Bürg crater, ~40 km in diameter, is located in Lacus Mortis and represents a fine example of a complex crater. On the Moon, complex craters form above diameters of about 15 to 20 km. Unlike most simple craters (diameters less than 15 km), complex craters often show a wide range of morphologies and geologic features. Overall, complex craters exhibit terraced walls, flat floors, and central peaks. However many factors, including bolide composition, bolide velocity, and target composition, influence the complex crater morphology - which is why we observe so many different complex crater varieties.
Bürg crater is unique from many other complex craters because instead of having a broadly circular rim, the crater's rim is scalloped and wavy. Sometimes, pre-existing geologic structures or features help shape a crater during crater formation. Meteor Crater on Earth has a slightly polygonal shape because of the joints and fractures that pervade the target sedimentary rocks. Could pre-existing joints in the mare basalts filling Lacus Mortis explain the scalloped nature of Bürg's rim? Possibly, but because the overall shape of the crater itself is circular and resembles other complex craters on the Moon, structural influences may have only affected portions of the crater rim, causing differential collapse and terrace formation. Looking closely at the portion of the LROC WAC image above, there seems to be greater terracing and wall-slumping on the western side of the crater, which also happens to be less circular than the eastern rim. However, before we use this observation to interpret the origin for the scalloped crater rim, we need to look at additional LROC NAC images and the LROC WAC image in detail to substantiate this hypothesis.
Explore Bürg's rim for yourself to see what geologic clues you can find that provide insight into the geologic features of this beautiful complex crater!
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