Lunar Enigma

[Barry Fitz-Gerald]

I like a good mystery, but I have to admit that this one has me slightly puzzled.  The Apollo 15 Metric and Panoramic cameras imaged the far side crater Paracelsus C, and it is now imaged in high resolution by LRO with the images available at the LRO Quickmap site hosted by the University of Arizona. Features in the image were identified and described in a 2016 Journal of Space Exploration article entitled  Image Analysis of Unusual Structures on the Far Side of the Moon in the Crater Paracelsus C by M.J. Carlotto and others (open source availability). Whilst I am not in complete agreement with their conclusions, I thought it worth bringing attention to the features discussed as they are interesting to say the least and I would like to hear some views on what they might represent.

The attached image is from taken from the LRO Quickmap NAC coverage of Paracelsus C and show a cluster of quasi angular apparently solid three dimensional structures on the southern crater floor. They appear to be partially buried by regolith, and indeed seem to lie within an apron of material that has slumped down off the southern crater wall. As can be seen they appear at first glance to be large slabs of rock, but the largest is some 70m long (the inset image of a Boeing 747-100 adds a touch of scale), whilst the others show a degree of angularity and the presence of straight line features that are at odds with the general run of the mill population of lunar boulders. Of course their composition is unknown, but if they are part of the normal lunar suite of rocks such as basalts or anorthosites (this is in highland terrain and so possibly dominated by the latter), then it is hard for me to understand how a 70m long slab of such rock with limited tensile strength could maintain its structural integrity and remain in one piece, especially if it originated higher up on the crater wall/rim. Of course this image is showing only one aspect of illumination and others, but other images with differing illumination angles and directions are available via the Quickmap site. Also such large blocks are big targets and would naturally be fragmented by meteorite/micrometeorite bombardment, but there is little in the way of spallation debris surrounding them.

So, if there is a structural engineer, igneous petrologist or someone who knows a thing or two about growing 70m long plagioclase crystals out there - please have a look and if possible offer an opinion.

Regards, Barry.

 

 

 

[Tiny Clanger]

That is fascinating, Barry,  but I am a bit worried about what the impressionable folk might make of it ! ("Airliner and airport terminal buildings seen on Moon !" 😉 , "Ryan Air announces Mars airport opens ... " 🙂 )

I am in no way a structural engineer, igneous petrologist or someone who knows anything about growing plagioclase feldspar crystals, but with a little bit of distant and vaguely remembered geological education, my initial thought was that looks too ...organised for igneous petrology to explain away. Columnar basalt is a pretty common phenomenon all over the Earth (Giant's Causeway etc ), but those are entire areas of vertical columnar jointed basalt, not isolated fallen specimens.

Then I thought of lava tubes , which are found on the Moon as well as the Earth and (on Earth) can have structures within them somewhat similar in appearance to the stalactites of limestone caves . Presumably the absence of atmosphere and lower gravity would change the likelihood and probable form of such a structure. But how such a thing might end up lying in the open ...

I'm intrigued.

Heather

[Barry Fitz-Gerald]

Well, I resisted mentioning things like monoliths and films set about 20 years ago for a reason!  But seriously - if there is an explanation for these objects I would be very interested to hear, hence the appeal to possible experts out there who know a thing or two about the behavior of crystalline rocks under stress. I suspect crystalline rock (basalts and their coarse grained equivalents as well as anorthosites) have limited tensile strength, and so a column of rock of volcanic origin is unlikely to remain intact if it is subject to much stress such as would be involved in a tumble off the crater rim - and in any case there are no potential parent exposures visible anywhere around the rim. In some plutonic setting (volcanic rocks that solidify very slowly underground) crystals can grow to large sizes in what are called pegmatites - and a single large crystal might have sufficient tensile strength to withstand mechanical disturbance - but 70m long - might be a bit ambitious. I did consider the regular jointing as is seen in columnar basalts as you suggest, and that may well be a potential explanation. The only problem with this is that lunar basalt lava flows tend to be thin (as opposed to the monsters represented in Fingals Cave and Devil's Tower in Wyoming which might fit the bill) due to their low viscosity, and the absence of mare type activity in this part of the lunar far side. However a counter argument might be that these are fragments of a deep volcanic intrusion where crystallisation rates were retarded due to the insulating properties of the overburden which allowed this type of jointing to develop. I really don't know. The absence of spallation debris has me particularly foxed as there are indications of small impact craters on the objects, which should have produced at least some fragments if they are crystalline rock.

The article quoted suggested that they were the result of mining activity of an unspecified origin, but without exploring more conventional explanations I would like to keep that idea on the back burner. And with the gas turned down quite low. For now.

Cheers, Barry.

[Drifter]

Quite like Tiny Clanger(Heather’s) ‘fragments of lunar column basalt’ idea … broken off initially by a deep impact in volcanic substrata? … then thrown up amongst loose debris, both larger pieces at a slight angle disturbed by a second impact … you can see evidence of this in the slight hint of a butterfly profile in the shape of Paracelsus C’s crater …. and the lumpy piles of surrounding ejecta and smoothed edges of the crater lip suggesting a multi-strike scenario?

…. One other option  … again a two-strike scenario … those angular boulders being fragments of 1st meteor thrown up amongst ejecta of a second angular strike? ….. fragments resistant to crumbling having been formed and tempered “in fire” elsewhere and en route to the lunar surface!

Looking at the shadow contours expanded slightly, it appears the rocks are fairly irregular compared to Earth’s ‘column’ examples generally formed in wetter conditions … but they still look a little unusual in a lunar context. 

Guessing variations in rock strength might occur when basalts are being formed under conditions of differing rates of lunar sublimation (drying and rapid/slow cooling) during the day/night lunar cycle? - likely also affected by the Moon’s magnetosphere strength/molten core activity at that time - which must have formed a semi-protective moisture-trapped atmosphere during some periods of lunar history when some basalts were formed. (The moisture being a high volume bi-product coming from the liquefaction of the rock).

 

[Barry Fitz-Gerald]

Well, Something along those lines - columnar volcanic rock - is a strong contender, though I am not aware of any research that has been done on the possible fracturing patterns in lunar plutonic bodies - those that cool very slowly underground and are the types to develop this form of structure. You might not expect much in the way of basalts on the lunar far side as they are more typical of the mare on the nearside, but you do have the mare filled Mare Ingenii,  Leibnitz and Van de Graaff nearby, so they must have some plumbing leading down into the crust that must be connected to large solidified rocks of a roughly basaltic composition. There are also a number of smaller craters nearby that have a very localised but very high olivine signatures which is odd, which suggest there is some unusual geology going on nearby and at shallower depths, but which could be a source for these strange creatures.  I think you may be on to something in your last sentence about 'variations in rock strength might occur when basalts are being formed under conditions'  maybe no one has looked in to how large a block could be produced and survive under different conditions.  Might be worth searching for further examples in the surrounding areas and trying to tie those in with unusual mineral abundances. Something to do this evening as there is nothing much on the TV.