Challenge No. 7
Concentric or Bullseye craters.
Time to get funky with the Moon!
I have to say I've only come across concentric craters in the last four weeks! All by chance after a sketch I did last month of Palus Epidemiarum, the Marsh of Epidemics. While working on the inner most part of the flat lava field, there was one really small crater that seemed to "wink". While all other features were quite static when seeing was stable, this little crater seemed to shimmer sooner and for longer when a band of thermal interference wafted across the field of view that makes the image blur just a little.
When I concentrated on that little crater, I got a heck of a surprise, it was a concentric crater! My first ever!
Yes, sure I had heard about concentric, or bullseye, craters, thinking that these were just coincidental double impacts in the same spot that created these. I also thought that the likelihood of finding these would be bugger all to none because of the probability of this happening is just soooo remote that these would also have to be very, very small craters to cope with the impact forces. So I didn't pursue these any further than a fleeting thought.
But then the crater Marth made an unexpected appearance, and set me on a new path of discovery with the Moon
Lunar 4 orbiter image of Marth. NASA photo.
Lunar Reconnaissance Orbiter image of Hesiodus A. NASA photo.
Both of the above concentric craters are targets in this Challenge.
Concentric craters are rare. However, their origin is not from double impacts as one would initially think as I did. Instead these are a combined result of first an impact followed then by subsequent magma intrusion caused uplift in the floor of the crater, but not enough to have lava flow into the crater, instead just enough to cause an upheaval within the crater floor.
A few things all concentric craters share is they are all smaller than 20km in diameter, most between 8km and 9km in diameter, and all are very old features. About 80% of concentric craters also occur around the edge of the Seas, with the balance deep inside the Seas and within the mountainous features.
Because of their relatively small size, concentric craters are not a commonly known feature and very rarely made a specific feature to chase down. Despite their small size, there are a few that are large enough for most amateur size scopes to resolve. And that they are rare makes them a worthy and challenging feature to chase down. And with the current new lunation phase of the Moon coming in, it is a timely Challenge to prepare for.
Below is a list of the larger concentric craters to look for. The list progresses from East to West. A good lunar atlas such as
“Virtual Moon Atlas” will show all of the concentric craters below. One tip to find them is to have the terminator one, two or three days past the target's location to take advantage of the shallow angle of the Sun to produce the longest shadows. The two smaller ones listed, Marth and Gambart J, will really test the quality of your optics to be able to resolve the very fine details/shadows.
Name & Diameter
Firmicus C, 15km
Apollonius N, 10.8km
Colombo B, 16km
Crozier H, 11km
Leakey, 13km
Pontanus E, 13.7km
Fontenelle D, 16.8km
Hesiodus A, 15km
Gambart J, 7.7km
Marth, 7km
Damoiseau D, 17km
Cavalerius E, 10.1km
Concentric craters are unique, small and very niche features. Just perfect for a lunar challenge!
Happy hunting,
Alex.