Lunar Gullies – slip sliding away….
Running water on Earth carves into soils causing gullies. These are particularly well seen on hillsides and steep slopes, often resembling ditches and small valleys. Similar features have been spotted on photographs of Mars (Fig. 1), but there is much scientific debate about whether such gullies are caused by running water, periodic release of snow or underground ice or frozen carbon dioxide or if they are formed in slope or debris collapses that do not have to involved water at all (e.g. landslides).
Fig. 1. Gullies on Mars.
In both cases the start of the gully is at the top of the image and material has flowed towards the bottom of the image. LEFT: Gullies in Nirgal Vallis. MGS MOC Release No. MOC2-535.This is a Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) narrow angle image of gullies carved into debris on the south-facing wall of Nirgal Vallis, an ancient martian valley. Scaled image width: ~3 km. RIGHT: MOC image S09-01005. Crater Wall in Noachis Terra. Scaled image width: ~1 km. Here the gullies are well developed with deep upper cut regions and fan like debris deposits at the bottom of the slope.
In 2006 Gwen Bart, a US planetary scientist searched through available images of the Moon that were collected by 1960’s Lunar Orbiter mission to see if she could find examples of gullies on the Moon. She suggested that if gullies could be seen on the slopes of lunar craters, and as the Moon typically has very very little water, it may provide evidence that gullies on Mars don’t have to have formed in the presence of water or ice. Her preliminary results are presented here and there is a nice summary of the implications of her study here.
As Moon Zoo users well know – the new images by the LRO LROC NAC are knocking the spots off the older Moon photos! For example, now we have these amazing new NAC images they are showing that gullies really are found all over the Moon…
Montage of LROC NAC image M127009259 (not map projected) showing view of Gambert C crater which is located on the nearside of the Moon near Copernicus crater.
Close up images of gullies in Gambert C crater shown in wider view above. Several gully networks can be seen in each image. In all cases the source of the gullies is at the top of the image and they flow down towards the base on the image. Flows and gullies show lobate tracks and channels, often with blocky rubble material having been pushed to the sides and end of the main channel.
Left: Montage of LROC NAC image M105185599E (not map projected) and close up section shown at right. Bright, elongate gully tracks can been seen in the photo eminating from bright rubbley regions on the crater wall. The gullies start in the bottom left hand corner and flowed towards the top right.
See also the dramatic examples in Marius crater and look at the wide view of the area.
And Moon Zoo users have already done a great job in spotting some really nice examples of landslides, gullies and channels on slopes. Check out the examples in Birt crater, Proclus crater and in other places on the Moon here and here.
So – we would like to issue a renewed challenge to keep a close eye out for gullies and landslides on the Moon. Hopefully your discoveries will help to provide a good database that scientists can use to address the diversity, shape and form of gullies on the Moon compared with those seen on Mars, the Earth and other planets.
Please remember, as well as posting examples on the Moon Zoo Forum under the landslides and gullies topic, if you find examples in the Moon Zoo user interface to flag these features as linear features so that we also have a record noted in our database!
Thanks for your help,
Katie
Moon Zoo Team
(Thanks to Allan Treiman and Amanda Nahm at LPI for drawing our attention to this interesting lunar science and Martian science topic).
The Moonometer™ Challenge
Did you know that Moon Zoo users have now classified an area three times the size of Wales? Or maybe you’d be interested to learn that, even on slow days, Moon Zoo users trawl a section of the Moon bigger than 500 Disneylands!
To celebrate International Observe the Moon Night we have launched the Moonometer™ – a fun way to understand how much work Moon Zoo is doing minute-by-minute. In addition, between September 15th and 19th we are challenging the Moon Zoo community to classify a huge chunk of the Moon: 20,000 images! This is roughly equivalent to an area twice the size of Chicago!
To take part in the challenge all you have to do is classify things on Moon Zoo using either the Crater Survey or Boulder Wars tools. The Moonometer™ keeps track of the number of LRO images that have been classified and converts them into approximate equivalent areas.
You can also keep track of activity on Moon Zoo via the Moon Zoo Live! page. Here you’ll find ever-updating maps that show how Moon zoo is connecting the Earth to the Moon thanks to our users.
International Observe the Moon Night is all about learning more about the Moon going and taking a look at it. You can use our Explore the Moon pages to find out more about our nearest neighbour in space. To get involved with the Moon Zoo community, visit our forum and see what we’re currently talking about and looking at.
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.
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Thanks to Katie for additional info and links. 🙂
Jules is a volunteer moderator for the Moon Zoo Forum.
The Moon Has Its Faults
At Moon Zoo, they have asked for your help in identifying linear features on the Moon, some of which are likely to be faults. So, what are faults? Faults are fractures in a planet or moon’s crust that accommodate some movement along them. If the movement along a fault is great enough, a quake can be felt.
On Earth, the majority of faults occurs at plate boundaries, like the San Andreas fault which separates the North American and Pacific plates (Figure 1) or along the boundary between the Indian and Eurasian plates which forms the Himalaya Mountains.
Figure 1. San Andreas fault in California. A) Aerial view of the Carizzo Plain. Note the offset drainages. B) Annotated image showing the fault trace (black line) and the arrows denote the direction each side of this strike-slip fault moved. Image A: IPGP, Paris.
However, a planet or moon does not need to have plate tectonics (as on the Earth) in order to have faults. Faults can form anywhere where there is enough stress (the amount of force distributed over an area) in the crust to make it break – or fault.
There are several ways in which faults form. On the Moon, the source of stress is most commonly associated with impact basins, so most of the faults are found in and around the large impact basins on the lunar nearside.
There are two types of faults found on the Moon. Normal faults occur where there is tension (stretching) of the crust. When this occurs, the crust in that area gets thinner and longer, which means that one side of the fault must move down relative to the other (Figure. 2). This happens when the weight of the mare in the basins pulls the crust apart as the middle of the basin subsides.
Figure 2. Cartoon showing cross-sectional and map views of a pair of inward dipping normal faults, resulting in the formation of a graben. The red arrows show that the crust is being pulled apart and white arrows show the movement directions along the faults. Image: Sue Selkirk/ASU.
Normal Faults: on the Moon, there are several types of normal faults: straight rilles normal faults and thrust faults.
1. Straight rilles are long, narrow troughs and they exhibit two fault-related surface morphologies: linear (Figure 3) and arcuate. These are formed by faults and are found in the highlands near mare basins as well as near their margins. They have steep sides, which are interpreted to be the normal faults, and flat floors. These structures with two inward sloping walls are called graben (meaning trench in German) (Figure 3).
Figure 3. Linear rilles or graben of Rima Ariadaeus is about 300 km in length and is located between Mare Tranquillitatis and Mare Vaporum. Image: WAC M116222025MC NASA/GSFC/Arizona State University.
2. Isolated normal faults also exist, but are rare compared to rilles (graben). Like some normal faults on Earth, these tend to be very linear as shown by the Rupes Recta (“Straight Wall or Cliff”) in Mare Nubium (Figure 4).
Figure 4. The Rupes Recta normal fault (shown by black arrows) is ~110 km long and is located in the southeastern part of Mare Nubium on the nearside of the Moon. The fault scarp is a few km high and is in shadow. Image: JAXA/NHK.
3. Thrust Faults: When stress acts in an opposite sense, compression of the crust occurs and thrust faults are formed. In this case, the crust gets thicker and one side of the fault moves upward over the other (Figure 5).
Figure 5. Cartoon showing a thrust fault. U: up, D: down. Large black arrows show compression direction and small half arrows show movement directions along the faults. The surface trace of the fault is what geologists would map. Image: American River College.
On the Moon, this happens most often in impact basin interiors where the weight of the mare causes the basin to subside, or bend downwards, causing compression in the mare. Thrust faults on the Moon have two morphologies: wrinkle ridges and lobate scarps. Wrinkle ridges (Figure 6) are found in the mare in the basins and are very complicated structures. They consist of a broad arch (the ‘ridge’ in wrinkle ridges) with narrow, often sinuous ridges superimposed (the ‘wrinkle’). Lobate scarps, which are much simpler in their morphology (Figure 7), are almost exclusively found in the lunar farside highlands far from the influence of basin subsidence. These structures are instead thought to have formed from the slow cooling of the lunar interior that led to a small amount of global contraction.
Figure 6. Wrinkle ridges in Mare Serenitatis. Note also that normal faults are present at the basin margin in the lower corners. Both wrinkle ridges and normal faults are primarily concentric to the basin. Image width is ~280 km. Image: mosaic of Apollo Metric camera frames AS17-450 and AS17-454. NASA.
Figure 7. Lee-Lincoln scarp in the Taurus-Littrow valley (Apollo 17 landing site). Image is 1.6 km across. Image: NAC frame M104318871LC. NASA/GSFC/Arizona State University
Why are faults important to lunar and planetary scientists? On the Moon, since most faults form in response to stresses from the lunar basins, they can tell us about the geologic and tectonic evolution of these basins, particularly the mare-filled ones on the lunar nearside. Wrinkle ridges and rilles can be used to determine the structure of the basin and in some cases how thick the mare are. The fact that lobate scarps are found primarily in the lunar highlands and wrinkle ridges are found only in the lunar mare suggests that these materials behave differently under stress, which is important when trying to puzzle out the geologic history.
The tectonic evolution of a planet or moon can also give us important information about what the interior is like since directly accessing the deep interior is nearly impossible. For example, the lobate scarps (along with the presence of mare) tell us that the Moon was initially hot and that it cooled some finite amount over its lifetime. This has been determined from the offsets along the faults to be about 70 m.
This post was written for Moon Zoo by by Amanda Nahm at the Lunar and Planetary Institute.
Looking for Change?
For centuries Earth-based astronomers have reported seeing transient changes on the Moon’s surface, known as Transient Lunar Phenomena, or TLP. These have taken the form of short term brightness changes, coloured and non-coloured glows, obscuration of detail, shadow anomalies and flashes of light. The duration of TLP can be from a fraction of a second up to several tens of minutes; then everything returns to normal as far as we can tell from Earth. Unfortunately many of these so called TLP are from inexperienced observers, or made under poor observing conditions, however some TLPs are more intriguing as they are confirmed by more than one observer.
Although the Moon is widely believed by planetary scientists to be effectively geologically dead, there are four currently accepted theories that could explain some of these mysterious TLP reports: (1) Flashes of light seen on the night side are simply due to energy released at optical wavelengths from meteorite impacts – these are being studied by project ALaMO at NASA’s Marshall Space Flight Center and also by amateur astronomers at the Association of Lunar and Planetary Observers (ALPO). (2) Eruptive out-gassing from residual radiogenic gases such as Radon and Argon that accumulate in deep seated cracks and are released during Moon Quakes – we would effectively see the dust kicked up from this out-gassing. A robotic telescope at Cerro Tololo, Chile, is being used to look for white light changes on the Moon every few tens of seconds, and amateur astronomers at ALPO and the British Astronomical Association (BAA) are searching for coloured events, and TLP in Earthshine or more shaded areas not easily accessible by the Cerro Tololo scope. (3) Electrostatic levitation of dust particles forming dust clouds in electric fields that build up around sunrise in the form of Moon dust fountains. (4) Internal reflectance off volcanic glass beads – analogous to rainbows. However the only attempt to prove a TLP was caused in this way, in Torricelli B crater, was unsuccessful at reproducing this effect.
Figure 1. (Left) part of an LROC image (M116282876RC) of the Ina formation with an enlarged inset (right) showing craters on the chaotic terrain
Here at Abewrystwyth University we have been analysing some of the statistics of past TLP reports. A preliminary report was given at the Lunar and Planetary Science Conference in Houston in 2009 and we will provide an update in September 2010 at the European Planetary Science Congress in Rome. However we are also very interested to hear from Moon Zoo users, via the on-line forum (e.g. Picture of the Week) of any interesting surface features that they think might have been freshly disturbed or perhaps a vent from which out-gassing has come from. You may find such features as you go through “Boulder Wars” or the “Crater Survey”. However for those really keen to find additional interesting objects, then they can dip into the Apollo Metric and Panoramic Camera or higher resolution LROC image archives, though the latter has a bit of a learning curve. Things to look out for include small craters which are completely shadow filled, whereas their neighbours are not – these could be ceiling vents into underground lava tubes. Also unusual disturbed surfaces that do not look like anything elsewhere in the vicinity such as Figures 1-4. We do not propose that any of these are examples of where out-gassing may have occurred, but they are the kinds of unique features that we would like to know about, just in case.
Figure 2 (Left) M104476560LE LROC image of a small part of the floor of Hyginus Crater containing some geologically young sunken areas. (Right) M111972680LE LROC image of bright ray crater (east of Reiner Gamma) with a remarkably featureless central floor patch.
Figure 3(Left) Unusual dark shaded depressions, or dark textured material, to the north west of Vallis Alpes from LROC image M122218486RE. (Right) deep seated fractures on the floor of Tycho from LROC image M114031031LE.
So as Jules says in an earlier Moon Zoo blog, please get out there, looking at the Moon close up and report your findings to the Forum. Just maybe you will be the first to identify a true TLP site, and if not then I am sure that your findings will interest the Moon Zoo geologists anyway!
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