J.),Springer, Berlin-Heidelberg.  [8] Amsden et al. (1980), LANL Rep. LA-8095. [9] Wünnemann et al. (2006) Icarus, 180, 514–527.