With all of the attention on NASA's Mars Perseverance rover, which landed on the Red Planet in February of 2021, and is scoping out the surface for signs of past microbial life, another important Mars mission is being overshadowed. This other mission, NASA's InSight lander, which touched down back in November of 2018, is focused on learning more about the interior structure of Mars using a seismometer.
Seismic waves, like those produced in earthquakes here on Earth, enable seismologists to map the interior structure of our planet. As the seismic waves pass through solid and liquid layers within the Earth, the different densities of the various layers cause the seismic waves to either speed up or slow down. Using this same principle, InSight's seismometer has been measuring the seismic waves produced during marsquakes. However, unlike Earth where many of its quakes are caused by tectonic plates colliding or scraping past each other, Mars' crust is not broken up into tectonic plates. So the smaller marsquakes are caused by volcanic activity.
After analyzing the data, an international team of researchers has discovered that the Martian crust near the lander is between 14 and 47 km (7.7-29.2 mi) thick. This is thinner than Earth's continental crust which can range from 35-70 km (21.7-43.5 mi) thick. Beneath the crust, comes the mantle, with the more solid part of the upper mantle and crust comprising the lithosphere. Mars' lithosphere was found to be thicker than Earth's at 400-600 km (248.5-372.8 mi) thick.
Unlike Earth, which has a liquid outer core of nickel and iron and a solid inner core of iron, the core of Mars is entirely liquid and composed of lighter elements like oxygen, sulfur, carbon, and hydrogen, in addition to iron and nickel. Mars' core was found to have a radius of 1,840 km (1,143 mi), which is 200 km (124 mi) larger than previously thought.
One of the main reasons to study the interior structure of Mars is to uncover why the planet lost its active magnetic field. The main theory is that large impacts on early Mars disrupted the convection currents in the liquid metal core of Mars, which are crucial to producing a magnetic field, like on Earth. Without the type of protective magnetic field that Earth has, the streams of ionizing particles coming from the Sun stripped away much of the atmosphere on Mars and would have subjected any surface life to harmful radiation.