InSight Mission: Detecting Mars | EurekAlert! science news

Image: Artist’s impression of the internal structure of Mars View More

Credit: © IPGP / David Ducros

Using information obtained from about a dozen earthquakes detected on Mars by the SEIS Very Wide Range Seismometer, developed in France, the international team for NASA’s InSight mission has revealed the internal structure of Mars. The three papers were published on July 23, 2021 in the journal Science, which includes several co-authors from French institutions and laboratories, including the CNRS, the Institute of Physics in the World in Paris, and the University of Paris, and supported in particular by the French. The CNES space agency and the French national research agency ANR provide, for the first time, an estimate of the size of the planet’s core, the thickness of its crust and the structure of its mantle, based on the analysis of reflected and modulated seismic waves by interfaces in the interior. It makes this the first-ever seismic exploration of the internal structure of a terrestrial planet other than Earth, and an important step toward understanding the formation and thermal evolution of Mars.

Before NASA’s InSight mission, the internal structure of Mars was still poorly understood. The models were based solely on data collected by Earth-orbiting satellites and on analysis of Martian meteorites that fell to Earth. On the basis of gravity and topographic data alone, the crust was estimated to be between 30 and 100 km thick. The values ​​of the planet’s moment of inertia and density indicate the presence of a core with a radius of 1400 to 2000 km. However, the detailed internal structure of Mars and the depth of the boundary between the crust, mantle, and core were not fully known.

With the successful deployment of the SEIS experiment on Mars in early 2019, mission scientists, including the 18 French co-authors from a wide range of French institutions and laboratories, along with colleagues from ETH Zurich, the University of Cologne and the Jet Propulsion Laboratory, in Pasadena collects and analyzes seismic data over the course of one Martian year (almost two years on Earth).

It should be noted that to define a structural model at the same time, more than one station, the time (arrival) of the earthquake, and its aftermath are usually required. However, scientists only have one station on the surface of Mars, and that is InSight. It was therefore necessary to search in the seismic records for the characteristic features of waves that interacted in different ways with the internal structures of Mars, to identify and validate them. These new measurements, along with mineralogy and thermal modeling of the planet’s internal structure, made it possible to overcome the limitations of having a single station. This method heralds a new era of planetary seismology.

One stop, multiple results

Another difficulty on Mars is the low seismicity and seismic noise generated by its atmosphere. On Earth, earthquakes are much stronger, while seismometers are most effectively located in basements or underground, which makes it possible to obtain an accurate picture of the planet’s interior. As a result, special attention had to be paid to the data. “But although Martian earthquakes have a relatively low intensity, less than 3.5, the very high sensitivity of the VBB sensor combined with the very low noise at nightfall enabled us to make discoveries that, two years ago, we thought were only possible with earthquakes of greater magnitude. of 4”, explains Philippe Lugnoni, Professor at the University of Paris and Principal Investigator of the SEIS Instrument at IPGP.

Every day, the data, processed by CNES, IPGP and CNRS, and transmitted to the scientists, was carefully cleaned of ambient noise (wind and distortion associated with rapid changes in temperature). The Mars International Seismic Service (MQS) team recorded seismic events on a daily basis: more than 600 of them have now been cataloged, more than 60 of which were caused by relatively distant earthquakes.

About a dozen of the latter contain information about the deep structure of the planet: “The direct seismic waves generated by an earthquake are somewhat like the sounds of our voices in the mountains: they echo. These echoes were reflected from the core, at the interface of the Earth’s crust or even at the surface Mars, which we looked for in signs, thanks to its similarity with direct waves, ”explains Lugnona.

Variable crust, exposed lid, large liquid pulp

By comparing the behavior of seismic waves as they travel through the crust before reaching the InSight station, several discontinuities in the crust have been identified: the first, observed at a depth of about 10 km, represents the boundary between a highly variable structure, caused by fluid rotation from a very long time ago. The crust is only slightly changed. A second discontinuity about 20 km down, and a third less visible at about 35 km, shed light on the stratification of the crust under Insight: “To identify these discontinuities, we used all modern analytical methods, with both earthquakes of tectonic origin with environment-induced vibrations (noise). seismicity),” says Benoit Touzin, senior lecturer at the University of Lyon and researcher at LGL-TPE.

In the mantle, scientists analyzed the differences between the latency of waves generated directly during an earthquake, and those of waves generated when these direct waves are reflected off the surface. These differences made it possible, using only one station, to determine the upper mantle structure, and in particular the variation in seismic velocities with depth. However, these differences in speed are related to temperature. “This means we can estimate the heat flux of Mars, which is probably three to five times less than that of Earth, and put constraints on the formation of the Martian crust, which is thought to contain more than half of the heat-producing radioactive elements found on the planet,” adds Henry Samuel, CNRS Researcher at IPGP.

Finally, in the third study, scientists looked for waves reflecting off the surface of the Martian core, the radius of which was one of the main achievements of the InSight mission. “To do this, we tested several thousand mantle and nuclei models against the observed phases and signals,” explains Mélanie Drilleau, a research engineer at ISAE-SUPAERO. Despite the low amplitudes of the signals associated with the reflected waves (known as ScS waves), an excess of energy was observed in cores with radii between 1,790 km and 1,870 km. This large size indicates the presence of photonic elements in the liquid core and has serious consequences for mantle minerals at the mantle/core interface.

Goals achieved, new questions appear

More than two years of seismic observations have resulted in the first model of the internal structure of Mars, all the way to the core. Thus Mars joins Earth and the Moon in a selection of terrestrial planets and moons whose deep structures have been discovered by seismologists. And, as often happens in planetary exploration, new questions arise: does the top 10 km of crust change in general, or is it limited to the InSight landing zone? What was the impact of these first models on theories of the formation and thermal evolution of Mars, particularly in the first 500 million years when Mars had liquid water on its surface and intense volcanic activity?

With a two-year extension of InSight’s mission and additional electrical power obtained after the successful cleaning of its solar panels by Jet Propulsion Laboratory, the new data should integrate and improve these models.

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About InSight and SEIS:

JPL manages the InSight mission on behalf of NASA’s Science Mission Directorate. InSight is part of the NASA Discovery Program, operated by the Marshall Space Flight Center (MSFC), a NASA facility in Huntsville, Alabama. Lockheed Martin Space of Denver has built the InSight spacecraft, including a cruise stage and lander, and supports the mission’s spacecraft operations. CNES is the main contractor for SEIS, for which the Institute of Physics in Paris (Université de Paris / IPGP / CNRS) has scientific responsibility. CNES finances French contributions, and the International Federation coordinates

He was responsible for integrating, testing, and providing the complete tool to NASA. IPGP has designed and tested VBB (Ultra Wide Band) sensors before they are delivered to CNES and contribute to the operation of VBBs on Mars.

SEIS and APSS operations are performed by CNES within FOCSE-SISMOC, with the support of Centro de Astrobiología (Spain). SEIS data is coordinated and distributed by the Mars SEIS Data Service of IPG Paris, as part of the InSight national monitoring service to which LPG also contributes, as well as GéoAzur for “Sismo at School” activities. Daily identification of earthquakes is performed by InSight’s Mars Earthquake Service, a collaborative operational service led by ETH Zurich, to which seismologists from IPG Paris, University of Bristol (UK) and Imperial College London (UK) also contribute.

Several other CNRS laboratories including LMD (CNRS/ENS Paris/Ecole polytechnique/Sorbonne University), LPG (CNRS/University of Nantes/University of Angers), IRAP (CNRS/University of Toulouse/CNES), LGL-TPE (CNRS/Ecole) Normal Superior de Lyon/Claude Bernard Lyon 1 University, IMPMC (Sorbonne University/French National Museum of Natural History/CNRS) and LAGRANGE (CNRS/University of Côte d’Azur/Observatoire Côte d’Azur) partner with IPGP and ISAE-SUPAERO in InSight mission data analyzes . These analyzes are supported by CNES and the French National Research Agency as part of the ANR MArs Geophysical InSight (MAGIS) project.

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