THE ORIGIN OF LUNAR CONCENTRIC CRATERS

We examined a class of craters unique to the Moon termed concentric craters. These craters exhibit one or more donut-shaped scarps, called an inner torus. Most lunar crater morphologies are attributed to impact processes. However, the crater formation process does not easily explain the inner torus of concentric craters. Consequently, five models have been previously proposed for the formation of the inner torus: successive impacts, extrusive doming, mass wasting, impact into layered targets, and igneous intrusion. Measurements and observations were made using topographic profiles along with optical, near infrared, and radar images obtained from Clementine, SELENE and the Lunar Reconnaissance Orbiter. Clementine derived FeO and optical maturity maps also were used to examine these craters.

We studied 49 concentric craters. Their global distribution is similar to floor-fractured craters, usually located along the margins of mare regions or near isolated mare ponds. They are also typically isolated, rarely occurring within 100 km of each other. Concentric craters range from 3-20 km in diameter and average ~9 km. The diameter of the inner torus ranges from 1.5-9.7 km from the crest, with an average of ~4.3 km. In contrast, the diameter of floor-fractured craters ranges from 15-200-km and average ~40 km.

Crater formation processes such as successive impacts and mass wasting do not explain the preferential distributed of concentric craters along mare-highland boundaries. An impact into layered targets require the interior of fresh craters near concentric craters to be modified as well, however, concentric craters are not found to cluster. Compositional maps do not show differences between the concentric crater and the local region, which is evidence against extrusion. Crater morphology and spatial relationship between floor-fractured craters and concentric craters suggest that process responsible for the crater floor modification was igneous intrusion. The idea suggests that dikes have propagated into impact-induced fractures producing a laccolith. The laccolith grows, uplifting the crater floor. The inner torus is created either because impact melt in the center of the crater is inhibiting uplift or the floor relative to the sides, or inflation is followed by a large deflation or collapse.