Caldera Collapse as the Trigger of Chaos and Fractured Craters on the Moon and Mars

Chaotic terrains are broad regions on Mars characterized by the disruption of the basaltic bedrock into polygonal blocks separated by deep fractures. To date, the proposed genetic scenarios often involve the occurrence of subsurface ice or liquid H2O. Nevertheless, similar features also occur within some craters on the Moon, namely floor‐fractured craters (FFCs), where water ice reservoirs are not present. We propose a new formation mechanism for Martian chaotic terrains as well as for lunar and Martian FFCs. The proposed mechanism does not require a major role of water but multiple cycles of inflation and deflation of a buried magma chamber. This process results in a particular type of caldera collapse, called the piecemeal (or chaotic) caldera collapse. A series of analog experiments show both geometrical and quantitative correspondence with natural case studies: Arsinoes Chaos (Mars), an unnamed FFC (Mars), and Komarov crater (FFC on the Moon). Plain Language Summary The formation mechanism of peculiar chaotic terrains on Mars remains at the center of a long debate. To date, the most supported theories involve a primary role of water, either liquid or ice. Our experiments’ goal was to demonstrate whether or not it was possible to reproduce the characteristic polygonal fractures of chaotic terrains through a volcano‐tectonic process that does not require an interaction with water. We found very strong resemblances between our experiments’ results and the morphology of chaotic terrains. The latter can also be compared to floor‐fractured craters, which also occur on the Moon, and are believed to be formed by volcano‐tectonic processes as well. Key Points The formation mechanism that produced chaotic terrains and floor‐fractured craters could be piecemeal caldera collapse The analog experimental models showed that H20 did not necessarily have a primary role in the collapse of chaotic terrains The faults due to piecemeal caldera collapse are consistent with the faults' geometry in chaotic terrains and floor‐fractured craters