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.
These small faults are typically less than 10 kilometres long and only tens of yards or meters high. They are most likely formed by global contraction resulting from cooling of the Moon’s still hot interior.
As the interior cools and portions of the liquid outer core solidify, the volume decreases; thus the Moon shrinks and the solid crust buckles.
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,” said 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,” said Watters, lead author of the research paper published in 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 modelled tidal and contractional forces is pretty striking,” said Watters.