Meteor impacts on Mars 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 Martian crust. Scientists have observed numerous earthquakes in the past, with waves propagating from the epicenter of the quake into the interior of the planet.

Since then, they anticipated a circumstance that would also result in waves traveling across the planet’s surface. On December 24, 2021, the moment had come when a meteorite hit Mars about 3500 kilometers away from InSight and created a crater with a diameter of more than 100 meters and the desired surface waves.

A meteorite impact less than 7,500 kilometers from InSight was also identified by the researchers as the cause of a second shock. In evaluating the information from these two incidents, Dr. Brigitte Knapmeyer-Endrun and Sebastian Carrasco from the Institute for Geology and Meteorology at the University of Cologne. Science has now published the results.

Researchers value surface waves because they reveal details about the composition of the Martian crust. The core, mantle and crust of Mars were previously revealed by body waves generated during tremors that travel through the planet’s interior. Although the data was collected for only one location on the planet, the crust is predicted to exhibit 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, “so far, our knowledge of the Martian crust has only been based on a point measurement under the InSight lander.”

The geophysicist was shocked by the surface wave analysis’ conclusion that, on average, the Martian crust had a fairly homogeneous structure and high density between impact sites and InSight’s seismometer. On the other hand, the scientists had previously found three layers of crust and determined a lower density directly below the lander. Notably, 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.

This is the first time researchers have been able to accurately verify that the seismic data obtained by InSight came from distant impacts, as the impacts created very clear craters that can be seen in photos taken from orbit. The rapid series of photos of the orbit also helped set useful time limits on how the craters formed. This is an exact match for the times when the seismic waves were recorded.

This study was the first to use seismic and photographic techniques to record impacts that did not occur on Earth. This could explain the lack of surface waves so far, as meteorite impacts take place on the surface of the planet. It’s possible that deeper seismic wave sources like marsquakes didn’t actually create these waves. Researchers can better identify, categorize, and model meteorite impacts in InSight data by knowing that specific seismic events are impacts.

“The new findings are so interesting because the crust of a planet provides important information about the formation and development of the celestial body. It is the result of early dynamic processes in the mantle and subsequent magmatic processes,” explains Dr. Brigitte Knapmeyer-Endrun. “Thus , it may shed light on the conditions billions of years ago and the history of impacts that 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, by measuring changes in velocity across different frequencies in seismic data, scientists can infer how velocity changes with depth. Since the seismic velocity also depends on the elastic properties of the material through which the waves penetrate, the mean density of the rock can be determined from this. This allowed the scientists to identify the structure of the crust between 5 and 30 kilometers below the planet’s surface.

The team tried 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 under the InSight lander. Is it caused by a variation in the composition of the surface rock or some other mechanism? The paths between the two meteorite impacts and the measurement site cross through one of the largest volcanic zones on Mars’ northern hemisphere, and volcanic rock often exhibits higher seismic velocities.

The development of surface lava or the closing of pore spaces through heating by volcanic processes are two examples of many mechanisms that could accelerate seismic waves. The crust beneath InSight’s landing site, on the other hand, may have acquired its characteristic structure as a result of material ejection during a major asteroid impact more than three billion years ago, according to the study.

If that’s the case, Kim says the lander’s bedrock is most likely not an accurate representation of the overall crustal structure of Mars.