Friday, June 07, 2019

How the Moon Got its Fault Scarps: Moonquakes and the Lunar Surface

The Moon is known for its varied landscape; impact craters dot its surface, many with picturesque rays extending from them across the highlands, and so-called mare, or basalt “oceans”, cover the lowlands. But one aspect of lunar geography that captured scientists’ interest early is the presence of fault scarps, short clifflike structures zigzagging across the surface of the Moon. Like the much larger-scale mountain ranges that characterize the joining of two tectonic plates on Earth, these features point to tectonic activity on the single-plate Moon.

The crispness of these faults indicates that little enough time has passed since their formation, so they haven’t been covered up or softened by more recent events, like meteorite impacts. Using a special dating technique based on this fact, scientists have given them an upper age limit of 50 million years—nothing compared to the Moon’s 4.5 billion-year history. The relative youth of the fault scarps led scientists to wonder: are there still active faults on the Moon?

Driven by this question, researchers recently reexamined data from four seismometers placed on the Moon during the Apollo missions. Using an algorithm specially designed for small seismic networks, they generated “clouds or cluster[s] of equally viable epicenters for each quake,” principal investigator Tom Watters explained in an email.

In all, 17 of the 28 recorded shallow moonquakes resulted in relocated epicenter clouds. They then compared epicenters in these “clouds” with the locations of the recently detected young fault scarps in images of the lunar surface from the Lunar Reconnaissance Orbiter Camera (LROC) to see if they fell near fault scarps.

Of these 17 clouds and clusters, 8 fault scarps lie within 30 km of a relocated epicenter, the distance strong seismic shaking is expected from a fault related moonquake. Although this sounds like a low fraction—just under half—the team used simulations to confirm that this would almost never occur just through chance. The LROC images also revealed tracks left by rolling boulders, a telltale sign of nearby surface activity from seismic shaking.

This LROC image shows a young fault scarp crossing the frame vertically. Left-pointing arrows indicate the step-like nature of the fault scarp, while right-pointing arrows show boulder fields. Both are evidence of recent seismic shaking. Credit: LROC NAC frame M190844037LR; NASA/GSFC/Arizona State University/Smithsonian

Their research indicates the young fault scarps likely resulted from many shallow moonquakes recorded between 1969 and 1977, which hadn’t been known until now. Its thought that as the Moon’s hot interior cools, this contracts its crust; causing faulting, which releases energy in the form of moonquakes. Adding to the contractional forces is the fact that the Earth exerts tidal forces on the Moon because of the change in its distance relative to the Earth. In fact, 18 of the 28 shallow moonquakes occurred when the Moon was less than 15,000 km from apogee, its farthest point from Earth.

The team’s findings help explain the origin of shallow moonquakes detected 40 to 50 years ago and indicates that the Moon has active faults, like Earth. This discovery comes on the heels of the first confirmed marsquake last month, capturing national attention. Although these celestial bodies are no Earth, it seems we may have more in common than we thought!

–Eleanor Hooke

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