The world “pole” tends to mean the “end” of something. If you and I are polar opposites, we’re as far from one another as the space we’re in will allow us to be — and that’s a common sense definition. After all, the most famous poles by far are the North and the South poles of the Earth, which are oriented at opposite ends of a slightly elongated sphere. These locations are by far the most noticeable things about the planetary poles — but it’s not what defines them. A new paper published this week in Geophysical Research Letters gets at this uncertainty with a startling new idea: not only have the Earth’s magnetic poles not always been at the ends of the planet, but there hasn’t even always been two of them!
What a planetary magnetic pole is is an area of a planet’s surface where the “lines” of that planet’s intrinsic magnetic field are perpendicular to its surface. In other words, it’s any place where the field lines that we draw to represent the force of a magnetic field are straight up and down, relative to the portion of the surface they’re crossing. In a two-lobed magnetic field like the Earth’s, called a “di-polar” system, the poles will naturally fall at the end — but a di-polar system is not the only system that can come about.
The Earth’s magnetic field, and all other planetary magnetic fields, come about as the result of something called the planetary dynamo. All this is is the layers of the Earth rotating a very slightly different speeds than one another — in particular, the relative motion between layers of the planet’s inner crust, so-called “convection” of molten iron. As the iron circulates through the molten portions of the planet, it moves around the planet’s solid core.
In an electrical generator, we have a magnetized system that we cause to move — by introducing motion into a electrically conductive system with a stable magnetic field, we get a constant flow of electrons. Power! In the Earth’s magnetic dynamo, it works the other way around — we have the movement of electrons in the electrically conductive iron, and we have the physical motion caused by convection of that iron within the Earth’s crust. The result is a stable magnetic field.
Now, notice that the Earth’s dipole is not perfectly aligned with the axis of rotation — it’s actually off by about 11 degrees. The reason for this is that the rotation and convection that cause this magnetic field are very complex, involving the interplay of heat, friction, and the Coriolis effect on rotating bodies. Today, with a fairly simple molten layer over a solid core, this results in nothing more than a slight misalignment with the planet’s rotational axis. But the Earth wasn’t always like this.
Back in the planet’s early history, its core wasn’t so simple, or stable. At a certain point, there had not been enough time elapsed to allow significant solidification of the core, meaning that the dynamo was entirely the result of a swirling storm of molten planet — much harder to model than the interior of the Earth today! Carnegie scientist Peter Driscoll tried, however, and by modeling the thermal lifetime of the Earth going all the way back to its inception, he was able to make predictions about what sort of magnetic field should have existed at each time period.
The geological record already showed that the planet’s magnetic field could possibly have gone haywire around a thousand years ago — and Driscoll’s model predicts that around this time, the slow solidification of the core should have been driving the magnetic field to some pretty crazy contortions. As the interior of the planet went through incredible changes, this produced a chaotic, many-lobed field that bears little resemblance to the planetary radiation shield we know and love.
Once the core solidified, Driscoll’s model says that the field should have settled down to a much more stable dipole. Previously, this was thought to have been the situation for the planet’s entire history. Now, it seems they have a much more complex history to dig into. All this syncs up quite well with the geological record, which shows a strange disturbance in the magnetic field 600-700 millions years ago. According to Discoll’s calculations, that’s right in the time period we’d expect the Earth to have multiple, oddly space magnetic poles.
The field of exo-planetology is advancing at a lightning pace, offering an obvious possible path for research into Earth’s history. We’ll never be able to see the Earth’s actual past, but astronomers are accruing an ever-growing list of Earth-like planets to study. Eventually, one with fall under their lens at the right time in its own planetary history to show us just how we got here.
In that spirit, there are already efforts at creating a method of measuring the magnetic field of a distant alien world — and the true next-gen planet hunting telescopes have yet to even go into service. Will we learn that there is a large diversity of ways for a planet to have a magnetic field?
Even in our own solar system, we have many non-metallic moons with no magnetic field, right alongside Ganeymede, which has a magnetic field so strong it can direct the path of incoming charged particles from space (called a magnetosphere). What will we find in the larger universe could be even more unique and alien, but in a way, we should hope for particularly mundane, Earth-like discoveries.