The Incredible Shrinking Moon

Back in the 1970’s Paleolithic age of lunar studies, scientists were busy using images of the Moon in an attempt to understand lunar processes and history.

A compressive thrust fault on the far side of the Moon: Is the Moon shrinking?

Back in the 1970’s Paleolithic age of lunar studies, scientists were busy using images of the Moon in an attempt to understand lunar processes and history.   In the rugged ancient cratered uplands of the Moon, they saw something curious.  Many small scarps dotted the highlands and were visible in only the very highest resolution images and even then, seen only under certain illumination conditions.   Moreover, these scarps appeared to cut or overlie small, very young impact craters – young in lunar terms (less than a few hundred million years) not some ancient feature recording an event that occurred billions of years ago.

These highland features looked similar to the Lee-Lincoln scarp found at the Apollo 17 landing site.  That scarp crosses the valley floor and trends up the slope of the North Massif.  Thrust faults are significant in that they are indicators of compression, where the crust is squeezed together due to some regional stress field.  Geologists interpreted Lee-Lincoln as a thrust fault, a zone of failure in which one slab of a planet is pushed over and on top of another.  Scientists had previously found abundant evidence for such compression in the dark, lowland mare basins (where piles of lava slowly sink under their own enormous weight) but they had not seen such evidence for compression in the highland crust.

This discovery, though interesting, was not revolutionary.  But it was curious – just what did the presence of these scarps mean?  Lunar scientist Alan Binder thought they could be highly significant and began searching all the panoramic photographs taken by the Apollo missions, mapping dozens of the small scarps scattered throughout the highlands, especially on the far side of the Moon.  He believed that these scarps indicated the Moon must have been completely molten at one time and even more startlingly, that their presence meant that after millions of years of quiescence, the Moon was becoming seismically active as its crust strained (“Moon-shaking”) under the compression of global contraction.

These were startling predictions and as expected, were challenged by other planetary scientists.  Some questioned Binder’s interpretation that the faults were real – such challenges are part and parcel of vigorous scientific research.  Others thought that while real, perhaps they were not as young and widespread as claimed, since the Apollo images covered a small zone around the equator of the Moon and only a few percent of the highlands surface was covered at resolutions and lighting conditions adequate to see the scarps.  Still others thought that the idea of a totally molten Moon did not jibe with what we knew from study of the lunar samples.

A new paper by Tom Watters and the Lunar Reconnaissance Orbiter camera (LROC) team has answered some of the questions about the presence and distribution of these enigmatic features.  The LROC produces high resolution images that can resolve features less than a meter across on the lunar surface.  These new images have produced spectacular views of a variety of landforms and show the surface of the Moon in incredible detail.  While mapping coverage is not yet global, it does extend our high-resolution vision well beyond the equatorial “Apollo zone” to the polar latitudes.  The new results indicate that these small scarps occur all over the Moon.   Moreover, they are indeed “young” in lunar terms; many scarps cut small craters that should have been eroded to invisibility on timescales longer than a few hundred million years.  Thus, as Binder claimed, the scarps must have been created relatively recently.

So, how much “shrinkage” of the Moon might these new findings indicate?  The LROC team suggests that the radius of the Moon might have decreased by about 100 m or so, not a very large amount for a body 1738 km in radius and not nearly as much as estimated in Alan Binder’s original paper (about 5 km).  This new number comes from an estimate of the amount of strain relieved by the formation of scarps found to date.  If there are substantially more of these features and of larger size, the strain estimate may be much greater and consequently indicate a greater contraction of the Moon.  We know from images returned from the Mariner 10 mission in 1973 and more recently by the MESSENGER spacecraft that Mercury (a much larger body than the Moon) also shows compressive thrust faults on its surface and consequently has decreased in size, on the order of about 1-2 km in radius.

Could the Moon still be seismically active?  Interestingly, the instrument network emplaced by the Apollo missions showed that while the Moon is extremely quiet compared to the continuous, restless trembling of the Earth, moonquakes do occur and some in the shallow levels of the crust are relatively strong (Richter magnitude 5).  Could these quakes be caused by the relief of stress in the crust accompanying the ongoing formation of highland thrust faults?  Unfortunately, the Apollo seismic network operated for only a few years.  Constant, global, decade-long monitoring of lunar seismic activity is needed to fully answer this question.  Such a global seismic network mission has been proposed, but the prospects of deploying this seismic network by robotic spacecraft within reasonable cost guidelines is challenging, to put it mildly.

Scientific studies continue to unravel the complex story of our Moon and the new flood of superb quality data from recent and ongoing orbital missions are showing us a range of new things that we have previously seen incompletely, if at all.  We’ve only “scratched” the surface of the Moon.  From the newest data, we’ve found evidence for the previous existence of water in the deep interior, ice at the poles, unsuspected (and unsampled) new rock types, and now, crustal compression on a global scale.  Imagine what we’ll discover when we do more than just look at images – when humanity’s destiny of again walking on our Moon comes to pass.

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