IN PICTURES | 'The Moon is shrinking and shaking; maybe generating moonquakes', says NASA

Moon is shrinking as its interior cools, getting more than about 150 feet (50 meters) skinnier over last several hundred million years, says NASA. Just as a grape wrinkles as it shrinks down to a raisin, Moon gets wrinkles as it shrinks. Unlike flexible skin on a grape, Moon’s surface crust is brittle, so it breaks as Moon shrinks, forming “thrust faults” where one section of crust is pushed up over a neighboring part.

“Our analysis gives first evidence that se faults are still active and likely producing moonquakes today as Moon continues to grually cool and shrink,” said Thomas Watters, senior scientist in Center for Earth and Planetary Studies at Smithsonian’s National Air and Space Museum in Washington. “Some of se quakes can be fairly strong, around five on Richter scale.”

This is a view of Taurus-Littrow valley taken by NASA’s Lunar Reconnaissance Orbiter spacecraft. valley was explored in 1972 by Apollo 17 mission astronauts Eugene Cernan and Harrison Schmitt. y h to zig-zag ir lunar rover up and over cliff face of Lee-Lincoln fault scarp that cuts across this valley. Credits: (NASA/GSFC/Arizona State University)

se fault scarps resemble small stair-step shaped cliffs when seen from lunar surface, typically tens of yards (meters) high and extending for a few miles (several kilometers). Astronauts Eugene Cernan and Harrison Schmitt h to zig-zag ir lunar rover up and over cliff face of Lee-Lincoln fault scarp during  Apollo 17 mission that landed in Taurus-Littrow valley in 1972.

Watters is le author of a study that analyzed data from four seismometers placed on Moon by Apollo astronauts using an algorithm, or mamatical program, developed to pinpoint quake locations detected by a sparse seismic network. algorithm gave a better estimate of moonquake locations. Seismometers are instruments that measure shaking produced by quakes, recording arrival time and strength of various quake waves to get a location estimate, called an epicenter. study was published May 13 in Nature Geoscience.

Astronauts placed instruments on lunar surface during Apollo 11, 12, 14, 15, and 16 missions. Apollo 11 seismometer operated only for three weeks, but four remaining recorded 28 shallow moonquakes – expected to be produced by se faults – from 1969 to 1977. quakes ranged from about 2 to around 5 on Richter scale.

Using revised location estimates from new algorithm, team found that eight of 28 shallow quakes were within 30 kilometers (18.6 miles) of faults visible in lunar images. This is close enough to tentatively attribute quakes to faults, since modeling by team shows that this is distance over which strong shaking is expected to occur, given size of se fault scarps. ditionally, new analysis found that six of eight quakes happened when Moon was at or near its apogee, farst point from Earth in its orbit. This is where ditional tidal stress from Earth’s gravity causes a peak in total stress, making slip-events along se faults more likely.

“We think it’s very likely that se eight quakes were produced by faults slipping as stress built up when lunar crust was compressed by global contraction and tidal forces, indicating that Apollo seismometers recorded shrinking Moon and Moon is still tectonically active,” said Watters. researchers ran 10,000 simulations to calculate chance of a coincidence producing that many quakes near faults at time of greatest stress. y found it is less than 4 percent. ditionally, while or events, such as meteoroid impacts, can produce quakes, y produce a different seismic signature than quakes me by fault slip events.

Taurus-Littrow valley is location of Apollo 17 landing site (asterisk). Cutting across valley, just above landing site, is Lee-Lincoln fault scarp. Movement on fault was likely source of numerous moonquakes that triggered events in valley. 1) Large landslides on of slopes of South Massif draped relatively bright rocks and dust (regolith) on and over Lee-Lincoln scarp. 2) Boulders rolled down slopes of North Massif leaving tracks or narrow troughs in regolith on slopes of North Massif. 3) Landslides on souastern slopes of Sculptured Hills. Credits: NASA/GSFC/Arizona State University/Smithsonian)