Wednesday, March 23. 2011
Typical highland lunar landscapes are heavily cratered, and the craters that mark the surface provide a record of the impact history for that area. Today's Featured Image highlights the effects of discontinuous ejecta (which forms secondary craters) on the lunar surface. When material is excavated during impact, much of that material is deposited very close to the crater - generally within 1 crater diameter. This material is called the continuous ejecta blanket. However, most impacts are so energetic that ejecta is deposited not only near the crater but also much, much farther away. This far-reaching ejecta is part of the discontinuous ejecta blanket and is responsible for forming secondary crater chains, clusters, and rays. The erosive energy of the secondaries is quite large - in the opening image, ejecta scoured the wall and rim of the crater, leaving behind deep grooves!
Looking at the full LROC NAC frame, secondary crater chains follow the grooves toward the northeast. Because secondary crater chains and streams represent the direction from which the ejecta came, scientists can use these features to try to determine which impact was responsible for them. The secondary crater chains trend southwest to northeast, determined by observing the direction of the majority of these features in this NAC frame. Furthermore, these secondaries probably originated from the southwest because many of the craters within the chains are tear-drop shaped. Looking around the region in the LROC WAC mosaic presents many possibilities for the source impact. Although not pictured in the WAC mosaic above, we probably can rule out Orientale basin as the source for these secondaries because the ejecta should be trending west to east and it is not. However, Byrgius A is a Copernican-aged crater (it still has visible rays) and lies to the southwest of the area in today's Featured Image; perhaps Byrgius A is the source of the secondaries scouring the landscape in the NAC frame.
Explore the scour marks and secondary crater chains for yourself in the full LROC NAC frame!
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