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Mercury's Complex Cratering History
Close up of Mercury's craters by Messenger
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On January 14, 2008, NASA’s MESSENGER spacecraft observed about half of the hemisphere not seen by Mariner 10. This image shows part of a large, fresh crater with secondary crater chains located near Mercury’s equator on the side of the planet newly imaged by MESSENGER. Large, flat-floored craters show terraced rims from post-impact collapse of their newly formed walls. The hundreds of secondary impactors that are excavated from the planet’s surface by an incoming object, create long, linear crater chains radial to the main crater. These chains, in addition to the rest of the ejecta blanket, create the complicated, hilly terrain surrounding the primary crater in the image. By counting craters that have formed since the impact event, the age of the crater can be estimated. This count can then be compared with a similar count for the crater floor to determine whether any material has partially filled the crater since its formation. With their large size and production of abundant secondary craters, these flat-floored craters both illuminate and confound the study of the geological history of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington