The moon might look like a dry, dead wasteland, but there’s a lot more activity happening on the gray, 4.5-billion-year-old rock than we give it credit for. In a study published this week in Nature Geoscience, a team of researchers illustrate how the moon is, in fact, shrinking—getting ever so smaller as its insides continue to cool down bit by bit, shriveling up and leaving behind geophysical faults. It’s along these lines scientists are now able to gauge how active the moon’s interior really is, and see evidence of moonquakes persisting even today.
“The story really starts with Apollo,” says Thomas Watters, lead author of the research paper and a senior scientist at the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington, D.C. NASA astronauts installed five seismic data collection stations on the moon during Apollo 11, 12, 14, 15, and 16. From 1969 to 1977, the stations recorded four different types of seismic activity on the moon ranging from magnitude 2 to 5: quakes that occurred from thermal expansion, quakes from meteorite impacts, deep moonquakes due to tidal stresses created from the orbit around the Earth, and shallow moonquakes that didn’t have a discernible cause.
It’s the shallow moonquakes that Watters and his team were most intrigued by, and they were spurred to see whether there might be some relation to another set of data that encapsulated its own set of mysteries: huge cliff-like fault scarps that thrust out of the ground and dotted the lunar surface.
Since the Lunar Reconnaissance Orbiter launched in 2009, we have finally seen how prevalent these features are globally, and what they looked like in finer detail, allowing Watters and his team to map the distributions, patterns, and orientations of the fault population. And they noticed those patterns were indirectly pointing to the sources of the scarps: global contraction.
Most scientists think that as the moon’s interior cooled down long ago, the crust began shriveling up like a raisin, encouraging crustal material to thrust upward in several locations and resulting in the fault scarps. The problem is, that process should result in a very unspecific fault scarp pattern. “That’s exactly not what we were seeing,” says Watters. “They were very organized in a specific way. Which meant something else was going on.”
The team took the Apollo seismic data for 28 moonquakes recorded from 1969 to 1977, and superimposed the location data onto the LRO imagery of the fault scarps. Through some computer modeling, Watters and his team found eight of the quakes had been produced from true tectonic activity occurring below the surface, right along the faults. The modeling suggests the epicenters were within 19 miles of the faults themselves, meaning the slippage along the faults likely created the quakes.
This means the scarps are effectively hotspots for geological stress, and are signs that the moon is still shrinking even today. It reinforces previous suggestions that the interior of the moon is still active, and still cooling down after billions of years.
This mechanism also explained why the scarp morphologies looked so young (tens of millions of years old), still looked crisp, and hadn’t been eroded away over time from meteorite impacts and other disturbances—the shrinking and quaking of the moon is an ongoing process, and it’s influenced by the thousands of scarps we’ve found so far.
“To me, the most interesting and enigmatic suggestions of the study is the whole notion that somehow, a silicate body or a rocky body the size of the moon has managed to retain its interior heat for 4.51 billion years,” says Watters. Conventional wisdom has always suggested a small body will quickly lose its heat and become fairly inactive. “The moon just hasn’t followed that path at all.”
“It’s all starting to make sense now,” says Nathan Williams, a postdoc at NASA’s Jet Propulsion Laboratory in Pasadena, California who recently authored a different study published in Icarus examining the lunar surface features created by the shrinking of the moon. “We’re putting the different pieces together now to look at this process globally, and it looks like everything has been shrinking, even fairly recently. And it looks like it’s still active thanks to the moonquakes.”
Besides helping us better understand the evolution of rocky celestial bodies in the solar system, the findings raise practical considerations as we ramp up for a return to the moon in the next decade. “We don’t really want to build anywhere that’s been found to be stressful” and exhibiting averse tectonic activity, says lunar scientist Clive Neal from the University of Notre Dame, who was not involved with the study. There are still plenty of questions surrounding how to accurately pinpoint the epicenter of these bits of activity, but like many others, Neal emphasizes the need to be cognizant about where we plan to situate infrastructure for a permanent outpost. You don’t want your moonbase to fall in on itself.
“The results of this paper, I hope, punctuate the need for a modern lunar geophysical network,” says Watters. “Many countries on the planet are interested in going to the moon, and interested in staying there. It’s important data for our long term goals on the moon.”
Not everyone is convinced the fault scarps explain these type of shallow moonquakes. “This is just one of those hypotheses that try to explain what really is causing the shallow moonquakes,” says Yosio Nakamura, a professor emeritus in geophysics at the University of Texas at Austin. “I appreciate their effort, but I am not convinced that this hypothesis is a valid one. I may be wrong, of course, but there are many problems with what they are presenting as evidence,” including the depth of the shallow moonquake hypocenters and the temporal distribution of the quakes. “We need to wait till we get more real data with further observations to find out what really is causing these enigmatic seismic events,” he says.
We might get those observations very soon, if NASA can meet its 2024 goal to return astronauts to the lunar surface. There’s certainly plenty for us to study when we get back, and it seems the moon is not nearly as lifeless as we once thought.
“I think there’s a general perception the moon is a dead, boring, place,” says Williams. “And that’s not entirely true. We have been to the moon and we’ve done some great science, but there is still a lot we don’t know. The moon is shrinking—we didn’t really realize that until recently. That’s huge. It’s a much more active and interesting place that we thought.”
