On the Moon, Linne F is a 5 km diameter crater located at 32.33°, 13.95°, and it displays a spectacular melt pond (now frozen) on its floor. Immediately after the impact event, melt pooled and eventually hardened to form the now lower reflectance flat deposit surrounding a central mound. The once molten material shares characteristics that are seen in many other impact craters: small mounds, blocky craters, and fractures. But what is the origin of the central mound? Compare the central mound in Linne F to the interiors of other similarly sized craters. Perhaps it is a proto-central peak or maybe the mound formed due to unusual target properties?
Do any other 5 km diameter lunar craters have central peaks? On the Moon, central peaks start to form in craters between 10-20 km in diameter, much larger than Linne F. If the mound isn’t a nascent central peak, perhaps the local target properties of this portion of Mare Serenitatis played a role. Target properties are important for small (<500 m diameter) craters as a loose regolith over solid bedrock can result in benches, mounds, and flat floors. It is unclear if target properties are also important for larger craters, but Mare Serenitatis has a thick layer of basalt overlying older basin material. Both hypotheses are possible, but hard to prove from remotely sensed data alone. Linne F is just one example of the uniqueness of each crater on the Moon. It is easy to generalize that all craters below 10 km in diameter are bowl shaped - but that generalization glosses over the variety of geologic forms seen in lunar craters. It is this richness of detail from crater-to-crater that scientists are studying to unravel the story of the surface, and subsurface, of our nearest neighbor!
Credit: NASA/GSFC/Arizona State University