By studying the signals of a dozen of marsquakes registered by the SEIS broadband seismometer deployed by the NASA's InSight mission, an international team of scientists has been able to determine the inner structure of Mars. The results of this research are published in three papers in the journal "Science" offering new details of the size of the planet's core and the structure and thickness of its crust and mantle by analyzing seismic signals. Martin Schimmel, a researcher at GEO3BCN-CSIC and collaborator of the team of the Institute du Physique du Globe de Paris (IPGP) coordinated by Philippe Lognonné, has co-authored two of the three released papers.
Mars has a liquid metal core with a calculated radius of 1.830 km (between 1.790 and 1.870 km), according to one of the released papers whose first author is Simon Stähler, a researcher from ETH Zurich. This size suggests, as the authors of the study explain, the presence of light elements (like oxygen, or hydrogen) inside the core which would be mainly composed of iron and nickel.
By studying and analyzing the seismic data from the SEIS, researchers were also able to constrain the thickness and structure of the Martian crust. This is on what the second paper published now is focused and whose co-lead author is Brigitte Knapmeyer-Endrun, researcher from the University of Cologne (Germany).
By analyzing the behavior of seismic waves, researchers were able to determine the thickness and structure of the Martian crust and were able to identify the discontinuities of this layer below the area where the InSight lander is placed. The first discontinuity in the crust is located about 10 km deep, the second discontinuity was detected at about 20 km deep, and the third discontinuity, which was less defined, was identified at 35 km deep. Researchers determined by combining seismic data with topographical and gravimetric measurements that the global thickness of Mars crust lies between 24 and 72 km.
To identify these discontuinities, the reseearches used the latest analytical methods to study the seismic signals, which included methods applied to the study of the tectonic earthquakes as well as methods applied to study the seismic noise.
"The Martian crustal thickness and crustal layering were unknown so far as no direct measurements existed. Thus this work provides the first direct measurements of crustal layering on another planet. This data is the key to further constraint the Martian structure and herewith its geological evolution and geochemistry among others.", said Martin Schimmel, a researcher at GEO3BCN-CSIC, and who has been collaborating with the IPGP researchers Eleonore Stutzmann, Zongbo Xu y Philippe Lognonné to develop seismic signal processing methods.
A single seismometer to unveil the inner structure of Mars
The models of the inner structure of Mars were based until now on the data provided by orbiters and the analysis of its surface. Based on the gravity and topographic measurements, the crust was estimated to be between 30 and 100 km thick. Moreover, the inertial movement data and the planet's density suggested a core with a radius between 1.400 and 2.000 km. But the exact details were unknown.
The SEIS seismometer which was deployed on the Mars surface at the beginning of 2019 has provided to the researchers the seismic data of a full martian year (equivalent to two terrestrial years). The SEIS's high sensibility enabled scientists to “hear” seismic events from hundreds to thousands of kilometres away. Seismic waves vary in speed and shape when traveling through different materials inside a planet. Those variations on Mars have given seismologists a way to study the planet’s inner structure.
The data was filtered to reduce the ambient noise generated by the wind and the deformation generated by changes in temperature. The "Mars Quake Service" of the InSight Mission registered and cataloged a total of 600 seismic events, from which approximately 60 were relatively distant marsquakes.
From all the seismic events registered, a dozen brought information on the deep structure of the planet. "The direct seismic waves from an earthquake are a bit like the sound of our voices in the mountains: they produce echoes. And it was these echoes, reflected off the core, or at the crust-mantle interface or even the surface of Mars, that we looked for in the signals, thanks to their similarity to the direct waves", said Philippe Lognonné.
One of the challenges the researchers had to face was that they only had one seismometer available. Usually, to carry out similar studies, the data acquired by several seismic stations are needed. Furthermore, Mars's low seismicity and the seismic noise generated by the planet's atmosphere were the other challenges that needed to be overcome. On the Earth, earthquakes have larger magnitudes and the seismometers are installed in quiet places to avoid the ambient noise. Therefore, careful signal processing work was needed to analyze the data registered by the SEIS. By using the wind sensors of the TWINS instrument, which is attached to the lander and was developed by the Centro de Astrobiología (CAB, CSIC-INTA), researchers were able to validate that the analyzed seismic signals were not associated to wind burst.
"The thickness of the crust constrains how the planet differentiated and evolved thermally and magmatically over time. Knowing the Martian core size and structure further provides information on the magnetic field generation which protected the atmosphere and controlled the climate through shielding the planet from the sun’s high-energy particles.", said Martin Schimmel. "The core size and structure play also a fundamental role for the mantle convection with surface manifestations as volcanic and tectonic activity. Understanding the evolution of Mars helps also to further constrain why Earth evolved as it did and to better understand the solar system."
Now, scientists have new data to refine their models of Mars and its formation, and new marsquakes are being detected every day.
More About the Mission
JPL manages InSight for NASA’s Science Mission Directorate. InSight is part of NASA’s Discovery Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver built the InSight spacecraft, including its cruise stage and lander, and supports spacecraft operations for the mission.
A number of European partners, including France’s Centre National d’Études Spatiales (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at IPGP (Institut de Physique du Globe de Paris). Significant contributions for SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; the Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the United Kingdom; and JPL. DLR provided the Heat Flow and Physical Properties Package (HP3) instrument, with significant contributions from the Space Research Center (CBK) of the Polish Academy of Sciences and Astronika in Poland. Spain’s Centro de Astrobiología (CAB) supplied the TWINS instrument, with temperature and wind sensors.
- B. Knapmeyer-Endrun, et al. (2021) Thickness and structure of the martian crust from InSight seismic data. Science. https://science.sciencemag.org/cgi/doi/10.1126/science.abf8966
- S.C. Stähler, et al. (2021) Seismic detection of the Martian core. Science. DOI: https://science.sciencemag.org/cgi/doi/10.1126/science.abi7730