by Mitch Battros

April 1, 2016

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The Earth’s magnetic field permanently protects us from the charged particles and radiation produced by our Sun. This shield is created by the geodynamo process, the rapid motion of huge quantities of liquid iron alloy in the Earth’s outer core. To maintain this magnetic field until the present day, the classical model required the Earth’s core to have cooled by around 3,000° C over the past 4.3 billion years.

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Now, a team of researchers from French National Center for Scientific Research (CNRS) and Université Blaise Pascal1 suggests that, on the contrary, its temperature has fallen by only 300° C. The action of the moon, overlooked until now, is thought to have compensated for this difference and kept the geodynamo active. Their work is published in the March 30 2016 journal Earth and Planetary Science Letters.

The classical model of the formation of Earth’s magnetic field has raised a major paradox. For the geodynamo to work, the Earth would have had to be totally molten four billion years ago, and its core would have had to slowly cool from around 6800° C at that time to 3800° C today. However, recent modeling of the early evolution of the internal temperature of the planet, together with geochemical studies of the composition of the oldest carbonatites and basalts, do not support such cooling. With such high temperatures being ruled out, the researchers propose another source of energy in their study.

The gravitational effects associated with the presence of the Moon and Sun on Earth induces cyclic deformation of the mantle and oscillations of the axis of rotation. This mechanical forcing applied to the entire planet induces strong currents in the outer core made of an iron alloy of very low viscosity. These currents are sufficient to generate the Earth’s magnetic field.

The Earth has a slightly flattened shape and rotates about an inclined axis that wobbles around the poles. Its mantle deforms elastically due to tidal effects caused by the moon.