Scientists have peered into the heart of Mars for the first time. NASA’s InSight spacecraft, sitting on the Martian surface with the aim of seeing deep inside the planet, has revealed the size of Mars’s core by listening to seismic energy ringing through the planet’s interior.

The measurement suggests that the radius of the Martian core is 1,810 to 1,860 kilometres, roughly half that of Earth’s. That’s larger than some previous estimates, meaning the core is less dense than had been predicted. The finding suggests the core must contain lighter elements, such as oxygen, in addition to the iron and sulfur that constitute much of its make-up. InSight scientists reported their measurements in several presentations this week at the virtual Lunar and Planetary Science Conference, based out of Houston, Texas.