Science & Technology

Moon is shrinking, causing moonquakes

In the past millions of years the Earth’s natural satellite has shrunk by almost 46 meters

Published: Wednesday 15 May 2019

The Moon is shrinking as it becomes cooler from the inside, according to latest research conducted by the National Aeronautical and Space Administration (NASA). In the past millions of years the Earth’s natural satellite has shrunk by almost 46 meters. When a grape shrinks it becomes wrinkly as its skin is stretchable. When a cold and brittle object like the Moon shrinks its surface breaks and develops thrust faults where one part gets elevated as compared to the surrounding area.

Now scientists have found evidence that “these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink,” Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington, said in a press release. “Some of these quakes can be fairly strong, around five on the Richter scale,” he added.

The earliest data about the occurrence of moonquakes came 50 years ago from the work done by the Apollo missions to the Moon. Astronauts aboard the Apollo 11, 12, 14, 15 and 16 missions had placed seismometers at various locations on the Moon’s surface. Between 1969 and 1977, four of these instruments, except the one planted by Apollo 11, remained functional and measured 28 shallow quakes.

The thrust faults were expected to produce such quakes and their intensities ranged from two to five on the Richter scale. This data has now been analysed by Watters and his team using an algorithm that can give a much better estimate of the locations of these moonquakes. With these locations in hand they have been able to conclude that eight of the 28 moonquakes happened close enough to the thrust faults to make them attributable to them. Six of them also occurred when the Moon was either at its apogee, the farthest point in its orbit around the Earth, or close to it.

This increases the tidal stress due to Earth’s gravity and makes the quakes more likely. The measurement of these quakes proves that the Moon is shrinking and tectonically active as the results obtained from simulations run by scientists have reduced the chances of it being co incidence to less than four percent.

Quakes on Earth happen due to a very different reason. The Earth’s lithosphere (uppermost layer of the planet’s interior) is broken up into multiple tectonic plates which swim on top of the layers beneath and collide with each other once in a while to cause earthquakes.

Moonquakes are also corroborated from the highly detailed images of the Moon taken by NASA’s Lunar Reconnaissance Orbiter (LRO) mission. It has taken images of more than 3500 thrust faults. Scientists observed landslides and boulders fallen near bright patches near the faults.

The surface of the Moon is known to darken over long periods of time when exposed to solar and other space radiation. The bright patches have been exposed to this radiation only recently hence the existence of boulders and landslides is evidence of recent moonquakes. Such boulders in bright spots on the Moon have been found in the Vitello cluster and the Gemma Frisius C and Mouchez L craters.

“It’s really remarkable to see how data from nearly 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 John Keller, project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

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