• Scientists have just managed to collect seismic data from Mars, according to our partner The Conversation.

  • Their analysis finally makes it possible to determine a model of the internal structure - crust, mantle, nucleus - of the red planet.

  • The analysis of this phenomenon was carried out by Philippe Lognonné, professor in Geophysics and Planetology at the Institut de Physique du Globe de Paris - University of Paris.

Before the NASA Insight mission operated by JPL, the internal structure of Mars was still poorly understood, studied thanks to images of satellites in Martian orbit and the analysis of Martian meteorites. Since early 2019 and the successful deployment of the first Martian seismometer, the SEIS experiment, scientists have collected and analyzed seismic data for a Martian year, almost two Earth years, which gives us direct indications of the structure of Mars. , and therefore on its formation and its history.

To date, almost 700 events have been listed in the data transmitted by CNES and IPGP to the international

Mars Quake Service team

led by ETHZ.

In this catalog, we find about sixty Martian earthquakes, including ten earthquakes sufficiently distinct so that we can, for the first time, determine a model of the internal structure of Mars, which this week is on the cover of the scientific journal.

Science

, with three articles co-authored by the InSight collaboration, on the crust, the mantle, and the core.

Detect a whisper under the hubbub of the Martian wind

On Earth, earthquakes are strong due to plate tectonics, and seismometers installed in caves or underground are deployed by the hundreds. On Mars, as soon as the sun rises, a strong seismic noise is generated by the atmosphere and its turbulence and the earthquakes are much weaker: it is as if you were trying to hear a whispered conversation in the middle of a room. hectic restaurant.

But unlike on Earth where, even far from the coast, seismic noise remains dominated by waves generated by ocean swells, the seismic noise observed by InSight is regional, even local. It therefore drops as soon as the sun sets and more particularly during the first part of the night, when the wind is so weak that InSight's weather sensor can no longer measure it. These few hours made it possible to detect small earthquakes, with a magnitude of less than 3.7 and up to several thousand kilometers from the InSight sensor. Some of these earthquakes have a sufficiently good "signal to noise ratio" (between 10 and 100) for information to be extracted.

Artist's impression of the seismic crustal propagation and conversion at the base of the 10 km discontinuity. Inlaid: Earthquakes 173, one of the largest Martian earthquakes, with indication of the P and S wave © IPGP / Nicolas Sarter

Before presenting this model, let us recall that to determine at the same time a model of structure, the time (arrival) of the earthquake and its distance, it is usually necessary to have more than one station.

However, on Mars, only the InSight station is available to scientists.

It was therefore necessary to research, identify and validate in the seismic recordings the signature of waves that interacted differently with the internal structures of Mars.

These new measurements, coupled with mineralogical and thermal models of the internal structure, have made it possible to overcome the single station constraint.

It is a method which opens a new era of planetary seismology.

Mars joins the very select club of celestial bodies whose structure we know

After more than two years of Martian seismic monitoring, the first model of the internal structure of Mars is obtained, right down to the nucleus.

Mars thus joins the Earth and the Moon in the club of planets and telluric satellites whose deep structure is explored by seismology.

And for the first time, it is possible not only to compare the internal structure of the Earth with that of another terrestrial planet, but also the state of their internal heat engine: the thickness of the crust in fact tells us about the quantity of radioactive elements found there, the thickness of the thermal lithosphere gives the depth of the possibly convective zone of the mantle and the existence or not of deep phases can have major consequences on the force of the convection.

With this first model of internal structure and current thermal profile, it is therefore all the theories of formation and thermal evolution of Mars that must now be adjusted, with the key to a better understanding of the evolution of the planet, then of the "loss of habitability" of Mars during the 500 million years that followed its formation.

Observe the surface envelope: the crust of Mars

Cross-section of the SEIS seismometer © NASA / JPL-Caltech / CNES / IPGP, CC BY (via The Conversation)

Before the InSight mission, models were based only on measurements collected by orbiting satellites or the analysis of Martian meteorites. The thickness of the crust, with the only measurements of gravity and topography, was estimated between 30 and 100 kilometers. The values ​​of the moment of inertia and the density of the planet suggested a nucleus with a radius between 1,400 kilometers and 2,000 kilometers. The details of the internal structure of the planet such as the depths of the borders between crust, mantle and core, and even more so the possible stratifications of the crust or mantle were therefore unknown.

Today, it is the analysis of the seismic signals collected by InSight that allows us to understand the internal structure of the planet. Before reaching the Insight station, at each "crustal discontinuity", the seismic waves will be partly converted into another type of wave, partly transmitted, and partly reflected. An S wave can thus be converted into a P wave, the higher propagation speed of which will ensure a faster arrival at the surface. The reverse is true for a P to S conversion, which will happen after the transmitted wave.

The demonstration of these "conversions" made it possible to identify several discontinuities in the crust: a first, observed at a depth of about 10 kilometers, marks the separation between a very altered structure which results from a very old circulation of fluid, and a slightly altered crust.

A second discontinuity around 20 kilometers then a less marked third, around 35 kilometers, indicate the stratification of the crust under the InSight seismometer.

The Martian crust under InSight is therefore between 20 and 35 kilometers thick.

Understand the structure and properties of the "Martian mantle"

In the mantle, we analyzed the differences in travel time between the waves generated directly by the earthquake (of types P and S) and those generated during the reflection of these direct waves on the surface (a single reflection gives the waves " PP ”and“ SS ”while two reflections generate the waves called“ PPP ”and“ SSS ”).

These differences make it possible with a single station to determine the structure of the upper mantle, and in particular the variation of seismic velocities with depth.

Artist's impression of the main P and S seismic waves and of the transmitted, reflected and converted waves © IPGP / David Ducros / Philippe Lognonné

However, these speed variations are linked to the temperature, the pressure and the mineralogy of the mantle. By crossing this information with the mineralogical constraints linked to our observations of Martian basalt meteorites, the reservoir of which could be the surface mantle, it is possible to directly relate the seismic speeds measured to the mantle temperature.

The results suggest that the temperature of the mantle of Mars increases linearly with depth until it reaches just over 1,500 ° Celsius between 500 and 600 kilometers deep.

This temperature increase characterizes a mantle where the internal heat diffuses without a slow convection of the latter being able to better balance these temperature differences.

On Earth, this zone where there is no convection called “thermal lithosphere” varies between 10 and 100 kilometers in thickness.

Where is the core of the red planet?

To find the nucleus, we determined the depth of the mantle base using the S waves reflected from its surface.

Despite the small amplitudes of the signals associated with these reflected waves (called "ScS"), an excess of energy is observed for models of nuclei with a radius between 1790 kilometers and 1870 kilometers, which gives us a size range for the Martian core.

Our “Mars” file

Such a size would imply the presence of light chemical elements in the liquid outer core.

This type of size excludes the existence at the base of the mantle of a layer of "bridgmanite", this iron and magnesium silicate with a perovskite structure which constitutes most of the earth's mantle between 660 and 2700 kilometers deep.

If it had existed on Mars, such a layer would have "boosted" convection and therefore volcanism, especially during the first 500 million years.

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This analysis was written by Philippe Lognonné, professor in Geophysics and Planetology at the Institut de Physique du Globe de Paris - University of Paris.


The author would like to thank Brigitte Knapmeyer-Endrun, Amir Khan, Mark P. Panning, Simon C. Stähler and the other co-authors of the three articles published in Science in 2021, as well as Charles Yana (CNES) and the entire operation SEIS team at CNES / JPL / IPGP.


The original article was published on The Conversation website.

Declaration of interests

Philippe Lognonné is the principal investigator of SEIS at the Institut de Physique du Globe de Paris.

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