Tug-Of-War Magnetism

I feel like I’ve been covering a lot of stories on magnetic fields over the past few months.  Fields around the Earth, the Sun, Mars, Jupiter’s Moons, and exoplanets are just some of the places in the universe where we are looking at magnetic field behaviour.  The intention is to use our understanding of magnetism to figure out what is inside these worlds, and how they interact with their space environment.

The gravitational effects associated with the presence of the Moon and Sun cause cyclical deformation of the Earth’s mantle and wobbles in its rotation axis. Credit: © Julien Monteux and Denis Andrault

You would expect us to understand the Earth’s magnetic field and interior very well, after all, we are stuck here.  But it turns out it’s very difficult to study the interior of any planet, even our own.  We have had working models of the Earth’s core for decades, but it wasn’t until recently that we had a satisfactory theory to explain its magnetic field.

The problem is finding the energy to keep Earth’s magnetic field going for so long.  The general idea is what we call magnetic induction, originally discovered by physicist Michael Faraday.  It simply states that a moving charge will create a magnetic field.  So if you have a liquid metallic core, made of Iron (like the Earth), it will continually move electrons around, inducing a magnetic field.

The only problem is that to power the magnetic field we measure, the core would have had to be 3,000 degrees hotter than it is today.  It certainly could have been that hot in the past, and it could have cooled since then. And I know that cooling something by 3,000 degrees doesn’t seem that difficult over a period of 4.3 Billion years, but when you have an entire planet to cool, it’s a tall order.  Luckily, like any other phenomenon, it has effects that can be measured.  By looking for this evidence, specifically in the composition of ancient volcanic rocks, scientists have shown that the cooling just hasn’t happened.  So where does the core get the energy to drive the magnetic field?

The answer lies in the Sun and Moon.  As the Moon orbits the Earth, and this system orbits the Sun, there is a gravitational tug-of-war that takes place.  We see this in the ocean tides, as the liquid water in the oceans is more easily distorted by gravity than the solid surface.  The reality is that the Sun and Moon also churn the liquid core of the Earth, driving the movement of charge and the resulting magnetic induction.  Researchers from France recently showed that this back-and-forth provides over 3,000 Billion Watts of power! This is more than enough to sustain the magnetic field of the Earth over its entire history.

Looking at the energy in these types of interactions puts the immensity of the solar system into perspective.  It’s one thing to understand how huge the Earth is, but another thing completely to think about the amount of energy released in its gravitational interactions.  For reference, humanity uses 18,000 Billion Watts of power on average (this was in 2013).  If you’ve been following along, that’s six times the power output of the Sun-Moon-Earth tug of war.  Humanity definitely has an impact on this planet, the signs are there. And our influence is growing along with our consumption.

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