The iron-rich core at the heart of our planet was an important part of Earth’s evolution. The core not only moves magnetic fields that protect the atmosphere and oceans from solar radiation, but also affects plate tectonics, which have continuously reshaped the continent.
However, despite its importance, many of the core’s most basic characteristics are unknown. We don’t know exactly how hot the core is, what it is, or when it started to freeze. Luckily, recent discoveries by me and my colleagues bring us much closer to answering all three of these mysteries.
The temperature of the inner core of the Earth is known to be approximately 5,000 Kelvin (k) (4,727°C). Once a liquid, it cooled and became solid over time, expanding outward in the process. When it cools it releases heat into the mantle above, driving the current behind the plate tectonics.
You might like it
This same cooling also generates the Earth’s magnetic field. Most of the energy in today’s field comes from freezing the liquid portion of the core and growing the solid inner core at its center.
However, since the core is not accessible, its properties must be estimated to understand how it is cooled.
An important part of understanding the core is knowing its melting temperature. We know that the boundary between the inner core of solids and the outer core of liquids is from seismology (seismic studies). The temperature of the core must be equal to the melting temperature at this location. This is because it is a frozen place. So, if you know exactly what the melting temperature is, you can get a closer look at the exact temperature of the core and what it is made of.
Mystical Chemistry
Traditionally, there are two ways to figure out what the core is made of: metstone and seismology. By examining the chemistry of metstones, which are thought to be fragments of unformed planets, or fragments of the core of planets like the destroyed Earth, we can see what our nuclei can make.
The problem is that this only gives us a rough idea. Metstones show that the core should be made of iron and nickel, and perhaps a few percent of silicon or sulfur, but it is difficult to be more specific.
On the other hand, seismology is much more concrete. As sound waves from an earthquake pass through the planet, they speed up and slow down depending on the material they pass through. From earthquakes to seismometers, you can learn what the interior of the Earth is by comparing the travel time of these waves, and the speed of waves passing through minerals and metals in experiments.
At these travel times, we can see that the Earth’s core is about 10% less denser than pure iron, and the liquid outer core is more denser than the solid inner core. Only known chemicals in the core can explain these properties.
You might like it
However, even within the selection of a small number of possible components, the potential melting temperature varies from hundreds of degrees.
New constraints
Our new study uses mineral physics to study the possibility that the core might have started to freeze first, and to discover new ways to understand the chemistry of the core. And this approach appears to be even more specific than seismology or metstones.
Studies simulating how liquid metal atoms bond to form solids have found that some alloys require more intense “supercooling” than others. Supercooling is when the liquid is cooled below the melting temperature. The stronger the supercooling, the faster the atoms bond together to form a solid, and the liquid freezes faster. Freezer water bottles can be supercooled to -5°C for several hours before freezing, but form a cleat in minutes where cloud droplets are cooled to -30°C in the cloud.
By investigating all possible melting temperatures of the core, we can see that the core may have been the most supercooled, below about 420°C above the melting temperature. However, pure iron requires something impossible to supercool 1000°C to freeze. If this had been cooled this far, the whole core would have been frozen, contrary to what a seismologist observed.
Adding silicon and sulfur can both metstone and seismology in the core, which only exacerbates this problem, and requires more supercooling.
Our new research explores the effects of core carbon. If 2.4% of the core mass is carbon, supercooling of about 420°C is required to begin freezing the inner core. This is the first time that cores have been shown to be freezing. If the core carbon content is 3.8%, only a supercooling 266°C is required. There are still a lot of this, but it’s much more plausible.
This new finding shows that seismology can narrow down the potential chemistry of the core to a number of different elements, while many of these cannot explain the existence of solid inner nuclei at the center of the planet.
The core cannot be made from iron and carbon alone, as at least one element is required for the seismic properties of the core. Our study suggests that it is likely to contain a bit of oxygen and perhaps silicon.
This marks an important step in understanding what the core is made of, how it began to freeze, and how it formed our planet from within.
This edited article will be republished from the conversation under a Creative Commons license. Please read the original article.
Inner Earth Quiz: Test your knowledge of the hidden layers of our planet
Source link
