Earth Tidal Forces Contribute to Orientation of Lobate Scarps on Moon

Summary: Earth tidal forces influence orientation of lunar lobate scarps, according to findings of a five-year study released Sept. 21 in Geology journal.

Prominent lobate fault scarp in Vitello Cluster, one of 3,200+ discovered via Lunar Reconnaissance Orbiter Camera (LROC) images (blues = lower elevations; reds = higher elevations): NASA/LRO/Arizona State University/Smithsonian Institution, via NASA press release @ NASA.gov

Gravitational forces from the moon and the sun reach out to the Earth to influence rising and falling tides. In turn, the Earth’s gravitational tidal forces reach back to the moon to contribute to the formation of solid body tides that reshape the lunar landscape and cinch its diameter.

Launched June 18, 2009, as a robotic spacecraft by the National Aeronautics and Space Administration (NASA), Lunar Reconnaissance Orbiter (LRO) carries six instruments, including the Lunar Reconnaissance Orbiter Camera (LROC) that consists of a pair of narrow-angle push-broom imaging cameras (NAC) and one wide-angle camera (WAC).

After reviewing the crisp topography revealed via NAC’s high-resolution images of 0.5 to 2 meters (1.64 to 6.56 feet) per pixel, a research team led by Thomas Watters, Smithsonian senior scientist with Washington D.C.’s National Air and Space Museum, reported in August 2010 via Science journal the discovery of 14 previously undetected lobate thrust-fault scarps, or ridges: Cabeus, Campbell, Gregory, Guyot J, Nansen F, Paraskevopoulos H, Poisson, Rozhdestvenskiy 1 and 2, Shoemaker, Simpelius, Slipher and Timaeus. About 70 lobate scarps were discovered via the limited high-resolution Apollo Panoramic Camera, which collected images covering about 15 percent of the lunar surface during the last three Apollo missions (Apollo 15, July 30–Aug. 2, 1972; Apollo 16, April 20-23, 1972; Apollo 17, Dec. 11–14, 1972).

After five years of viewing over 70 percent of the lunar surface via over 1 million lunar surface images generated by LRO’s narrow and wide angle cameras, the team reported in September 2015 in Geology journal the discernment of over 3,200 thrust scarps. The tally identifies lobate scarps as the Moon’s most common, broadly distributed landform.

Forming as breaks in near-surface materials from pressure on the lunar crust as the moon contracted during cooling of its core, lobate thrust faults present lobed outlines with their curved or scalloped edges. Thrust faults uplift crustal materials to form ridges known as scarps.

Lobate scarps are present as common landforms on Earth and the solar system’s three other inner, terrestrial or earthlike, planets of Mercury, Venus and Mars. Lobate scarps on the moon are small-scale features, with typical lengths of less than 10 kilometers (6.2 miles) and relief of only tens of meters (32.8+ feet). Contrastingly, on scale with Earth’s largest thrust fault structures, lobate scarps on Mars and Mercury may extrude over 1 kilometer (0.62 miles) above the surface and measure lengths of hundreds of kilometers (62.1+ miles).

In their global distribution over the moon’s surface, fault scarps reveal an overall orientation pattern of north-south at low- to mid-latitudes and east-west at high latitudes. Global contraction, which produces random orientations, is the force responsible for the scarps’ creation but not for their orientation pattern.

The overall pattern is reflective of local illumination bias, whereby features perpendicularly oriented to local light direction are enhanced. Illumination direction is east or west at the equator and north or south at the poles. Nevertheless, significant deviations to local illumination bias exist at all latitudes, such as north- to south-oriented scarps amid dominant east-west orientations in high latitudes, and thereby eliminate the illumination effect as the dominant factor in orientation patterns.

Following through on early suspicions of Earth’s tidal forces as a contributing factor, Thomas Watters’ team superimposed a model of tidal force-influenced predictions onto mapped fault orientations and achieved a striking fit. According to the team’s model, peak stresses occur at apogee, the orbital point of furthest distance between moon and Earth.

Based upon such youthful features as crisp appearance, crosscutting with small-diameter impact craters, infilling rates of associated graben (downthrown-produced valley flanked by scarps), and small scale, Thomas Watters’ team places the lobate scarps’ timeline as less than 50 Ma (50,000,000 million years ago) and suggests that they may still be undergoing active formation today. The tidal force-scarp orientation interaction between Earth and moon reflects the intricate dynamism of the solar system in general and of the Earth and moon specifically.

The Earth continues to emerge as more than the moon’s placeholder, and the moon continues to emerge as more than the Earth’s nightlight and tidal puppeteer.

Locations of over 3,200 lobate thrust fault scarps (red lines) with orientation vectors (black double arrows) on Moon: NASA/LRO/Arizona State University/Smithsonian Institution, via NASA press release @ NASA.gov

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

locations of over 3,200 lobate thrust fault scarps (red lines) with orientation vectors (black double arrows) on Moon: NASA/LRO/Arizona State University/Smithsonian Institution, via NASA press release @ http://www.nasa.gov/sites/default/files/thumbnails/image/watters_et_al_press_photo_2.jpg

prominent lobate fault scarp in Vitello Cluster, one of 3,200+ discovered via Lunar Reconnaissance Orbiter Camera (LROC) images (blues = lower elevations; reds = higher elevations): NASA/LRO/Arizona State University/Smithsonian Institution, via NASA press release @ https://www.nasa.gov/sites/default/files/thumbnails/image/watters_et_al_press_photo_3.jpg

For further information:

Neal-Jones, Nancy, and William Steigerwald. “NASA’s LRO Discovers Earth’s Pull is ‘Massaging’ our Moon.” NASA > Press Release > Solar System and Beyond. Sept. 15, 2015.
Available @ http://www.nasa.gov/press-release/goddard/shrinking-moon-tides

Neal-Jones, Nancy, and William Steigerwald. “NASA’s LRO Reveals ‘Incredible Shrinking Moon.’” NASA > Missions > Lunar Reconnaissance Orbiter. Goddard Release No. 10-072. Aug. 19, 2010.
Available @ http://www.nasa.gov/mission_pages/LRO/news/shrinking-moon.html

Watters, Thomas R., et al. “Evidence of Recent Thrust Faulting on the Moon Revealed by the Lunar Reconnaissance Orbiter Camera.” Science, vol. 329, no. 5994 (Aug. 20, 2010): 936–940.
Available @ http://www.sciencemag.org/content/329/5994/936.full.pdf

Watters, Thomas R., Mark S. Robinson, Geoffrey C. Collins, Maria E. Banks, Katie Daud, Nathan R. Williams, and Michelle M. Selvans. “Global thrust faulting on the Moon and the influence of tidal stresses.” Geology, vol. 43, no. 10 (Sept. 21, 2015): 851–854.
Available @ http://geology.gsapubs.org/content/43/10/851.full.pdf