Moonquakes, have you heard of them? And yes, I’m taking seism, because the Moon, sometimes shakes too…
This detail changes a lot about what we thought of living there. For many years, experts believed that the surface was transformed mostly by impacts coming from the space itself. However, the Taurus–Littrow valley (where Apollo 17 landed), show signs of shallow terrains that moved slopes, displaced large rocks and left dust slides over millions of years.
That valley—investigated by the last astronauts of the Apollo program in 1972—became a good place to understand the risks of living long-term at the Moon.
The study, published in ScienceAdvances looked at samples taken from Apolo 17 site. And with it, scientists determined how long some materials had been exposed to the surface. That sort of “exposure age” works as a natural “terrain clock” that allowed rebuilding a sequence of events that goes back to about 90 million years.
Small earthquake, big moonquake
For decades, fallen rocks and extensive dust slides near Apollo 17 were attributed to meteorite impacts. However, the new study suggest that it has a lot more to do with internal shocks.
When comparing block sizes, slopes and distances traveled, the team estimated the minimum ground acceleration necessary to set them in motion. The result pointed out to seisms of a magnitude close to 3: On Earth, they are considered small, but on the Moon, they can happen very close to the surface, so the local effect can be quite big.
Identifying the cause change the whole picture, because planning a trip to the Moon will not be the same if the main risk comes from impacts or shocks that spread through a very fractured cortex.
Fallen rocks and slips work like “natural indicators” of past vibration. In many cases, the shock was comparable to a big fraction of lunar gravity, which eliminates the theory that only mild movements happen in the Moon, and shows clear seismic events.
Also, the way in which the waves travel in the lunar crust, indicates that a moderate seism can explain the footprints we see.
The real risk of lunar shakes
The Lee-Lincoln fault is a young fracture, that’s visible on the surface and cuts the ground like a scar. The study, points out to this fault as the responsible for the old seism at the Valeey.
Researchers also found out that this fault activated on multiple occasions. Although the magnitudes are around 3, this is enough to overturn blocks or destabilize slopes in the Moon.
Could it have been dangerous for the astronauts? The research addresses this question with numbers. Since Apollo 17 lasted a few days, the probability of a harmful event during that window was extremely low.
The stability of the lunar module was also evaluated: only a strong shock and very close to the fault would have generated a movement capable of putting in trouble a relatively high structure and supported on granular regolith.
Returning to the Moon, with a more detailed plan
Today, we know that placing infrastructure and machines near young escarpments creates a risk that can easily be avoided, since the shaking decreases fast with the distance.
Therefore, choosing zones that are further away from faults, could make a big difference in terms of safety. Also, centers of gravity might need to be lower, and bigger bases and cushioning systems will have to be adapted to granular surfaces.
Modern solutions like seismometers tools could make it possible to identify active areas and calculate the risks more accurately. If they can predict even the smaller moonquakes, astronauts will be safer to take the next trip to the Moon.