Ejecta Facies at Lonar Crater, India: Inventory of Expected Impactites, Thermal Infrared Spectroscopy, and Implications for Mars

Introduction: While a common goal of the study of terrestrial impact sites and their impactites is for insight into the impact process, another goal is the application of these data and lessons learned to the understanding of the surfaces of other planetary bodies. Here, the ejecta structure of one unique terrestrial ejecta blanket is described in detail along with descriptions of impactite sample analyses and how these data can be applied current and future rovers of Mars and perhaps the Moon. Lonar Crater ejecta structure: Whereas field data of any preserved terrestrial ejecta blanket is relevant to understanding the impact process and products, studies of Lonar Crater, India have implications for Mars [1-4]. The target Deccan basalt provides an opportunity to examine terrestrial shocked basalt similar to shergottites from Mars [4-5]. Further, Deccan basalt has been labeled as an excellent analog for Surface Type 1, a thermal infrared spectral type identified from orbital and Rover observations [6]. Field geology at Lonar Crater (diameter = 1.8 km) reveals a DLE structure with two distinct layers of ejecta [1,7] (Figure 1). The lower unit is lithic breccia extending to the limits of the continuous ejecta blanket (CEB), or 1.4 km (~1 1⁄2 crater radii) from the crater rim and measuring ~8 m at maximum thickness. The clasts in the lithic breccia are angular, highly fractured, and either unshocked (mineralogically) or Class 1 shocked basalt (0-20 GPa) [1], which consists of fractured grains but no melting or mineral phase changes [1]. As shock pressures are typically 1-2 GPa near the crater rim, no intense shock metamorphism has occurred. From comparisons to basalt flows exposed in the crater walls, the clasts originated from both the oldest flows that have more secondary mineralization of groundmass and from the youngest flows that lack this feature. This is attributed to the level of the pre-impact water table of the ~65 Ma Deccan basalts [7]. The matrix consists of finely pulverized basalt. In theory, this unit grades into what would be overturned or inverted strata near the crater rim, but the crater has degraded since its initial diameter (1.7 km, based on gravity surveys [2]), meaning that ~50 m of the original crater rim has eroded to contribute to the post-crater fill. The upper ejecta unit is a suevite breccia containing clasts shocked to all degrees of shock pressure from unshocked up to Class 5 (> 80 GPa) of Kieffer et al. [1]. The suevite layer measures ~1 m in thickness and extends to ~0.5 km (~1⁄2 crater radii) from the rim. The matrix is finely pulverized basalt but with the addition of local glass spherules, beads, and dumbbells [1,2] reportedly not found in the lithic breccia unit. This DLE structure described for Lonar Crater is similar, if not identical, to the “throw out” and “fall out” layers observed at Meteor Crater [8] or the Bunte Breccia and Suevite at Ries Crater [9], and suggests that two processes are responsible [10,11] for the ballistic emplacement of the lithic breccia moments before the “falling out” of the fall-out suevite layer. Shoemaker [8] identified a suevite layer within Meteor Crater, then later surmised that this unit likely used to be thicker in near-rim regions of the CEB, but has eroded, as the fine matrix of this unit is easily transported by the SW winds of the Colorado Plateau to leave behind the clasts of the suevite layer as a lag [8]. Whereas the lower, more weathered basalt flows have not been identified as protoliths for Classes 2 through 5 in the suevite breccia, heavily fractured basalt corresponding to these basalt flows is observed that are either unshocked or Class 1, implying that these deeper strata are incorporated into the suevite layer. At Meteor Crater, the Coconino Sandstone serves as a lithologic tracer, as highly shocked lechatlerite is distributed in the Meteor Crater fall-out layer [8], suggesting material deep in the target sequence is incorporated into this unit. This provides data to “ground truth” modeling of Lonar Crater, Meteor Crater, and similar small (1-2 km diameter) craters [12].