Earth is Shaping the Shrinking Moon

Posted on Fri, September 18, 2015

Planetary science is one of those fields of research where you can always count on being surprised. The remarkable terrain of Pluto and Charon in images being sent back by the New Horizons spacecraft certainly qualifies. One of my all-time big surprises is from a recent discovery on an object much closer to home—the Moon. You might think the Moon has given up all its big secrets by now, after centuries of study by telescope, then robotic orbiters and landers, and ultimately by the Apollo astronauts, but that isn’t the case.

I have the good fortune to be involved in one of the recent missions to the Moon, the Lunar Reconnaissance Orbiter (LRO).  LRO was launch in June 2009 and has been returning amazingly detailed images of the Moon ever since. One of my main goals as a member of the Lunar Reconnaissance Orbiter Camera (LROC) team was to look for evidence of small fault scarps, or cliffs. Before LRO’s arrival, only a relatively few of these fault scarps had been discovered in the very limited high resolution Apollo era photographic coverage available. After about a year in orbit and with only about 10% of the surface covered by high resolution LROC Narrow Angle Camera (NAC) images, we reported the discovery of 14 fault scarps widely distributed on the lunar surface. These lobate thrust fault scarps, formed when the lunar crust is pushed together, breaks, and is thrust upward, are evidence that the Moon is shrinking. Now, after more than six years in orbit, the LROC NAC images cover nearly three-quarters of the lunar surface. The first big surprise is how many fault scarps have now been found—over 3,000!  In fact, these globally distributed fault scarps have emerged as the most common tectonic landform on the Moon.

Lobate scarps like the one shown here are like stair-steps in the landscape formed when crustal materials are pushed together, break, and are thrust upward along a fault forming a cliff. Image: NASA/GSFC/Arizona State University/Smithsonian

Topography derived from the LROC Narrow Angle Camera (NAC) stereo images shows a degraded crater has been uplifted as the fault scarp, in the Vitello Cluster, has formed (blues are lower elevations and reds are higher elevations). Boulders in the crater have aligned in rows that parallel the orientation of the fault scarp. Image: NASA/GSFC/Arizona State University/Smithsonian

The map shows the locations of over 3,200 lobate thrust fault scarps (red lines) on the Moon. The black double arrows show the average orientations of the lobate scarps sampled in areas with dimensions of 40° longitude by 20° latitude and scaled by the total length of the fault scarps in the sampled areas. The pattern of the black double arrows (orientation vectors) indicates that the fault scarps do not have random orientations as would be expected if the forces that formed them were from global contraction alone. Image: NASA/GSFC/Arizona State University/Smithsonian

The large number of thrust fault scarps is not the only big surprise. We think the fault scarps are formed by cooling of the Moon’s still hot interior.  This causes the volume of the interior to decrease and the Moon to shrink. A shrinking Moon should generate a global array of thrust faults scarps with no particular pattern in the orientations of the faults because the contracting forces are equal in all directions. The second big surprise is the fault scarps don’t have random orientations! Something else is influencing their formation and that something else has to be acting globally.

There are other forces acting on the Moon, but they don’t come from the lunar interior; they come from Earth. These are tidal forces. The Moon is tidally locked—this is why we always see the same hemisphere—and is slowly receding from Earth. This slow recession causes stress to build up over time. Just as the Sun and Moon raise tides on Earth, Earth’s gravitational pull causes tides on the Moon. As the Moon orbits the Earth, the lunar crust rises and falls by a small amount generating stresses. These tidal stresses are relatively small compared to the stress from global contraction, and here’s the third big surprise: when we modeled the tidal stresses and superimposed them on stresses from global contraction, the combined stresses predicted an array of faults with orientations in a distinct pattern. That predicted pattern of faults is remarkably similar to the observed fault orientations! In fact, this close relationship between the Earth and the Moon goes back to their origins, but what a surprise to find the Earth is still helping to shape our shrinking Moon. 

Interested in reading more? Findings from this research are reported in the paper Global Thrust Faulting on the Moon and the Influence of Tidal Stresses, published in the October issue of the journal Geology. Or simply admire the Moon from afar this Saturday for International Observe the Moon Night.