On Earth, we can feel and see the often terrifying results of plate tectonics moving under our feet. As they are ground, they generate earthquakes that produce seismic waves that reverberate through layers of rock, magma, and metal deep within our planet.
Scientists can monitor these seismic waves using a variety of instruments that pick up even the faint vibrations that pass through the Earth’s crust and core. Studying how the behavior of these waves changes as they pass through the interior of our planet reveals details about what is deep within the Earth, far from our sight.
But Earth is not the only place in our solar system that experiences seismic activity. Both Mars and the moon also experience earthquakes, although for different reasons than here on Earth. Seismometers deployed on the Moon and, more recently, on Mars, are allowing researchers to probe the interior of these two distant worlds.
The results show that while on the surface of the Earth, Mars and the Moon are not the same, underneath they have more in common than might be suspected, but with some surprising differences.
Moonquakes or lunar earthquakes
Lunar earthquakes occur as a result of meteorites striking the surface or by the gravitational pull of the Earth that squeezes and stretches the interior of the moon, similar to the pull of the moon’s tide on Earth’s oceans. . As the lunar interior cools, it also causes the moon to shrink and wrinkle like a raisin, causing other tremors as the crust bends and breaks. Heat from the sun can also cause thermal earthquakes due to the difference in temperature in the lunar crust when the moon emerges from its night.
Five seismometers have been deployed on the moon, dropped off by astronauts during the Apollo missions between 1969 and 1972. The first lunar seismometer was installed by Neil Armstrong and Buzz Aldrin on the Apollo 11 mission. After deploying the instrument, Aldrin stepped onto the lunar surface. to check that it worked, with the instrument picking up the waves produced by his foot.
The other four seismometers were abandoned by later missions and were in operation until 1977, five years after the last Apollo astronauts set foot on the lunar surface. But some 43 years later, scientists are still analyzing their data.
SeisMo is a project that recently re-analyzed the data. “We were trying to apply a technique that is used quite often on Earth,” said Ceri Nunn of NASA’s Jet Propulsion Laboratory in California, USA, the lead scientist on the project. “If you correlate the noise between stations, you can see waves traveling between them. The first station is a source and the second station is a receiver.”
Unfortunately, Nunn was unable to detect similar patterns in the moon data. But that glitch revealed something else about the moon, namely that it doesn’t appear to have surface waves, which get trapped in the upper layers of rock and bounce off. “That wave does not appear to exist on the moon,” the researcher commented.
This suggests that the top layer of the moon’s surface is probably heavily fractured and up to 100 kilometers thick, disturbing the movement of seismic waves across the surface. “This heavily fractured layer is changing the way seismic waves behave,” Nunn clarifies.
There are currently no active seismometers on the moon. But there are proposals to send new seismometers back to the lunar surface on future missions.
“We are interested in using much smaller seismometers, possibly launched by penetrators, which are almost like missile-shaped objects,” continues the expert. “You put a very small seismometer in the back and then you launch it from a descending lander or directly from Earth.”
Questions
Putting new seismometers on the moon could answer several outstanding questions, such as why there are large structural differences between the near side of the moon pointing toward us and the far side pointing outward.
“(That could be) related to the internal structure. There’s a theory (the moon) was hit again after it was formed by another moon, and that’s why you get this strange asymmetry. Exploring the internal structure would be interesting. And besides of that, we would like to limit the thickness of the core, “he says.
Understanding this could help test theories for how these early cataclysmic impacts around the time the Earth and Moon formed helped determine the structures they have today.
On Mars, however, things are a bit different. Earthquakes are not produced by tidal interactions, but by the cooling and contraction of the planet, which produces deep stresses. Meteorite impacts are believed to play a role as well, just like on the moon, sending seismic waves around the planet.
The existence of Martian earthquakes had never been proven until researchers landed a seismometer on the Red Planet in 2018 as part of NASA’s InSight mission. The InSight Mars lander detected the first definitive Martian earthquake on April 6, 2019 using its Sismic Experiment for Interior Structure (SEIS) instrument, which had been gently placed on the surface by the lander’s robotic arm shortly after it. It will land on November 26, 2018. Since then, some 500 subsequent events have also been detected.
Volcanic activity
While most of the Martian earthquakes have been relatively small, some of them have been large enough, almost equivalent to a magnitude 4 earthquake, to be traced back to their origin, an area known as Cerberus Fossae, about 1,600 kilometers away. east of InSight. The earthquakes are believed to be caused by stress build-up as fractures in the Martian crust stretch, possibly from volcanic activity.
While the largest earthquakes appear to originate in the mantle beneath the Martian crust, the smallest earthquakes are believed to begin in the crust itself. However, the speed of seismic waves in the upper Martian crust in the first eight to 11 kilometers appears to be about 50% slower than in similar rocks on Earth.
Researchers who are part of the GeoInSight project have been studying the surface geology around the InSight landing site to understand more about what might be happening. They used images and data from NASA’s Mars Reconnaissance Orbiter (MRO) to study the area of Elysium Planitia before InSight arrived.
The images revealed that there are lava flows 200 to 300 meters below the lander, according to Dr. Lu Pan of the University of Copenhagen, Denmark, the project coordinator at GeoInsight. “But underneath those lava flows, we have sedimentary rocks and rocks that contain clay a few kilometers deep,” he said.
This stratification is one explanation for the slower speed of seismic waves, Dr. Pan says, because sedimentary rocks have high porosity that could slow down the waves. Another possibility is that the upper crust has been badly damaged and fractured by meteorite impacts and other processes, producing more resistance for the waves.
The findings also have implications for some of the other InSight results, Pan noted . “For example, one of InSight’s exciting discoveries was the magnetic field, (which was) ten times what we observe from orbit. Having established the stratigraphy (the bedding of the rocks), we could help put some restrictions on the origin of the magnetic field: stratigraphy from before 3.9 billion years ago “.
While InSight will continue to probe the interior of Mars with its SEIS instrument, scientists are also eager to unravel the mystery of a strange reading it has been capturing.
“There’s a buzzing with a specific frequency that happens when there’s another event,” says Pan. “We don’t really understand what it is. Sometimes when there’s an earthquake, we see that the buzzing comes after. We don’t really have a good analog on Earth. “.
As InSight and its instruments listen to the inner workings of the red planet, it could help reveal the source of this hum and reveal what really lies deep within this alien world .
Original article
This article was originally published in Horizon, the EU Research and Innovation Magazine
