"Clusters"

Cluster of secondary craters about 12 kilometers southeast of farside mid-latitude crater Rayet Y. LROC Narrow Angle Camera (NAC) observation M182796787R, LRO orbit 12041, February 2, 2012; field of view 1.68 km, centered on 46.892°N, 113.807°E, resolution 1.68 meters from 171.53 km overhead [NASA/GSFC/Arizona State University].

Hiroyuki Sato
LROC News System

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.

Rayet Y crater and surrounding area from LROC Wide Angle Camera (WAC) monochrome mosaic (100 meters/pixel). Original image centered near 46.56°N, 113.42°E, cropped frame above covers a field of view approximately 168 kilometers across. The blue rectangle shows the full LROC NAC footprint of observation M182796787R and the white arrow indicates the location included in the LROC Featured Image released November 15, 2012 [NASA/GSFC/Arizona State University].

Explore the typical secondary craters in full NAC frame HERE.

Related Posts:
Stream of Secondary Craters
Regolith on Basalt
Impacts on the Melts
Dark Secondary Crater Cluster
Bench Crater
Cluster of farside secondary craters

A different perspective on secondary craters, a definite cluster of "secondaries" from an oblique LROC NAC observation of the Mee crater group far to the south of Mare Humorum on the near side. LROC NAC observation M127232206 [NASA/GSFC/Arizona State University].

A small chain of small secondary craters 12 kilometers southeast of Rayet Y are radiant from Giordano Bruno, nearly 400 km to the southwest [NASA/GSFC/LMMP/Arizona State University].