Lunar Reconnaissance Orbiter Camera

South Massif Landslide

By lunar standards South Massif is a relatively modest mountain, but with a rich history (geologic and exploration). The massif is about 7000 meters across at the base and 2300 meters tall; M1266925685LR, incidence angle 33°, slew angle 65°, phase angle 104° [NASA/GSFC/Arizona State University].

South Massif is located at the edge of the Serenitatis impact basin on the Moon's nearside and borders the Taurus Littrow Valley where the Apollo 17 astronauts landed and explored. This oblique view drmatically shows the north-facing slope of South Massif, with the 3.6 km diameter Ching-Te crater in the background. From summit to base, the massif's relief exceeds that of the Grand Canyon.

The distinct high reflectance deposit that spreads across the Taurus Littrow valley floor formed as a giant landslide from the north face of South Massif. Apollo era scientists proposed that the landslide was caused by ejecta from Tycho crater landing on the summit and south side of the massif. The resulting seismic jolt sent regolith sliding down the steep north slope resulting in the distinctive landslide we see today. Look closely at the summit, you can see what appears to be dark and blocky material that may be a deposit of now solidifed impact melt from the Tycho event.  Sampling the landslide was an objective of the Apollo 17 mission. By  determining how long rocks had sat on the surface of the slide scientists could know the timing of the formation of Tycho crater, which is more than 2000 kilometers to the southwest. 

South massif and landslide mosaic
Map view of north slope of South Massif, edge of summit seen in lower left; a two lobe landslide spreads across the relatively flat floor of the valley, partially covering the mighty Lee Lincoln scarp (which snakes across the middle of the image from top to bottom). Mosaic is 10 km wide [NASA/GSFC/Arizona State University].

As it turns out, the exposure ages of the samples brought back from the landslide were about 110 million years - thus Tycho crater was assigned that age. However, new mapping and analysis of the area brought forward a second hypotheisis, reported this spring at the 49th Lunar and Planetary Science Conference. The alternative idea is that the landslide was caused by motion along the Lee Lincoln fault. It is certainly logical that seismic shaking along such a massive fault could cause a landslide. In fact, the paper (authored by Apollo 17 astronaut Harrison Schmitt) proposes that there were actually two large quakes and thus two landslides, the younger covering most of the older. The older deposit (smaller lobe on the right) is distinguished by its lower reflectance relative to the brighter younger slide (left side). If Dr. Schmitt is correct then we do not know the age of Tycho crater which in turn has implications for how scientists estimate the ages of other young craters across the Moon (and other inner Solar System bodies).

Explore the area and see if you can determine which hypothesis is correct. How could the debate be settled? Mapping and analysis from existing observations and samples? Or is a new mission required to gather a different type of measurement?

 

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