Ejecta Blocking Boulders
All credit for this entry goes to forum regular kodemunkey who wrote this article:
Hello, and welcome to what will hopefully be the first of many IOTW posts from me.
I was exploring the LRO Data using the WMS Browser and I came across Maginus crater.
(Maginus crater, as seen in the WMS browser, latitude -48.992774 longitude -5.149416)
This is what Wikipedia has to say about the crater:
Maginus is an ancient lunar impact crater located in the southern highlands to the southeast of the prominent crater Tycho. It is a large formation almost three quarters the diameter of Clavius, which lies to the southwest. Just to the north of Maginus is the smaller crater Proctor, and to the southeast is Deluc.
The rim of Maginus is heavily eroded, with impact-formed incisions, and multiple overlapping craters across the eastern side. The wall is broken through in the southeast by Maginus C, a worn crater. Little remains of the original features that formed the rim of Maginus, and it no longer possesses an outer rampart. The floor is relatively flat, with a pair of low central peaks.
The thing about Maginus that interested me at the time was the unnamed crater near Maginus A, as it has a lot of NAC frame coverage. I’m certain that like me you prefer to look at areas with a lot of coverage, if only out of sheer nosiness!
The thing that first caught my eye about the crater was this large, and probably quite deep crack:
The next thing to catch my eye are these huge boulders which are blocking the flow of material down the slope.
These things are quite large, probably at least the size of a house, I wonder where they came from?
Sources and more information:
https://en.wikipedia.org/wiki/Maginus_crater
http://the-moon.wikispaces.com/Maginus
NAC frame: http://wms.lroc.asu.edu/lroc/view_lroc/LRO-L-LROC-2-EDR-V1.0/M175308014LE
Your task, should you choose to accept it (even if you don’t ) is to try and figure out how the boulders came to be in their present positions.
Exploding Boulders!
We are used to seeing boulder tracks on Moon Zoo and often come across (or actively go hunting for!) the boulder that caused them. Usually we find something like these large intact boulders having come to rest at the end of their tracks.
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highlighted by placidstorm and kodemunkey
Moon Zoo team member Dr Anthony Cook recently sent me this picture of two boulder tracks in Schiller crater:
In this case the boulders are far from intact and appear to have “exploded” at the end of their journeys. What might have caused these boulders to fracture and fragment? One theory Tony suggested was that due to being under tension the boulders might have fractured before they rolled down the slope and that the movement further weakened them. Then over time the extreme temperature variations between lunar day and night could have fragmented the weakened rocks resulting in the appearance we see in the image.
Tony said:
I’m a bit puzzled though why the one on the top left has rock debris so far away from the centre. The boulder that looks like a skull rock on the bottom right has debris a lot closer to it, that could simply be explained by bits falling off as one would expect from the explanation above.
An alternative theory is that the boulders did roll down the hill intact, but were of sufficient size, area and age to be impacted by later meteorites, and these high velocity impacts split the rocks into many pieces. However, as Tony points out, the chances of this happening to two large rocks next to each other seem a bit remote.
Here is the NAC image M109502471L and the LROC article “A Recent Journey.”
In order to study this process in more detail we need more examples. So if you find any exploded (or partly exploded) boulders please post them on the forum here.
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
Stratified Ejecta Blocks (The Stripy Boulder Hunt!)
Another hunt….and this time it’s stripy!
Katie Joy from the Moon Zoo team says:
We would like you to take a closer look at large boulders in Moon Zoo images. We want people to spot boulders that have layers cutting across the rock. There are some examples of the features we are interested in this LROC post and in this fascinating Lunar and Planetary Science Conference abstract.
Please look for layers that are as clearly spotted as those in the examples – ideally more than five obvious bands in the boulder (dark to light layers). Look at boulders that have rolled down slopes, those that are sitting in rubble-filled gullies and even boulders that are just sitting on their own. Please provide images of features you find and if you can state the location of the NAC frame where the boulder was found would be a big help.
Finding such boulders is really exciting as they suggest that large blocks of ‘bedrock’ are exposed and that could potentially be sampled by astronauts of robotic missions to the Moon. Lunar bedrock is normally hidden from view under a soil-like covering called regolith – the Apollo missions never sampled rocks from bedrock units (although layers of rock were seen from afar in the walls of Hadley Rille at Apollo 15 – see the astronaut’s comments at 165:23:26 available here!). Sampling bedrock layers that have a stratigraphic sequence (layers that have built up in a time-sequential manner) will provide unique information about how lunar rocks have formed with time. They will likely contain a temporal archive of lunar and Solar System processes (see this research about accessing a record of the Moon’s interaction with Space), and therefore are time capsules that provide a view to processes occurring millions, if not billions of years ago.
Happy stripy boulder hunting, and thank you once again for all your help,
Katie Joy
Well to kick things off, forum members Half65 and Tom128 found these examples of stratified bouders in Aristarchus. I think this is the type of boulder we should look out for as the LPI paper Katie links to shows some similar boulders in Aristarchus.
Hopefully a team member will come along and comment. The NAC image is M111904494RE
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Jules is a volunteer moderator for the Moon Zoo forum
Making Sense of Shadows
Forum member Caidoz13 posted this picture last week:
AMZ10006ni
and commented:
“There seems to be something really tall, casting a long, thin shadow. There are boulders nearby that are casting similar, but much shorter shadows. It looks like the object on the right is a really tall, thin rock, almost a column.”
Another forum member jumpjack took this further and suggested that rather than simply being a tall rock casting a long thin shadow in low sun what we were looking at was the rock shadow cast over a dip in the terrain. He explained with a sketch:
“If you look closer, you can notice that shadows describe a crater to the right of the rock, hence the terrain goes down as much as far it is from the rock, thus causing the long shadow.”
jumpjack remembered seeing a similar effect somewhere else and it reminded me of the shadow we found at Milichius A Crater. Phil Stooke LRO image scanner extraordinaire also agreed that although the Milichius shadow did look unusual it was likely to be nothing more than a linear shadow of an appropriately placed and shaped rock near the terminator. Although the Milichius image isn’t as clear it does look as if the shadow might be cast over a dip in the terrian too though this particular feature would benefit from further scrutiny when additional images are available. Several examples of this type of shadow taken when there is a very low sun (lunar sunrise / sunset) have appeared in the “Interesting Terrain” and “Spacecraft or Space Debris” threads as at first glance they do look unusual and, as jumpjack points out, the effect is exaggerated if the shadow is cast over a drop in the terrain. This combination of a low sun shining on a tallish boulder – especially one at the edge of a dip – can give the illusion of a tall, thin, man made mast-like structure. So I thought it worth highlighting this long thin shadow effect to help people sort the rocks from the space debris.
Here are a few more examples:
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And for comparison here’s Apollo 14. OK the tracks give this away too!
Keep looking closely at these low sun angle pictures though – as any monolith out there will cast a very similar shadow. 
Jules is a volunteer moderator for the Moon Zoo Forum
Boulder Tracks
The Boulder Tracks thread is one of the most popular within the Moon Zoo forum and we have some amazing tracks posted there.
Boulder tracks are important to the Moon Zoo project – the following quote is from one of the Moon Zoo team members:
One of the main reasons we are asking Moon Zoo users to search for scars left behind by tumbling boulders is to help support future lunar exploration initiatives. Boulders that have rolled down hillsides from crater walls, or massifs like the Apollo 17 landing site, provide samples of geologic units that may be high up a hillside and thus difficult to access otherwise by a rover or a manned crew vehicle. If mission planning can include traverses to boulders that have rolled down hills, and we can track these boulders back up to the part of hillside from where they have originated, it provides a neat sampling strategy to accessing more geological units than would have been possible otherwise… Thus we hope to use Moon Zoo user data to produce a map of known boulder tracks (and terminal boulders) across the Moon. – Katie Joy
Recently ElisabethB (Els) posted the tracks shown below. Quite an amazing variety of track sizes and shapes! The track on the bottom appears to have mounds inside the track caused by the shape of the boulder that created the track.
Also, some of the tracks have craters overlapping them which may have been caused by the same impact. The original impact would have sent boulders bouncing and rolling along the regolith but would also have sent boulders upwards and they would have eventually fallen back and created craters.
The area where these tracks are found is Montes Alpes / Vallis Alpes.
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ID: AMZ4003tvl
Latitude: 48.611°
Longitude: 1.70272°
Sun Angle: -62.71°
Scale: 0.51 meters / pixel
Zoom Level: 3
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