Largest planetary core

The largest planetary core (as a proportion of total volume) is that of Mercury, the innermost planet of the solar system. Research published in Geophysical Research Letters on 15 March 2019 revealed that Mercury's core makes up 85 per cent of the planet's total volume, and has a diameter of approximately 4,000 km (2,480 mi). For context, Mercury's total diameter is only 4,879.4 km (3,031.9 mi).

The 2019 paper – authored by Antonio Genova of the University of Sapienza in Italy and an international team of researchers – was also able to make the most accurate estimates yet of the internal composition of Mercury's core. The authors suggest that the planet has an inner solid core with a diameter of around 2,000 km (1,260 mi) – roughly the same size as Earth's – surrounded by an active molten layer. The research suggests that Mercury's core has been cooling faster than Earth's, diminishing the strength of its magnetic field.

The new model for Mercury's core is based on readings taken by the MESSENGER probe during the last phase of its mission in 2015. During its final year over the planet, MESSENGER was lowered in altitude from around 200 km (125 mi) to roughly 100 km (62 miles), providing more detailed measurements than before.

The team behind this discovery used information from MESSENGER's sensors to track tiny variations in the spacecraft's orbital parameters. From these almost imperceptible increases or decreases in acceleration, they were able to map out variations in the planet's gravitational field, which, in turn, could be used to infer the density and composition of the planet's interior.