Largest recorded meteorite impact on Mars sheds unexpected details on planet's interior structure

Largest recorded meteorite impact on Mars sheds unexpected details on planet’s interior structure

In December 2021, a meteor impacted the surface of Mars, generating powerful seismic waves on the planet’s surface. It was enough for the InSight mission’s seismometer. The analysis of these seismic waves has made it possible to establish the structure of the Martian crust, for the first time on a large scale.

On December 24, 2021, an important earthquakeearthquake shook the surface of Mars, an event immediately detected and recorded by the InSight mission’s seismometer, perched on the surface. The origin of this tremor quickly intrigues scientists, because the Red Planet, which has no tectonic platestectonic plates, very rarely produces large earthquakes. And indeed, the earthquake has nothing to do with a geological event intrinsic to the planet. It would have been caused by the impact of a meteorite on the surface, a hypothesis confirmed by the probe MROMRO in orbit around the planet revealing the presence of a new crater 150 meters in diameter!

For scientists, this type of event was simply unexpected. Because the force of the impact generated a very particular type of wave, which propagated on the surface. The characteristics of these waves, which had never been detected on Mars at the time, made it possible to study the structure of the Martian crust as a whole.

Hear the meteor impact on the ground on Mars recorded by the InSight lander. © NASA, JPL-Caltech, Cnes, Imperial College London

Very special waves that allow to probe the Martian crust

Surface waves are in fact very special waves that, on Earth, are produced during strong earthquakes. For this reason, they have never before been observed on Mars, which produces only low-magnitude earthquakes. Although these waves only propagate along the surface, they are the ones that transmit most of the energy of an earthquake. On Earth, they are particularly feared because they do the most damage.

Unlike the waves of volumesvolumes (P and S), the speedspeed surface waves depends on their frequencyfrequency. However, low frequencies are more sensitive at greater depths. This is a particularly interesting feature for the study of the Martian crust. For the first time, the measurement of the dispersion of these waves has made it possible to directly observe the variations in speed as a function of depth and to build a complete image of the structure of the Martian crust.

The meteorite impact carved out rock-sized blocks of ice from the subsurface of Mars. The presence of ice so close to the surface has surprised and questioned scientists. Overview of the affected region created from images captured from space by the MRO probe. © NASA, JPL-Caltech, University of Arizona

The structure of the crust of Mars, for the first time on a large scale

As the Earth’s crustEarth’s crustThe Martian crust shows important variations in topography, thickness, and age, but it is also variably impacted by craters, erosion, and volcanismvolcanism. The crust of Mars also exhibits an important difference between theNorth HemisphereNorth Hemisphere and thesouthern hemispheresouthern hemisphere, whose origin is still hotly debated. A dichotomy that is characterized by a topographic unevenness of about five kilometers. The higher southern part also has a highly cratered surface compared to the northern surface, marked by vast plains.

the seismometerseismometer Until now, InSight had allowed better limiting the thickness of the crust in the region where it is located (Eliseo Planitia), that is, 39 +/- 8 km. Despite the importance of this result, it is only one specific point and the Martian crust remained, as a whole, largely unknown. The impact of the meteorite, however, made it possible to build a global model. The analysis of surface waves has thus enabled scientists to study the structure of the crust down to depths of 30 kilometers below the surface.

An a priori uniform crust between north and south

The results, published in the journal Sciences, suggest that the Martian crust exhibits a relatively uniform structure between the meteor impact site and InSight’s location. As a whole, the crust also has a high density, contrary to what the observations made at the seismometer level suggested. The crust directly below InSight appears to be low-density and could have been structured by a large meteorite impact 3 billion years ago. In any case, it seems that this part of the Martian crust is not representative of the general structure of the Martian crust.

Regarding the origin of the Martian dichotomy, the study suggests that, contrary to previous theory, the northern and southern crusts are probably relatively similar, either in composition or structure. Variations in density could explain this impression of difference in crustal thickness. However, the origin of the dichotomy is still debated and could be related to major meteorite impacts or a anomalyanomaly on the convectionconvection of CoatCoator a combination of the two processes.

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