Mars has been explored by space probes for years and many details of its surface and geology are already known. But what it looks like has so far remained hidden. Seismic data from NASA’s Mars InSight space probe are now providing initial information. Among other things, they reveal that the Martian core is larger and less dense than expected – it takes up almost half of the interior of the planet. The mantle of Mars is less differentiated than the earth’s mantle and it lacks a counterpart to the lower mantle. The Martian crust, on the other hand, is rather thin and could be divided into two or even three layers of different compositions.
Mars, Venus, Mercury or Earth – the inner planets of the solar system all emerged in the inner region of the primordial cloud and in a similar way: From initially small chunks they grew into larger and larger celestial bodies. In the initially glowing planets, a differentiation gradually set in – the layers were formed. As a result, all inner planets developed an iron-rich core, a silicate-containing rock mantle and a solid rock crust. But while the interior of the earth has already been investigated relatively well thanks to seismic data, the structure of our neighbor Mars has so far been largely unknown. Although models can be used to roughly estimate the dimensions of the core, mantle and crust, without seismic measurements on site, more precise information about the structure and composition of the interior of Mars is missing.
Differences to the inner life of the earth
NASA’s Mars InSight space probe has now supplied these measurement data. Thanks to its seismometer standing on the surface of Mars, it can for the first time capture the seismic tremors of the Martian earthquakes and record their reflection on the internal structures. Since landing in November 2018, it has recorded several hundred Marsquakes. What these reveal about the anatomy of the red planet is now reported by international research teams in three publications in the specialist magazine “Science”. According to this, the rough layer structure of Mars corresponds to the basic scheme, but in the details there are clear differences to the inner workings of the earth. These start with the Martian crust: on Mars, it is apparently divided into two or even three layers of different compositions. “In any case, we can rule out that the entire crust consists of the same material that is known from surface measurements and from Martian meteorites,” says Brigitte Knapmeyer-Endrun from the University of Cologne.
According to the seismic data, the upper crustal layer at the Mars InSight site is around eight kilometers thick, followed by a second layer around twelve kilometers thick. “Then the coat could already follow. This would be a surprisingly thin crust, also compared to the continental crust on earth, ”says Knapmeyer-Endrun. But it could also be that Mars has a third crustal layer that extends another 19 kilometers deeper. Overall, the Martian crust would be between 20 and 39 kilometers thick at the probe’s landing site near the equator. The seismic measurements also suggest that the Martian crust contains more radioactive, heat-producing elements than expected. “Determining the crust thickness based on the InSight data not only helps us to understand what Mars looks like today, but also provides us with important information about its thermal development,” says Ana-Catalina Plesa from the German Aerospace Center.
No undercoat and a big core
There are also differences in the mantle of Mars. Its lithosphere extends to a depth of 400 to 600 kilometers – more than twice as deep as on Earth. With us, this more solid, 100 to 200 kilometer thick layer of rock is part of the drifting continental plates. Mars, on the other hand, has no real plate tectonics according to common assumptions. “The thick lithosphere fits in well with the model of Mars as a ‘one-plate planet'”, explains Amir Khan from ETH Zurich. What Mars lacks, however, is a counterpart to the Earth’s lower mantle. This begins with us at a depth of 660 kilometers and is characterized by particularly dense mineral forms such as bridgemanite, which are solid due to the high pressure despite the heat. Such perovskites only arise under extremely high pressure. However, the new measurements suggest that this pressure will not be reached in the Martian mantle – even at the core-mantle boundary it is almost ten times lower than on Earth.
According to the new data, the core of Mars is smaller than that of Earth, but significantly larger than previously assumed. It could have a radius of 1,830 kilometers and thus occupy half of the interior of Mars. “That is more at the upper end of the size range that all previous estimates had shown,” explains Ana-Catalina Plesa from DLR. However, this means that the density of the Martian core, at around six grams per cubic centimeter, must be lower than the Earth’s core density, which is nine to 13 grams per cubic centimeter. “The core must therefore contain a large proportion of lighter elements in addition to iron and nickel,” explains Khan. In addition to sulfur with a share of ten to 15 percent, there could also be oxygen, hydrogen and carbon in the Martian core, according to the preliminary assumptions of the research team. The current investigations also confirm that the Martian core is liquid, as previously suspected.
The data from the InSight probe has already revealed some unexpected details of the interior of Mars. “But we are far from over with the evaluation of all the data – Mars still puzzles us, especially the question of whether it was formed at the same time and from the same material as our earth,” explains Khan.
Source: Brigitte Knapmeyer-Endrun (University of Cologne) et al., Science, doi: 10.1126 / science.abf8966; Amir Khan (ETH Zurich) et al., doi: 10.1126 / science.abf2966; Simon Stähler (ETH Zurich) et al., doi: 10.1126 / science.abi7730