Moon's Shrinking May Be Causing Moonquakes

NASA calls for Apollo stories for 50th anniversary oral history

The Moon is shrinking - study

In this image, a mosaic composed of many images taken by Nasa's Lunar Reconnaissance Orbiter (LRO), and released by Nasa on May 13, 2019, shows new surface features (outlined) of the Moon, discovered in a region called Mare Frigoris.

The researchers' findings led to improved estimates of the epicenters of the shallow moonquakes, finding that eight of them fell within about 18 miles (30 km) of young lunar faults.

New research from seismometers that were placed on the moon between 1969 and 1977 tells a different story.

"It's really remarkable to see how data from almost 50 years ago and from the LRO mission has been combined to advance our understanding of the moon while suggesting where future missions intent on studying the moon's interior processes should go", said Lunar Reconnaissance Orbiter (LRO) Project Scientist John Keller.

It's believed these slip-events - much like landslides - were caused by additional tidal stress heightened by the Earth's gravity.

On Mars, scientists suspected that quakes were triggered by the Martian core slowly cooling over millions of years, triggering sporadic quakes as the lost energy swept through the interior of the planet. These faults resemble small stair-shaped cliffs, or scarps, when seen from the lunar surface; each is roughly tens of yards high and a few miles long.

"Unlike the flexible skin on a grape, the Moon's surface crust is brittle, so it breaks as it shrinks". The scarps form when one section of the moon's crust (left-pointing arrows) is pushed up over an adjacent section (right-pointing arrows) as the moon's interior cools and shrinks. "Some of these quakes can be fairly strong, around five on the Richter scale".

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These freakish moonquakes were detected by five seismometers placed on the lunar surface during the Apollo 11, 12, 14, 15, and 16 missions.

Six out of the eight tectonically active moonquakes occurred when the Moon was at or close to its apogee, the point where it's most distant from Earth and where the diurnal and recession stresses create the most compression near the tidal axis. These tracks are evidence of a recent quake because they should be erased relatively quickly, in geologic time scales, by the constant rain of micrometeoroid impacts on the Moon.

The Lunar Reconnaissance Orbiter first helped NASA learn that the moon was shrinking in 2010, the agency reported at the time. Since it began operation a decade ago, over 3,500 of these faults have been identified by the LRO.

The researchers noted other evidence in the orbiter's photos of landslides and boulders at the bottom of bright patches, signaling recent activity.

The Apollo missions also detected about one moonquake per day resulting from space rocks hitting the lunar surface.

"For me, these findings emphasize that we need to go back to the moon", said researcher Nicholas Schmerr of the University of Maryland. "It is also a testament to how much can be gained by human spaceflight to the surface of other worlds and underlines the unbelievable potential for future missions back to the moon and, hopefully someday, Mars".

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