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Holy double doughnuts, Batman! A pair of concentric craters.

Maksutov Catadioptric CMOS Moon
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#1 maroubra_boy

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Posted 28 July 2021 - 07:15 PM

Hi all,

 

One of the more unique and rare features of volcanic origin are concentric craters.  These are essentially domes that have collapsed in on themselves due to magma displacement.  While there was at first a push up by magma, the pressure was reversed and that material was then pulled down.  The result of this is a double ringed concentric structure.  Whether or not it was an impact that led to the initial formation of the structure I do not know.  But all of these concentric craters all exist along known volcanic areas, namely because they hadn’t been obliterated by the Great Bombardment.

There would appear to be an upper and lower limit to the size of these.  The largest concentric crater is only 30km, but most are between 6 to 15km, with a few smaller and a few larger.

 

Hesiodus A is one of the easiest to find & one of the largest, and Marth is just a little west of it.

The pic was done using a 9” Santel Mak with a 224 colour camera.  Seeing conditions were not good on the night.  Still working on my processing skills.  Processed using Autostakkert and Registax6, and touched up using PhotoDirector.

Thanks for looking,

Alex.

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Edited by maroubra_boy, 28 July 2021 - 08:07 PM.

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#2 TOMDEY

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Posted 28 July 2021 - 09:00 PM

Nifty! I gota wonder if an occasional pair would just be two "regular" impact craters that just happened to line up. Like the bigger one first and then the smaller one eons later from less energetic impactor that hit the original one right in the middle.   Tom 


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#3 maroubra_boy

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Posted 28 July 2021 - 09:13 PM

Probability would suggest yes.  But the nature of these of volcanic origin are very specific and have specific characteristics, for instance the internal depression is noticeably not an impact crater.

 

A little more reading hints at their starting point could well have been an impact.   The following article also suggests not subsidence but the welling up of magma following the impact, but there is still uncertainty.  Always something new to learn smile.gif   See here:

 

http://www.psrd.hawa...ic-craters.html

 

And here's a great list of these:

 

https://commons.wiki...ers_on_the_Moon

 

Alex.


Edited by maroubra_boy, 28 July 2021 - 09:17 PM.

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#4 frank5817

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Posted 28 July 2021 - 09:34 PM

Here is another article worth looking at that I read last year; that goes into some detail on the subject from 3 years and some months back. It is from the Wiley Library here

 

Another more recent article from Arizona State University with a brief explanation and also has a nice map for locating  concentric craters. copy or paste in your browser  A Lunar Donut: Bell E Crater | Lunar Reconnaissance Orbiter ...  or  here

 

Frank


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#5 John_Moore

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Posted 29 July 2021 - 09:45 AM

Their formation is still misunderstood, but I fancy the layered target rock hypothesis where rebound and hardness properties of each separate layer may be responding differently to the initial impact and impactor.  

 

In 2018, I put together a list of images on the MoonWiki (when it was around back then) - some of which were never seen before, and so I classed them as uncertain or unconfirmed (for example, this potential CC east of crater Stark on the farside).

 

John Moore


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#6 Tom Barnacle

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Posted 30 July 2021 - 02:47 AM

The article and paper by Trang et.al cited above concluded that concentric craters were probably formed as a result of igneous intrusions into the crater floors, which is plausible. There is very little evidence however for volcanism associated with any of these craters with the exception of one, Firmicus C, where there is a small deposit of pyroclastic material on the ring, but then again pyroclastic vents are not uncommon over much of the lunar surface, so not much in the way of supporting evidence given the volcanic products seem to find their way to the lunar surface everywhere else. An argument however can be made for the less glamorous origin as a simultaneous rim collapse as the crater experienced extensional forces such as during an uplift - and Hesiodus A which is the classic example of a concentric crater has been distorted by uplift (See attached profile taken from LRO Quickmap). A collapse origin for the ring explains why these rings are compositionally identical to the crater rims with generally no indications of volcanic rocks being present.  The floor Fracture Craters Lavoisier (see image attached) and Humboldt have concentric craters very similar to Hesiodus A on their floors, and these smaller craters are cut by fractures that formed as the floors of the larger craters were uplifted. Younger simple craters very nearby (just to the N of the Lavoisier CC) are not cut by these fractures, showing that the concentric craters pre-date the uplift and would have been affected by it, but the younger simple craters post-date the uplift so would not have been subject to deformation. Small simple craters are surrounded by concentric faults formed during the impact process, and you can imagine that forcing the crater floor upwards would open these faults up, destabilising the rim and resulting in rim collapse - which on a small enough scale could occur simultaneously around the entire circumference of the crater. The observation that concentric craters do not form above a certain diameter (15kms diameter) is also consistent with a collapse scenario, as any bigger and the crater starts to enter the complex crater range (>20kms) and the concentric faulting probably becomes more complicated and divided up into arc like sections that result in the scallops common in larger craters. Some concentric craters also have only a partial ring such as the crater Leaky, which is more likely in a case of collapse than igneous intrusion.

 

I think the jury is still out on this one, but the link to volcanism is not strong, and collapse is more consistent with what we see.

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Edited by Tom Barnacle, 30 July 2021 - 02:53 AM.

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#7 maroubra_boy

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Posted 30 July 2021 - 03:42 AM

We just need to go up there with a bloody big chainsaw & cut one of the suckers open to find out. Going deep, roots and all.

There are also some clues that have to be noted, such as the upper limit of these & the similar location of close proximity to the edge of mares/seas where igneous intrusion would not only still have been possible at the time of the formation of these , but that these concentric craters can hence be seen as a possible incomplete intrusion of magma into the crater. There was just insufficient penetration of the impactor to allow for flooding to takes place.

We see ghost craters, flooded & none-flooded craters. It could well be that at least some of these concentric craters are a half-way point between flooded & non- flooded craters. Their age is another strong indicator to this in the timing of volcanic activity on the Moon.

The discussion reminds me of the debate that existed in the origin of what we know today as asteroid impact craters here on Earth & those who believed these were all volcanic in origin even though no volcanic/igneous material was ever found around & in these that was not already in the parent rock. Not until the detonation site of the first nuclear weapons was examined & found the mineral proof that linked the craters to the catastrophic energy of an asteroid strike.

A complex & intriguing topic.

Alex.

PS, anyone got a big enough chainsaw they can lend?

Edited by maroubra_boy, 30 July 2021 - 03:49 AM.


#8 Tom Barnacle

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Posted 30 July 2021 - 06:23 AM


We see ghost craters, flooded & none-flooded craters. It could well be that at least some of these concentric craters are a half-way point between flooded & non- flooded craters. Their age is another strong indicator to this in the timing of volcanic activity on the Moon.

 

Marth that you show in your pic is a CC that has been partially inundated by mare lavas - so almost a Ghost Crater, but not quite. It also has been cut by a graben (one of many hereabouts) showing that it has been subject to uplift as well. It also sits on the highest point of Palus Epidemiarum which has something of a shallow dome thing going on. As with most other CC's there is no trace of volcanism in the spectral data, the rim and ring or torus being of a highland composition - which suggests the mare lavas here are thin enough that the crater penetrated down to highland rocks below.

 

Good luck with that chainsaw.

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