The inner structure of the red planet revealed after two years of seismic monitoring

Before important vision From
NASA operated by
Jet Propulsion Laboratory, the internal structure of Mars is still poorly understood, as it has been studied thanks to satellite images in the Martian orbit and the analysis of Martian meteorites. Ago
early 2019 and the successful publication of the first Martian seismometer, and
SEIS ExperienceScientists have collected and analyzed seismic data for a Martian year, roughly two years on Earth, which gives us direct indications of the structure of Mars, and thus of its composition and history.

So far, nearly 700 events have been recorded in the data sent by CNES The IPGP for Team Mars earthquake service Run by ETHZ. in a
This catalogThere are about sixty Martian earthquakes, including about ten distinct enough to be able, for the first time, to model the internal structure of Mars, which appeared this week on the cover of the scientific journal. Sciences, with three articles co-authored with the InSight collaboration, about
Shell, the
Coat, and the
Nucleus.

Discover the whisper under the noise of the Martian winds

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, the atmosphere produces great seismic noise and turbulence and earthquakes cause much weaker : It’s like trying to hear a whispered conversation in the middle of a noisy dining room.

But unlike land where seismic noise is still dominated by waves from ocean swells, even far from the coast, the seismic noise observed by InSight is regional, even local. Therefore, it falls as soon as the sun goes down and more specifically during the early part of the night, when the winds are 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 Signal to Noise Ratio » Good enough (between 10 and 100) to extract information from it.

Artist’s impression of seismic crust spreading and shifting at the base of the 10 km discontinuity. Studded: Earthquakes 173, one of the largest on Mars, with reference to the P and S waves © IPGP / Nicolas Sarter

Before introducing this model, let us remember that to determine the model of the structure at the same time, the time (arrival) of the earthquake and its aftermath, 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 verify in the seismic recordings the signature of waves that interacted differently with the internal structures of Mars. These new measurements, along with metallurgical and thermal modeling of the internal structure, made it possible to overcome the limitations of the single station. It is a method that opens a new era of planetary seismology.

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Mars joins a very select group of celestial bodies whose structure we know

After more than two years of seismic observations of Mars, the first model of the internal structure of Mars, all the way to the core, has been obtained. Thus Mars joins Earth and the Moon in the club of planets and satellites whose deep structure is being explored by seismology.

For the first time, it is possible not only to compare the internal structure of the Earth with that of another planet, but also the state of the internal heat engine: the thickness of the crust actually tells us how much radioactive substances there are, the thickness of the thermal lithosphere gives the depth of the potential convective zone of the mantle and can The presence or absence of deep phases has serious consequences for the strength of convection.

With this first model of the internal structure and current thermal profile, all theories of the formation and thermal evolution of Mars must now be modified, with a key to a better understanding of the evolution of the planet, then ” Loss of habitability Mars during the 500 million years after its formation.

Watch the surface cover: the crust of Mars

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

Prior to the InSight mission, models were based solely on measurements collected by orbiting satellites or analysis of Martian meteorites. The thickness of the crust, with only gravity and terrain measurements, was estimated to be between 30 and 100 km. The values ​​of the moment of inertia and the density of the planet indicate the presence of a core with a radius of between 1,400 km and 2,000 km. Details of the planet’s internal structure such as the depths of the boundaries between the crust, mantle, and core, and even more, the possible layers of the crust or mantle were unknown.

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Today, it is the analysis of the seismic signals collected by InSight that allows us to understand the internal structure of the planet. Before arriving at the Insight station, at each “cut-off” the seismic waves will partly transform into another type of wave, partly transmitted, and partly reflected. Thus the S wave can be converted into a P wave, as its higher propagation speed ensures that it reaches the surface more quickly. The opposite is true for the transformation from P to S, which will occur after the transmitted wave.

Highlighting these “transformations” has allowed the identification of several of them discontinuities in the cortex : The first, observed at a depth of about 10 km, indicates the separation between a highly variable structure resulting from a very ancient circulation of the fluid, and a slightly altered crust. The second discontinuity about 20 km and then the less visible third, about 35 km, indicates the stratification of the crust under the InSight seismometer. Thus the thickness of the Martian crust under InSight is between 20 and 35 kilometres.

Understanding the Structure and Characteristics of the “Martian Mantle”

In the mantle, we analyzed the differences in travel time between waves generated directly by the earthquake (types P and S) and those generated during reflection of these direct waves at the surface (one reflection gives “PP” and “SS” waves while two reflections generate waves called “PPP”). and “SSS”). These differences make it possible through 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 the transmitted, reflected and transduced waves © IPGP / David Ducros / Philippe Lognonné

However, these differences in velocity are related to the mantle’s temperature, pressure, and mineralogy. By crossing this information with the mineral constraints associated with our observations of Martian basaltic meteorites, whose reservoir could be the surface mantle, it is possible to correlate seismic velocities measured directly to mantle temperature.

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The results indicate that the temperature of Mars’ mantle increases linearly with depth until it reaches just over 1,500 °C between 500 and 600 km in depth. This rise in temperature characterizes the mantle in which the internal heat diffuses without slow convection because the latter is able to better balance these temperature differences. On Earth, the thickness of this zone of no convection called the “thermal lithosphere” varies between 10 and 100 km.

Where is the heart of the red planet?

To find the core, we determined the depth of the mantle’s base using the S waves reflected from its surface. Despite the small amplitudes of the signals associated with these reflected waves (termed “ScS”), an excess of energy was observed for models of cores with radii between 1,790 km and 1,870 km, giving us a size range of the Martian core.

Such a volume implies the presence of light chemical elements in the liquid outer core. This type of size excludes the presence of a layer of “bridgemanite” at the base of the mantle, an iron-magnesium silicate of the perovskite structure. Which forms the main part of the Earth’s mantle Between 660 and 2,700 km. If it were on Mars, such a layer would “enhance” convection, so the
volcanoes, especially during the first 500 million years.

This analysis was written by Philippe Legnone, Professor of Geophysics and Planetary Science at the Institute of Physics of the World in Paris – University of Paris.
The author would like to thank Brigitte Knappmayr Andron, Amir Khan, Mark B. Operation of the SEIS team at CNES/JPL/IPGP.
The original article was published on the website of
Conversation.

Stan Shaw

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