Four Leaf Clover

Several large depressions dot the surface of Mare Imbrium, giving the impression of a four leaf clover. LROC NAC M190780929RE, image width is 1500 m [NASA/GSFC/Arizona State University].

These large, ~500 m diameter, depressions are characteristic of secondary impacts on the Moon. When a bolide (asteroid or comet) hits the surface of the Moon a crater forms at the impact site. To create a secondary crater material is ejected from the impact site at about a 45° angle. If the ejecta travels less than the escape velocity, it falls back down to the Moon. Since the escape velocity on the Moon (~2.4 km/s) is much lower than that at which bolides typically impact the Moon (10-20 km/s) secondary craters often have a distinctive appearance. These lower velocity impacts result in irregularly shaped craters. Sometimes secondaries land in clumps and create distinctive patterns, such as the "four leaf clover" whimsically identified in today's Featured Image.

White arrow points to location of today's Featured Image at 28.239° N, 332.895° E. Red arrows point in the direction that several other secondary craters are sourced. Image width is 100 km, LROC WAC mosaic [NASA/GSFC/Arizona State University].

If the secondaries featured today were formed in another impact, which impact created them? The number of craters in our secondary group is fairly large, so the parent crater cannot be small. In the context image, several other secondary chains (red arrows) appear to point to the southeast. Maybe zooming out further will reveal the mystery parent crater!

Larger context image of today's Featured Image. The white box locates the previous context image. Image width is 650 km, LROC WAC mosaic [NASA/GSFC/Arizona State University].

It looks like Copernicus crater is the parent crater! That makes sense as Copernicus fits our criteria. These secondary chains have been previously identified, but the fact that they were sourced from Copernicus crater hundreds of kilometers away is remarkable. The impact cratering process really is amazing. 

Can you identify other secondary craters in the full LROC NAC? 

Related Posts:

A Scar in the Highlands

The Rays of Messier A

Stream of Secondary Craters

Cluster of farside secondary craters

Published by Drew Enns on 21 August 2012