An earthquake struck Tokyo, Japan on April 18, 1889. 64 minutes after the earthquake, its seismic waves were detected by two horizontal pendulums installed at two observatories in Postdam and Wilhelmshaven (Germany). It was the first time that the passage of telluric disturbances inside the planet had been recorded. 132 years later, a large group of scientists discovered what Mars is like, thanks to a seismograph that is somewhat more sophisticated than these oscillators.
NASA’s InSight spacecraft (see graph below) detected more than one hundred so-called marsquakes in their first year on the surface of Mars. The aim of this expedition is to explore the interior of the red planet using, among other indicators, seismic waves. As with sound, these oscillations are modulated by the medium through which they pass. And it is these changes that allow us to know the thickness, density or even the type of material they go through. Since InSight landed in a crater on the Elysee Lowlands in November 2018, its SEIS seismograph has recorded more than a thousand events. Although none exceeded size 4, a dozen of them left a clear enough signal to see the internal structure of Mars with all the similarities and differences with Earth.
The first results have just been published in a scientific journal in three different jobs. Like Earth, the interior of Mars is structured into three large layers, the crust, mantle, and core. Outer layer is between 20 and 39 kilometers thick, at least in the area below the probe. When extrapolating data to the entire planet, they estimate a thickness of between 24 and 72 kilometers. The last figure would more than double the 33 km in diameter of the earth’s crust. In addition, they have estimated that there are up to 20 times more materials in the Martian shelter that generate radioactive heat, such as uranium and thorium, than previously thought.
The mantle is relatively thinner on Mars than on Earth. Thanks to the signal from the tremor, scientists believe that it also differs in its composition, which emphasizes the absence of bridgmanite, the most abundant mineral on Earth, concentrated mainly in the lower part of the Earth’s mantle, and that it plays a key role in geothermal energy and dynamics.
There are also differences in the innermost part, the core. The radius of Mars is about 1840 kilometers, which is just over half the Earth’s endosphere. Remember that the red planet is much smaller than Earth. Iron is the main element that makes up both nuclei, but there are more light materials in Mars, such as sulfur or oxygen. The reflection of the semic waves confirms that the center of Mars has a layer in a liquid state, but they have found no evidence of the existence of another solid interior, as is the case on Earth.
For a seismologist specializing in Mars Simon Stähler, from the Institute of Geophysics of the Federal Polytechnic School in Zurich (Switzerland) and co-author of these studies, the main difference between Earth’s core and Mars’s core is related to density: “The Earth’s core weighs on average more than 10 grams per cubic centimeter, much more than iron [7,7gr/cm³]. It’s so hard because iron, the main component, is compressed due to the high pressure at this depth. ” On the other hand, “the Martian core is only 6 grams per cubic centimeter, so it is much lighter than iron. It must contain light elements, namely sulfur, oxygen, carbon or hydrogen. But how did he get there? Why was so much sulfur available (> 10%)? “Stähler wonders. “For him, it could” point to the early rise of Mars compared to Earth. “
But the peculiarities of the interior of Mars are also the key to understanding the current situation of its exterior. Thus the seismologist z Barcelona Institute of Geosciences-CSIC Martin Schimmel, also co-author of two of the studies: “Mars was an Earth-like planet with a range of temperatures, an atmosphere. It now suffers from temperature fluctuations of up to 80 °, extreme sunlight and the absence of life. How did it happen? “
“Mars was an Earth-like planet with a temperature range and an atmosphere.” It now suffers from temperature fluctuations of up to 80 °, extreme sunlight and the absence of life. How did it happen? “
Martin Schimmel, seismologist from the Barcelona Institute of Geosciences-CSIC
Iron in a rotating core is nothing more than geodynamics that generate a magnetic field that is strong enough on Earth to protect life on the planet from excessive radiation. It was on Mars in the past, but not now. “Knowing the size of the nucleus and its liquid state helps to limit the explanation for what happened to the magnetic field,” said Schimmel, a teammate at the Institut du Physique du Globe in Paris, which is conducting this triple investigation into the corona, Martian mantle and nucleus.
Cambridge University seismologist Sanne Cottaar, who did not participate in these studies, points to a possible story of what happened: “The observed core of Mars is on the same scale [en proporción a las menores dimensiones de Marte] radius than Earth, but is larger than suggested by most previous estimates. Therefore, the mantle is thinner than previously thought, and because gravity is also weaker on Mars, the pressures in the mantle are insufficient to keep the bridgmanite stable. Bridgmanite provides a blanket above our core that limits cooling. Its absence on Mars suggests that in the beginning it may have cooled so rapidly that it generated a geodynamic and short-term magnetic field. “
A similar idea is advocated by Miguel Herráiz, who studies the composition and structure of Mars at the University of Complutense in Madrid (UCM). This professor recalls that Mars had a global magnetic field like Earth about 4.2 billion years ago. “From that magnetic field, there are archaeological remains in magnetism observed in part of the southern crust of the planet.” “Factors for maintaining geodynamics are not well known for Earth either,” he says, but adds, “the presence of so many sulfides [azufre] in the nucleus, instead of heavier materials confirmed by these investigations, it could accelerate cooling and slow down the movement of the nucleus. “
Diego Córdoba, a seismologist and colleague from Herráize at the UCM Faculty of Physical s, remembers that there are seismographic networks with hundreds and thousands of seismographs to explore the interior of the Earth. “They only have one on Mars.” With multiple devices, such as the SEIS tool, they were able to better determine both the thickness and density of the various layers and their composition. For this reason, the data obtained must be considered preliminary and studies with other tools will be needed to reinforce these results.
Increasing earthquakes are also needed to confirm these first results and to obtain many more data on the origin, evolution and fate of Mars. Schimmel is still waiting for a big earthquake that will multiply the information he gained with these ten small martemots.