Billions of years ago, Earth's Moon formed vast basins called "mare" (pronounced MAR-ay). Scientists have long assumed these basins were dead, still places where the last geologic activity occurred long before dinosaurs roamed Earth.
But a survey of more than 12,000 images reveals that at least one lunar mare has been cracking and shifting as much as other parts of the Moon - and may even be doing so today. The study adds to a growing understanding that the Moon is an actively changing world.
Taken by NASA's Lunar Reconnaissance Orbiter Camera (LROC), the images reveal "wrinkle ridges" - curved hills and shallow trenches created by a lunar surface that is contracting as the Moon loses heat and shrinks. The features are described in a study published in Icarus on March 7, 2019, and led by Nathan Williams, a post-doctoral researcher at NASA's Jet Propulsion Laboratory in Pasadena, California.
Previous research has found similar surface features in the Moon's highlands, but wrinkle ridges have never been seen in basins before now. For this study, Williams and his co-authors focused on a region near the Moon's north pole called Mare Frigoris, or the Cold Sea.
The study estimates that some of the ridges emerged in the last billion years, while others may be no older than 40 million years old. That's relatively fresh in geologic terms; previous studies have estimated these basins all stopped contracting about 1.2 billion years ago. ...
Evidence for Recent and Ancient Faulting at Mare Frigoris and
Implications for Lunar Tectonic Evolution ~ Nathan R. Williams et al
Lee Lincoln Scarp at the Apollo 17 Landing Site ~ Credit: NASA GSFC/David Ladd This visualization of Lee Lincoln scarp is created from Lunar Reconnaissance Orbiter
photographs and elevation mapping. The scarp is a low ridge or step about 80 meters
high and running north-south through the western end of the Taurus-Littrow valley, the
site of the Apollo 17 Moon landing. The scarp marks the location of a relatively young,
low-angle thrust fault. The land west of the fault was forced up and over the eastern
side as the lunar crust contracted. Credits: NASA/Goddard/SVS/Ernie Wright
The Moon is shrinking as its interior cools, getting more than about 150 feet (50 meters) skinnier over the last several hundred million years. Just as a grape wrinkles as it shrinks down to a raisin, the Moon gets wrinkles as it shrinks. Unlike the flexible skin on a grape, the Moon’s surface crust is brittle, so it breaks as the Moon shrinks, forming “thrust faults” where one section of crust is pushed up over a neighboring part.
“Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink,” said Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington. “Some of these quakes can be fairly strong, around five on the Richter scale.”
These fault scarps resemble small stair-step shaped cliffs when seen from the lunar surface, typically tens of yards (meters) high and extending for a few miles (several kilometers). Astronauts Eugene Cernan and Harrison Schmitt had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp during the Apollo 17 mission that landed in the Taurus-Littrow valley in 1972.
Watters is lead author of a study that analyzed data from four seismometers placed on the Moon by the Apollo astronauts using an algorithm, or mathematical program, developed to pinpoint quake locations detected by a sparse seismic network. The algorithm gave a better estimate of moonquake locations. ...
Shallow Seismic Activity and Young Thrust Faults on the Moon ~ Thomas R. Watters et al