Astronauts and Jeff Bezos-types hoping to set up shop on the Moon might have another challenge to worry about: moonquakes caused by tectonic activity.
Apollo mission seismometers measured 28 shallow moonquakes between 1969 and 1977 that seemed to lack origin. Researchers re-calculated the epicentres of these quakes and found that eight of them occurred near small cliffs produced along fault lines. Combined with evidence of dust and boulders moving near these faults, the researchers conclude that the Moon is tectonically active.
“We’ve got these possibly active faults on the Moon, which means it isn’t this dead body,” Tom Watters, study first author from the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington, DC, told Gizmodo. “It flies in the face of conventional wisdom, that the smaller a rocky body, the quicker it loses interior heat and becomes geologically inactive.”
The Apollo seismometers measured lots of seismic activity, from meteorite-induced vibrations to rumbles as the cold crust expanded once the Sun began shining at the end of a lunar night. But 28 of those quakes were relatively shallow and powerful — one had a magnitude of 5.5, according to a NASA factsheet. The source of these quakes has remained a mystery for decades.
But in 2010, high-resolution images taken by the Lunar Reconnaissance Orbiter (LRO) mission revealed Moon-wide fault scarps, or cliffs tens to hundreds of feet high produced by motion along fault lines. These fault scarps appeared to be young — less than 50 million years old. Scientists wondered: How young were they, really? Are they still active?
The researchers started by applying a new algorithm — one that was designed for working with inaccurate data from Earth-based seismic networks with few sensors — in order to re-find the epicentres of the 28 Apollo quakes. They calculated at what distance from the fault scarp the shaking would be strongest, and compared this distance to the re-calculated epicentres. They built shake maps to show what forces would be experienced during moonquakes along these faults.
They also compared the quakes’ timing to the distance between Earth and the Moon, to test how tidal forces between the two orbs could have influenced the seismic behaviour.
The team found that eight of these quakes seemed to occur close to these fault scarps, according to the paper published Monday in Nature Geoscience. Taking all the models together and examining images showing evidence of recent moving debris on the Moon, the researchers hypothesized that the Moon must still be tectonically active.
To be clear, the Moon does not have tectonic plates like Earth does. Instead, the Moon would contract as it loses heat, causing the land to rift and crack along faults. Scientists have already demonstrated evidence that Mercury experiences such tectonic behaviour. But evidence is beginning to show that these rocky bodies stay warm, and continue to remain seismically active, for a long time, Watters said.
This is still a model-backed correlation (and others have analysed and re-analysed the Apollo moonquakes to try and find the source), and there’s more work to do to truly confirm the source of the seismic activity. Hopefully scientists will put more seismometers on the Moon to better localise the source of these quakes. Watters hopes the LRO team will be able to re-analyse fault scarps imaged by the orbiter since 2009 to find evidence of movement—that would be a smoking gun signature, he said.
It shows just how much we have left to learn about our closest cosmic neighbour. And if we’re planning to built outposts on the Moon, we’ll probably want to know a bit more about these moonquakes.