The Incredible Shrinking Moon
A recent article that appeared in the journal Science led to a flurry of shrinking Moon headlines. Don’t worry, the Moon is still there, and will be for some time yet. But seeing as the images for this study came from the LROC camera that we all know and love, I thought it’d be worth re-telling the story here.
Figure 1. A perspective image looking across the Taurus-Littrow valley, made from LROC stereo images M104311715LE and M104318871LE. The arrow shows the Apollo 17 landing site, with the Lee-Lincoln lobate scarp in the left of the image. This scarp runs across the valley floor mare and up through highland material. Credit: NASA/Goddard/Arizona State University/Smithsonian
Lobate scarps are probably amongst the smallest tectonic features on the Moon, and so they are too small to be visible in telescopic images, but rather only the highest resolution images from orbiting spacecraft. In the past they were only visible in Apollo Panoramic Camera high resolution images, which only covered a tiny portion (about 20%) of the Moon near the equator.
So there wasn’t really a detailed understanding of where these features occurred as the image coverage wasn’t good enough. We had basically been limited to the 40 year old data that were available.
The general consensus is that lobate scarps form by contraction of the upper crust and so are a form of low-angle thrust fault (see Amanda Nahm’s excellent blog post on lunar faulting here), which are common geological features on the Earth. But we have a slightly harder time explaining their formation on other planetary bodies, mainly because we can’t rely on plate tectonics to provide the driving force. So what does cause the contraction that is responsible?
On relatively small planetary bodies, like Mercury and the Moon, it’s long been thought that the original cooling of the body very early in its history could cause a global contraction in the size of the body (this is a fairly familiar concept to anyone that has played as much winter sport in England as I have – some things get smaller as the temperature decreases). The original higher temperatures are probably the result of a combination of the heat left over from accretion (formation) of the original body, and from the decay of short-lived radio-isotopes.
Alternatively, tidal forcing can also cause tectonics, both extensional and compressional, on planetary bodies when there is a tidally-locked moon involved. For example, Europa is covered in so-called cycloid fractures thought to be the result of the massive tidal forces occurring in the inner Galilean satellites. In this case the body is pumped and squeezed by the tidal forces, causing fracturing (Europa) or even frictional melting (Io).
However, these two different possible causes of stress in the upper crust would produce different tectonic features at different places on the surface. Tidal forcing would cause contraction and thrust faults (including scarps) at the tidal bulge and its antipode (so near the equator), but would also cause extension and normal faulting at the poles. In contrast a global period of cooling would cause contraction over the entire globe of the body, rather than just at or near the equator.
So the increased coverage offered by LROC meant that the search for lobate scarps could, for the first time, be extended beyond the equatorial region and in effect test the hypotheses for the cause of the crustal stress on a global scale. By scanning through LROC images the authors saw not only the previously identified lobate scarps, mostly near the equator, but also crucially identified lobate scarps close to the polar regions (see Figure 2)
By observing these lobate scarps at high latitudes, this study has shown that the major tectonic force near the poles was probably compressive, rather than extensional, and so doesn’t fit well with the idea that tidal forcing (alone) has caused lobate scarps to form, but instead supports the idea that they are primarily the result of contraction over the entire lunar globe.
Figure 2. This map illustrates the distribution of lobate scarp features located thus far. Black dots indicate previously known scarps while white dots depict newly detected scarps found in images from the Lunar Reconnaissance Orbiter Camera. Credit: NASA/Arizona State University/Smithsonian
The apparent age of these scarps is also very interesting. Although it is notoriously difficult to estimate the age of structural features like these (as they are so small), it is possible to estimate an upper age from dating the material in which the scarps have formed. In planetary science this dating is most often done by looking at the relative age of different features (so, what came first?) and also by counting the number of impact craters in a given area (hence the crater survey in Moon Zoo!). The authors estimate that the material that these scarps form in is less than a billion years old, which is relatively young for a geological process on the Moon.
This study has then taken this idea further and compared the estimated stress observed at the lobate scarps and compared it to the amount of stress predicted near the surface throughout the history of the Moon by different models of global cooling and contraction. What the authors suggest is that the models that fit best with the young age of the scarps predict that the Moon has undergone relatively little global contraction when compared to Mercury. I guess we’ll be able to compare better after NASA’s Messenger mission goes into orbit around Mercury in March 2011.
I think it’ll be really interesting to see the distribution of lunar lobate scarps once LRO has completed a few more years of its mission, when there is much better coverage of the lunar surface. For example, if these scarps occur at all latitudes, but only in specific regions, then could that be evidence that it wasn’t a global cooling and contraction, but rather a more regional process? And, bearing in mind the difficulty in dating these features, are all these scarps the same age? That surely would be a good thing to know, as that would really test the idea that it is a global process.
I couldn’t let this blog post pass without giving in to my love for all things Apollo. The study mentions a specific lobate scarp called Lee-Lincoln, which is in the Taurus-Littrow valley explored by Apollo 17 (shown in Figure 1). The astronauts Gene Cernan and Harrison (‘Jack’) Schmitt even drove up the lower slope of part of this scarp, and Schmitt (the geologist) described it as “very rolling and relatively smooth”. So, although these features are pretty obvious to look at from orbit (when you have the right lighting conditions), they probably wouldn’t be all that dramatic to look at from the surface. That said, Cernan claims the lunar speed record for coming down off the slope of the scarp in the rover. So maybe it wasn’t that shallow after all!
Figure 3. Apollo 17 traverse map of the Taurus-Littrow valley. Lee-Lincoln scarp is to the left of the image. Note that the illumination is from the right. The image is about 12.5 km across. Credit: Lunar and Planetary Institute.
If you want to read more about Apollo 17, or the other missions, I heartily recommend visiting the Apollo Lunar Surface Journal, which is an absolute fantastic resource. For the EVA near to the Lee-Lincoln scarp, head to the second EVA and geology stop 3, for transcripts, photos and much more.
Figure 4. Photography of the Lee-Lincoln scarp taken by Jack Schmitt (Apollo 17) taken between geology stops 2a and 3 during EVA-2. The scarp can be seen running through the left of the image, with multiple lobes, and up into the highland material making up the North Massif. 70 mm Hasselblad camera image AS17-138-21118, magazine 138/I. Credit: NASA/Apollo Lunar Surface Journal.
So, although this isn’t a call-to-arms to go lobate scarp hunting, please do keep an eye out for them in future images in MoonZoo.