This entry is part 1 of 3 in the series Structure of the Earth Theme Week

The Inner Inner

Most children glean a bit about the geologic makeup of our planet in primary school.

We learn the Earth is comprised of four layers: the crust, the mantle, the outer core, and the inner core.

The basic construction of our planet by layer - image by BBC

The crust is the layer on which we live and walk. It is typically between 5 and 70 kilometers thick. The thinnest parts are the oceanic crust; conversely, the thickest regions are the continental areas that rise miles above the oceans.

Below the crust is the mantle, the thickest layer, extending to depths of 2,890 kilometers. Though solid, the mantle is far hotter than the crust, which enables it to flow on very long timeframes. Convection of the mantle is responsible for plate tectonics. You can think of the crust, divided into plates, as “floating” on the mantle, which slowly moves due to convection.

Then we have the inner and outer core, the center of the planet. The inner core is solid and has a radius of about 1,220 kilometers. The outer core, on the other hand, is liquid and extends to a radius of about 3,400 kilometers.

For many moons, this basic model has been widely accepted as an accurate representation of the planet.

A pretty picture of the dynamics of the core portions interacting with the mantle - image by Andrew Colvin

How do we know the inner core is solid and the outer core is liquid?

The deepest we’ve ever drilled is just over 12 kilometers, so we’ve never even come close to sampling the core. Yet scientists are certain part of it is liquid and part is solid. The answer arrives thanks to earthquakes.

Seismic activity is so strong that the waves travel back and forth through the entire planet. By monitoring the waves as they move through the planet, seismologists can actually study what happens to them as they make laps through the material of the Earth. The wave action tells us the inner portion of the core is solid, while the outer part is liquid.

Differences in seismic wave paths through layers of Earth - image by Stephenson et al.

Based on some of the data coming from seismic waves, certain scientists theorized we might not have just four layers inside the planet. Specific waves hinted at a third section of the core, specifically the inner core.

A new study from the Australian National University recently confirmed the existence of what they are calling Earth’s “innermost inner core.”

The study utilized data from decades past but featured new technology to unlock an unclear puzzle. The study’s lead geophysicist, Joanne Stephenson, said, “We got around this [puzzle] by using a very clever search algorithm to trawl through thousands of the models of the inner core.”

This graphic might need an extra layer - image by forplayday

While the difference between four layers and five layers might seem trivial or inconsequential, the discovery of the “innermost inner core” might actually have significant implications for any scientific branch that deals with the history of the earth.

Two separate portions of the inner core suggest at least two different phases of evolution for the planet, in terms of cooling and construction.

“We found evidence that may indicate a change in the structure of iron, which suggests perhaps two separate cooling events in Earth’s history,” Stephenson said. “The details of this big event are still a bit of a mystery, but we’ve added another piece of the puzzle when it comes to our knowledge of the Earth’s inner core.” 

Read the study and some more about the new old inner inner core below!

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