Not all planets move the needle. But whatever planet you take a magnetic compass to, it’s sure to point out clues to secrets underfoot.

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byBas den Hond 21 December 202028 September 2021
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Credit: Goodstudio/stock.adobe.com

Congratulations! With the IP9, the new interplanetary model in mmsanotherstage2019.com’s signature line of magnetic compasses, you’ve chosen a travel companion that will serve you as best it can on the many GPS-challenged bodies of our solar system—be your plans a hike on Mercury, a ride on Mars, or a glide over Neptune.

Before you start using your compass, please note that your warranty is voided when you drop your IP9 onto a hard surface or into a high-pressure or high-temperature environment, or store it unshielded from magnetic fields during extended periods of interplanetary travel.

Other warnings and pointers, specific to select extraterrestrial destinations, are as follows.

Destination: Mercury

On Mercury, using the compass will be straightforward. The structure of Mercury’s magnetic field is much like Earth’s, so your compass will behave approximately as if a huge bar magnet rests at the planets center, aligned with its rotational axis. Or—a bit closer to the mark—as if electric currents are girding that axis.

Do give your needle time to adjust. Mercury’s magnetic field, which was measured by the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft that orbited the planet from 2011 to 2015, is only 1.1% the strength of Earth’s.

Your compass may actually measure fields in the magnetosphere of Mercury that are caused by interaction with the solar wind.

And pay attention to space weather: “Because Mercury is much closer to the Sun,” said Sabine Stanley, a professor in the Department of Earth and Planetary Sciences at Johns Hopkins University, “and because the planet’s magnetic field is much weaker than Earth’s field, there are times when the solar magnetic field gets really important, even very close to the planet. Your compass may actually measure fields in the magnetosphere of Mercury that are caused by interaction with the solar wind. We call them external magnetic fields because they are due to currents flowing outside of the planet, not inside of it.”

In rocky planets like Mercury and Earth, any such internal electric currents flow in the iron cores they obtained when they were young and hot, and their materials separated out according to density.

“The biggest thing Mercury’s field tells you is it has an iron core, and that core is still partly liquid and moving around,” said Stanley. “Before we can really understand what the field tells us about the planet, we need to understand what the composition is of the core, what’s mixed in with the iron, what are the temperatures. We learn a little bit about that from the composition of the surface.”

Those assumptions about composition eventually go into modeling, which is what Stanley does. The goal is to predict how an iron core, wholly or partially fluid, sheds its primordial heat. If this happens fast enough, convection will occur. As swirls of electrically conducting fluid both create and are moved around by magnetic fields, they become a self-sustaining source of such fields: a dynamo. But modeling that process realistically is not really possible just yet, said Stanley. “Because the viscosity of the iron is so low, the flows are turbulent at a small scale, so in our simulation we would really need high resolution, a lot of grid points.”

Bruce Buffett, of the Department of Earth and Planetary Science at the University of California, Berkeley, agreed. Models, he said, are characterized by how the friction forces associated with viscosity compete with the Coriolis forces associated with the rotation of the planet. “When we first started , the viscous forces were about 1,000 times less important than the Coriolis forces. In a realistic model, they are 1015 times less. In our current models, 105 or 106 is possible.”

Achieving realistic conditions means that modelers like Buffett and Stanley need computers that are about 2,000 timefs faster than what they can currently get their hands on. If Moore’s law, which says computer power doubles roughly every 2 years, keeps working, scientists will get those computers in 11 years.

In the meantime, researchers studying Mercury’s magnetic field have to work with approximations, which “do produce magnetic fields,” said Buffett. “There are some people who believe that if you go to lower viscosity, you stay in the same dynamical regime, and others say there could be something different, a change of phase almost. I’m not sure who’s right. But the results that we are getting now are useful.”

Destination: The Moon

Although all major rocky bodies in our solar system have iron cores, your IP9 compass is unfortunately not suitable for use on Venus and the Moon and is of only limited use on Mars.

The time when the Moon had a global magnetic field is long past. Your compass will at most pick up remanent magnetization in some lunar rocks.

That the field is absent tells us that the lunar core is fairly quiescent, said Sonia Tikoo, an assistant professor of geophysics at Stanford University.


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Rocks like the Contingency Sample, above, the very first sample picked up from the Moon, provide clues to the Moon’s magnetic past. Credit: NASA/Astromaterials 3D

The ages of magnetized rocks constrain the time when a dynamo was active inside the Moon. But there are large uncertainties to those constraints, due to the limited samples of rocks that Apollo astronauts brought back to Earth.

“At least prior to 3.5 billion years ago, the field appears to have been as strong as Earth’s,” Tikoo said. “After that it was an order of magnitude weaker. It lasted at least until 1.9 billion years ago, and very likely was turned off by 0.9 billion years ago.”

These numbers pose hard questions for modelers. The early magnetic field seems too strong to have been generated by the sort of dynamo the Moon’s heat budget could sustain. “So people are looking at alternatives that are mechanical in nature,” Tikoo said.

One possible energy source is precession, with the core and mantle, and perhaps a liquid outer core and a solid inner core, rotating around different axes. “That can generate turbulence in the fluid core and power a dynamo,” said Tikoo. “But what is missing is any magnetohydrodynamic simulation. Nothing yet has been published that says, ‘Yes, you can do this.’”

Destination: Venus

For Venus, information about any past magnetic field is even scarcer. “We don’t know what we would see your compass do,” admitted Joe O’Rourke, an assistant professor in the School of Earth and Space Exploration at Arizona State University. “One possibility is that it would do nothing, because there never was a magnetic field of any kind. The second is that it would occasionally behave erratically as you encounter regions of the crust that are magnetized.”

“If there is a magnetic field at Venus, it has to be 100,000 times weaker than Earth’s mag-netic field.”

Such regions would prove that Venus did have a magnetic field and that it was preserved in rocks. But whether those rocks exist is anyone’s guess. “The mission that provided the tightest constraints on the magnetism of Venus was the Pioneer Venus Orbiter, which launched in the late ‘70s,” said O’Rourke. “All we really know is that if there is a magnetic field at Venus, it has to be 100,000 times weaker than Earth’s magnetic field.”

The most likely explanation for the absence of a magnetic field now, according to O’Rourke, is that the Venusian lithosphere is not broken up into wandering continental plates. “Because there is no plate tectonics, silicate mantle probably cools down slowly, and the mantle is what is insulating the core. So Venus could have a core that is exactly the same size, exactly the same composition, as Earth, but cooling down more slowly over time.”

Venus could also differ from Earth in radical ways that would reduce the strength of its magnetic field or prevent it from having one at all. A chemical gradient in the core, for instance, or an insulating ocean of molten rock surrounding the core may prevent convection in the planet’s interior.

If Venus never had a magnetic field at all, it would be a very special planet indeed. Research on the remanent magnetism of meteorites suggests that even some planetesimals, the building blocks of planets like Earth and Venus, had iron cores that for a time produced dynamos.

Stanley has studied a class of very old meteorites called angrites. “They’re dated very close to the beginning of the solar system,” she said, “and they have a magnetic signature in them that seems to suggest they formed on a body that had a dynamo. Nothing else, like solar wind or flares, is strong enough . And the field has to be very stable, for about tens of thousands of years, because rocks cool very slowly, and they imprint an average field over that time.”

“So we did some modeling to ask the question, Could a planetesimal, something that’s maybe 100 kilometers to a few hundred kilometers in radius, generate a dynamo?” Stanley continued. “And we found that yes, in certain circumstances you could have enough power available to generate those motions you need in the cores of these planetesimals.”

That power would have come from a radiogenic isotope of aluminum, Al-26. “It has a very short half-life,” said Stanley, “so all of it has already decayed today, but very early in the solar system it was an available heat source.”

The decay of Al-26, according to Stanley’s calculations, could completely melt a planetesimal, allowing an iron core first to form in its center and then to cool down through convection, creating a short-lived dynamo.

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Many planetesimals are still around, for instance, as Kuiper belt objects, and two have been visited by a spacecraft: the contact binary Arrokoth and the dwarf planet Pluto. When New Horizons came calling, however, it didn’t bring a magnetometer. Its designers didn’t think such an instrument would measure anything while passing Pluto. Pluto’s small size and slow rotation—its day takes almost an Earth week—work against any dynamo activity. Stanley is quite sure: “Pluto does not have a magnetic field.”

Whether Venus ever had a dynamo can be established only by a new magnetometer-equipped mission.