On April 18, 1889, an earthquake occurred in Tokyo (Japan).
64 minutes after the earthquake, its seismic waves were detected by two horizontal pendulums installed in two observatories in Postdam and Wilhelmshaven (Germany).
It was the first time that the passage of telluric disturbances had been recorded through the interior of the planet.
132 years later, a large group of scientists has revealed what Mars is like inside thanks to a seismograph that is somewhat more sophisticated than those oscillators.
NASA's InSight probe (see graph below) detected more than a hundred so-called martemots in their first year on the Martian surface. The objective 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 that they pass through. Since InSight landed in a crater in the Elysee Plain in November 2018, its SEIS seismograph has detected more than a thousand events. Although none have exceeded a magnitude of 4, a dozen of them have left a clear enough signal to glimpse the internal structure of Mars, with all its similarities and differences with Earth.
The first results have just been published now by the scientific journal
Science
in three different works. Like Earth, the interior of Mars is structured in three great layers, crust, mantle, and core. The outer layer is between 20 and 39 kilometers thick, at least in the region below the probe. When extrapolating the data to the entire planet, they estimate a thickness of between 24 and 72 kilometers. The last figure would be more than doubling the 33 km that the earth's crust has on average. In addition, they have estimated that in the Martian cover there are up to 20 times more materials that generate radioactive heat, such as uranium and thorium, than previously believed.
Recreation of what the interior of Mars looks like and how seismic waves generated by a 'martemoto' bounce off the core and are captured by the seismograph.Chris Bickel / Science
The mantle is relatively thinner on Mars than on Earth.
Thanks to the signal from the tremors, scientists believe that it is also different in its composition, highlighting the absence of bridgmanite, the most abundant mineral on Earth, concentrated especially in the lower part of the Earth's mantle, and that it plays a key role. in the geothermal energy and dynamics of the planet.
There are also differences in the innermost part, the core. The radius of that of Mars is around 1,840 kilometers, just over half of the Earth's endosphere. Keep in mind that the red planet is much smaller than Earth. Iron is the main element that forms both nuclei, but in Martian there is a greater abundance of light materials, 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 not found evidence of the existence of another solid interior, as it happens on Earth.
For the seismologist specialized in Mars Simon Stähler, from the Institute of Geophysics of the Federal Polytechnic School of Zurich (Switzerland) and co-author of these studies, the main difference between the Earth's core and the Martian core has to do with density: “The core of the Earth weighs on average more than 10 grams per cubic centimeter, that is, much more than iron [7.7gr / cm³]. It is so heavy because iron, the main component, is compressed due to the high pressure at that depth ”. On the other hand, “the Martian nucleus has only 6 grams per cubic centimeter, so it is much lighter than iron. So there must be light elements in it, specifically sulfur, oxygen, carbon, or hydrogen. But how did they get there? Why was there so much sulfur available (> 10%)? ”Stähler wonders. For him,"This could point to an early formation of Mars, compared to Earth."
But the peculiarities of the interior of Mars are also key to understanding the current situation on the outside.
The seismologist from the Barcelona-CSIC Institute of Geosciences puts it this way, Martin Schimmel, also co-author of two of the studies: “Mars was a planet similar to Earth, with its temperature range, its atmosphere.
Now it suffers thermal variations of up to 80º, extreme solar radiation and absence of life.
How did this happen? "
“Mars was a planet similar to Earth, with its temperature range, its atmosphere.
Now it suffers thermal variations of up to 80º, extreme solar radiation and absence of life.
How did this happen? "
Martin Schimmel, seismologist at the Barcelona Institute of Geosciences-CSIC
The iron in the rotating core is nothing more than a geodynamic that generates a magnetic field that, on Earth, is strong enough to protect life on the planet from excessive radiation. On Mars it was in the past, but not now. "Knowing the size of the nucleus and its liquid state helps to restrict explanations about what happened to the magnetic field," says Schimmel, a collaborator of the team at the Institute du Physique du Globe in Paris, who is leading this triple investigation on the corona, the Martian mantle and core.
The Cambridge University seismologist Sanne Cottaar, who has not participated in these studies, points to a possible story of what happened: “The observed nucleus of Mars is in the same range [in proportion to the smallest dimensions of Mars] of radius than Earth's, but it is larger than most previous estimates suggested. Therefore, the mantle is thinner than previously thought, and since gravity is also weaker on Mars, the pressures in the mantle are insufficient for the bridgmanite to be stable. Bridgmanite provides a blanket over our core that limits cooling. Its absence on Mars suggests that such rapid cooling could have occurred in the early days that it generated a geodynamic and a short-lived magnetic field ”.
A similar idea is defended by Miguel Herráiz, who investigates the composition and structure of Mars at the Complutense University of Madrid (UCM). This professor recalls that Mars had a global magnetic field, like Earth's, until about 4.2 billion years ago. "From that magnetic field there are archaeological remains in the magnetism observed in part of the southern crust of the planet." How did it get lost? "The factors for the maintenance of the geodynamic are not well known even for the Earth," he says, but adds, "the presence of so many sulfides [sulfur] in the core instead of heavier materials confirmed by these investigations could accelerate the cooling. and slow down the movement of the nucleus ”.
Diego Córdoba, a seismologist and colleague of Herráiz in the Faculty of Physical Sciences of the UCM, remembers that to know the interior of the Earth there are seismograph networks with hundreds and even thousands of seismographs.
"On Mars they only have one."
With more devices such as the SEIS instrument, they could better determine both the thickness and density of the different layers as well as their composition.
For this reason, the data they have obtained must be taken as preliminary and studies with other instruments will be necessary to reinforce these results.
To confirm these first results and obtain many other data on the origin, evolution and destiny of Mars, more and more intense earthquakes are also needed.
Schimmel is still waiting for a great earthquake to occur that multiplies the information they have obtained with these ten small martemots.
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