Meteorite impacts on Mars reveal new information about the planet's crust

NASA’s InSight mission collected data on two meteorite impacts on Mars that shed new light on the composition of the Martian crust. In the past, scientists had seen many earthquakes whose waves propagated inside the planet from the epicenter of the earthquake.

Since then, they had been anticipating a circumstance that would also result in waves propagating across the surface of the planet. The moment had arrived on December 24, 2021, when a meteorite struck Mars at a distance of about 3,500 kilometers from InSight, creating a crater over 100 meters in diameter and the desired surface waves.

A meteorite impact that occurred at a distance of less than 7,500 kilometers from InSight was also determined by researchers to be the cause of a second shock. Dr. Brigitte Knapmeyer-Endrun and Sebastian Carrasco from the Institute of Geology and Meteorology of the University of Cologne participated in the evaluation of the information provided by these two incidents. Science has now published the results.

Researchers like surface waves because they reveal details about the composition of the Martian crust. The Martian core, mantle and crust have already been revealed by body waves generated during earthquakes and traveling inside the planet. Although data was only collected for one location on the planet, the crust should have the highest degree of heterogeneity, similar to Earth.

According to Dr. Doyeon Kim, lead author of the study and assistant professor at ETH Zurich’s Institute of Geophysics, “Until now, our knowledge of the Martian crust has been based on a single point measurement beneath the lander. Insight.”

The geophysicist was shocked by the conclusion of the surface wave analysis that the Martian crust had, on average, a fairly homogeneous structure and high density between impact points and InSight’s seismometer. On the other hand, scientists had previously found three layers of crust and assessed a lower density just below the lander. The near-surface layer, about 10 km thick under InSight and characterized by low seismic velocities and low density, was not present in the new data, which is remarkable.

This is the first time researchers have been able to accurately verify that seismic data obtained by InSight came from distant impacts because the impacts generated very distinct craters that can be seen in photographs taken from orbit. The quick series of photos from orbit also helped establish useful temporal boundaries on when the craters were produced. This corresponds exactly to the times when the seismic waves were captured.

For the first time, seismic and photographic techniques were used in this investigation to record impacts that did not occur on Earth. This may explain the absence of surface waves so far, as meteorite impacts take place on the surface of the planet. It is possible that deeper seismic wave sources like the Marsquakes did not actually produce these waves. Researchers will be better able to recognize and categorize meteorite impacts in data obtained by InSight and use them for models if they are aware that specific seismic events are impacts.

“The new findings are so exciting because a planet’s crust provides important clues to the formation and evolution of the celestial body. It is the result of early dynamic processes in the mantle and later magmatic processes,” said explained Dr. Brigitte Knapmeyer-Endrun. , it can provide information about conditions billions of years ago and the history of impacts, which were particularly common in the early days of Mars.”

The frequency of surface waves determines the speed at which they propagate. Since low frequencies are sensitive to deeper depths, measuring the variation in velocity over different frequencies in seismic data allows scientists to infer how velocity changes with depth. Since the seismic velocity also depends on the elastic properties of the material through which the waves pass, this can be used to determine the average rock density. This allowed scientists to identify the structure of the crust between 5 and 30 kilometers below the surface of the planet.

The team was trying to figure out why the average speed of the observed surface waves was so much higher than they would have predicted based on an earlier spot measurement taken under the InSight lander. Is this due to a variation in the composition of the surface rock or to another mechanism? The pathways between the two meteorite impacts and the measurement location cross one of the largest volcanic areas in the northern hemisphere of Mars, and volcanic rocks often have higher seismic velocities.

The development of surface lava or the closure of porous spaces due to heating caused by volcanic processes are two examples of the many mechanisms that can accelerate seismic waves. The crust beneath InSight’s landing site, on the other hand, may have taken on its distinctive structure as a result of material being ejected during a large asteroid impact more than three billion years ago. years, according to the study.

If so, the lander’s foundation likely isn’t an accurate representation of the overall structure of Mars’ crust, Kim said.