My own take on Boulder Repellent Craters
I’ve been enjoying the boulder repellent thread and the project very much, and after reading Jules’ excellent IOTW last week, I’ve decided to do something different with this week’s. Simply continuing the theme with some of my own ideas and questions.
One thing that seems to be promoting a lot of discussion is the nature of the central feature of these craters that appear to be flat pools of impact melt.
Looking at the latest images in the thread, I personally believe that these central melt pools are not entirely flat/level. To this end I went through the whole thread; I looked at every image, I read every post and many of the links.
Both Anthony Cook and Tom128 have suggested that the melts may not be flat; these were the suggestions I was looking for to follow up this idea.
I’ll briefly explain that as a teen at college I was asked to create seemingly flat surfaces using a thermo-plastic material. One could stand on the surface and assume it was flat. These surfaces were then simply tested with a marble to reveal the direction of the fall or gradient. In the same way, roads and many sidewalks/pavements or sealed exterior ‘foot traffic’ surfaces are not actually level. The gradients are slight (as in, we’re not all struggling to walk in a straight line, falling towards the road), the gradients are revealed in heavy rain where the water can be clearly seen running towards the drains. Of course, this isn’t secret knowledge; I write only to make a point.
The gradient, or fall, does not have to have a high value and can easily be imperceptible to a high flying camera looking straight down but it will have noticeable effects to any rolling rocks and boulders.
Do they all look the same?
If all the features looked the same, I would agree there they may be flat; but there are many obvious differences. Some of the images in the thread clearly show signs of a slight ‘bulge’ in the melt pool that is revealed by the light and dark shades on both sides. So I think it’s fair to assume that at least some are raised slightly towards the centre.
Images posted by Geoff
Another thing that attracts me to the idea of a raised centre is the circular pattern of the boundary around the melt pool in many images. In my opinion, these boulders are simply ‘jammed’ into the ‘low point’. Indeed, many more rocks and boulders can be seen ‘backing up’ the craters walls, being blocked by the embedded ones.
Tom128 points out the circular boundary line
Some rocks and boulders ‘backing up’
Posted by Aliko
I think, also, that the amount of boulders and activity in the area can play another important role in the appearance of these features; assuming the ‘low point’ is a roughly circular area defined in many of these images. If there are enough boulders in the region then the low point can be filled allowing more to simply roll over, closer to the centre. This would explain the jagged shaped boundary lines around the centres with an apparently smaller central melt pool. In some cases, it appears that a few have bounced over the boundary to land quite close to the centre.
posted by Half65
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Posted by Cruuux
I’m certainly not an expert in the mechanics of melt pools and my knowlege of lunar volcanism is limited. If these features were volcanic then I suppose it would be fair to expect some bulging towards the centre. However, as these pools are are formed from impact melt, different processes are at work.
Now, all of the above is written in my humble opinion to throw out an idea, get some feedback, promote discussion and/or take a beating.
Thanks for taking the time to read this.
Thomas 
Thomas J is a volunteer moderator for the Moon Zoo forum.
Boulder Repellent Craters
Something is puzzling us on the Moon Zoo forum. Ever since Tony Cook set us a challenge last year to find craters with floors cleared of boulders we have been collecting these boulder repellent craters with melt pool floors. Tony Cook said of one of the examples:
“Why have the numerous boulders within this crater avoided filling the centre of this crater? Why is the central area so featureless – presumably it is younger than the main crater? Or is it that the solar altitude of 56° is preventing us from seeing craterlets on the floor of the flat patch. Is this central patch the reverse of a central peak, perhaps a central dimple and was filled with impact melt?”
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So why is this a feature of just some craters and not others? Discussions on the forum have raised several questions. Some of these melt pools appear to show signs of impacts before the melt had solidified. So did the melt pool solidify at a slower rate so that some of the boulders that did roll into the centre sank from view? Although lunar temperatures suggest that melt pools would have solidified quickly. Are these cleared areas just very flat so the boulders stop rolling when they meet it? Does this feature correlate with a particular size of crater, impactor or type of rock being hit? And what role does space weathering play?
Some guidance was provided by a forum member xitehtnis whose work includes boulder clustering on Mars. He advised us that there are similar craters on Mars and offered some current thinking stating that different reasons applied depending on the age and environment of the craters:
“ For fresh craters, some amount of melt is generated in the course of an impact that takes diffusivity dependent cooling timescales for different depths of melt (which scales to crater size) (see Melosh, 1989).
For highly degraded craters it is likely the regolith has all been broken up to pieces beyond the limit of resolution due to impact gardening (very small impactors break up boulders and generally resurface the moon at small scales) ……..
For craters in the middle there could be a wide array of things going on. My preferred hypothesis comes from previous studies I worked on regarding glacial moraines. Basically, the idea is that fine particles are more easily mobilized during any erosive process. Since most erosive processes are gravity driven and craters generate slopes you would expect small particles to migrate to the lows in slopes while leaving the large particles (boulders) behind (check out Putkonen, Connolly, and Orloff 200?).”
xitehtnis also pointed us to this paper Impact Melt In Small Lunar Highlands Craters (Plescia et al, 2011) which notes that such melt deposits are very rare in small (km to sub-km) simple craters and concludes that:
“It may be that the small craters for which well defined melt pools are observed represent a special case – a vertical or near vertical impact.”
Could this also be the case for larger craters? OK we have more questions than answers right now! But that’s good. It allows us to research and learn – the forum is a great place for that. So we will continue to look for these intriguing craters, maybe map them out – and continue to debate their formation.
With thanks to the following forum members for their contributions: Tom128, JFincannon, astrostu, xitehtnis, Geoff, IreneAnt, Caro, matt.vader, jules, Half65, ElisabethB, Cruuux, Aliko, Thomas J, claymore, khearn. Read more and contribute here.
jules is a volunteer moderator for the Moon Zoo forum
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