Earth's Core
How fast does it rotate?
Artist's impression of Earth's core and inner layers
Scientists from the University of Cambridge (UoC), for the very first time, have come up with the first accurate estimation of how fast the Earth's inner core is rotating compare to the rest of the planets in our solar system. Although previous researches have shown that Earth's core is rotating but, this recent study, of which details have been published in the 20th February edition of the journal Nature Geoscience, depicts a more exact picture. It has been discovered by the researchers from UoC that the previous estimates of the rotation of Earth's core by 1 degree every year were in fact inaccurate and the core of our home planet is actually moving much slower than that. To be precise, the rate is approximately 1 degree, every million years. The inner core is 5200 km beneath our feet and the effects of its presence is one of great importance on Earth's surface. Because, as the inner core grows, the heat released during solidification drives convection in the fluid in the outer core and this convection generates Earth's geomagnetic field. And without this magnetic field, the surface of Earth would not be protected from solar radiation and hence life on this 3rd planet from the Sun would not be able to exist. It is also key to mention here that the inner core grows very slowly over time as material from the fluid outer core solidifies onto its surface and during this very process, an east-west hemispherical difference in velocity is frozen into the structure of the inner core. And according to Lauren Waszek, lead author of the paper and PhD student from the UoC's Dept. of Earth Sciences, "The faster rotation rates are incompatible with the observed hemispheres in the inner core because it would not allow enough time for the differences to freeze into the structure". The researcher further explains that, this very thing of nature has been a major problem since the two properties cannot coexist. And this prompted the researchers to derive the rotation rates from the evolution of the hemispherical structure which exclusively made this study the very first one in which the hemispheres and rotation are inherently compatible. For the purpose of the research (which was a rigorous one), the scientists used seismic body waves which pass through the inner core that is 5200 km beneath the surface of Earth and compared their travel time to waves which reflect from the inner core surface. And the difference between the travel times of these waves provided the team with the velocity structure of the uppermost 90 km of the inner core. And this particular information was reconciled with the differences in velocity for the east and west hemisphere of the inner core. At first, the scientists observed the east and west hemispherical differences in velocity and this was followed by constraining of the two boundaries which separate the hemispheres. And this process of examination discovered that they both shifted consistently eastward with depth. And since the inner core grows over time, therefore the deeper structure is older and the shift in the boundaries between the two hemispheres results in the inner core rotating with time. And the rotation rate is consequently calculated by the research team from the shift of the boundaries and the growth rate of the inner core which eventually lead to the research finding. In conclusion, Lauren Waszek thinks "This result is the first observation of such a slow inner core rotation rate. It therefore provides a confirmed value which can now be used in simulations to model the convection of the Earth's fluid outer core, giving us additional insight into the evolution of our magnetic field".
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