Wrinkle Ridges? Sort of!
Recently, a user found an enigmatic feature (see forum thread here) and several suggestions were given for how it formed, including (1) impact melt flows, (2) lava flow fronts / thick lava flows, and (3) wrinkle ridges.
Figure 1. Enigmatic feature found by Moon Zoo user Tom128 and discussed on the Moon Zoo Image of the week here from LROC strip: M117732304RE
Let’s address each hypothesis individually to see if we can determine which is the most likely scenario for the formation of the structures.
(1) Impact Melt Flows. A few things are important to note about impact melt flows. First, they are always associated with impact craters. Second, like water, the flows will always head downhill. Third, the flows often have lobate morphologies at their termini. Here are a few a beautiful examples of impact melt flows from the crater Giordano Bruno (Figs. 2). In these images, we see the characteristic lobate shape of flows. The features in the original post do not appear to either be near or closely associated with large impact craters (though the field of view afforded by NACs is admittedly small) or formed as the material flowed downhill. Evidence for this latter point comes from the observation that the structure has two steep scarps to it (one on either side) which would be difficult to get if there was a dominant flow direction.
Figure 2. Left: Image of impact melt flow on the southern flank of the crater Giordano Bruno. From Denevi et al. (2010). Portion of NAC frame M101476840LE. Illumination is to the west. The flow moves downhill to the lower right hand side of the image. North is up. Resolution 1.5 m/px. Right: Impact melt flows on the flanks of Giordano Bruno. Portion of NAC frame M110919730L (centered at 35.84°N, 102.72°E). Downhill is roughly towards the lower left corner of the image. Note the multiple different flows (if you are having trouble seeing these flows as positive relief features look at the white features – these are boulders – hopefully the impact melt flow should pop out into a flow!). Image resolution is 0.6 m/pixel. See the full frame here.
(2) Lava Flows. Similar morphological features are also characteristic of volcano lava flow fronts (flow downhill, lobate terminus), except lava flows do not typically emanate from impact craters. Lava flows on the Moon are typically much wider and longer (up to 800 km long and 20-40 km wide in places!) than the narrow impact melt flows seen in Fig. 2, as they are formed from highly effusive sources (vents) when there was lot of lava erupted to produce large-scale flows. However, there is a possibility that the feature could represent a localized, highly viscous flow unit, as suggested by Irene in the Image of the Week post. As much of the Moon has not been seen in such high-resolution before, we may well be viewing geological constructs that are completely new to lunar science!
(3) Finally, we should address the final formation hypothesis: wrinkle ridges. (See also a previous blog entry for some background structural geology.) So, what is a wrinkle ridge and how does it form? Since their discovery on the Moon (and elsewhere), there has been much debate about these questions. In the last 10 years or so, there has been a convergence in the community toward the “right answer:” that these structures are blind thrust fault anticlines. That’s structural geology speak for ridges or convex-upward shapes formed by a thrust fault that does not yet reach the surface. They form in response to horizontal squeezing (or compression) of the material they form in and this compression is usually related to regional-scale processes, like sagging of the crust under large impact craters filled with mare. Wrinkle ridges seem to preferentially form in layered material, where there are mechanically weaker layers (like ash or regolith) separating the mechanically stronger layers (the lava flows). This mechanical juxtaposition means that when the material is compressed, three things happen: folding, sliding, and faulting. As the thrust fault propagates toward the surface, the material folds above it and slip occurs along weak interfaces. Sometimes a thrust fault will form which slopes opposite to the main thrust fault (the backthrust, like this y) and this is manifested at the surface as the wrinkle.
The structures shown in the Image of the Week thread do not necessarily have what I would call typical wrinkle ridge (WR) morphology (except those shown in Tranquilitatis). This is what is interesting! If these are wrinkle ridges, it tells us something different is going on here and that difference will tell us about the material in which these structures formed and the geologic evolution of the area where they are found. When I first looked at these images, I wasn’t sure what to make of them. In the NAC frame to the left of the first image (see Figure 3 for both NAC frames montaged together), I saw a structure with more typical WR morphology and that really clinched the identification for me. I also took a look at the “WR” in Aitken crater from the LROC site. So, are they wrinkle ridges? Well, the answer is…sort of.
Figure 3. Section of montage of NAC frames M117732304RE and M117732304LE showing wider field of view of the area of interest (the image width is ~5 km). Note that the original section of image is at top right.
If you allow me to get on my soapbox for a few sentences, I can explain what I mean. To structural geologists like me, there is a connotation associated with terms like wrinkle ridge or lobate scarp telling us about the mechanical properties of the material, the morphology of the structures, and how they formed. Since these structures do not have the typical morphology of a wrinkle ridge, I would not call them wrinkle ridges, though they likely formed in a similar manner: compression of layered lava flows separated by weak layers, forming thrust faults and folds. The narrowness of the sinuous ridges implies that the interbedded basalt is fairly thin (maybe less than a kilometer). The lack of a broad ridge is something for which I do not yet have an explanation.
As Drew Enns suggests in his blog post on the LROC Picture of the Day site, the size of the ridges implies that the thickness of the basalt in both Aitken crater and near Bessarion W is thin. It may also suggest that the faulted layer is thin. That is, there may be more basalt at depth, but the weak mechanical layer may only be a few tens to hundreds of meters below the surface. It is in this way that structures like wrinkles ridges can tell us about the physical properties of the materials they deform and the geologic history of the areas where they form.
Dr. Amanda Nahm is a tectonic-fault expert at the Lunar and Planetary Institute
Splosh!
Latitude: 49.2394°
Longitude: 3.63271°
Back in June Moon Zoo forum member Toban posted this unusual looking crater in the Alpine Valley east of Plato from the LRO strip M104497175LE. Toban wondered if what we were seeing was “old” lava. He asked:
“Was the impact so deep, that this has “opened” the ground a long time ago?! I think there is no liquid under the moon surface… so maybe it is very old or it’s not lava…”
It reminded me of another unusual crater posted by Geoff a week earlier
which IreneAnt identified as “an impact into a not-completely-solidified melt sheet” and she also provided a link to a paper showing experimental impacts into viscous targets and noted that there were more examples of this type of crater in the same area (Aristarchus – strip M111904494RE.)
Alternatively could Toban’s crater be a very eroded crater or a “ghost” crater? Eroded craters have been worn and eroded by a history of micrometeorite impacts so that their original form is hard to make out. Ghost craters are craters which have subsequently been filled with lava leaving only a “ghost” of a rim visible when the sun is at a sufficiently low angle. Here are a couple of examples of ghost craters:
Prinz crater wiki |
. | Stadius Crater astrosurf.com |
Toban thought this unlikely because his crater has a very definite unbroken ring structure with a clear boundary which has captured some of the boulders that have rolled down into the ring area.
Close up of Toban’s crater – the section around 10 o’clock
IreneAnt confirmed that Toban’s crater was another impact into viscous material and provided a link to another paper which describes laboratory experiments to study the morphologies of craters produced by impacts into various viscous materials using different impact velocities. IreneAnt draws our attention to Figure 4b of this paper which shows an impact into a clay-oil mixture producing a crater with no central peak and poorly defined rim topography. A similar type of crater, known as a “splosh crater” is much more common on Mars where the geology, history, atmosphere and gravity produce craters which are multi-lobed with splashed rather than rubbly ejecta. This type of Martian crater was formed when a meteorite hit an area rich enough in water to turn the impact site into runny mud. The lunar version, however, did not involve water but was formed because the impact site rocks had melted enough to behave like a viscous liquid. IreneAnt, therefore, suggests the term “melt splosh crater” to describe the lunar version to avoid confusion with the Martian water based splosh craters.
Although not quite the same thing the lunar versions were still formed more from a splash than a crash. Geoff found more examples of lunar viscous impact craters around King Crater on LROC strip M115529715LE.
These craters were formed when parts of the Moon were covered in molten lava and the splashes have been “frozen” as the lava cooled. So we should be able to find craters in various stages depending on the consistency of the lava impacted. Look out for them and post your finds in Toban’s thread.
Jules is a volunteer moderator for the Moon Zoo Forum.
Moon Bridges
Moon Zoo team member Katie sent me an e-mail with a link to an article about a newly discovered lunar feature!! The feature is a natural bridge likely formed by a dual collapse into a lava tube. The one Katie linked to is one of 2 found in the impact melt around King Crater on the lunar farside. This and a smaller neighbouring bridge (also in impact melt material) are the only examples of natural bridges that have been found on the Moon and she asked if we could look out for some more.
These are similar to the lava tube skylights we are already looking for and might be quite tricky to spot. Look out for 2 or more “holes” and some evidence of sunlight shining through from one hole to another showing that it really is a larger cavity spanned by a “bridge.”
You might come across bridges in impact melts (like the King Crater bridge) or in rilles (valleys carved out by lava flows or created by the collapse of lava tubes) running over the mare basalts. Rilles may have some uncollapsed roofed over sections. These are the natural “bridges” we should look out for.
The area around King crater seems a good place to start. Have a look at these LROC strips showing the King Crater bridge under different illumination:
M103725084L, M103732241L, M106088433L, M113168034R.
Other locations rich in impact melt would also be good hunting grounds. In large impacts the impact itself and the damage caused by shock waves raises the temperature of the surrounding area and significant amounts of lunar rock melt or vaporize together with the original impactor. Some of this impact melt rock is ejected, but most remains in or around the crater. Impact melt can be seen as flows or ponds.
More information about about impact melt can be found here:
BYRGIUS A CRATER IMPACT MELTS – AN LROC PERSPECTIVE. There is more about Byrgius A here.
IMPACT MELT MOVEMENT IN LUNAR CRATERS.
IMPACT MELT FLOWS ON GIORDANO BRUNO.
These features are interesting to lunar scientists because they are essentially covered caves and ideal places to build future Moon bases as the bridge roof will offer protection from solar wind and cosmic ray radiation. There’s a paper on how bridges may form here: A Search for Intact Lava Tubes on the Moon: Possible Lunar Base Habitats.
More information in Katie’s link: Natural Bridge on the Moon. And here’s another article with a 3D image.
This is the King Crater Bridge from LROC image number M113168034R with what I think is the second bridge. Can you find a better candidate for the second one? The small blue rectangle in the inset shows the locations.
 |
 |
Thanks to Katie for additional info and links. 🙂
Jules is a volunteer moderator for the Moon Zoo Forum.
Dark Haloed Craters
Dark haloed craters are windows to the volcanic history of the Moon. This blog entry was inspired by Thomas’s choice of Image of the Week where he highlights impact craters that have dark material surrounding the crater hole itself.
Dark haloed craters provide us with key insights into what must have been a dramatic and violent volcanic period of lunar history. To probe a little further here is a little background about eruptive volcanism on the Moon:
Between about 4 billion years ago and 3 billion years ago the lunar mantle underwent a period of partial melting where magma was generated at depth and then propagated up through fracture networks and magma conduits towards the lunar surface. It has been proposed that some of this lava was very rich in gases like carbon monoxide that may have caused rapid upward movement (maybe on the scale of one to several days) and caused dramatic pyroclastic eruptions at the lunar surface. These fire-fountaining events are similar to, but on a much larger scale than, eruptions witnessed at volcanoes in Hawaii, with some plumes of lava being thrown up to 40 km in height above the surface of Moon!
Fire fountaining: Volcanic eruption in Hawaii. Droplets of molten lava are thrown up into the air, where they rapidly cool to form glass beads called Pele’s tears. This is a good analogy to how volcanic dark haloed mantling deposits were formed on the Moon. Image: USGS.
Other lavas that were less gas-rich would have migrated to the lunar surface more slowly, and could have erupted more gently, forming long lava flows that travelled great distances from their volcanic vent site. These lava flows are thought to have the consistency of runny motor oil and easily flowed into topographic lows like impact craters. Sometimes large quantities of lava must have flowed in channel networks – forming rivers of fire across the lunar surface. It must have been a dramatic time, but as available heat sources were diminished in the lunar interior, less magma was generated and by about 1 billion years ago we believe the Moon’s eruptive volcanic history came to a close.
Lunar glass beads: Orange and black glass beads collected from a pyroclastic deposit at the Apollo 17 landing site. These types of beads form mantles around the volcanic vent site from which they were erupted, forming dark mantling deposits. Image: NASA.
So how do dark haloed craters fit into this story? Well they actually help address two separate lunar science questions as there are two types of dark haloed craters to keep an eye out for in Moon Zoo images (although we would please like you to classify them using the same button!)…
1. Volcanic eruption sites – these are rare places where pyroclastic beads, the ‘airfall’ deposits of lunar volcanoes, are concentrated on the lunar surface and form mantles around their source vents. These beads are a little bit like the volcanic ash or the Pele’s tears glass that gets erupted from volcanoes on Earth and you can read more about these types of dark halo mantles at:
- Explosive Volcanic Eruptions on the Moon.
- The Pyroclastic Volcanism Project at the US Geological Survey.
- Moon Zoo Thomas’s guide to pyroclastic craters.
- Pyroclastic bead features seen in LROC NAC images.
2. Crater excavation sites – you are far more likely to spot these types of dark haloed craters in Moon Zoo images. The dark haloed craters provide us with a neat view down through a series of geological layers. These are formed when an asteroid or comets smashes into the Moon, punches through an overlying light coloured layer (probably an ejecta blanket material from a nearby highland impact crater) and excavates material from below that is darker in colour. This darker material is likely to be a lava flow that was buried at depth and that is now revealed by the impact cratering process.
Here’s a nice diagram of this process can be seen at where the lower image shows a schematic of what two dark haloed impact craters look like from side-on. You can also view a 3D perspective of this process.
Impact revealing buried lava flows: This LROC NAC image (taken from M112183669LE) is a good example of a dark haloed impact crater that has punched through a light surface deposit and has excavated darker material from an underlying lava flow. Image: LROC/NASA.
We call these types of buried lava flows cryptomaria as they would otherwise be hidden from view if we had not have spotted the tell tale signs of dark haloed craters. By mapping the location and extent of dark haloed craters we can therefore map out the distribution of buried lava flows at depth across the Moon and get a much better idea of the amount of ancient volcanism on the Moon. This in turn helps to shed new light on the Moon’s thermal and magmatic history, helping us to understand geological processes on small rocky planetary bodies.
Good examples of these types of impact formed dark-haloed craters spotted by Moon Zoo users include:
- http://www.moonzoo.org/examine/AMZ100185s (top right)
- http://forum.moonzoo.org/index.php?topic=209.msg4111#msg4111 (lovely collection of small craters at left centre)
- http://forum.moonzoo.org/index.php?topic=209.msg4270#msg4270 (right centre)
- Other good examples are provided on the Moon Zoo How to Take Part Tutorial.
So please do keep an eye our dark haloed craters on your Moon Zoo lunar exploring! Thanks to Irene Antonenko for providing helpful guidance about this topic.
Moon Zoo 
