Sound is now our best tool for looking deep inside our world, but scientists have long been confused by some of their observations – what we’re hearing isn’t quite what the theory says we should be hearing. A team at the University of Texas has come up with a theory that may explain these contradictions, suggesting there is something akin the iron ‘snow’ at the core.
Obviously, scientists are unable to observe the Earth’s core directly. It is found about 3000 km below the planet’s surface (with the centre even further down, at more than double that), and has temperatures ranging from 4400°C (the outer core) to 6100°C (near the inner core). The deepest hole humans have ever created on the surface is the Kola Superdeep Borehole in Russia, and that’s only 12.3 km deep, so we have got a lot of mining to go.
The deepest hole humans have ever created on the surface is the Kola Superdeep Borehole in Russia
So, if we can’t get down there, how do we know what’s there? One way is to use the Earth’s mass, which can be calculated by observing how the planet’s gravity interacts with objects on the surface – this gives you a mass of 5.9 sextillion tonnes (59 followed by 20 zeroes). Because the material on the surface is much lower than the Earth’s average density, it logically means the centre must be much denser. From this, we can figure out which materials make up the core – given how common it is in the universe, the answer is likely a load of iron, crushed to an extremely high density.
The other major question (and the most pertinent for our purpose) is how we know the size of the core, and the answer is seismology. When an earthquake happens, it sends shockwaves throughout the planet, which scientists record. Depending on the route these vibrations take, they will pass through different parts of the Earth, which affects how they ‘sound’ at the other end. Early in the history of seismology, a number of waves went missing – it transpires that these S-waves could only reverberate through solid material and not liquid, thus confirming a molten core in the Earth’s centre. Later observations refined this, revealing a solid inner core and a molten outer core.
When an earthquake happens, it sends shockwaves throughout the planet, which scientists record.
It is here that discrepancies have started to show between seismic data and current models of the Earth’s centre. Waves appear to move slower than expected the base of the outer core and faster than expected through the top of the inner core in the eastern hemisphere. The observational data suggests that the core is covered in something viscous which doesn’t coat the core all the way around. Cue the Texas team, who have posited the iron ‘snow’ theory – iron-based crystals could fall down and pile up on top of the solid inner core, some of which could be up to 200 miles thick. The uneven distribution means that the inner-core boundary is not a simple and smooth surface, affecting how waves pass through, as well as the thermal conductive and convection properties.
This ‘snow’ observation has several benefits – it helps explain the wave discrepancies and could explain why the core isn’t a perfect sphere. The core influences the Earth’s magnetic field and radiates the heat that drives the movement of tectonic plates, so more knowledge of how it operates will factor into our knowledge of how the Earth works and according to other geoscientists, it could also shed light on the formation of the Earth’s core. Understanding how the Earth came to be is an important factor in understanding how it developed. The idea of snow in the Earth’s centre may sound a bizarre one, but it could fundamentally change how we perceive our world.