30 replies to this topic

[Darren Drake]

Image captured 2/19 with 18 inch dob..

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Edited by Darren Drake, 20 February 2024 - 04:55 PM.

[gstrumol]

A most unusual crater! 

[vtornado]

I saw this last night.  Thanks Darren

[ButterFly]

Try for Marth too, nearby.  It's all about the lighting with concentric craters.

[alvarete]

I like this little cráter.
With your 18-incher It is really spectacular.
Thanks Darren

[coopman]

That is a weird crater, isn't it?  Thanks.

[Lightbucket12]

Most interesting and intriguing crater.  Being still new to “everything lunar” I’ve never seen a double wall crater like this as its concentric rings are similar to shock waves emanating in still water when an object strikes the water perpendicular to its surface.  In that case the concentric wave is of equal distance all the way around the circumference as it projects forward from the center point of impact.

 

So, are we looking at the equivalent for fluid displacement dynamics at play here?

 

Very strange but intriguing.

 

Bert

[ButterFly]

Most interesting and intriguing crater.  Being still new to “everything lunar” I’ve never seen a double wall crater like this as its concentric rings are similar to shock waves emanating in still water when an object strikes the water perpendicular to its surface.  In that case the concentric wave is of equal distance all the way around the circumference as it projects forward from the center point of impact.

 

So, are we looking at the equivalent for fluid displacement dynamics at play here?

 

Very strange but intriguing.

 

Bert

Chapter 4 of Trang, A REMOTE ANALYSIS OF THE LUNAR LANDSCAPE, is a good place to start.  You can also find Wood's original list on the wikicommons page: Concentric craters on the Moon

[12BH7]

I was going to look at that crater last night. But of course I got distracted and didn't.

[Lightbucket12]

Chapter 4 of Trang, A REMOTE ANALYSIS OF THE LUNAR LANDSCAPE, is a good place to start.  You can also find Wood's original list on the wikicommons page: Concentric craters on the Moon

Thank you.

 

Just browsed the link, interesting stuff.

 

Bert

[Teemu Ohman]

That's a great shot of a wonderful crater!

 

What has always bothered me about the naming of these things is that the term "concentric crater" was first used in many papers for smaller craters formed in layered targets (nowadays typically called "bench craters", which I strongly dislike) seen in Ranger images and only subsequently for these volcanically modified concentric craters (at least as far as I have been able to track the history of the terminology). Oh well, it's just one example of very bad and illogical ways that terms are used in planetary geology.

 

Teemu

[Tom Barnacle]

This is an old post, but may be relevant here:

 

'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 and is capable of explaining aspects of CC's that the volcanism model can not.'

 

One aspect of CC distribution that has been cited as being evidence of a link with volcanism is their location around the edges of maria where Floor Fracture Craters are found – which are volcanic in origin. This is not a strong link however as these areas have all been subject to uplift due to sub surface magmatic activity – and the formation of CC's is probably only related to volcanism because of this uplift and not as a direct consequence of intrusion into the crater itself. The volcanic hypothesis is worth considering, but in my opinion is just not strong enough to explain what we see and collapse is a far more parsimonious model.

 

*David Trang, Jeffrey J. Gillis-Davis, B. Ray Hawke, The origin of lunar concentric craters, Icarus, Volume 278, 2016, Pages 62-78, ISSN 0019-1035, https://doi.org/10.1...rus.2016.06.001.

 

Cheers, Tom.

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[12BH7]

Thanks, I enjoyed reading that article. 

[scottinash]

Thanks, Darren, for the excellent image!   Also, Tom, for the very good article/post!  

 

KAGUYA captured a fly-over view of Pitatus where you can see a nice oblique view of Hesiodus A to the west (left) at around 49 seconds

Edited by scottinash, 21 February 2024 - 11:28 AM.

[Lightbucket12]

This is an old post, but may be relevant here:

 

'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 and is capable of explaining aspects of CC's that the volcanism model can not.'

 

One aspect of CC distribution that has been cited as being evidence of a link with volcanism is their location around the edges of maria where Floor Fracture Craters are found – which are volcanic in origin. This is not a strong link however as these areas have all been subject to uplift due to sub surface magmatic activity – and the formation of CC's is probably only related to volcanism because of this uplift and not as a direct consequence of intrusion into the crater itself. The volcanic hypothesis is worth considering, but in my opinion is just not strong enough to explain what we see and collapse is a far more parsimonious model.

 

*David Trang, Jeffrey J. Gillis-Davis, B. Ray Hawke, The origin of lunar concentric craters, Icarus, Volume 278, 2016, Pages 62-78, ISSN 0019-1035, https://doi.org/10.1...rus.2016.06.001.

 

Cheers, Tom.

Thank you Tom, very informative and interesting.

 

Much appreciated,

 

Bert

[macpurity]

The topography of Hesiodus A is really remarkable with subtle differences from perfect symmetry. Here is a detail from the SLDEM2015 digital elevation model. This map has north up.

 

Darren's photo has south-up and he has picked-up the double rise in the central peaks. The height of those central peaks is only on the order of 150 meters, relative to low spots in the main crater. The concentric ring is about 500 meters above the main crater basin. The main rim is about 1000 meters above the "moat." Trang et al's paper will be interesting to read.

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[Teemu Ohman]

This is an old post, but may be relevant here:

 

'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 and is capable of explaining aspects of CC's that the volcanism model can not.'

 

One aspect of CC distribution that has been cited as being evidence of a link with volcanism is their location around the edges of maria where Floor Fracture Craters are found – which are volcanic in origin. This is not a strong link however as these areas have all been subject to uplift due to sub surface magmatic activity – and the formation of CC's is probably only related to volcanism because of this uplift and not as a direct consequence of intrusion into the crater itself. The volcanic hypothesis is worth considering, but in my opinion is just not strong enough to explain what we see and collapse is a far more parsimonious model.

 

*David Trang, Jeffrey J. Gillis-Davis, B. Ray Hawke, The origin of lunar concentric craters, Icarus, Volume 278, 2016, Pages 62-78, ISSN 0019-1035, https://doi.org/10.1...rus.2016.06.001.

 

Cheers, Tom.

Great to have this type of discussion here!

 

It would be interesting to see a systematic study of the regional topography of the concentric craters. I had a quick look at about eight of them with something like four to six profiles each, and couldn't see any systematics (no wonder, given such a tiny sample). It also (obviously) depends heavily on the orientation of the profiles; Marth, for example, is on an uplift in one profile but in a broad depression in another.

 

The concentric faults around simple craters should be there. However, in reality we have a very poor understanding of them, and the most comprehensive studies that I can recall come from just a few terrestrial complex craters. In order to achieve the morphology of, e.g., Hesiodus A, through collapse, you'd need an almost continuous concentric fault in the upper part of the crater wall, and I'm not sure how plausible that would be, given the brecciated nature of the wall material.

 

There's also quite a bit of variation in the morphology of the concentric craters. I'm having a hard time envisioning the formation of those features through collapse. To me, concentric craters just don't look like collapse features.

 

Admittedly the lack of obvious volcanic material is troublesome. But I could believe a scenario where the magma is fairly viscous and what we see is mostly the viscous magma bulging the floor without necessarily bursting through the floor material.

 

I'm willing to agree that we don't necessarily know how the concentric craters are formed, but I'll put my money on the magmatic intrusion model. In any case, they are fascinating targets to observe as well as to study using remote sensing data!

 

Teemu

[Tom Barnacle]

Great to have this type of discussion here!

 

It would be interesting to see a systematic study of the regional topography of the concentric craters. I had a quick look at about eight of them with something like four to six profiles each, and couldn't see any systematics (no wonder, given such a tiny sample). It also (obviously) depends heavily on the orientation of the profiles; Marth, for example, is on an uplift in one profile but in a broad depression in another.

 

The concentric faults around simple craters should be there. However, in reality we have a very poor understanding of them, and the most comprehensive studies that I can recall come from just a few terrestrial complex craters. In order to achieve the morphology of, e.g., Hesiodus A, through collapse, you'd need an almost continuous concentric fault in the upper part of the crater wall, and I'm not sure how plausible that would be, given the brecciated nature of the wall material.

 

There's also quite a bit of variation in the morphology of the concentric craters. I'm having a hard time envisioning the formation of those features through collapse. To me, concentric craters just don't look like collapse features.

 

Admittedly the lack of obvious volcanic material is troublesome. But I could believe a scenario where the magma is fairly viscous and what we see is mostly the viscous magma bulging the floor without necessarily bursting through the floor material.

 

I'm willing to agree that we don't necessarily know how the concentric craters are formed, but I'll put my money on the magmatic intrusion model. In any case, they are fascinating targets to observe as well as to study using remote sensing data!

 

Teemu

Thanks - lots of good thoughts!

 

There is a huge diversity of form amongst CC's most of them being on the maria or their edges. The problem, as you point out, is the lack of a ground truth regarding the fracture patterns around small craters (< 15kms) with only hydrocode modelling and terrestrial examples to go on. If the radius of curvature of the fractures and faults is about the same as the crater circumference, then a symmetrical and concentric rim collapse is at least plausible, in larger, complex craters the radius of curvature is less than the crater circumference, and rim collapse associated with these fractures produces the scallops we see in larger craters. A possible example of the former is the 12mile diameter Silverpit crater in the North Sea (see image) where the fractures form kind of a continuous concentric pattern, albeit with some partial sections and en-echelon overlaps. Of course this structure is not confirmed as an impact one, and even if it was, it may not be possible to scale this terrestrial example to the Moon with its different gravity regime. It is also possible that the homogeneous layered maria form a target that is conducive to symmetrical faulting – thus increasing the chances of a CC forming.

 

There are a few CC's that have an partial torus such as Leakey, and a topographic profile shows that the crater rim height where the torus is most well developed is lower than the rim section above where the torus is absent. This suggests the torus is formed by the inwards collapse of the rim/crater wall. But one of the better examples of a torus formed by collapse is the CC in the Apollo Basin, where an original torus (probably initially a lot like the Hesiodus A one) has been distorted by subsequent rim collapses that produced short, partial tori between the original one and the crater wall, which have the same textures and optical properties as the original torus. And most of these collapses can be traced to scars on the inner walls from which the material has collapsed – so no intrusion necessary. This crater does not show any obvious signs of being involved in uplift, but then again the present surface level may not reflect previous periods of inflation/deflation as a consequence of magmatic injection or migration.

 

The lack of any volcanism in these CC's is for me the Achilles Heel of the Trang argument. Sure, there are examples of viscous lavas of basaltic composition such as the in the Marius Hills, or more silicic such as the Gruithuisen domes, but why in the case of CC's would the activity stall before reaching the surface when the overburden would presumably be a thin and highly fragmented breccia lens forming the floor of the crater? And with the exception of Firmicus C, no associated pyroclastic activity?

 

I should point out that these comments solely my private musings - so opinion mostly!

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[macpurity]

This is a really thought provoking thread. I'm sure many of us have pondered the origins of concentric craters. Tom & Teemu raises some valuable questions about current hypotheses on mechanisms behind CC formation.

 

That oblique LROC image of the multiple-partial torus of the CC in the Apollo Basin is very intriguing. Tom, could you provide coordinates of that location? Thanks...

 

The Leakey crater is a great example of a partial torus CC. On Leakey we can see that the north rim is about 150 meters lower than 3/4 of the rest of the rim (I believe "a" and "b" are reversed in Tom's profile plot). The concentric portion (NW "shelf") corresponds somewhat to this lower rim topography. The "shelf" is located about 1000 meters below the western rim, and about 800 meters above the lowest level found on the crater floor, or about 1/3 the way up from floor to rim. There is an interesting "ramp" on the WSW portion of the "shelf."

 

In my most certainly untrained and under-educated eye, I can't help but wonder whether the incident angle of the impactor played a role in some fashion? But the crater, itself, lacks much horizontal elongation consistent with an oblique impact. Is it possibly related to a post-impact slumping activity? NAC images show some striations on the inner northern rim. Very curious formation, indeed. High resolution SLDEM2015 map attached.

 

With respect to Hesiodus A, on the NAC imagery, you can see a huge boulder directly on top of the central peak. Okay, who put that up there?

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Edited by macpurity, 22 February 2024 - 08:40 AM.

[Tom Barnacle]

Doh!  Yes you are right the label on the profile is wrong - I would claim it was a deliberate mistake but it was just me being sloppy. Leakey is an odd one for sure - and your high resolution SLDEM2015 are just fabulous - thanks!  How on earth do you prepare them?

 

Anyway - Leakey is not in a typical mare setting - more highland really, but I guess there may be some cryptomare deposits hidden away at the impact site, as some of the rim shows an elevated olivine signature and some of the small nearby craters also have elevated olivine levels in their ejecta. I wondered if the final form of the torus was the result of a slightly more diverse target lithology here as compared to the typical mare CC where the geology is more homogeneous - but I guess your suggestion of post impact modification is along the right lines.

 

There are some more CC's in marginal highland/mare settings such as Apollonius N, Firmicus C  and Crozier H that have very prominent tori with the area around Dubyago having quite a few examples. Some of these tori are quite symmetrical despite the crater rim varying in height quite a bit, potentially an argument against the collapse model - but at the moment I am hitching my waggon to that hypothesis! 

 

As for the boulder - odd it should end up there, but not as odd as the rocks on the floor of Paracelsus C - but that's a rabbit hole best avoided.

 

Location of Apollo CC attached.

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[Borodog]

Bullseye!

 

Nice catch.

[macpurity]

The Apollo Basin CC is beautifully captured in the oblique LRO view that Tom posted a couple of posts ago.

 

The high-resolution SLDEM2015 view (20m contours) provides the actual heights of the partial tori. The whole crater is about 12km in diameter. The inner crater appears slightly offset, towards the NW, from the center of the rim.

 

The inner-most torus is about 560 meters above the crater floor, but is about 1000 meters below the higher parts of the rim. The partial outer-torus to the SE is slightly higher by 100 to 200 meters. These CCs are really something to behold under close examination.

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Edited by macpurity, 22 February 2024 - 12:02 PM.

[Teemu Ohman]

Thanks - lots of good thoughts!

 

There is a huge diversity of form amongst CC's most of them being on the maria or their edges. The problem, as you point out, is the lack of a ground truth regarding the fracture patterns around small craters (< 15kms) with only hydrocode modelling and terrestrial examples to go on. If the radius of curvature of the fractures and faults is about the same as the crater circumference, then a symmetrical and concentric rim collapse is at least plausible, in larger, complex craters the radius of curvature is less than the crater circumference, and rim collapse associated with these fractures produces the scallops we see in larger craters. A possible example of the former is the 12mile diameter Silverpit crater in the North Sea (see image) where the fractures form kind of a continuous concentric pattern, albeit with some partial sections and en-echelon overlaps. Of course this structure is not confirmed as an impact one, and even if it was, it may not be possible to scale this terrestrial example to the Moon with its different gravity regime. It is also possible that the homogeneous layered maria form a target that is conducive to symmetrical faulting – thus increasing the chances of a CC forming.

 

There are a few CC's that have an partial torus such as Leakey, and a topographic profile shows that the crater rim height where the torus is most well developed is lower than the rim section above where the torus is absent. This suggests the torus is formed by the inwards collapse of the rim/crater wall. But one of the better examples of a torus formed by collapse is the CC in the Apollo Basin, where an original torus (probably initially a lot like the Hesiodus A one) has been distorted by subsequent rim collapses that produced short, partial tori between the original one and the crater wall, which have the same textures and optical properties as the original torus. And most of these collapses can be traced to scars on the inner walls from which the material has collapsed – so no intrusion necessary. This crater does not show any obvious signs of being involved in uplift, but then again the present surface level may not reflect previous periods of inflation/deflation as a consequence of magmatic injection or migration.

 

The lack of any volcanism in these CC's is for me the Achilles Heel of the Trang argument. Sure, there are examples of viscous lavas of basaltic composition such as the in the Marius Hills, or more silicic such as the Gruithuisen domes, but why in the case of CC's would the activity stall before reaching the surface when the overburden would presumably be a thin and highly fragmented breccia lens forming the floor of the crater? And with the exception of Firmicus C, no associated pyroclastic activity?

 

I should point out that these comments solely my private musings - so opinion mostly!

Very good points there and questions to which I have no answers. One comment, though, about Silverpit. I'd be very careful about drawing too many conclusions based on it. As you mentioned, we don't know if it's an impact structure or not because there's no solid evidence. In addition, if it is an impact structure, the peculiar target lithology and the interpreted morphology (after all, it's an interpreted image from seismic data that we're looking at) make it analogous to the Valhalla-type multi-ring impact basins seen on the icy satellites, not smaller impact craters. So from the point of view of the geologic processes, Silverpit is probably an example of ring tectonics (as championed by Bill McKinnon and late Jay Melosh), which I would say is quite different from the excavation of a simple crater and its possible later modification as seen in concentric craters.

 

Teemu

[Teemu Ohman]

This is a really thought provoking thread. I'm sure many of us have pondered the origins of concentric craters. Tom & Teemu raises some valuable questions about current hypotheses on mechanisms behind CC formation.

 

That oblique LROC image of the multiple-partial torus of the CC in the Apollo Basin is very intriguing. Tom, could you provide coordinates of that location? Thanks...

 

The Leakey crater is a great example of a partial torus CC. On Leakey we can see that the north rim is about 150 meters lower than 3/4 of the rest of the rim (I believe "a" and "b" are reversed in Tom's profile plot). The concentric portion (NW "shelf") corresponds somewhat to this lower rim topography. The "shelf" is located about 1000 meters below the western rim, and about 800 meters above the lowest level found on the crater floor, or about 1/3 the way up from floor to rim. There is an interesting "ramp" on the WSW portion of the "shelf."

 

In my most certainly untrained and under-educated eye, I can't help but wonder whether the incident angle of the impactor played a role in some fashion? But the crater, itself, lacks much horizontal elongation consistent with an oblique impact. Is it possibly related to a post-impact slumping activity? NAC images show some striations on the inner northern rim. Very curious formation, indeed. High resolution SLDEM2015 map attached.

 

With respect to Hesiodus A, on the NAC imagery, you can see a huge boulder directly on top of the central peak. Okay, who put that up there?

Regarding Leakey, a slightly oblique impact is always a possibility. There isn't much of a regional slope, so I'd suppose that doesn't play a role. There are plenty of grabens in the vicinity, so a pre-existing structural weakness is also a distinct possibility, although there aren't any visible faults immediately surrounding Leakey. Such a "blind" fault could have influenced the collapse (if one chooses Tom's preferred model) or it could have influenced the magmatic intrusion (if one likes that model more).

 

Btw., these topographic maps you produce are really amazing and much appreciated!

 

Teemu 

[Lightbucket12]

The Apollo Basin CC is beautifully captured in the oblique LRO view that Tom posted a couple of posts ago.

 

The high-resolution SLDEM2015 view (20m contours) provides the actual heights of the partial tori. The whole crater is about 12km in diameter. The inner crater appears slightly offset, towards the NW, from the center of the rim.

 

The inner-most torus is about 560 meters above the crater floor, but is about 1000 meters below the higher parts of the rim. The partial outer-torus to the SE is slightly higher by 100 to 200 meters. These CCs are really something to behold under close examination.

I totally agree with Teemu, these maps are amazing.

 

Now questions from the people who get the report card grade of “L” (still learning).  Are these topo maps pubic domain, open source?  By what method or means is the benchmark elevation, zero, established or defined?

 

”L” grade student,

 

Bert