When building a remote outpost, it’s usually a good idea to stay away from active fault lines. Yet for people planning human habitation on the moon, it hasn't always been clear whether our tiny natural satellite is a geologically dead world. After all, you need internal heat for tectonic activity, and small rocky worlds like the moon are thought to cool much more rapidly than larger ones like Earth.
But now, fresh analysis of Apollo-era data suggests that the moon is actually more tectonically active than previously presumed.
In a study out today in Nature Geoscience, researchers may have finally pinpointed the epicenters of mysterious moonquakes recorded by Apollo-era seismometers, and the tremors seem to originate from cliff-like features called fault scarps.
“The whole idea that a 4.6-billion-year-old rocky body like the moon has managed to stay hot enough in the interior and produce this network of faults just flies in the face of conventional wisdom,” says study coauthor Thomas Watters of the Smithsonian Institution in Washington, D.C.
This summer will mark 50 years since the remarkable achievements of Apollo 11. Back during the space race, the United States was more focused on getting its astronauts to the lunar surface and back before the Soviets than on what to do with their moonwalkers while they were there.
“They were like, Oh crap, we need to come up with some stuff,” says coauthor Renee Weber, a planetary scientist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.
That “stuff” ended up being science, such as collecting a wealth of geological data, including rocky samples brought back to Earth. And on four of the Apollo missions, astronauts placed seismometers at their landing sites, which recorded thousands of deep quakes over the eight years that they remained functional.
Seismicity on the moon relies on quite different underlying mechanisms than what’s happening on our own world, Weber says. Most of the moon’s activity comes from gravitational forces that Earth exerts on its small gray satellite, essentially the converse of how the moon pulls on our oceans to cause the regular rise and fall of the tides. Without water, the lunar surface instead gets deformed, stretching from a spherical shape into a more oblong one and back again.
Enormous day-to-night temperature shifts, which can exceed 500 degrees Fahrenheit, also drive some lunar seismic activity. A few of the detected quakes were even humanmade, created when mission control ordered expendable spaceship parts to crash into the moon to calibrate the seismometers.
But 28 of the recorded events seemed to originate in the upper few miles of the lunar crust. These events were as powerful as magnitude 5.5 earthquakes on our planet, and they have defied explanation for more than 40 years.
Since 2009, Watters has been using images from NASA’s Lunar Reconnaissance Orbiter to map thousands of scarps all around the moon. Based on how fresh the lunar material around them looked, he could tell that they had formed relatively recently, perhaps less than 50 million years ago, “which is young geologically, but not young, young,” he says.
Still, Watters suspected that the scarps might be the sites responsible for the shallow moonquakes, but he needed further evidence. With four seismometers that produced relatively low-quality data, the Apollo instruments could only triangulate the tremors to within around a hundred miles.
So the researchers used an algorithm normally employed to determine the location of quakes on Earth when seismometer networks are sparse, creating a grid of putative origin points for the lunar quakes. Of the 28 recorded events, eight might have occurred within around 18 miles of a scarp, and six of those happened when the moon was at its farthest point from Earth, just when tidal stresses on the lunar surface should peak. (Find out how advanced seismic networks revealed that California gets an earthquake roughly every three minutes.)
To check whether the correlation might just be due to random chance, the team simulated 10,000 seismic events to see how often they might produce a pattern like this. They found that there were roughly one-percent odds that all these factors could line up in this way. For Watters, this suggests that the scarps are the most likely epicenters of the moonquakes.
“That means, for all intents and purposes, the moon is tectonically active,” he says. “To me, that’s an amazing result.”
Sights on a scarp
Yet the uncertainties in the results mean that other researchers aren’t considering this a done deal just yet.
“They use a lot of statistical arguments, and I think they do good science, but I wouldn’t say it’s definitely there,” says Ceri Nunn, who works on lunar seismology at NASA’s Jet Propulsion Laboratory in California. Still, given the non-ideal quality of the original data, she thinks the team seems to have pinned down the locations for the epicenters as well as possible for now.
Weber and others are gearing up to propose a mission that would place a new network of state-of-the-art seismometers on the moon, perhaps with one touching down on or near a scarp.
And in the meanwhile, given the renewed interest in lunar exploration from countries and private companies around the world, the findings provide a good map for places that upcoming landers might like to avoid.
Editor's Note: This story has been updated to clarify that there was previous uncertainty about whether the moon might be geologically active.