Exploring the peak-ring to multiring basin transiton on the moon

Introduction: The formation of impact basins played a dominant role in the evolution of ancient planetary crusts. Despite their importance, the formation of large basins, and especially the transition from peakring to multiring basin morphologies, is poorly understood. The prevalence of impact craters on the Moon and wealth of geophysical data present an opportunity to better understand basin formation and the peak-ring to multiring basin transition. Hertzsprung Basin, the smallest basin classified as a multiring basin on the Moon, has a distinct inner depression and 3 rings at 256 km, 408 km, and 571 km diameter [1]. FreundlichSharonov, the largest identified peak-ring basin, is slightly smaller with a peak-ring at 200 km and an outer ring at 582 km diameter [1, 2]. We consider both basins to be at the transition between peak-ring and multiring basins. The intermediate ring of Hertzsprung is much lower in relief than the inner or outer rings, unlike the intermediate rings of well-developed multiring basins. A similar intermediate ring in Freundlich-Sharonov could have been obscured by the large craters within the basin. In high-resolution crustal thickness models [1], both basins show a cap of crustal material in the center of the basin, with the cap of Hertzsprung being thicker than that of Freundlich-Sharonov (Figure 1). Hertzsprung also shows a bench-like structure in the crustmantle morphology, defined by a step-like geometry of the crustal thickness profile (yellow star) which is not observed in Freundlich-Sharonov or in larger multiring basins. This difference in crust-mantle shape raises the question of whether bench development is related to the transition from peak-ring to multiring basins, and what accounts for the difference between FreundlichSharonov and Hertzsprung basins. Here we simulate lunar basin formation to understand how pre-impact conditions affect basin formation and the transition between peak-ring basin and multiring basins, with a focus on the bench structure of Hertzsprung and reproducing the crustal caps of both basins. Methods: We model impact crater formation on the Moon using the shock-physics hydrocode iSALE2D [3-7]. Our models consist of a dunite impactor hitting a granitic crust overlying a dunite mantle, testing crustal thicknesses between 30-60 km and lithospheric thermal gradients from 10-20 K/km. All models had 1 km resolution and an impactor diameter of 40 km. We are testing a variety of crustal thicknesses and thermal Figure 1: Topography and crust-mantle boundary profiles for Freundlich-Sharonov (A) (after [2, 8]) and Hertzsprung (B) (after [11]) basins derived from GRAIL data. Crustal composition is gray and mantle composition is white. The bench morphology, highlighted by a yellow star, of the crust-mantle interface of Hertzsprung is not seen in the comparably-sized Freundlich-Sharonov basin.