Peering into the history of Earth’s formation
Although we walk on it every day, very few people consider what exactly our planet is. Earth’s composition can be reduced to four main layers. Working from the outside in, these layers are the crust, the mantle, the outer core, and the inner core. This last layer, the inner core, is a superhot solid sphere of iron that sits at the center of Earth. The mechanism of the inner core’s formation has recently come under scrutiny, and a newly posited explanation provides insight into Earth’s genesis.
The conventional theory about the formation of the Earth’s inner core, also known as the nucleation or accretion event, suggests that a massive pool of molten iron spontaneously crystallized into a giant, solid, spherical core surrounded by a layer of liquid metal. This idea has been generally accepted for approximately 80 years. It is based on the assumption that, at some point in time, the temperature of the molten iron dropped to a point lower than its melting point, allowing the molten metal to undergo a spontaneous phase transition from liquid to solid.
However, researchers at Case Western Reserve University recently published a new study that questions this traditional explanation. They attempted to answer the so-called “inner core nucleation paradox,” which asserts that the energetic barrier to the nucleation of the molten iron—the amount of energy necessary for this phase transition—is too high for spontaneous crystallization to have occurred in the way that the conventional theory suggests.
The new study agrees with convention in that nucleation requires the molten iron to have been supercooled well below its melting point, a chemical process in which a compound can exist in a liquid state even at a temperature below its freezing point. It argues, however, that in order for spontaneous accretion to occur, the molten core must have been supercooled by nearly 1000 Kelvin, a temperature that is not possible for a body as massive as the Earth’s core. Thus, the researchers reasoned that there must be another mechanism at play.
The study posits that the introduction of a low-energy substrate, such as a piece of solid iron, into the supercooled molten metal is a plausible event that could have occurred simultaneously with supercooling. This substrate could adequately lower the energetic barrier to nucleation and initiate crystallization without supercooling the core by 1000 Kelvin.
“The nucleation event could be similar to the introduction of an ice cube into supercooled water” said Dr. Ludovic Huguet, principal investigator in this new study. In this scenario, the ice cube serves as the substrate that initiates the phase transition of the supercooled water to solid ice.
The study hypothesizes that this substrate could have been an iron nugget that broke off from the Earth’s mantle and made its way into the molten iron at the center of the planet. However, the researchers admit that while this event is plausible, it is extremely rare. The iron nugget must have been large enough to withstand disintegration over the course of its trajectory towards the center of the Earth and enter the molten core intact. More specifically, it must have had a minimum radius of about 9 kilometers, or 5.6 miles.
With this study completed, Dr. Huguet intends to broaden the scope of his research and look past planet Earth, into the extensive universe. He is now doing research on energy barriers and nucleation events on other planets in his quest to understand how exactly planets’ cores are formed. “Presently, I am investigating the consequences of the nucleation barrier for other planets where their cores have a regime of crystallization different than that of the Earth” he added.
Although this new nucleation theory comes with its probabilistic limitations, it marks a next step forward in understanding the true geologic history of the Earth. “The formation of the inner core is only one piece of the puzzle of the thermal history of the Earth,” said Dr. Huguet. In other words, a solid understanding of this event could allow geologists to unlock more secrets about our planet’s rich past.