The Earth's deepest layer, the inner core, is capped by snow made of tiny particles of iron, which are much heavier than the snowflakes seen in the atmosphere, a new study says. According to the study, published in the journal JGR Solid Earth, the iron-snow falls from the molten outer core, and piles up on top of the inner core, creating stacks up to 200 miles thick which cover the innermost layer of the planet.
The researchers, including those from the University of Texas at Austin in the US, studied this layer by recording and analysing signals from seismic waves as they passed through the Earth. In doing so, they found variations between recent seismic wave data, and the values they expected based on mathematical models of the Earth's core, raising new questions.
One of their observations was that the seismic waves moved much slower than expected when they passed through the base of the outer core, and they moved faster than estimates as they passed through the eastern hemisphere of the top inner core. Based on these findings, the study proposed the iron snow-capped core as an explanation for these aberrations.
Earlier studies had pointed that a slurry layer exists between the inner and outer core, however, the existing knowledge at that time about heat and pressure conditions in the core quashed that theory, the researchers said.
With new data from experiments on core-like materials, the scientists found that crystallisation was possible and that about 15 per cent of the lowermost outer core could be made of iron-based crystals. They said these could eventually fall down the liquid outer core and settle on top of the solid inner core.
"It's sort of a bizarre thing to think about. You have crystals within the outer core snowing down onto the inner core over a distance of several hundred kilometres," said study co-author Nick Dygert from the University of Tennessee in the US.
According to the researchers, accumulated snowpack may be the cause of the seismic aberrations. They said the slurry-like composition of the snowpack slows the seismic waves in such a way that the variation in snow pile size -- thinner in the eastern hemisphere and thicker in the western -- affected the speed of the waves.
"The inner-core boundary is not a simple and smooth surface, which may affect the thermal conduction and the convections of the core," study co-author Youjun Zhang from Sichuan University in China.
Since the core has significant influence over processes that affect the entire planet -- from generating its magnetic field to radiating the heat that drives the movement of tectonic plates -- the scientists said, understanding more about its composition could help in unravelling mysteries about these larger processes.
