Earth's Magnetic Field May Not Have Existed Without The Moon

Our silent guardian, our watchful protector. Gregory H. Revera/Wikimedia Commons;CC BY-SA 3.0

Earth’s magnetic field is our greatest vanguard: It protects us against dangerous incoming solar radiation that, left unchecked, would make the existence of life on our world all but impossible.

And now a new study in the journal Earth and Planetary Science Letters has suggested something truly remarkable: our world's magnetic field could not be sustained without the help of the Moon. This has enormous implications for how scientists not only view planetary formation, but how they may look for life elsewhere in the cosmos.

“In this scenario... the habitability on Earth appears to require the existence of a large satellite,” wrote the team of researchers, led by Denis Andrault from the Université Blaise Pascal.

The physics of Earth’s magnetic field is not entirely understood, but it’s almost certainly generated within the outer core layer of the planet, which contains broiling, liquid metallic iron and nickel. The heat escaping from the depths up to the surface sets up cycling convection currents within the outer core, and this movement of magnetic materials generates a powerful magnetic field that extends far beyond the surface of the planet.

When charged particles from the Sun travel towards us – a phenomenon known as “solar wind” – our magnetic field deflects these particles, preventing them from stealing away our atmosphere. Without an atmosphere, these particles would bombard Earth’s surface, making life far less likely to exist, if not impossible.

Earth’s magnetic field is one of the many reasons life is possible at the surface. Marc Ward/Shutterstock

As this study reveals, however, there’s a catch. In order for there to be convection in the outer core, there needs to be a large temperature difference from the inner core to outer core, and the outer core to the mantle, the semi-molten layer that makes up around 84 percent of Earth’s volume. If there is no major temperature difference, there will not be efficient convection, and there will be no magnetic field.

Conventional models state that the Earth's core needs to have very slowly cooled from 6,800°C (12,272°F) to 3,800°C (6,872°F) for there to be efficient convection. After carefully sifting through experimental studies, mathematical models, and long-touted arguments about the current condition of Earth’s innards, the authors conclude that Earth's depths have only cooled by around 300°C (572°F) since its fiery birth 4.54 billion years ago. This implies that Earth’s magnetic field isn’t being generated by conventional convection.

So where is it coming from? Well, as it turns out, our Moon may be our silent guardian. Although it’s weak, its gravitational pull on Earth may be responsible for not only generating tides at the surface, but also deep within the outer core by mechanically “stirring” its liquid contents.

This tidal mechanism has already been observed causing powerful volcanism on Jupiter’s Io and Saturn’s Enceladus. On Earth, the pull of the Moon is far less powerful, but still manages to generate around a trillion watts of power within the outer core.

The new model: Over time, the convection switches from being thermally driven to being driven by the Moon's tidal forces. Andrault et al./EPSL

This would be enough to mechanically force efficient convection in the outer core, meaning that our Moon – working with the metallic, liquid core – may be responsible for sustaining our magnetic field, without which we would not exist. “We found out that this possible effect of the Moon had already been proposed about 50 years ago,” Andrault told IFLScience. “However, no one since then had argued that this effect could be important.”

The enigmatic lack of a magnetic field on Mars may be one of the reasons that complex life failed to evolve on the Red Planet. Perhaps, as the study suggests, it didn’t have the right kind of moon watching over it.

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