Today's Featured Image displays a classic set of aligned craters, most likely formed as a swarm of secondary impactors hit the surface. The location of this cluster is only about 10 km southeast of Rayet Y crater (14.5 km in diameter). But since the cluster extends in a northeast-southwest direction that does not point back to Rayet Y, the source of these secondary craters must be another crater. Giordano Bruno is one possible candidates in terms of the direction and maturity (similar or younger in age than Rayet Y), even though it is over about 450 km away.
For age dating small and young surfaces with crater counts, secondary craters introduce a serious problem. As the image resolution increases, we can count more small craters from a small portion of the ground, giving the impression of an increase in the accuracy of age estimates. However, secondary craters are more common at small diameters and their distribution is not random over small areas, violating one of the principal assumptions of age dating via crater counting. Counting secondaries in addition to the normal random population of primary craters can result in an artificially older age estimate for a surface.
So how can we determine if secondaries are present? One of the definitive signs of secondary craters is the clustering shown in the opening image. Random impacts over time typically don't result in such local high densities. Secondary craters can also have a V-shaped pattern in their rays, known as a "herringbone" pattern, seen for some of the craters above. Counts of craters thus try to exclude clusters and irregularly shaped craters to minimize errors in age estimates introduced by secondaries.
Explore the typical secondary craters in the full NAC frame yourself!
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