Spudis reported in 2014 that on the Moon is key to understanding its geological history. In northern Arizona, a spectacular region of exposed, layered rocks over 6,000 feet thick was carved by the Colorado River. Aptly called the Grand Canyon, it represents over a billion years of Earth’s history. Geologists are able to study the history of past ages in exquisite detail by reading the historical record found in that well-known natural landform. No matter the planet, geologists are always searching for layered rocks. The study of rock layers (stratigraphy, from strata, meaning rock layers) allows scientists to reconstruct the geological history of a region and over time, an entire planet.
The nature of the Moon does not lend itself well to the display of rock layers, yet considerable effort has been expended searching for outcrops. Most layered rocks on the Earth are created from water-laid or wind-blown sediments, and neither of those processes occurs on the Moon. Still, the lunar surface has been built up piecemeal by the sequential deposition of blankets of ejecta—the ground-up rock thrown out radially from the center of impact craters and basins during formation. The overlap relationship of these ejecta deposits allows scientists to reconstruct the history of the Moon, i.e., younger impact craters overlie older ones. This simple methodology has allowed us to decipher the stratigraphy of the Moon.
Given that geologic history, one might expect that some evidence of rock layering was found in the abundant data returned from the Moon, but such evidence is limited and ambiguous. One of the most startling finds during the Apollo missions was a breathtaking view of Mt. Hadley, a lunar mountain north of the Apollo 15 landing site. Astronauts Dave Scott and Jim Irwin were startled to see evenly spaced, sub-horizontal lines in the mountain, similar in appearance to fine-scale layering present in some terrestrial strata. It looked as though the mountain was a single, gigantic crustal block, uplifted and overturned by the impact that created the nearby Imbrium basin. The layering described by the astronauts greatly intrigued the mission scientists, who were unable to clearly see it in real time in the TV pictures sent to Earth.
When the crew returned to Earth, images taken on the surface dramatically showed this layering (above, left). But this presented scientists with a puzzle. Because large impacts are highly energetic, chaotic events, how could they generate evenly spaced, regular layering? Some team members began to suspect that something else was going on. Ed Wolfe and Red Bailey of the U.S. Geological Survey made scale models of the mountain and dusted it with cement powder. They then photographed it under low, oblique illumination, similar to the lighting conditions of the landing site during the mission. Surprisingly, fine-scale linear features were evident in the laboratory “mountain” (above, right), suggesting that the “layering” seen by the astronauts on the Moon may have been an illusion, caused by the low-angle illumination of a particulate, granular surface.
Other layered deposits at the Apollo 15 site were less amenable to explanation as an artifact of lighting. A ridge southeast of the landing site named Silver Spur displayed a set of topographic “benches” associated with its apparent layering (below). On Earth, the formation of a bench indicates differential erosion, with hard rocks making up the cliff-forming units and softer rocks being expressed as more gently sloping units. However, such an erosive pattern on the airless, waterless Moon is difficult to envision. To this day, we do not have a good explanation for the origin of Silver Spur. As an example of layering in the highlands, it remains problematical.
Read on ...
Managed & Built by I2M Consulting, LLC | © 2010-
