The Earth is vibrating. And I am not referring to the kind of unspecified vibration to which the most esoteric circles allude from time to time, but that literally the material that makes up our planet is continually being shaken by geological forces. In fact, between the great masses of molten rock that move in the depths of the Earth and the friction between the tectonic plates, the interior of the Earth is flooded at all times by the echoes of the around 500,000 earthquakes that take place every year (although most are so weak that they are hardly recorded by seismometers.)
Well, since the Apollo missions and the Viking 2 probe installed seismometers on the surface of the Moon and Mars, and, more recently, with the arrival of the InSight probe on the red planet, we now know that Earth is not the only planet that trembles.
Measuring extraterrestrial earthquakes
Many characteristics of a planet can be studied from afar, such as its relief or its approximate atmospheric composition, but, if you want to measure the shocks that its interior experiences, you have no choice but to place a seismometer on its surface. This is precisely what the Apollo missions did, installing five on the lunar surface that recorded thousands of seismic movements between 1969 and 1977.
In 1977, a possible earthquake was also detected on Mars thanks to the Viking 2 probe seismometer. However, unlike the Moon, the red planet has an atmosphere that has a bad habit of generating air currents. Since the Viking 2 instruments were not measuring the wind speed at the time the alleged earthquake was recorded, it cannot be ruled out that the cause of that abnormal vibration was a momentary gust of wind that shook the instruments, in instead of an earthquake.
Luckily, the InSight probe landed on the Martian surface in 2018, and its most advanced seismometer managed to unequivocally record Martian seismic activity, almost 40 years after the last attempt. Curiously, this probe did not detect anything during the first months it spent on the red planet, but today it is registering an average of two earthquakes a day. This suggests that InSight landed at a time when Mars was especially “quiet” and that the planet is more seismically active than originally thought.
Now, the fact that the Moon and Mars experience earthquakes might seem inconceivable at first glance because, unlike Earth, they are two worlds that do not have tectonic activity. So what is shaking the lunar and Martian surface?
What makes a planet rumble
Tectonic activity is not the only phenomenon that is capable of causing the mass of a planet to vibrate. For example, lunar seismometers recorded vibrations that originate between 700 and 1200 kilometers deep and that seem to be synchronized with the lunar cycle, suggesting that our satellite could contain masses of liquid material inside (such as water or molten rock ) and that the tidal forces that the Earth exerts on them are capable of moving them. Furthermore, the same instruments also detected more frequent, weak, and external vibrations that are likely a result of the expansion and contraction of rocks on the lunar surface, whose temperature increases during the day and decreases at night.
In the case of Mars, the InSight probe has detected some 450 seismic signals and most are likely to actually come from within it, unlike the ambient vibrations recorded by the Viking 2 probe. In fact, a couple of earthquakes appear to have been originating in the Cerberus Fossae region, an area in which satellite images show large rocks that have slid down the slope and on whose surface lava flows flowed only 10 million years ago. These details suggest that the tremors produced in this region could be volcanic in origin.
Now, the shocks that are detected on the surface of a celestial body can also have an external cause. For example, many of the vibrations detected by seismometers installed on the Moon are attributable to the impact of meteorites. In this case, rock and metal fragments crashing onto the lunar surface at speeds of the order of kilometers per second produce seismic waves strong enough to be detected at points on the surface far removed from the collision site. .
“And what’s that for?”
Whatever the source of the extraterrestrial earthquakes, the truth is that they are very useful. The reason is that the speed at which waves propagate through a planet depends on the properties of the material they travel through. Therefore, by measuring the arrival times of the different vibrations, their frequency and their duration, a large amount of information can be deduced about the composition and internal structure of a planet. In fact, the effectiveness of this technique is more than proven on Earth, where we have been using the seismic waves produced by the great earthquakes for almost a century to investigate what the interior of our own planet is like.
With this in mind, it is no wonder that those responsible for the InSight mission are crossing their fingers for a major earthquake to break out on Mars. The most intense detected so far had magnitude 4 on the Richter scale, but that intensity is insufficient for the waves caused by the event to reach the core of the planet. Therefore, as soon as Mars experiences an earthquake energetic enough for InSight to detect the echo of waves reaching the depths of the planet, we can get many clues about what the interior of the red planet is like … Without even having to break it in half to investigate it.
DON’T NECK IT:
- We have reached the Moon and far beyond. Some of the devices that we have left on its surface can be seen directly or indirectly from the Earth’s surface.