Do magnetic fields really care about what's in a planet's core?

Posted by Jake Parks
on Monday, January 29, 2018

Guest blog by Sharmila Kuthunur.

In this artist concept from NASA, Earth’s magnetic field extends through field lines eminating from the North Pole (orange lines) and connecting to the South Pole (blue lines). Though Earth’s molten iron core contributes to the strength of our magnetic field, research indicates that spin may be an even more important factor. //NASA Goddard's Scientific Visualization Studio

Although the geodynamo residing in Earth’s core is critical to maintaining our protective magnetic field, the composition of the core itself seems to have little to do with the structure and alignment of the field. Earth’s rotation, on the other hand, plays a surprisingly important role.

The very existence of a magnetic field depends on there being enough energy to drive the dynamo, which in Earth’s case comes from the convection currents that carry heat from the core to the surface. But previous research has shown that in addition to needing an energy source for the dynamo, a planet must be rotating.

“The fact that Earth is rotating doesn’t affect the energy budget — but likely it does affect what kind of magnetic field is generated,” said Phil Livermore, a professor at the University of Leeds in the UK, whose research interests focus on predicting the internal magnetic field of our planet.

As a result of Earth’s rotation, the Coriolis force acts on objects in motion and aligns the molten iron in Earth’s core so that it is parallel to Earth’s rotation axis, which gives rise to a dipolar field. However, the fact that Earth’s core is iron has scant relevance when it comes to the generation of its magnetic field. In the chunk of our solar system where ices and volatiles dominate rocky materials (Jupiter and beyond), non-iron cores also contribute to the magnetic fields.

“Gas giants do likely have a solid silicate region at their center that can be referred to as a core, but that is not where they generate their magnetic fields,” said Jonathan Mound, Associate Professor in Geophysics at the University of Leeds in the UK.

In Jupiter and Saturn, the magnetic fields are produced in places where high pressure causes the hydrogen to “metallize,” whereas, for Uranus and Neptune, the magnetic fields are generated in fluid regions that are mainly composed of methane and ammonia. The conductivity of these elements, along with the rotation of the planets, gives rise to a strong, steady, and a complex magnetic field.

A schematic of Jupiter's magnetosphere. Magnetic field lines are shown in blue. //John Spencer

There’s also good news for the lazy planets out there too!

“In practice, planets do not need to be rotating very fast. They just need to be rotating fast enough for the Coriolis force to be important. Even if the Earth was rotating a million times more slowly than it currently is, it would still easily be rotating fast enough for the Coriolis force to be important in its outer core,” explains Professor Mound. Astronomers can witness this effect by observing planets that are tidally locked to their host star.

For example, the recently discovered TRAPPIST-1 System has all its planets tidally locked to an ultra-cool red dwarf star. These planets should not be producing magnetic fields because they do not rotate in the classical sense. Yet, in the frame of reference of the planets, they do rotate once about their axes each time they orbit their star. That minimal rotation is enough for the Coriolis force to present itself. However, the kind of magnetic fields that this produces can be completely different than Earth’s.

“For bodies which do not rapidly rotate, the magnetic field may be much more complex and have no obvious preferred alignment,” said Professor Livermore.

Though the planets do not rotate substantially, the little rotation that is there swirls their magnetic fields in random directions, giving rise to multiple poles of a complex nature. Since the innermost planets of the TRAPPIST system are similar to Earth in almost every aspect, including the iron cores, it is plausible that they could generate considerable magnetic fields through a geodynamo like Earth’s.

So there you have it: Iron cores or otherwise, as long as a planet is rotating, a magnetic field will likely exist. And since a strong magnetic field is essential to hold an atmosphere in place, this means life has the chance to thrive on even the most slowly rotating of planets.

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