NASA’s InSight provides valuable data on global thickness and density of the Martian crust

"From this quake, the largest quake recorded during the entire InSight mission, we observed surface waves that circled Mars up to three times," said Doyeon Kim, a seismologist and the lead author of the study, in an official statement.

Martian crust thickness

By studying the speed at which these waves propagated at different frequencies, researchers could determine the interior structure at various depths.

The combination of seismic data with existing information on Mars' gravity and topography allowed the team to calculate the average thickness of the Martian crust, which ranges from 42 to 56 kilometers (26–35 miles). 

Notably, the crust is thinnest at the Isidis impact basin, measuring approximately 10 kilometers (6 miles), while it reaches its greatest thickness in the Tharsis province at around 90 kilometers (56 miles). 

In comparison, the Earth's crust has an average thickness of 21 to 27 kilometers (13–17 miles), while the lunar crust is between 34 and 43 kilometers (21–27 miles) thick, as determined by the Apollo mission seismometers.

"This means that the Martian crust is much thicker than that of the Earth or the moon," said Kim. Surprisingly, smaller planetary bodies tend to have a thicker crust than larger ones. Mars, despite being smaller than Earth, effectively transports seismic energy, allowing scientists to glean valuable information about its crust. 

Kim adds, "We were fortunate to observe this quake. On Earth, we would have difficulty determining the thickness of the Earth's crust using the same magnitude of quake that occurred on Mars."

Planet's thermal history

One intriguing aspect of this research is the difference observed between the northern and southern hemispheres of Mars. This contrast, known as the Martian dichotomy, has long been observed but remained poorly understood. 

The team's findings suggest that the crust density in the northern lowlands and southern highlands is similar, indicating that the composition of the rocks in both regions is likely the same. 

However, the southern hemisphere has a deeper crust compared to the northern hemisphere, suggesting a variation in thickness.

These discoveries provide insights into the Martian crust. They also offer clues about the planet's thermal history. Analysis of the seismic observations and gravity data led to the conclusion that 50% to 70% of the heat-producing elements, such as thorium, uranium, and potassium, are concentrated in the Martian crust.

As a result, Mars generates much of its internal heat through the decay of radioactive elements. The high accumulation of these elements in the crust explains why certain regions beneath the Martian surface may still experience melting processes today.

"This finding is very exciting and allows an end to a long-standing scientific discussion on the origin and structure of the Martian crust," said Kim. 

The study is led by researchers from  ETH Zurich, and the results have been published in the journal Geophysical Research Letters.

Study abstract:

We report observations of Rayleigh waves that orbit around Mars up to three times following the S1222a marsquake. Averaging these signals, we find the largest amplitude signals at 30 s and 85 s central period, propagating with distinctly different group velocities of 2.9 km/s and 3.8 km/s, respectively. The group velocities constraining the average crustal thickness beneath the great circle path rule out the majority of previous crustal models of Mars that have a >200 kg/m3 density contrast across the dichotomy. We find that the thickness of the martian crust is 42-56 km on average, and thus thicker than the crusts of the Earth and Moon. Together with thermal evolution models, a thick martian crust suggests that the crust must contain 50-70% of the total heat production to explain present-day local melt zones in the interior of Mars.