Deep below Earth’s surface, at a staggering depth of over 5,100 km, lies the inner core a solid ball of iron and nickel. Despite its size and influence, this mysterious part of Earth still puzzles scientists. While we know it plays a crucial role in shaping the conditions necessary for life on Earth, how and when the inner core formed remains a topic of ongoing debate.
The Earth’s inner core is responsible for generating the planet’s magnetic field, which shields us from harmful solar radiation. This magnetic field is likely key to creating the conditions that allowed life to flourish billions of years ago. Without it, our planet may not have been able to sustain life.
Interestingly, the inner core wasn’t always solid. As Earth has cooled over time, the once-liquid core has been gradually solidifying, with its temperature still estimated at over 5,000 K (around 4727°C). This slow freezing process continues today, as the inner core expands outward while surrounding iron-rich liquid crystallizes.
Freezing and Supercooling: The Puzzle of the Inner Core’s Formation
When the inner core freezes, it releases elements like oxygen and carbon into the liquid outer core. This dynamic process generates electric currents, creating Earth’s magnetic field through dynamo action. Yet, understanding the exact process of this freezing and the time at which the inner core solidified is complex.
One of the key challenges lies in the concept of supercooling a phenomenon where a liquid cools below its freezing point without turning into a solid. In the case of Earth’s core, mineral physicists have calculated that the freezing of iron would have required supercooling up to 1,000 K below its melting point. This presents a major hurdle since such extreme supercooling is unlikely, and would imply that the inner core has been below its melting point throughout Earth’s entire history.
What We Know From Seismology
Because humans have only drilled 12 km into the Earth far from the core we rely heavily on seismology to understand the deep Earth. The discovery of the inner core in 1936 provided vital clues. Its size, about 20% of Earth’s radius, is one of the most well-constrained properties of our planet’s interior. Seismologists use this information to estimate the temperature at the boundary between the solid inner core and the liquid outer core, offering insight into when and how the core may have solidified.
However, models predicting the degree of supercooling have struggled to align with seismological data. If the core had been supercooled by 1,000 K before freezing, the inner core should be far larger than observed. Conversely, if it never reached that level of supercooling, the inner core shouldn’t exist at all. Clearly, a more nuanced explanation is required.
Recent studies by mineral physicists have started to shed light on this enigma. Testing different iron alloys and mixtures, researchers have found that unexpected crystal structures and the presence of carbon might reduce the amount of supercooling needed to initiate freezing. These results suggest that certain chemical properties may allow the core to freeze with less than 400 K of supercooling, explaining the current size of the inner core.
Implications for Earth’s Magnetic History
The age of the inner core is another aspect clouded by uncertainty. Previous estimates ranged between 500 and 1,000 million years, but these don’t account for the supercooling issue. Even modest supercooling could suggest the inner core is significantly younger than previously thought.
This has far-reaching implications for our understanding of Earth’s magnetic field and the conditions necessary for life. The paleomagnetic record Earth’s historical magnetic data preserved in rocks can offer insights into how solar radiation may have impacted life and mass extinctions. To fully comprehend the role of the magnetic field in Earth’s habitability, we must first unravel the inner core’s formation.
While we are closer than ever to understanding the mysteries of the inner core, significant questions remain. The combination of mineral physics, seismology, and geophysical modeling will continue to provide clues to how this vital part of Earth evolved. Unraveling the formation of the inner core may ultimately unlock key answers about the Earth’s past and its role in the development of life as we know it.
Reference: This article is republished from The Conversation under a Creative Commons license. Read the original article.