Why It’s So Hard To Date a Crater

Panorama of a sunset-lit Giordano Bruno crater, imaged by LRO (NASA/GSFC/ Arizona State University)

The 13-mile (21-km) wide Giordano Bruno crater on the Moon’s far side was recently imaged by NASA’s Lunar Reconnaissance Orbiter at an angle at a time when the setting sun cast long shadows, creating the high-relief image seen above. It’s known that the brightly-rayed crater is relatively young (see the video below) but how young? It could be anywhere from 834 years old (if some Medieval accounts are to be accepted as accurate descriptions of the crater’s formation) to 2 to 4 million years old, up to even 10 million years old — of which there would obviously be no written documentation. So why is it so hard to date a crater?

Basically, without a sample of the material within the crater itself there’s no way to know for certain when it was formed. The superimposition of younger craters on top of older craters helps the dating process, based on an accepted rate of impact collisions during the history of the Earth-Moon system, but it’s difficult to know (again, without samples) if the smaller craters that surround Giordano Bruno are the result of independent impact events or the result of secondary impacts resulting from ejected material — either from the same event (forming a “self-secondary” crater) or others nearby.

Only a mission to land a rover or human explorers into a crater like Giordano Bruno and collect samples for radiometric dating would solve this mystery for certain. Regions where exposed impact melt exists would be the best option, not only for landing but also for sampling, as it’s when rock is melted and recrystallizes that its “clock” is reset, and can most accurately be dated.

“This new knowledge [would] help crater counting experts understand the importance of self-secondary craters on very young craters. A new calibration for these youngest craters could be obtained, thus making age estimates for all other young craters on the Moon more reliable.”
– Mark Robinson, Arizona State University

Learning crater dates would not only give us more insight to the evolution of our satellite world, but also provide more clues into the types and rates of impact events that occurred during the history of the Moon — as well as the Earth.

Read more on the LROC site maintained by ASU here, and check out a previous LRO mosaic of Giordano Bruno in its entirety as seen from above.

(Video: JAXA)