In December 2022, NASA announced the end of its InSight Mars mission. After more than four years on the surface of the Red Planet, the lander, on whose panels a thick layer of dust had settled, no longer had enough energy to work. Death by suffocation, as it were. However, in space exploration, long after the robots stop working, the data recorded continues to feed science, and InSight is no exception. The anomaly in the internal structure of Mars was resolved on Wednesday, October 25, in the journal Nature, which published two studies, independent of each other but based on measurements from the lander's main instrument, the SEIS seismometer supplied by France.
Two years ago, in the summer of 2021, the InSight team described the internal structure of Mars for the first time. To achieve this, they deciphered the seismic waves created by "Marsquakes" and meteorite impacts. The speed at which these waves propagate depends on the properties (such as density, temperature and compressibility) of the medium they pass through. This reveals its chemical composition and whether it is solid, soft or liquid. The result obtained in 2021 shows three simple layers: a thin crust overlying a rocky, homogeneous and unstratified mantle, which itself surrounds a fairly large metallic and liquid core, with a radius of 1,830 kilometers (the Martian radius measures 3,390 kilometers).
However, there was a snag, an anomaly summarized by Henri Samuel, a CNRS researcher working at the Institut de Physique du Globe de Paris (IPGP) and lead author of one of the two studies: "The nucleus we had was not very dense. To achieve this low density, we had to add light elements to the iron, such as sulfur, carbon, oxygen and hydrogen. The problem was that we had to add a lot, too much compared to what was allowed."
Of course, there's no instruction manual for a planetary core, but the laws of physics and chemistry do set constraints. "Experiments have been carried out in the laboratory, under high temperature and pressure, attempting to incorporate light elements into iron alloys," said Samuel. "The limits that these tests established were somewhat exceeded by data from the Mars core."
What's more, the quantities of light elements assumed to be present in the Mars core did not tally with the materials available at the time of planet formation, 4.5 billion years ago. Since the anomaly could not be solved, "we learned to live with it," said Samuel with a smile.
However, on September 18, 2021, an unexpected event shed new light on the matter. On that day, a meteorite crashed on Mars, some distance from InSight. This generated waves that were different from the previous ones and capable of propagating in liquid, where their speed was halved. As Samuel pointed out, "the propagation time recorded for this event was far too slow to be explained by a homogeneous, solid mantle." The 2021 model no longer held water: the mantle had to be heterogeneous and based on a liquid layer, which is stable because it is weighed down by iron, which had previously been confused with the upper part of the core.
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