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Not Your Average Complex Crater
The small, irregular terraces on the walls of Bürg Crater and the debris piles and outcropping wall material, with strong variations in reflectance, only hint at the geologic diversity of this complex crater. Bürg's crater rim is on the upper left, with downslope direction toward the lower right. Illumination is from the right, image width is 870 m, LROC NAC M116139887R. Credit: NASA/GSFC/Arizona State University
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.
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Subset of a map-projected LROC WAC monochrome context image of Bürg crater. Notice the terraced crater walls, the smooth crater floor, and the well-developed central peaks. The arrow points to the location of the area highlighted in the opening LROC NAC image. Credit: NASA/GSFC/Arizona State University
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.
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