Of the three broad stratifications that Earth can be divided into, the crust (outermost) and the mantle (in the middle) are rich in silicates and act as something like a shell around the inner core, which is highly metallic. While we have known that for long, there is no consensus as to how the planet got a metallic core.

In fact, all terrestrial planets in the solar system have metallic cores and scientists have struggled to explain how that came to be. A new theory, published Monday in the journal Proceedings of the National Academy of Sciences, suggests it happened as molten metal percolated to the inner depths of rocky planets, through gaps between grains of rock.

Earlier theories about the behavior of metals under the intense heat and pressure conditions of planetary formation proposed that at the time when planets were forming, some molten metal remained trapped in the gaps between rocky grains that were isolated from each other. The new research posits that once the isolated pores grow large enough to connect with each other, the trapped molten metal starts to flow and, making its way along the grain boundaries, trickles down through the mantle and accumulates in the core.

"What we’re saying is that once the melt network becomes connected, it stays connected until almost all of the metal is in the core," study co-author Marc Hesse, an associate professor at the University of Texas, Austin, said in a statement Monday.

The research was the doctoral thesis of Soheil Ghanbarzadeh, who earned his Ph.D. at the university. For his thesis, he developed computer simulations of molten rock distribution between grains of rock as the porosity increased or decreased. The researchers found that even when porosity reduced significantly, metal continued flowing, which was in stark contrast with earlier simulations and theory.

Melt Networks Images of melt networks in irregular grains (a) used in a study by The University of Texas at Austin and melt networks in regular grains (b) used in previous studies. Their simulations show that irregularity of grains promotes connectivity of the melt. Photo: UT Austin

For the simulation, Ghanbarzadeh used irregular grain geometry, thought to better reflect real-life conditions. Earlier simulations relied on regular, identical grains that produced a geometrical pattern. The new model showed only 1-2 percent of the initial metal remaining in the silicate mantle after the flow and percolation stopped, which is consistent with how much metal Earth’s mantle has.

"What we did differently in here was to add the element of curiosity to see what happens when you drain the melt from the porous, ductile rock," Ghanbarzadeh said.

The study was titled "Percolative core formation in planetesimals enabled by hysteresis in metal connectivity" and was also co-authored by Maša Prodanovic, who along with Hesse, was Ghanbarzadeh’s adviser.