Researchers at ETH Zurich made a groundbreaking discovery during the final year of the NASA Mars InSight mission. A powerful earthquake on Mars provided them with the opportunity to determine the thickness and density of the planet’s crust on a global scale. The findings revealed that, on average, the Martian crust is considerably thicker than both the Earth’s and the moon’s crust. Additionally, it was established that Mars primarily derives its heat from radioactive sources.
After diligently monitoring Mars for over three years and with the InSight seismometer’s power levels dwindling, researchers finally received valuable data from a significant Marsquake in May 2022. Surface waves resulting from this quake, estimated to be of 4.6 magnitude, not only traveled from the epicenter to the measuring station but astonishingly circled the entire planet multiple times. This seismic data not only provided detailed information about specific regions of Mars but also allowed for a comprehensive understanding of the planet as a whole.
Lead author of the study published in the journal Geophysical Research Letters, Doyeon Kim, who is a seismologist, remarked, “From this earthquake, which happened to be the most substantial one recorded during the entire InSight mission, we observed surface waves that propagated around Mars up to three times.” By measuring the speed at which these waves traveled at different frequencies, the researchers were able to glean insights into the interior structure of Mars at various depths.
While previous observations of surface waves resulting from two major meteorite impacts had offered limited regional findings along their specific paths, Kim emphasizes the significance of the latest seismic observations, stating, “Now, we possess seismic data that represents the global structure of Mars.”
Comparing data from Mars with that of the Earth and the moon
By combining their recent findings with existing data on Mars’ gravity and topography, the researchers at ETH Zurich successfully determined the thickness of the Martian crust. On average, the crust measures approximately 42 to 56 kilometers (26–35 miles) in depth. Notably, the crust is thinnest at the Isidis impact basin, measuring around 10 kilometers (6 miles), while it reaches its greatest thickness at the Tharsis province, estimated at about 90 kilometers (56 miles). To provide a comparison, the Earth’s crust has an average thickness of 21 to 27 kilometers (13–17 miles), while the lunar crust, as determined by Apollo mission seismometers, ranges from 34 to 43 kilometers (21–27 miles) in thickness.
Kim highlights the remarkable disparity, stating, “This means that the Martian crust is much thicker than that of the Earth or the moon.” It is a common trend among smaller celestial bodies in our solar system to possess thicker crusts compared to larger bodies. Kim further explains the significance of their seismic observations on Mars, expressing gratitude for the opportunity to study such a quake. Determining the thickness of Earth’s crust using a similar magnitude earthquake would pose challenges due to the larger size of our planet. Mars, despite its smaller size, efficiently transmits seismic energy, facilitating their measurements.
One of the key outcomes of this research pertains to the contrasting features of Mars’ northern and southern hemispheres. This dichotomy has been apparent since the advent of telescopic observations and is particularly evident in satellite images of the planet. The northern hemisphere comprises vast lowland regions, while the southern hemisphere is dominated by elevated plateaus. The boundary between the northern lowlands and southern highlands is referred to as the Martian dichotomy, which holds significant importance in the study of the planet.
Similar crust density and radioactive heat
Kim explains, “One might assume that the discrepancy in thickness between the northern and southern Martian crust could be attributed to different rock compositions, with one type being denser than the other.” However, despite the possibility of similar compositions, the researchers have discovered that the density of the crust is comparable in the northern lowlands and southern highlands. Instead, the variation lies in the depth of the crust. In the southern hemisphere, the crust extends to a greater depth compared to the northern hemisphere. Kim expresses excitement, stating, “This significant finding brings an end to a longstanding scientific debate regarding the origin and structure of the Martian crust.” Last year’s analysis of meteorite impacts on Mars already provided evidence supporting the notion that the crust in both the north and south consists of the same material.
The implications of the thick Martian crust extend beyond settling the debate. Kim elaborates, “Our study sheds light on how the planet generates its heat and provides insights into Mars’ thermal history.” Being a single-plate planet, Mars relies on the decay of radioactive elements such as thorium, uranium, and potassium as the primary source of heat in its interior. The study reveals that 50% to 70% of these heat-producing elements are concentrated within the Martian crust. This significant accumulation potentially explains the existence of localized regions beneath the crust where ongoing melting processes may still occur to this day.
Source: ETH Zurich