We were able to understand what Mars is like inside

We were able to understand what Mars is like inside

Thanks to the data from the lander Insight seismograph, scientists were able to create a cross-section of the internal conformation of the planet

(Image: © IPGP / D. Ducros) While the NASA Perseverance rover goes around probing the surface of Mars, collecting samples of rocks in the hope of also finding traces of past life, the Insight lander stands still and “listens”. Since February 2019, his highly sophisticated seismograph, Seismic Experiment for Interior Structure (Seis), has recorded the weak Martian earthquakes, allowing scientists to understand how the Red Planet looks like inside. And it's the first time in history that we've got a map of another planet's internal structure in record time. The research results are described in three articles published by Science and a press conference is scheduled to be streamed at 6pm today, July 23, on YouTube.

Red and Blue Planet: Similar but different

Mars is in some ways similar to Earth but also very different. It has no magnetic field, for example, and for this reason its surface is swept by strong solar winds. It has no plate tectonics and its volcanoes (some really huge) are concentrated in a few areas, while on Earth they are widespread a little everywhere. Martian earthquakes, therefore, are few and of low intensity (so far no magnitude greater than 4 has been recorded) compared to those on our planet and are probably originated not by geological activity but by the fact that Mars, cooling down, is contracting.

But to really understand the differences between Earth and Mars in terms of geology and evolution we needed to obtain an internal map. The company, designed just about ten years ago, has now achieved important results. And in record time, if you think that it took centuries to understand the interior of the Earth and it took us forty years to understand the Moon.

From earthquakes to the map

The lander Insight is equipped with a very sophisticated seismograph, called the Seismic Experiment for Interior Structure (Seis). This instrument is capable of capturing and recording the P (primary) and S (secondary) seismic waves on the red planet (actually it also picks up a lot of wind, which doesn't make analysis easier), which just like on Earth they change speed and shape based on the characteristics of the medium in which they propagate.

(Nasa / Jpl-Caltech)

In this way the scientists were able to draw an internal map of the planet. "P waves are compression waves, like those produced by sound in air, and they are the fastest waves we see moving through any planetary body," explains Cologne University seismologist Brigitte Knapmeyer-Endrun, lead author of article that described the crust of Mars. “And then we have the secondary waves, the S waves, the shear waves. Their movement is more like that produced by the pinch of a swinging guitar string, ”she added. S waves are slower and travel only through solids, faster P waves also travel through liquids and gases. By analyzing the traces, in summary, it is possible to understand the position of the earthquake that originated the waves and what kind of medium they crossed.

It was not an easy job, however: with a single seismograph it is not possible to enter too much detail, not to mention that Martian earthquakes are few and of low intensity.

(Nasa / Jpl-Caltech)

The Martian crust

Like Earth, Mars has a crust, mantle and core, and the goal of Insight's mission was to try to measure the size, depth and structure of these inner layers.

According to NASA scientists, the measurements derived from Insight testify that the Martian crust, the outermost and thinnest layer, has different sizes in different areas of the planet, but overall it is thinner and less dense than previously thought: between 24 and 72 kilometers deep, with two or three sublevels. The most superficial level of the crust, about ten kilometers thick on average, is unexpectedly light, perhaps made of rocks shattered by the impact of meteorites; the second level drops by about twenty kilometers on average, while it is not yet certain whether there is another level below or if at that point it is already the mantle.

The mantle of the red planet

As the team of Amir Khan, a geophysicist from Eth from Zurich, explains in detail in the second article on Martian anatomy, under the crust there is a mantle that extends for 1,560 kilometers and is different from that of the Earth. In particular, there does not appear to be an inner mantle, i.e. the lower layer on Earth that acts as a blanket that traps heat around the core. The absence of this layer, according to the researchers, may have contributed to the rapid cooling of the Martian core and to the exhaustion of convective motions, those currents that on Earth originate the magnetic field that protects the atmosphere from solar winds. Mars, in fact, does not have a magnetic field and is swept by solar winds.

It doesn't stop there. Analyzes of Insight's data also “saw” that the mantle's lithosphere is thick, dense, rigid and cold. One possible explanation for why Mars doesn't have plate tectonics like Earth, but crustal fractures are thought to be due to the planet contracting as it cools. Some data on the magnetization of the Martian crust suggest, however, that perhaps Mars in the first moments of its history had dynamics similar to those of Earth, which could have allowed the origin of life forms (which we are looking for).

The core of Mars

Thanks to the study of S waves, the team of Simon Stähler from Eth in Zurich, and author of the third article in Science, deduced that the core of Mars is liquid (not solid as it was thought) and was able to derive the depth at which it is located. The S waves produced by an earthquake, in fact, bounce at the boundary between the mantle and the core (which is why it is assumed to be liquid) and take about ten minutes to be re-captured by Insight. From this the scientists deduced that the core must be 1,550 kilometers deep, with a radius of 1,830 kilometers, which is almost half the planet. It also appears to be much less dense than expected, which, comments Stähler, “still remains a bit of a mystery”.

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