The Earth's global magnetic field is generated by the flow of liquid metals in its core and protects the planet from dangerous particles. While the moon contains magnetic material under its crust, it does not have an encompassing field. In a recent study, Xi Yang and collaborators at the Department of Earth and Planetary Sciences, ETH Zurich studied a Dewar region in hopes of answering the question of whether the Moon used to have a global magnetic field or if large impacts deposited magnetized material into its crust that we see today.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about the Moon's magnetic field.
But let's start with how magnetic fields are produced on planets or moons, and that's from a dynamo. For the Earth, it's called a geodynamo and like all the terrestrial or rocky planets and moons, it roughly requires the same ingredients — A liquid flowing magnetic material.
As you travel deeper into the Earth through the crust, the material becomes made of heavier, more metallic stuff. Earth's mantle contains about 14% of its iron, while the core has 85 to 90% of the total iron. So, most of the iron and nickel on Earth is locked in its core — and the Earth has two core regions — a solid inner core surrounded by a liquid molten core. As it's immersed in liquid, the solid inner core spins more or less independently of the Earth's surface — sometimes a little faster and then others, a little slower.
The different spin, along with heat-driving convective currents, keeps the liquid core flowing, and that flowing metallic liquid generates massive electric currents. Those flowing electric currents then generate our magnetic field that penetrates the mantle crust and even extends out into space, where it protects us from lots of harmful particles. While the bottom line to generating a planet-wide or moon-wide magnetic field is to have a subsurface liquid layer that is metallic and moving around.
Mercury, a much smaller planet but with a larger fraction of core, has basically the same dynamo method as our Earth, and it generates a global magnetic field just at 1% the strength of ours. Venus, our closest twin of a planet, does not have a global magnetic field, and it's a bit of a mystery. Possibly it's very slow rotation, taking 243 Earth days to spin once, or it's lack of plate tectonics is the reason. Interestingly, Mars once had a global magnetic field like the Earth and Mercury, but no longer does even though it has a fairly large molten core. But that core is infused with lighter elements, meaning less magnetic material, and the lighter elements may have stratified, likely ending, convective currents, and Mars' global magnetic field.
So our Earth has a global magnetic field, as does Mercury, and Mars once did. What about our moon?
Its structure of crust, mantle, and core made of rock is the same as terrestrial planets. Thanks to seismographs placed on the moon by Apollo astronauts, we have actually measured thousands of moonquakes, revealing that our moon also has a solid inner core of nearly pure iron surrounded by a liquid core. But our moon does not currently have a global magnetic field. Instead, it has pockets of magnetized material frozen into its crust, and the question bugging scientists is,
"Did it ever have a global field?"
There are two competing ideas. One that, yes, the moon did once have a dynamo generating a magnetic field, and the other is that large impacts — and the moon has had many early in its history — could have created localized magnetic processes.
A new article by Xi Yang of the Department of Earth and Planetary Sciences at ETH Zurich and collaborators uses an interesting region of the moon to answer that question. The region is on the far side of the moon and is known as the Dewar region. It is named after the British physicist James Dewar, and it is not the darker regions we see on the moon, but similar to the lighter grey regions known as the Highlands. In fact, the backside of the moon has very few darker mare regions.
The Dewar region is interesting because it has both a stronger magnetic field and what's called a gravitational anomaly, that is, its material is more dense than in its surroundings. You might wonder how we know that as we've never landed on the far side of the moon, but satellites can easily measure magnetic fields, and cleverly, using high-speed signals sent back to Earth, the positional height above the lunar surface can be measured. NASA also sent twin satellites that were called EB and FLOW that flew in formation specifically to measure gravitational anomalies.
The special Dewar region is about 40 miles wide and 6 miles deep and is much denser than its surroundings. Xi and collaborators determined that this was solidified magma that rose from a deeper volcanic region that they aged to 4.2 billion years old. Observed cratering above the higher density material rules out large impact events and subsequently that rules out impacts forming the magnetism locked into the Dewar region. Their paper concludes that our moon definitely had a global magnetic field likely driven by a dynamo like the Earth's, but probably about 5 times weaker than Earth's.
The question now shifts from, "Did the moon have a dynamo", to "How did it work?" and that question has yet to be answered.