2023-04-24 19:24:59
Even though the Insight mission ended last December, the data collected by the seismic station on the surface of Mars continues to bear fruit. The analysis of the waves recorded during two particular seismic events has thus made it possible to characterize the core of the Red Planet more precisely: it would be entirely liquid and would include a significant proportion of light elements. Characteristics that significantly differentiate it from the Earth’s core.
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A few months ago, in December 2022, the Insight lander was breathing its last breath, unable to recharge its battery, its solar panels being completely covered in dust. But the analysis of the countless data collected by this small seismic station, placed on the surface of Mars since November 2018, has however not been interrupted, on the contrary!
Thanks to the analysis of the seismic waves recorded by the instrument, we now know much better the internal structure of the Red Planet. The Insight data, and in particular those collected at the time of a strong meteorite impact in December 2021, have indeed made it possible to establish more clearly the structure of the Martian crust on a large scale. But these data also made it possible to investigate the depths well beyond the crust of Mars. For the first time, seismic waves having passed through the core have been recorded, allowing an unprecedented characterization of the deepest levels of the planet. Result: the core of Mars would be mainly liquid and composed not only of iron, but of a significant part of sulfur and light elements. Observations that should help to better understand the formation of the planet and its evolution, compared to that of the Earth.
Listen to the distant tremors of Mars to image its depths
To achieve this, not one, but two seismic events were used. To have a chance for seismic waves to travel through the core, events had to occur at a great distance from the seismic station and be of sufficient magnitude to be detected by the seismometer. The much longer than expected duration of the mission fortunately allowed the recording of two events presenting these conditions. The first (S0976a) corresponds to an earthquake of magnitude 4.2. The second (S1000a) corresponds to the meteorite impact. Its magnitude was estimated at 4.1. The epicenters of both events were located on the opposite hemisphere from Insight.
In both cases, the seismometer was able to detect the arrival of so-called SKS waves. However, this type of seismic waves is well known on Earth. They are the ones that made it possible to determine the liquid nature of the Earth’s outer core. S waves are indeed shear waves. In fact, they cannot propagate in liquid media. But SKS waves refer to waves that travel as shear waves in the mantle and as compressive (P) waves in the liquid core before emerging into the mantle once more as S waves. Their detection by Insight is therefore confirmation that Mars does indeed have a liquid core.
A completely liquid core?
Moreover, unlike the Earth, which has a liquid outer core and a solid inner core, the core of Mars would be mainly, if not totally, liquid. ” For the moment, we have not collected any clue suggesting the existence of a solid seed in the heart of Mars “, explains Jessica Irving of the University of Bristol, first author of the study published in the journal PNAS. « We currently only have two seismic events, which did not allow us to image the exact center of the planet. It is possible that there is a very small solid core that we have not yet been able to see thanks to seismic signals. The seismic data nevertheless reveal that if the planet has a solid seed, it must necessarily have a radius of less than 750 kilometers (compared to 1,221 kilometers for the Earth). The liquid core would have a thickness of 1,799 to 1,814 kilometers (compared to 2,300 kilometers for the Earth). The liquid nature of the Martian core had previously been proposed, but the new results suggest it would be slightly smaller than previously assumed. It would also be a little denser than what previous studies had suggested. The speed of propagation of seismic waves depends on the density of the medium, itself intrinsically linked to its composition.
A very different composition of the Earth’s core
The speeds of the seismic waves having crossed the core thus suggest that it is not composed only of liquid iron, but would contain a substantial part of light elements and in particular sulfur in large quantities as well as a little oxygen, of carbon and hydrogen. The fraction of light elements in the core of Mars would also be twice as high as that of the outer core of the Earth. This composition might in particular explain the absence of a solid nucleus, ” the light elements lowering the temperature at which the core can maintain itself in a completely liquid state », explique Jessica Irving.
These differences in the composition and structure of the core between Earth and Mars might lead to a better understanding of the internal formation of the planets of the Solar System. It is indeed possible that the planets located furthest from the center of the original solar nebula, like Mars, incorporated more light volatile elements at the time of their formation. ” A more precise determination of the quantity of these elements is however necessary to better understand which differences between the Earth and Mars result from the material accreted during their formation and which are linked to the physical processes at play during planetary differentiation. “, explain the authors in the study.
An impact on the Martian magnetic field
The difference in internal structure between the two planets might also have played a role in the habitability of the two worlds. While on Earth, the existence of a solid inner core and a liquid outer core has allowed the establishment of a magnetic field that durably protects the surface of the planet from the harmful action of solar winds and allows it to keep its water, the structure of the Martian core would not have been favorable. However, the planet would have had a magnetic field during its youth before it gradually decreased, making Mars then hostile to life. This evolution might be related to the completely liquid structure of the core.
Data collected by Insight continues to be analyzed. ” Seismic records might still contain a great deal of informationajoute Jessica Irving. Combined with other scientific results, such as new measurements of the properties of iron alloys under high pressure and temperature, they might make it possible to better constrain the conditions for the formation of Mars and other planets in the Solar System. »
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