From Earthquakes to Moonquakes and Marsquakes

All of us are familiar with earthquakes as they occur almost everywhere on the planet Earth. But surprisingly as it may sound, the Earth isn't the only planet in our solar system that experiences seismic activity. As there are earthquakes that occur on Earth, there are also moonquakes occurring on the Moon and even marsquakes occurring on the planet Mars.

Although the idea of quakes occurring on the Moon or Mars might sound odd to you, Earth is not the only place where seismic activity represents a common phenomenon. And not even the only planet with seismometers delivering various data that enable us to understand the structure, surface, and seismic activity better. This is even more important because distant planets, such as the Moon or Mars, really are far away, which is why we can't feel what's going on beneath their surface, as we do on Earth, for example.

The seismic activity on Earth presents an already well-known phenomenon as we've been living with it and experiencing it for ages. The fact that the earth shakes is indeed terrifying, which doesn't necessarily mean that it can't be explained. The origin of earthquakes on planet Earth is well connected to plate tectonics: the tectonic plates that Earth consists of are constantly slowly moving, causing friction and producing seismic waves reverberating through different layers of rock, metal, and magma. And it's the seismic waves traveling through the Earth's crust that we usually describe and feel as earthquakes. The best part of it, of course, is the fact that these seismic movements can be monitored by using a variety of instruments, e.g., seismometers, picking up even faint vibrations. This not only gives us a better insight into our planet's interior but also enables us to forecast the next possible future seismic event.

But what goes for Earth could also be said for the Moon and Mars which also experience seismic activity although not that often and for different reasons. Recent experiments as well as seismometers deployed on the Moon and Mars allow us to probe the interior of both these distant worlds and to be able to recognize patterns in their seismic activity. "The results show that while on the surface Earth, Mars and the Moon are not alike, beneath it they have more in common than might be suspected, but with some striking differences¹."

The First Seismometers on the Moon

The Earth's surface is pretty different from that found on the Moon, which is also true for the Moon's interior as well. But that's only one of the reasons why seismic activity on the Moon is different. The fact that we're even able to explore the Moon's surface and the processes that are going on beneath it is due to the first seismometers that were deployed on the Moon by astronauts during the Apollo missions between 1969 to 1972. The first lunar seismometer was, for example, set up by Neil Armstrong and Buzz Aldrin, and picked up the first wave, e.g., the movement of Aldrin's foot, just a few seconds later as Aldrin stamped on the lunar surface to check it was working. Later, another four seismometers were left on the moon by subsequent missions and were operational until 1977. Even though their data was being pored over by scientists ever since the project SeisMo recently re-analyzed the data and came up with some impressive findings.

Picture 1: Moon's surface.

One of them, for example, addresses the origin of moonquakes. These "[...] are produced as a result of meteoroids hitting the surface or by the gravitational pull of the Earth squeezing and stretching the  Moon’s interior, in a similar way to the  Moon’s tidal pull on Earth’s oceans. As the lunar interior cools, it is also causing the  Moon to shrink and shrivel like a raisin, causing other quakes as the crust buckles and breaks. The heat from the sun can also produce thermal quakes due to the temperature difference in the lunar crust as the Moon emerges from its night²."

One further finding also reveals that the  Moon doesn't really have surface waves as these usually get trapped in the upper layers of rock and bounce around, which also reveals that the Moon's surface is highly fractured and is therefore changing the behavior of the seismic waves. Nevertheless, between 600 and 3,000 moonquakes were detected on the  Moon each year the seismometers were in operation, even though the seismic events were more or less very small.

Picture 2: Each year, between 600 and 3,000 moonquakes are detected on the Moon.

Seismic Activity on Mars

Both earthquakes and moonquakes are produced by tidal interactions, however, that's not the case with marsquakes. Marsquakes also represent a relatively new phenomenon as their existence had never been shown until researchers that were a part of NASA's InSight mission landed a seismometer on Mars in 2018. The first-ever marsquake was detected in 2019 and since then, more than 1,313 marsquakes have been detected.

Picture 3: More than 1,313 marsquakes have been detected since 2019.

On Mars, deep stresses are produced by the planet cooling and therefore contracting. And similar to the Moon, meteor impacts are believed to play a part in sending seismic waves around the planet too.

Marsquakes remain relatively small but some of them have the magnitude of what would have been the equivalent of a magnitude 4.0 earthquake on Earth. What's more interesting, though, is the fact that some of the stronger marsquakes "[...] are being caused by the build-up of stress as fractures in the Martian crust are stretched, possibly by volcanic activity³." Moreover, the larger quakes are believed to originate from the mantle beneath the crust while the smaller quakes begin in the crust itself. "The velocity of seismic waves in the upper Martian crust, however, in the first eight to 11 kilometers, seems to be about 50 % lower than in similar rocks on Earth⁴."

Picture 4: Mars is also known for its volcanic activity.

According to the findings made by GeoInSight, the lower velocity of the seismic waves is probably connected to the layering of the surface as sedimentary rocks have a high porosity that could slow the waves down. On the other hand, the waves' resistance could originate from the heavy damage done to the upper crust by meteorites and other processes.

Either moonquakes or marsquakes – all the observations and findings help us understand the inner workings of both planets but also reveal what really lies deep inside. This not only brings us closer to a proper understanding of our solar system but also to a revelation that our planet and other planets are not as far apart as they might seem.

Sources:
1) Horizon. Moonquakes and marsquakes: How we peer inside other worlds. Accessed on 18-Oct-2022. Available at: https://ec.europa.eu/research-and-innovation/en/horizon-magazine/moonquakes-and-marsquakes-how-we-peer-inside-other-worlds
2) Horizon. Moonquakes and marsquakes: How we peer inside other worlds. Accessed on 17-Oct-2022. Available at: https://ec.europa.eu/research-and-innovation/en/horizon-magazine/moonquakes-and-marsquakes-how-we-peer-inside-other-worlds
3) Horizon. Moonquakes and marsquakes: How we peer inside other worlds. Accessed on 17-Oct-2022. Available at: https://ec.europa.eu/research-and-innovation/en/horizon-magazine/moonquakes-and-marsquakes-how-we-peer-inside-other-worlds
4) Horizon. Moonquakes and marsquakes: How we peer inside other worlds. Accessed on 17-Oct-2022. Available at: https://ec.europa.eu/research-and-innovation/en/horizon-magazine/moonquakes-and-marsquakes-how-we-peer-inside-other-worlds