Moonquakes rattle the Moon as it shrinks like a raisin

Posted: May 14, 2019 by oldbrew in Gravity, moon, research, solar system dynamics, Tides
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View from the Moon [credit: NASA]


Moons don’t generally ‘shrink’, so what’s going on here? The abstract of the research paper speaks of compressional stresses, but the only potential source of compression would seem to be the Earth. It’s known that ‘the crust on the far side is a lot thicker than it is on the near side’, as discussed here.

The moon is still tectonically active, like Earth, generating moonquakes as our planet creates earthquakes, a new study based on Apollo mission data found.

These moonquakes likely happen because the moon is quivering as it shrinks, researchers added.

On Earth, tectonic activity, such as earthquakes and volcanism, results from shuffling of the crust’s tectonic plates driven by the churning of the planet’s molten interior, says Charles Quoi at Space.com.

However, the moon is much smaller than Earth and therefore largely cooled off long ago, so one might not expect much, if any, tectonic activity.

However, in 2010, high-resolution images from NASA’s Lunar Reconnaissance Orbiter revealed a network of thousands of faults on the moon that appeared to be less than 50 million years old. Researchers suggested these faults were evidence of lunar tectonics, although it was unclear how recent this activity was.

Seismometers at four Apollo landing sites on the moon recorded 28 shallow moonquakes between 1969 and 1977, ranging from magnitude 1.5 to 5 on the Richter scale. Some of these shallow quakes might in theory result from activity on lunar faults, but the locations and depths of the sources of these quakes were uncertain.

Moonquakes can have origins other than tectonic activity in the lunar crust. For example, the Apollo missions detected about 11,000 moonquakes happening about 500 to 680 miles (800 to 1100 kilometers) beneath the lunar surface.

Previous research suggested that these deep moonquakes resulted from Earth’s gravitational pull on the lunar interior, much as how the moon’s gravitational pull on Earth’s waters results in the tides, said study co-author Nicholas Schmerr, a planetary seismologist at the University of Maryland at College Park.

The Apollo missions also detected about one moonquake per day resulting from space rocks hitting the lunar surface. Those missions even saw artificial moonquakes from the impacts of the spacecraft used to bring astronauts to the moon, Schmerr added.

The researchers in the new study wanted to see if the shallow moonquakes that the Apollo missions detected were linked with faults on the lunar surface, and thus ongoing tectonic activity on the moon. To do so, the scientists relied on analytical techniques developed to interpret data from sparse networks of seismometers on Earth.

Full report here.
– – –
NASA press release: Shrinking Moon May Be Generating Moonquakes

Comments
  1. oldbrew says:

    Abstract says:
    We found that the epicentres of eight near-surface quakes fall within 30 km of a fault scarp, the distance of the expected strong ground shaking. From an analysis of the timing of these eight events, we found that six occurred when the Moon was less than 15,000 km from the apogee distance.

    NASA adds: This is where additional tidal stress from Earth’s gravity causes a peak in the total stress, making slip-events along these faults more likely.
    https://www.nasa.gov/press-release/goddard/2019/moonquakes


    – – –
    six occurred when the Moon was less than 15,000 km from the apogee distance

    That amounts to saying there’s more stress the further away the Moon is from the Earth – but gravity is known to fall off by the inverse square. 😐

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