Mars' surface is known in great detail through exploration using orbiting spacecraft. But until now its interior structure could only be derived indirectly or simulated using computational models. With the participation of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), NASA's InSight mission has provided surprising new discoveries. The core of our planetary neighbour is larger than previously thought, and the overlying mantle has a structure similar to Earth's upper mantle. Finally, the crust of the Red Planet's surface is thinner than previously estimated. The Seismic Experiment for Interior Structure (SEIS), a seismometer developed under French leadership, acquired the data to address this geological puzzle over a period of more than two years. The results were published on 22 July 2021 in the journal Science.
Martin Knapmeyer, a geophysicist at the DLR Institute of Planetary Research in Berlin who was involved in the evaluation of the data, is excited by the breakthrough: "Models of the structure of Mars have existed for almost 100 years. However, the last decades have seen little progress with regard to the dimensions of the crust and core. Only seismology can measure these directly. In the past, they had to be inferred from other observations."
"According to our measurements, the Martian core has a diameter of approximately 3700 kilometres," explains Ana-Catalina Plesa, also from the DLR Institute of Planetary Research. "That is about half the diameter of Earth's core and is rather at the upper end of the size range indicated by all previous estimates," explains the scientist, who was involved in two of the three studies that have been published. Mars' diameter is 6770 kilometres, about half that of Earth's.
"A larger core also means a lower density than previously assumed," Plesa adds. "The lower density shows that the iron-nickel melt has a larger proportion of lighter elements such as sulphur, carbon, oxygen, or perhaps even hydrogen mixed into it." The scientists have determined the density of the core to be six grams per cubic centimetre. At nine to 13 grams per cubic centimetre, the density of Earth’s core is much higher. In addition, analysis of the seismograms shows that the Martian core must be molten – at least in its outer zone. The investigation of the core, led by Simon Stähler of the Swiss Federal Institute of Technology (ETH) in Zurich, thus confirms earlier measurements from satellite geodesy, which, however, were unable to determine the core size precisely.
Mars, like Earth, has a shell structure
We know that Earth is made up of concentric shells. A thin crust of light, brittle rock is followed by the thick mantle of heavy, plastically deformable rock, whose circulatory movements shift the continental plates around the globe. Below this is Earth's core, which consists largely of iron and nickel. A similar structure was assumed for the other Earth-like bodies of the inner Solar System, such as the Moon, Mercury, Venus and Mars. One of the main scientific goals of NASA's InSight mission is to study the planet's shell structure. InSight is a geophysical station that has been located in the Elysium Planitia region near the Martian equator since November 2018. The seismic data form InSight now allows absolute quantifcation of the thicknesses of the inner shells of Mars, and puts constraints on their possible compositions.
Crust thinner than previously thought
"There are also interesting new findings about the Martian crust and the mantle underneath, located between the crust and the core," emphasises Brigitte Knapmeyer-Endrun from the Bensberg Earthquake Station at the University of Cologne, the first author of the Science study on the thickness of Mars' crust, in which Plesa and Knapmeyer are also involved. As recently as 2018, a crustal thickness somewhere between 19 and 90 kilometres had been predicted for the landing site of the InSight mission. Knapmeyer-Endrun can now narrow this down: "The data now only allow two possibilities: Either the crustal thickness at the landing site is approximately 20 kilometres, or it is just under 40 kilometres, which is indicated by an additional weak signal."
Global maps of the gravity field and topography of Mars allow this point measurement at the InSight landing site to be extrapolated to the entire planet. This shows that the average thickness of the Martian crust is between 24 and 72 kilometres. "Mars research has been waiting decades for an 'anchor point' to calibrate the global maps," says Martin Knapmeyer, explaining the significance of the result. The wide range between minimum and maximum values for crustal thickness is related to the distribution of radioactive elements in the planet's interior, which generate heat through their decay and ultimately drive the geological processes. A 'thicker' crust is more consistent with the abundance of heat-generating radioactive elements in the crust as observed at the surface, while a 'thinner' crust can be explained by a greater concentration of such elements at depth. "Determining the thickness of the crust based on the InSight data not only helps us understand what Mars looks like today, but also provides us with important information about its thermal evolution," says Ana-Catalina Plesa.
Mantle of Mars 'a simpler version of Earth's mantle'
In Earth's mantle, the pressure required for the formation of the mineral bridgmanite is present from a depth of approximately 700 kilometres. Bridgmanite is a silicate magnesium oxide (MgSiO3) from the mineral family of perovskites. Such perovskites make up four fifths of Earth's mantle and are only formed under extremely high pressures. The new measurements now show that this pressure is only reached in the iron core of Mars, and thus Mars’ entire mantle should be dominated by the mineral olivine [(Mg,Fe)2SiO4], similar to Earth's upper mantle.
Marsquake waves show layer boundaries
The InSight scientists obtained the new results by analysing various seismic waves generated during quakes. During marsquakes, as in earthquakes, energy is released in the form of waves, referred to as 'P' and 'S' waves for historical reasons. P-waves are pressure waves, like sound waves in the air. S-waves, on the other hand, oscillate perpendicular to the direction of propagation, like a guitar string. Since P-waves propagate at greater speed, the distance to the quake's origin can be calculated from the time interval between the arrival of the different wave types. Seismic waves pass through planets and are reflected and refracted at different layer boundaries in the interior.
The InSight mission seismometer has detected more than 700 identified marsquakes since the beginning of 2019. The P- and S-waves travelling directly from the source to the landing site are easily identified. However, to study the inner structure of the planet, the scientists need other signals. Since the liquid core is impermeable to S-waves, thus they produce strong echoes. The thickness of the crust can be determined by means of an effect that only occurs in solid bodies. At the boundary between two types of rock, such as that between the crust and the mantle, there is a partial conversion of P-waves into S-waves, where the delay between the two is larger for a thicker crust.