When did Earth’s inner core begin to form? There has been much debate over this question with estimates ranging from 0.5 billion years ago to four billion years ago. A new assessment of ancient rocks suggests that the Earth’s molten core began to solidify around 1.3 billion years ago.

The inner core is the deepest layer of the Earth- the center or pit of the planet. It is slightly larger than the planet Pluto.

The emergence of the inner core had profound effects on Earth’s development. As the core cooled, the iron-nickel alloy it is made of started to generate a greater and greater magnetic field. This magnetism exists to this day and is responsible for the North and South Poles.

Scientists have long searched for some sign of magnetism in rock formations- any sort of recording of the inner core’s solidification.

“The timing of the first appearance of solid iron or ‘nucleation’ of the inner core is highly controversial, but is crucial for determining the properties and history of the Earth’s interior,” said lead author Dr Andy Biggin of the University of Liverpool, UK.

“Studying the magnetism of ancient rocks is a huge scientific challenge, because old rocks can lose their magnetic memory, or the magnetic signals they carry can become overwritten and corrupted (just like the files on your hard drive),” said Dr Richard Harrison, University of Cambridge. “However, it is one of the best ways to look for concrete evidence of when the core started to solidify.

It is not hard to imagine what would have happened to our planet had the inner core not developed in this way. Our planetary neighbor, Mars, saw the opposite transition- from the start it had a strong magnetic field. Yet over time, the magnetism weakened then died out all together around four billion years ago. As far as we know, Mars is a barren and desolate planet.

Our inner core is a perpetual motion machine of sorts, generated in part by the constant turbulence of the molten iron outer core. In turn, much of the outer core’s turbulence is caused by excess heat escaping upwards from the inner core by convection.

The outward core is also affected by the slow solidification of the inner core. As the iron and nickel of the inner core begins to freeze, it expels light and buoyant impurities into the outer core. This accelerates the convection and strengthens the magnetic field.

“The theoretical model which best fits our data indicates that the core is losing heat more slowly than at any point in the last 4.5 billion years and that this flow of energy should keep the Earth’s magnetic field going for another billion years or more,” said Dr. Biggin.

The resultant magnetic field not only aligns our compasses but also protects the planet from many solar storms produced by the sun. The field acts as a deflector of the Sun’s harmful ultraviolet radiation. Without it, the UV rays would strip away the ozone of Earth’s upper atmosphere.

These cosmic storms can be seen where the magnetic field is weakest, around the North Pole. Indeed, Aurora Borealis is a display of UV rays sent from the Sun.

It is only because our planet had this protective shield that life has managed to survive over hundreds of millions of year.

“Although data are scarce, this study applied strict quality controls to decide which data were the most reliable and then used statistics to demonstrate that a boost to Earth’s the magnetic field occurred 1,300 million years ago. If this turns out to be the elusive signature of inner core growth, then we may have to revise our ideas about the core yet again!” said Dr. Harrison.

The paper was reported in Nature Journal.