NASA about Doughnuts on the MOON

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Written by David Trang
Hawai'i Institute of Geophysics and Palaeontology :-

Nearly all lunar impact craters smaller than 15 kilometers in diameter come in one basic shape, a circular bowl with a rim around it. However, there is a category, called concentric craters, with the basic crater and rim, plus a dough nut-shaped feature within the crater that is concentric to the crater rim. Jeffrey Gillis-Davis (University of Hawai‘i), the late B. Ray Hawke, and I (still at the University of Hawai‘i) decided to take on the task of determining how these concentric craters formed. After long meticulous measurements of their shape, determining their composition, examining their spatial relationship on the lunar surface, and calculating their age, we find that these craters are regular impact craters that experienced intrusions of magma beneath them. Each intrusion pushed on the material around the crater causing the construction of the concentric dough nut-shaped topographic ridge.

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