Geologic processes, like your mother, are both fundamentally responsible for life as we know it and probably something you don’t think about very much. (You did call your mother on Sunday, right?) Geologists are interested in the geological activity going on across the solar system because we know these processes on Earth have fundamentally shaped its structure and ability to support life. Understanding how they function on other planets and moons (when they exist at all) helps understand how these environments evolved and why they look the way they do. For decades, conventional thinking has been that the Moon is essentially a dead rock in space. But a new analysis of Apollo-era data suggests these conclusions are inaccurate. The Moon may still be geologically active, right up to the present day.
During the Apollo missions, astronauts placed seismometers on the surface to measure seismic activity. While these seismometers have long since gone silent, they returned up to eight years of data while active. In the process, they recorded 28 shallow moonquakes. At the time, however, we didn’t have enough data to map those quakes to specific features on the Moon itself. A new analysis of the data has tracked eight of the quakes to proposed epicenters near known lunar geological features known as fault scarps. A fault scarp is an area where the ground in one area has moved vertically with respect to another as a result of geological processes like earthquakes. A video of the Lee-Lincoln fault scarp is shown below:
A new paper in Nature Geoscience discusses these findings, which could collectively demonstrate that the Moon is not, in fact, a geologically dead world. From the abstract:
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. Analytical modelling of tidal forces that contribute to the current lunar stress state indicates that seven near-apogee events within 60 km of a fault scarp occur at or near the time of peak compressional stresses, when fault slip events are most likely. We conclude that the proximity of moonquakes to the young thrust faults together with evidence of regolith disturbance and boulder movements on and near the fault scarps strongly suggest the Moon is tectonically active.
Unlike Earth, the Moon lacks plate tectonics. It isn’t volcanic like Io or Venus (Venus is not known to be volcanically active at present, but the planet contains more volcanoes than any other location in the solar system). The Moon doesn’t undergo cryovolcanism that we’ve observed (cryovolcanoes may have been observed on Pluto, Titan, and Ceres), and it has no regular ice geyser activity like Enceladus. Instead, any ongoing lunar geological resurfacing would be produced by the slow dissipation of heat from the Moon’s core and the tidal stresses that Earth puts on Luna as it orbits. The fact that these moonquakes occur at or near-apogee, when the Moon is most distant from us, imply that they are related to tidal forces exerted by gravity.
Like the Earth, the Moon has a core, mantle, and crust, but the core of the Moon is much smaller and only partially liquid (approximately 40 percent of the lunar core is believed to have solidified). As the Moon cools, it shrinks — and the Moon’s diameter has shrunk approximately 150 feet over the past several hundred million years, according to NASA. This shrinkage creates “wrinkles” on the Moon’s skin, except the rock isn’t flexible. Eventually, it breaks, forming thrust faults.
“Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink,” said Thomas Watters, a senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington. “Some of these quakes can be fairly strong, around five on the Richter scale.”
As an aside, Mercury’s geology appears to have been shaped by similar processes, though at a larger scale. That planet may have shrunk by 1-7km since formation, and the wrinkle ridges and thrust faults found on Mercury are larger and more extensive than those found on the Moon.
President Trump has talked about NASA returning to the Moon and possibly constructing a long-term moonbase on the lunar surface. These findings suggest moonquake surveys should absolutely be part of any long-term planning, to avoid any associated risks.