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.

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