London: Earth’s inner core was formed 1-1.5 billion years ago as it “froze” from the surrounding molten iron outer core, scientists from University of Liverpool have revealed.

The inner core — Earth’s deepest layer — is a ball of solid iron just larger than Pluto which is surrounded by a liquid outer core.

The inner core is a relatively recent addition to our planet and establishing when it was formed is a topic of vigorous scientific debate with estimates ranging from 0.5 billion to two billion years ago.

“This finding could change our understanding of the Earth’s interior and its history,” said Dr Andy Biggin, palaeomagnetism expert and the study’s lead author.

Researchers analysed magnetic records from ancient igneous rocks and found that there was a sharp increase in the strength of the Earth’s magnetic field between 1 and 1.5 billion years ago.

This increased magnetic field is a likely indication of the first occurrence of solid iron at Earth’s centre and the point in Earth’s history at which the solid inner core first started to “freeze” out from the cooling molten outer core.

“The results suggest that the Earth’s core is cooling down less quickly than previously thought which has implications for the whole of Earth Sciences,” Dr Biggin added.

It also suggests an average growth rate of the solid inner core of approximately one mm per year which affects our understanding of the Earth’s magnetic field.

The Earth’s magnetic field is generated by the motion of the liquid iron alloy in the outer core, approximately 3,000 km beneath the Earth’s crust.

These motions occur because the core is losing heat to the overlying solid mantle that extends up to the crust on which we live producing the phenomenon of convection.

The theoretical model which best fits the new data indicates that the core is losing heat more slowly than at any point in the last 4.5 billion years.

This flow of energy should keep the Earth’s magnetic field going for another billion years or more.

“This contrasts sharply with Mars which had a strong magnetic field early in its history which then appears to have died after half a billion years,” Dr Biggin pointed out.

The study was published in the journal Nature.