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Description
Proximal ejecta blankets of very small (<200 m in diameter) impact craters can contain fragments of charcoal (Losiak et al. 2022). Those charcoals can be used to: precisely and accurately date impact structures, determine environmental effects of impacts of small asteroids, and potentially, better understand the energy distribution during formation of very small impact craters or identify impact origin of yet unknown structures. Previously, these charcoals were interpreted to represent pre-impact forest fires.
However, because the charcoal assemblages: 1) are found within the same stratigraphic and geomorphological context within numerous craters, 2) have 14C ages consistent with the timing of crater formation and 3) have reflectance properties unlike the wildfire charcoals, they appear to be produced by the impact events. However, their exact formation mechanism is unknown. The aim of the current study was to test whether impact charcoals can be formed by a shock wave passage as suggested by [10], and if so, under which conditions this occurs.
We have conducted 10 impact experiments using the University of Kent’s horizontal two-stage light-gas gun [11]. Impactors were 2 mm stainless steel. We varied the following parameters: 1) impact velocity (1.0 to 7.0 km/s) and, thus, peak pressure (~1 to >15 GPa), 2) wood moisture content of the spruce target (40% fresh/green and 10% dried), and 3) air pressure (near vacuum: 1 mbar, and at 1 bar).
No production of charcoal, like that observed in the natural impact craters, was observed during any of the experiments. This was true even if conditions were the most favourable: velocity of 5 km/s, dry wood, with 1 bar atmosphere. However, there were slight signs of very small thermal damage. The latter may be related to condensation organic vapour plume (e.g. soot), which is distinct from charcoal.
