Martian meteorite impacts reveal new information about the planet’s crust

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

Since then, they have been waiting for an occasion that would also lead to waves traveling across the surface of the planet. The moment had arrived on December 24, 2021, when a meteorite struck Mars at a distance of about 3500 kilometers from InSight, creating a crater more than 100 meters in diameter and the desired surface waves.

A meteor impact that occurred less than 7,500 kilometers from InSight was also identified by researchers as the cause of a second shock. Dr Brigitte Knapmeyer-Endrun and Sebastian Carrasco of the Institute of Geology and Meteorology of the University of Cologne were involved in evaluating the information provided by these two incidents. Science has now published the findings.

Researchers value surface waves because they reveal details about the composition of the Martian crust. Mars’ core, mantle, and crust have previously been revealed by body waves generated during earthquakes that travel through the planet’s interior. Although the data has been collected for only one part of the planet, the crust is predicted to have the highest degree of heterogeneity, similar to Earth.

According to Dr. Doyeon Kim, lead author of the study and assistant professor at the Institute of Geophysics at ETH Zurich, “Until now, our knowledge of the Martian crust has been based on measuring only one point below the InSight lander.”

The geophysicist was shocked by the surface wave analysis’ conclusion that the Martian crust had, on average, a fairly homogeneous structure and high density between the impact sites and InSight’s seismometer. On the other hand, scientists had previously found three layers of crust and had estimated a lower density just below the vessel. The near-surface layer, which is about 10 km thick beneath InSight and is characterized by low seismic velocities and low density, was notably absent from the new data.

This is the first time researchers have been able to accurately verify that seismic data taken by InSight came from distant impacts, because the impacts created very distinct craters that can be seen in photos taken from orbit. The rapid series of orbital photographs also helped establish useful temporal constraints on when the craters formed. This exactly matches the times when the seismic waves were recorded.

For the first time, seismic and photographic techniques were used in this survey to record impacts that did not occur on Earth. This may be due to the absence of surface waves so far, as meteor impacts take place on the planet’s surface. It is possible that deeper sources of seismic waves, such as markers, did not actually produce these waves. Researchers will be better able to identify and categorize meteor impacts in the data taken by InSight and use them for models if they know that specific seismic events are impacts.

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

The frequency of surface waves determines how fast they propagate. Because lower frequencies are sensitive to greater depths, measuring how velocity varies at 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 travel, this can be used to determine the average density of the rock. This enabled scientists to identify the structure of the crust between 5 and 30 kilometers below the planet’s surface.

The team was trying to understand why the average speed of the observed surface waves was much higher than they would have predicted based on an earlier point measurement taken below the InSight lander. Is it caused by variation in surface rock composition or by some other mechanism? The paths between the two meteor impactors and the measurement site cross one of the largest volcanic belts in Mars’ northern hemisphere, and the volcanic rocks often have higher seismic velocities.

Surface lava growth or closure of pore spaces due to heating caused by volcanic processes are two examples of many mechanisms that may accelerate seismic waves. The crust beneath the InSight landing site, on the other hand, may have acquired its characteristic structure as a result of material being ejected during a major asteroid impact more than three billion years ago, according to the study.

If so, the vessel’s foundation is likely not an accurate representation of the overall structure of the Martian crust, according to Kim.