Numerical simulations of Lockne impact crater formation using iSALE-2D

Introduction: The Lockne impact structure, located in central Sweden, is a well-preserved marine-target crater that was formed ~470 million years ago in the epicontinental sea that covered great part of the Baltoscandia [1][2]. The water depth at the time of the impact is interpreted to approximately have been 500 meters, or possibly more [3]. The nearly horizontal target rocks were comprised of, from top to bottom, ~50m of limestone, ~30m of dark, organic rich, shale, and Proterozoic crystalline basement [1]. The Locke structure consists of a concentric 7.5 km wide nested crater, in the basement, surrounded by a 14 km wide outer crater, where most of the sediment material was excavated [2]. Core drilling in the interior of the nested inner crater revealed crater-fill breccias composed mostly by sedimentary material, interfingered with crystalline breccia lens (Tandsbyn breccia) and resurge deposits (Lockne breccia and Loftarsone). Lockne-Målingen crater doublet has been hypothesized to have formed by a rubber-pile “pancake” shaped impactor [4]. This study aims to understand, through numerical simulations, the crater formation based on different asteroid parameters such as density, shape, and velocity.

Methodology: The formation of Lockne is being simulated by iSALE, an extension of the SALE hydrocode   developed   to   model   impact   crater   formation [5,6,7,8]. Current study focuses on iSALE-2D simulations with an axisymmetric approximation of the original impact problem and a resolution of 20, 30 and 60 CPPR (cells per projectile radius), depending on the simulation. The main question to be explored is the influence of the impactor parameters on crater development.

In the 60 CPPR simulations, we consider a four-layer target represented by different equations of estate (EoS): (1) crystalline basement as granite, (2) 30 meters of mudstone as wet tuff, (3) 50 meters of limestone as calcite, and (4) 500 meter of sea water. In this case we set a 600-meter wide massive asteroid traveling at 20km/sec.

For the 30 CPPR simulations the model comprises a three-layer target: (1) crystalline basement, (2) ~80 meters of limestone, and (3) 500 m of sea water. In the first set of 30 CPPR, we consider a massive 600-meter wide asteroid with velocity of 20 km/sec, and also a 7km/sec simulation where, in order to keep the same kinetic energy released by the impact, the asteroid diameter was expanded to 1200 m. To compensate the doubled volume, we set damage value to the maximum of 1, suggesting less cohesive, and more fragmented, material.

The 20 CPPR simulations consider a “pancake” shaped asteroid based on the original and doubled asteroid size. We kept the original volume of the asteroid and calculated the new diameter/height in an approximately 8:1 ratio. For these simulations, we consider the three-layer target (similar to 30 CPPR simulations), asteroid damage set to 1, and impact velocity of 7 km/sec.

Results and Discussion: Cratering processes were observed mostly in 30 CPPR simulations, which have reached up to ~900 seconds. Simulations show a maximum transient crater at approximately 15 seconds, and the ejecta curtain starts to collapse over the water layer around 25-30 seconds, forming a tsunami wave that moves outwards. Some amount of water is kept in the crater interior without being ejected and then, at 52 seconds, the sea water starts to move back into the crater, gradually filling the structure, bringing ejecta sediments and crystalline material. At about 650 seconds the water layer is stable and covering the entire 8km wide structure.

In the 30 CPPR simulations, the higher velocity simulation revealed a pressure peak of ~90GPa at 0.1 seconds (same was observed for 60 CPPR), whereas the lower velocity simulations show a peak of ~50GPa, even with the enlarged projectile. Temperature peaks are also higher for faster impact, being ~8000K for 20km/sec (similar in 60 CPPR) and 1850K for 12km/sec. The amount of kinetic energy on both cases is similar but the velocity itself seems to play an import role in pressure and temperature conditions. If not, these significant differences can be attributed to different asteroid damage values. New simulations, with similar asteroid damage values, are being prepared to better understand the influence of damage on crater formation. Other differences are related to volume of excavated material. Higher speed simulations show a maximum transient crater with 8km in diameter and 2.0km in depth, whereas lower speed impact show a 7 km wide crater with 2km in depth.

The 20 CPPR simulations with a “pancake” shaped asteroid were performed keeping exactly the same parameters as previous 30 CPPR simulations, just changing the asteroid shape. Peak pressure and temperature were lower for spherical asteroids (Table 1), being the difference in temperature more significant than the difference in pressure.

As future work, we intend to perform simulations where the asteroid density is decreased by addition of porosity properties to the material. Also, to perfom different combinations in order to explore the actual role of projectile velocity and damage.

Peak	Spherical projectile	Ellipsoid projectile
Pressure	45 GPa	52 GPa
Temperature	1850 K	11500 K

Table 1. Peak pressure and temperature for two identical simulations except for the  projectile shape. Speed: 7km/sec, Damage=1

References: [1] Lindström et al. (2005) Impact Studies, Springer 357-388 [2] Ormö et al. (2007) Meteoritics & Planet. Sci. 42, 1929-1943 [3] Ormö et al. (2002) JGR, 107, 31-39 [4] Sturkell E. and Ormö J., EPSC abstracts 2020, EPSC2020-956. [5] Melosh H.J. et al. (1992) JGR 97, no. E9, 14735-14759. [6] Ivanov B.A. et al. (1997) Int. J. Impact Eng. 20, 411-430. [7] Collins G. et al. (2004) MAPS 39, 217-231. [8] Wunnemann K. et al. (2006) Icarus 180, 514-527.

Acknowledgements: The authors are grateful to the CSIC financial support for international cooperation: I-LINK project LINKA20203 “Development of a combined capacity of numerical and experimental simulation of cosmic impacts with special focus on effects of marine targets”.