Mercury, the Second Densest Planet

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., John Hopkins University

Mercury is the second densest planet of our solar system, second only to Earth. Mercury, surprisingly, also has a global magnetic field, which exists due to an even stranger phenomenon called antifreeze.

A high resolution image of our solar system displaying Mercury.
Mercury is small enough to be a moon, yet it’s the second densest planet after Earth. (Image: Michelangelus/Shutterstock)

Mercury is small enough to be a moon. Jupiter’s moon Ganymede and Saturn’s moon Titan are both bigger than Mercury. Compared with Earth, all of Mercury is even smaller than just the iron core that makes up the center of the Earth. Although Mercury is small, it’s unusually dense.

If we calculate Mercury’s average density by taking its mass and dividing by its volume, we come up with a density of about 5430 kilograms per cubic meter. Any density in that ballpark suggests a planet that’s mostly rocks and iron.

Doing the same calculation for the Earth gives an average density of about 5500 kilograms per cubic meter. Iron is denser than rocks, so you might be tempted to conclude that Earth has a slightly larger fraction of iron compared to Mercury.

But hold on, here’s where the tricky part comes in. You see, because Earth is 18 times more massive, the pressures in the interior are higher, and these higher pressures cause the inner regions of Earth to compress, effectively taking up less volume than the same materials would in a smaller planet.

In short, density isn’t just determined by what a material is made of, but also by what pressure it experiences.

So even though Earth has a slightly higher density than Mercury, if we were to take all the material in Earth and uncompress it then Earth’s uncompressed density would be 4200 kilograms per cubic meter. For Mercury, uncompressing a much smaller interior pressure doesn’t have such a big effect. Mercury’s uncompressed density would be only slightly lower, about 5400 kilograms per cubic meter.

The fact that Mercury has an uncompressed density much larger than Earth tells us that Mercury has a much larger fraction of iron in its interior. In fact, Mercury has the largest fraction of iron of any planet in our solar system. The radius of Mercury’s iron core is about 1800 kilometers, which is almost 75% of the planet’s radius. By volume, that means Mercury is over 50% iron core, whereas Earth is only 17% core.

So how did Mercury end up with such a large core? Or in other words, what was happening early in solar system history to result in a planet with such a large iron core? The leading theory is that Mercury actually used to be much bigger, with a thicker rocky mantle surrounding its iron core. If so, something must have happened billions of years ago, early in Mercury’s history, to remove the outer mantle layer and leave the iron-rich planet we see today.

This could be accomplished by a giant impact that gave a glancing blow to the planet. An impactor just sliding by, together with some outer layers of Mercury, could have escaped the system or crashed into the Sun. This would mean that Mercury is really the iron-core remnant of a much larger planet.

An image showing the different layers and cores of Mercury.
Mercury, the second densest planet, has a large iron core at its center. (Image: A loose necktie/CC BY-SA 4.0/Public domain)

Explanations like this are sometimes uncomfortable for scientists because it seems to suggest really special, rare circumstances for Mercury’s formation. If Mercury had been just a little to the left, it wouldn’t have been hit by that object, and we wouldn’t see the planet we have today. That makes it seem like an unlikely event. However, although it’s true that large impacts are unlikely, we know that they occurred in the early solar system since we have evidence for them in the large impact craters all over the solar system.

Even the fact that Earth has a large Moon is understood to have involved a giant impact. Such collisions early in our solar system’s history were not as rare or special as they seem from the perspective of today’s solar system.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Does Mercury Have a Magnetic Field?

Mercury’s large, metallic core is also home to another surprising discovery. In the mid-1970s, the first spacecraft to visit Mercury, Mariner 10, discovered that Mercury has a global-scale magnetic field. Before the Mariner 10 mission, scientists didn’t think Mercury had the right ingredients for dynamo action to produce a magnetic field.

So what ingredients does a dynamo need? Think of how pedaling a bicycle can power a bike light. Dynamo action occurs when materials that are good electrical conductors can vigorously move around in such a way to create electromagnetic energy from the kinetic energy of the motions. This is the same process at work in a generator. Basically, electrical currents can be generated in moving electrical conductors. And these currents can generate magnetic fields.

In a terrestrial planet like Mercury, the metallic iron core is a good candidate for an electrically conducting region. But in order to have the vigorous motions necessary to generate magnetic fields through dynamo action, the iron core needs to be liquid.

Early on, scientists didn’t think it would be possible for Mercury’s core to be liquid. This is because Mercury is a small planet, and small planets cool faster than big planets because of their larger surface area to volume ratio. Thermal models for Mercury showed that the temperatures in the interior would be below the freezing temperature of iron, which is about 2800 Fahrenheit. So Mercury’s core would be solid.

But then, of course, along comes the Mariner 10 mission and the more recent MESSENGER mission in 2011, both of which demonstrated that Mercury has a global magnetic field, which is only possible if the core is at least partially liquid.

Learn more about how the solar system family is organized.

How Could Mercury’s Core Still Be Liquid?

How do we reconcile Mercury’s small size with the fact that we know at least some of Mercury’s core is liquid? The answer lies in realizing that Mercury’s iron core must have an antifreeze.

Iron’s freezing temperature can be greatly reduced by adding sulfur to the mix. We know that the cores of planets aren’t made of pure iron from our studies of meteorites. Seismology has also told us that Earth’s core is not pure iron. It contains about 10% lighter elements, like sulfur, silicon, oxygen, and others. Scientists have determined that just a few percents of sulfur in Mercury’s core could act as a good enough antifreeze to keep a portion of Mercury’s core liquid.

Another key ingredient for a dynamo is that the liquid conductor has to have vigorous motions. This can occur inside a planet if it’s cooling fast enough to transport heat through convection. Because Mercury’s a small planet with rapid cooling, the turbulent churning motions from heat transport can generate the electrical currents that produce magnetic fields.

And this magnetic field partially shields Mercury from solar radiation and from high-energy particles emitted from other stars and galaxies. That’s better than Mars can offer.

So next time you’re trying to spot Mercury in the sky, or wondering how worthwhile it is to explore Mercury, keep this in mind: Mercury is, on average, the closest planet to Earth.

Since Mercury’s orbit is so close to the Sun, it is never very far away from us. As a result, Mercury averages about 8.5% closer to Earth than Venus. So yes, this makes Mercury, on average, our nearest neighbor.

What’s surprising is just how extreme it is. Mercury’s orbit around the Sun is less circular—more elliptical—than any other planet. Its surface has the highest and coldest temperatures, and thanks to the cold, there’s even a lot of frozen water, right next to the Sun!

It’s small enough to be a Moon, yet it has a big-planet core and even a big-planet magnetic field. It’s amazing how much this little planet has to tell us.

Learn more about Venus, the veiled greenhouse planet.

Common Questions about Mercury, the Second Densest Planet

Q: Which is the densest planet is the Solar System?

Earth is the densest planet in our Solar System, but Mercury’s uncompressed density is even higher than the Earth’s.

Q: Which is the least dense planet?

The least dense planet in our Solar System is Saturn.

Q: Why is Mercury so dense?

Mercury is dense because its heavy iron core amounts to nearly two-thirds of the planet’s mass, more than twice the ratio of core to mass for Earth, Venus, or Mars.

Q: How cold is Mercury at night?

In the night, on Mercury can get as cold as minus 290 F; however, the planet has an average temperature of 332 F.

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