Impact excavation processes studied by Ground-penetrating radar (GPR) on terrestrial analogues for Mars.
Authors: Wilk Jakob , Wulf Gerwin 
Published: July 2021

Introduction: Recent studies have shown some terrestrial impact structures to possess ejecta ramparts similar to martian impact craters, e.g. Lonar crater in India [1], and Ries crater in Germany [2]. Nonetheless, the formation of ejecta rampart structures is still controversial and several formation models have been proposed to explain the rampart formation process (see review by [3]).The 26 km diameter Ries crater in Germany is a relatively pristine, complex impact crater characterized by two distinct ejecta lithologies. The built up of the continuous ejecta blanket by a polymict lithic breccia, called Bunte breccia, generally composed of mainly unshocked to weakly shocked sedimentary target clasts, minor amounts of crystalline basement fragments, and reworked surficial sediments [4], overlain by patches of shocked and partly melted crystalline basement material, called suevite [6, 7], is well exposed in several outcrops within the outer rim (see Aum&#252;hle Fig.1A).&#160;This study focuses on the trackability of the ejecta layers transition and the documented thickening of the ejecta deposits [2] by ground-penetrating radar (GPR) in Aum&#252;hle. We plan to assess the capability of the GPR method for an analogue of particular importance, Lonar crater. Since, Lonar crater occurred in basalts of the Deccan Plateau [5,6], it represents a target lithology that is comparable to the massive basaltic plateaus on Mars.Methods: The GPR study for Ries crater is investigating the subsurface contact between Bunte breccia and suevite with a bi-static 50MHz-200MHz antenna configuration for constant separation traversing profiles. The spatial arrangement of the profiles in grids allows a reconstruction of the boundary layer (contact plane). In addition, the attenuation of the radar signal has to be studied for layered ejecta deposits, as present in Aum&#252;hle, and in a not yet determined basaltic analogue.Studies prospective: The continuous ejecta blanket of Lonar crater extends to an average distance of 1 km from the crater rim (Fig.1B) and is reported to be divided into two ejecta types as well [8, 9, 10].&#160; An apparent thickening of the ejecta deposits is observable in its distal parts similar to rampart structures on mars [1, 11]. If the Ries study provides a feasible concept, this study will be extended to the distal ejecta parts at Lonar.Acknowledgments: We thank the &#8220;Wissenschaftliche Gesellschaft, Freiburg im Breisgau&#8221; for funding the field trip to Ries crater.References: [1] Maloof et al. 2009 Geological Society of America Bulletin 122:109&#8211;126. [2] Sturm et al. 2013 Geology, 41, 531-534. [3] Weiss and Head 2018 Meteoritics &amp; Planet. Sci., 53, 741-777. [4] H&#246;rz et al. 1983 Reviews of Geophysics and Space Physics, 21, 1667-1725. [5] Osae et al. 2005 Meteoritics &amp; Planet. Sci., 40, 1473&#8211;1492. [6] St&#246;ffler 1977 Geologica Bavarica, 75, 443-458. [7] Von Engelhardt 1997 Meteoritics &amp; Planet. Sci., 32, 545-554. [8] Fredriksson et al. 1973 Science, 180, 862&#8211;864. [9] Kumar et al. 2014. Journal of Geophysical Research: Planets 119, 2029&#8211;2059 [10] Kiik et al. 2020 Geosciences 10, 417. [11] Wulf et al. 2019 Earth. Planet. Sc. Lett., 506, 209-220. [12] USGS Earth Explorer website.