Astronomical Society of Coonabarabran

Damoiseau: a crater chain?

by Harry Roberts

On the Moon a crater chain is termed a "catena", and they come in different shapes and sizes. Some crater chains are nice strings of regular sized craters, and Davy Catena is a good example. (I had to admit recently that I've never observed Davy Catena when asked about it.) Some catenas (plural cateni?) are less regular than Davy, like the Vallis Rheita, which is thought to have been formed by debris ejected from Mare Nectaris. Most crater chains are created by material ejected during crater formation, when a high velocity impacter creates a crater.

Recently when observing with the terminator near the west limb I noticed a nice, (but short,) chain of craters just south east of the large Grimaldi basin. The attached sketch (Fig 1) shows a detailed view of the chain. Checking Rukl[1] I found that some of the craters bore letter designations as secondary craters of the Damoiseau crater (90 km to the north), though the chain itself was not named as a lunar feature. The chain appears in two views (Consolidated Lunar Atlas images) on pages 182 and 183 of Woods' "Modern Moon".[2] At the time I could see a central pair of overlapping bright fresh craters, these were Damoiseau B and BB. In the deep shadow cast by their rims I could detect a short curved bright-lit ridge, suggesting that BB at least had an irregular floor. On the southwest (left) side of this pair two smaller overlapping craters intersected the rim of BB. And on the northeast side I detected three overlapping craters that also formed a short valley. Re-checking Rukl, I was surprised to see only two craters mapped at this spot, where I saw three. On the referenced photos, I could also clearly see three overlapping craters, in the short northeast chain.

Since Lunar Orbiter photos are available on the Net, I downloaded a Grimaldi image, and there (Fig 2) was a nice view of my "catena". I now saw that the two narrow chains to the left and right of B and BB were comprised of multiple overlapping impacts. And the right hand chain is divided into two parts by a short valley that crosses it, giving the impression of only two craters in contact. The Orbiter image shows an overlap of possibly even four craters!

Interestingly, the large elongated crater at the left (south west) end of the catena looks fresher than the rest of the chain, and may be only a chance alignment. But the rest of the chain, including the two craters B and BB, all seem comprised of multiple overlapping craters, and reminiscent of the larger Vallis Rheita. A short "tail" on the far left looks to be a part of the main chain too. It seems the whole chain was formed simultaneously. And where would such a string of impacters have come from? The chain points towards the Mare Orientale multi-ring basin, behind the Moon's southwest limb; and similar ejecta features can be found surrounding the giant Basin.

So it seems that Damoiseau "Catena" is a true crater chain. I have not found any mention of it in my references. Being such a well-defined feature, I think it deserves to be better known. So take a look when the phase is suitable and let me know what you think.

Happy Moon-watching.

[1] Rukl, A. "Atlas of the Moon." Sky Publishing. 2004. Map 39.
[2] Wood, C. "The Modern Moon - A Personal View". Sky Publishing. 2003.

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