AS17-2120(M)

Figure 124

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This photo taken at low Sun angle emphasizes how common secondary impact craters are on the Moon. The primary craters whose ejecta formed most of the secondaries in this part of southeastern Mare Imbrium can be identified by observing the orientation of the secondary crater chains and of the "herringbone" ridges that splay outward from  individual secondary craters. The chains are radial to the primary or parent crater and the apexes of  the "herringbone" ridges point toward it. The most conspicuous chain and the chain at the left center of the picture are secondary to Eratosthenes, 250 km to the southeast in the direction of the arrows labeled E. Lying athwart these chains is a large younger cluster (arrow C) secondary to Copernicus, 400 km to the southwest. The chains in the extreme upper right corner (arrow T) are secondary to Timocharis whose rim is only 35 km northeast of the pictured area. Most of the other, smaller chains, clusters, and "bird's-foot" gouges can also be traced to Eratosthenes, Copernicus, or Timocharis, but some probably were created by fragments from more distant sources. The largest crater in the scene is Timocharis A, 8 km in diameter.   -D.E.W.