Unraveling Double-Layered Ejecta: Lessons from Lonar Crater and Beyond

Abstract

Comparative planetological studies suggest that subsurface fluidization during ejecta emplacement may play a significant role in the formation of double-layered ejecta (DLE) craters, not only on Mars but potentially on other planetary bodies like Ceres or Ganymede. On Mars, DLE crater formation likely involved water or water-ice in the near-surface substrate. For the formation process it is hypothesized that volatile-rich rock debris flows driving the outer layer's emplacement and the inner layer moving as a translational slide.This process results in two superimposed ejecta morphologies with striated surfaces, rampart, and moat structures. Although morphological features, especially the ejecta blankets, have largely been destroyed for most terrestrial impact structures, recent studies have revealed that Lonar crater in India, Ries crater in Germany, and Bosumtwi crater in Ghana possess ejecta ramparts similar to Martian DLE impact craters.In this study we can show the typical striated DLE morphologies to be preserved in the subsurface contact of the different ejecta layers for the Ries crater and suggest that this might be the case for other terrestrial DLE craters as well, as long as multiple layers can be found. In a recent field study, we were able to confirm the presence of two distinct ejecta layers at least for Lonar crater in India, similar to the Ries crater in Germany. Additionally, we conducted a detailed study involving drone photogrammetry, rock magnetics, micro-fracture network analysis, and geophysical investigations using a 100 MHz bi-static ground-penetrating radar (GPR) system at Lonar. These findings were compared to similar observations made for the ejecta of the Ries crater in Germany, where additional electric resistivity tomography (ERT) corroborates the findings of the GPR.By studying terrestrial analogs, such as Lonar crater, we aim to constrain boundary conditions for DLE crater formation on other planets, specifically regarding the volatile content of the affected crust. Lonar, with its smaller diameter compared to Martian DLE craters, offers valuable insights into DLE formation variability. Moreover, Lonar can serve as an analogue laboratory for Martian DLE craters in basalt regions such as Hesperia Planum. Lastly, our findings shed light on the temporal variability of landforms in connection to the formation conditions required for DLE craters on Mars, which mostly occur at mid-latitudes and scarcely in the southern hemisphere, mirroring past climatic evolution for both Ries and Lonar crater.