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. Cooling of the still hot lunar interior is causing the Moon to shrink, but the pattern of orientations of the scarps indicate that tidal forces are contributing to the formation of the young faults. Credits: NASA/LRO
Earth's gravity has influenced the orientation of thousands of faults that form in the lunar surface as the moon shrinks, according to new results from NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft.
In August, 2010, researchers using images from LRO's Narrow Angle Camera (NAC) reported the discovery of 14 cliffs known as "lobate scarps" on the moon's surface, in addition to about 70 previously known from the limited high-resolution Apollo Panoramic Camera photographs. Due largely to their random distribution across the surface, the science team concluded that the moon is shrinking.
Now, after more than six years in orbit, the Lunar Reconnaissance Orbiter Camera (LROC) has imaged nearly three-fourths of the lunar surface at high resolution, allowing the discovery of over 3,000 more of these features. These globally distributed faults have emerged as the most common tectonic landform on the moon. An analysis of the orientations of these small scarps yielded a surprising result: the faults created as the moon shrinks are being influenced by an unexpected source—gravitational tidal forces from Earth.
Global contraction alone should generate an array of thrust faults with no particular pattern in the orientations of the faults, because the contracting forces have equal magnitude in all directions. "This is not what we found," says Smithsonian senior scientist Thomas Watters of the National Air and Space Museum in Washington. "There is a pattern in the orientations of the thousands of faults and it suggests something else is influencing their formation, something that's also acting on a global scale -- 'massaging' and realigning them." Watters is lead author of the paper describing this research published in the October issue of the journal Geology.
The other forces acting on the moon come not from its interior, but from Earth. These are tidal forces. When the tidal forces are superimposed on the global contraction, the combined stresses should cause predictable orientations of the fault scarps from region to region. "The agreement between the mapped fault orientations and the fault orientations predicted by the modeled tidal and contractional forces is pretty striking," says Watters.
The fault scarps are very young – so young that they are likely still actively forming today. The team's modeling shows that the peak stresses are reached when the moon is farthest from Earth in its orbit (at apogee). If the faults are still active, the occurrence of shallow moonquakes related to slip events on the faults may be most frequent when the moon is at apogee. This hypothesis can be tested with a long-lived lunar seismic network.