In the final year of NASA’s Mars InSight mission, a powerful earthquake allowed researchers from ETH Zurich to determine the global thickness and density of the Martian crust. The crust of Mars turned out to be much thicker than that of the Earth or the Moon, and the main source of the planet’s heat is radioactivity.

In May 2022, the ETH Zurich Earthquake Service detected the strongest earthquake ever recorded on a celestial body other than Earth. This event, with an estimated magnitude of 4.6, was documented on the Martian surface by the NASA Mars Seismometer. InSight task.

“This earthquake produced powerful seismic waves that propagated on the Martian surface,” says Doyon Kim, a seismologist at the ETH Institute for Geophysics in Zurich.

Surface waves offer a global perspective

Researchers found evidence of a significant earthquake on the InSight seismometer after more than three years of daily monitoring and low power levels. The surface waves observed during this great earthquake not only spread from the earthquake’s source to the measuring station, but also continued to circle the entire planet several times. These data not only provided information about specific regions of Mars, but also provided a global overview of the planet.

“From this earthquake, which was the largest recorded during the entire InSight mission, we observed surface waves reaching Mars up to three times over,” says the seismologist and lead author of the study, newly published in the journal Geophysical Research. Literature .

The researchers measured how fast these waves propagated at different frequencies to get information about the structure through which the waves were traveling.

A topographic map of the Martian surface (left) and an image of the thickness of the earth’s crust (right). Image credit: MOLA Science Team / Doyon Kim, ETH Zurich

These seismic velocities allow us to understand the internal structure at different depths. Surface waves previously observed from the impacts of the two large meteorites also allowed regional findings along certain propagation paths.

“We now have seismic observations that represent the global structure,” says Kim.

Comparison of data from Mars with data from Earth and Moon

By combining the newly obtained results with existing data on Mars’ gravity and topography, the researchers were able to determine the thickness of the Martian crust. On average it is 42 to 56 kilometers (26 to 35 miles). On average, the crust is thinnest at ∼10 km (6 mi) in the Isis impact basin and thickest in Farsis Province at ∼90 km (56 mi).

To put this into perspective, seismic data shows that Earth’s crust averages 21 to 27 kilometers (13 to 17 miles) thick, while the lunar crust is 34 to 43 kilometers (21 to 17 miles) thick, as measured by the Apollo seismometers. 27 miles) stay.

“This means that the Martian crust is much thicker than that of the Earth or the Moon,” says Kim. As a general rule, smaller planetary bodies in our solar system have thicker crusts than larger bodies. Kim explains: “We were lucky enough to witness this earthquake. On Earth, using the same magnitude of the earthquake that occurred on Mars, it would have been difficult for us to determine the thickness of the Earth’s crust. Although Mars is smaller than Earth, it carries seismic energy more efficiently.”

One of the most important results of this study concerns the difference between the northern and southern hemispheres of Mars. This contrast has been observed as long as telescopes have existed; this can be seen especially in images of Mars moons. The northern hemisphere of Mars consists of flat plains, and the southern hemisphere consists of high plateaus. The separation between the plains in the north and the mountainous areas in the south is called the Martian dichotomy.

Similar crustal density and radioactive heat

“It’s conceivable that this difference could be explained by two different compositions of the rocks,” says Kim. “One breed will be busier than the other.” Although the composition may be the same in the north and south, the thickness of the earth’s crust is different. If the crust is thicker in the south, there will be less dense Martian mantle material underneath, while the thinner crust in the north will have more of this dense, heavier material.

What exactly did the researchers manage to prove? “Based on seismic observations and gravitational data, we show that the density of the Earth’s crust is similar in the northern plains and southern highlands,” they write. In contrast, the crust in the southern hemisphere extends deeper than that in the northern hemisphere. “This discovery is very exciting and ends the long-standing scientific debate about the origin and structure of the Martian crust,” says Kim. After all, analysis of meteorites that fell on Mars last year provided evidence that the crusts to the north and south were made of the same material.

Other conclusions can be drawn from the thick Martian crust. “Our work shows how the planet generates heat and explains the thermal history of Mars,” says Kim. As a single-plate planet, the main source of heat produced in the interior of Mars today is the decay of radioactive elements such as thorium, uranium and potassium. The study found that between 50 and 70 percent of these heat-producing elements are found in the Martian crust. Such a large accumulation may explain why there are still local areas where smelting can occur today. Source