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?”

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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