Earth's magnetic poles drift over time. This is something that every airplane pilot or navigator knows. They have to account for it when they plan their flights.
They drift so much, in fact, that the magnetic poles are in different locations than the geographic poles, or the axis of Earth's rotation.
Today, Earth's magnetic north pole is 965 kilometres (600 miles) away from its geographic pole. Now a new study says the same pole drifting is occurring on Mercury too.
Earth's magnetic poles anchor our planet's magnetosphere. The magnetosphere extends out into space around our planet, and protects us from the Sun's radiation. The magnetosphere and its poles are artifacts of Earth's molten core, and scientists think that Mercury has a molten core, too.
But what, exactly, makes the poles drift? The phenomenon is called polar drift, and on Earth it's caused by variations in the flow of molten iron in the planet's core.
On Earth, the north magnetic pole drifts about 55 to 60 kilometers (34 to 37 miles) per year, The south magnetic pole drifts about 10 to 15 kilometers (6 to 9 miles) each year. The poles also flip, and that's happened about 100 times in the planet's history.
The study shows that the same polar drift is likely happening on Mercury, and that the story behind pole drift on that planet is more complicated than thought.
Above: The Earth's magnetic poles drift around in relation to the geographic poles. The drift is caused by variations in the flow of the Earth's liquid core.
The new study is published in the American Geophysical Union's Journal of Geophysical Research: Planets. It's titled "Constraining the Early History of Mercury and its Core Dynamo by Studying the Crustal Magnetic Field." The lead author is Joana S. Oliviera, an astrophysicist at the European Space Agency's European Space Research and Technology Centre in Noordwijk.
The authors relied extensively on data gathered by NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft. It orbited Mercury from 2011 to 2015, and it was the first spacecraft to orbit the planet.
One of MESSENGER's instruments was a magnetometer that measured Mercury's magnetic field in detail. The spacecraft's elliptical orbit took it to within 200 km above the surface. MESSENGER acquired data showing weak magnetic anomalies in Mercury's crustal surface associated with impact craters.
The authors assumed that these anomalies were due to iron in the impactors that created the craters. They also assumed that as this molten material cooled it was shaped by Mercury's magnetic field.
Scientists know that as igneous rock cools, it preserves a record of the planet's magnetic field at the time. As long as those rocks contained magnetic material, they will align with the planet's field. It's called "thermoremanent magnetization".
As different rock at different locations on Earth cooled at different times, it created a historical record of Earth's drifting poles. This is how we know that Earth's poles have flipped in the past, the last time almost 800,000 years ago.
They key to this is the thermoremanent magnetization. As lead author Oliviera said in a press release, "If we want to find clues from the past, doing a kind of archaeology of the magnetic field, then the rocks need to be thermoremanent magnetized."
Scientists have been able to study Mercury's magnetic field, but no rock samples have ever been collected. No spacecraft has ever landed on Mercury.
To get around this, the authors of this study focused on five impact craters on the surface and on the magnetic data that MESSENGER collected when it got close to Mercury's surface.
Five craters showed different magnetic signatures than MESSENGER measured throughout Mercury. These craters are ancient, between 3.8 and 4.1 billion years old. The researchers thought that they might hold clues to the position of Mercury's ancient poles, and how they've changed over time.
"There are several evolution models of the planet, but no one has used the crustal magnetic field to obtain the planet's evolution," Oliveira said.
These impacts melted rock, and as the rock cooled it retained a record of the planet's magnetic field. They used the magnetic data from those five impact craters to model Mercury's magnetic field over time. From that, they were able to estimate the location of Mercury's ancient magnetic poles, or "paleopoles."
Their results were surprising, and point to Mercury's complicated magnetic nature. They found that the ancient poles were far from the current south magnetic pole, and that they likely changed through time. That much they expected.
But they also expected that the poles would cluster at two points that were close to Mercury's rotational axis, much like Earth's. But the poles were randomly distributed, and, shockingly, were all in the southern hemisphere of the planet.
As the press release says, "The paleopoles do not align with Mercury's current magnetic North pole or geographic South, indicating the planet's dipolar magnetic field has moved."
This evidence supports the idea that Mercury's magnetic history is much different than Earth's. It also supports the idea that Mercury shifted along its axis. That's called a true polar wander, when the geographic locations of the North and South Poles change.
While Earth has a dipolar magnetic field with a distinct northern pole and southern pole, Mercury is different. It currently has a dipolar-quadrupolar magnetic field with two poles and a shift in the magnetic equator. In ancient times, according to this study, it may have had the same field. Or, it may have had a multipolar field, with twisted magnetic "field lines like spaghetti" according to Oliviera.
That's where our knowledge of Mercury's magnetic field lines stands for now. What scientists really need to do is study multiple rock samples from Mercury. But no spacecraft has ever landed there, and no landings are planned.
Mercury is a tough place for a spacecraft to visit and orbit, much less land on. It's proximity to the Sun means any mission to Mercury has to contend with the Sun's powerful gravitational pull. It takes a lot of fuel to do much more than fly past Mercury rapidly, and only two spacecraft have ever visited the planet: MESSENGER and Mariner 10.
For now, scientists are looking ahead to BepiColombo, the ESA's first mission to visit Mercury. It'll arrive at Mercury in 2025 and will spend one or two years there. It's actually two orbiters in one, but there's no lander.
One of the orbiters is called MMO (Mercury Magnetospheric Orbiter.) As the name implies, its role is to study Mercury's magnetic field, which is rare among planets. Data from that mission may well build on studies like this one, and could shed more light on Mercury's complicated magnetic history.