Wednesday, August 22, 2012

LROC: Looking over a four-leaf clover..

Several shallow depressions, secondary craters, dot the surface of Mare Imbrium, in this case near a rocky ext Mons la Hire (near Euler and Lambert), and giving the impression of a four leaf clover. LROC Narrow Angle Camera (NAC) M190780929RE, spacecraft orbit 13158, May 4, 2012; resolution 1.5 meters and field of view 1500 meter across. View a larger cropped image HERE.  [NASA/GSFC/Arizona State University].
Drew Enns
LROC News System

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.

Smaller scale context image shows the relationship of the out-cropping above with the larger Mons La Hires 30 km to the southeast.  Image width is 650 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 covering a slightly broader field of view below, other secondary chains (red arrows) appear to point to the southeast. Maybe zooming out further will reveal the mystery parent crater!

A quick look over the 605 kilometers from the southwestern tip of  the northwest Mons La Hire outcrop and the center of Copernicus, courtesy of the ILIADS application released by NASA/LMMP. The immediate and long-range legacy of the Copernicus event was lasting.
It looks like Copernicus is the parent crater! That makes sense. 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 frame, HERE?

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