Science

The Moon is Shrinking And Shaking

Moon is considered as the archetypal “dead” world as no life exists on it and virtually all its volcanic activity died out billions of years ago. Even the youngest lunar lava is so old that it’s scarred by numerous impact craters created over eons of cosmic debris crashing onto its surface. But signs that the Moon is not geologically dead have been observed for the last 50 years when the Apollo missions 12, 14, 15 and 16 left operational “moonquake detectors” on the lunar surface. These working seismometers recorded vibrations caused by internal “moonquakes” and transmitted this data to Earth until 1977.

But it was not clear whether any of these were associated with actual moving faults breaking the surface of the Moon or just internal movements that triggered tremors. Now a new study, published in Nature Geoscience, submits the Moon may indeed have active faults at present.

Another clue that the Moon was active came in 1972 when Apollo 17 astronauts Gene Cernan and Jack Schmitt inspected a step in the terrain, a few tens of meters high and named it “the Lee-Lincoln scarp”. Their team of experts back on Earth assumed this to be a geological fault but they weren’t sure.

Similar cases were spotted in photographs captured by Apollo craft orbiting near the Moon’s equator, but in 2010 the Lunar Reconnaissance Orbiter Camera, capable of recording details less than a meter across, finally revealed that such scarps were scattered across the whole Lunar surface. It is now commonly agreed that these scarps are thrust faults that form as the Moon cools down from its hot birth. During the cooling process, “thermal contraction” causes its volume to shrink and compresses the surface, Implying that the Moon is shrinking slightly. Analysis reveals that these faults are relatively young and not older than about 50 million years. But it was unknown whether they were active and still moving today.

For this new study, Tom Watters of the Smithsonian Institution in the US and his colleagues worked out a new way to isolate the locations of the near-surface moonquakes in the Apollo data more precisely than ever before. The team found that among the 28 detected shallow quakes, eight are similar to (within 30km of) fault scarps, suggesting that they may indeed be active. Six others formed when the Moon was almost at the greatest distance from Earth in its orbit. At this point of peaking contraction stress across the surface, quakes were most likely to be triggered.

The team also investigated fresh looking tracks left by dislodged boulders that presumably fell out due to the ground shaking as they were also spotted close to fault scarps having rolled or bounced down a slope. They also discovered traces of landslide deposits which as per the team, all add up to a strong case that fault movements are still taking place on the Moon.

The US recently announced a mission to the Moon in the next five years to set up a lunar base. Fortunately, none of the new findings show that the Moon is a center of ground tremors and unsafe for human exploration.  Moonquakes are rarer and weaker than on Earth, but a few locations close to the identified geological faults may best be avoided when planning Moon bases.

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