Were Scientists Wrong About the Planet Mercury? Its Big Iron Core May Be Due to Magnetism!

Mercury Planet Core

New research shows the sun’s magnetic field drew iron toward the center of our solar system as the planets formed. That explains why Mercury, which is closest to the sun has a bigger, denser, iron core relative to its outer layers than the other rocky planets like Earth and Mars. Credit: NASA’s Goddard Space Flight Center

Proximity to the sun’s magnetic field determines a planet’s interior composition, according to new research from the University of Maryland.

New research disputes the prevailing hypothesis on why Mercury has a big core relative to its mantle (the layer between a planet’s core and crust). For decades, scientists argued that hit-and-run collisions with other bodies during the formation of our solar system blew away much of Mercury’s rocky mantle and left the big, dense, metal core inside. But new research reveals that collisions are not to blame—the sun’s magnetism is.

William McDonough, a professor of geology at the University of Maryland, and Takashi Yoshizaki from Tohoku University developed a model showing that the density, mass and iron content of a rocky planet’s core are influenced by its distance from the sun’s magnetic field. The paper describing the model was published on July 2, 2021, in the journal Progress in Earth and Planetary Science.

“The four inner planets of our solar system—Mercury, Venus, Earth, and Mars—are made up of different proportions of metal and rock,” McDonough said. “There is a gradient in which the metal content in the core drops off as the planets get farther from the sun. Our paper explains how this happened by showing that the distribution of raw materials in the early forming solar system was controlled by the sun’s magnetic field.”

McDonough previously developed a model for Earth’s composition that is commonly used by planetary scientists to determine the composition of exoplanets. (His seminal paper on this work has been cited more than 8,000 times.)

McDonough’s new model shows that during the early formation of our solar system, when the young sun was surrounded by a swirling cloud of dust and gas, grains of iron were drawn toward the center by the sun’s magnetic field. When the planets began to form from clumps of that dust and gas, planets closer to the sun incorporated more iron into their cores than those farther away.

The researchers found that the density and proportion of iron in a rocky planet’s core correlates with the strength of the magnetic field around the sun during planetary formation. Their new study suggests that magnetism should be factored into future attempts to describe the composition of rocky planets, including those outside our solar system.

The composition of a planet’s core is important for its potential to support life. On Earth, for instance, a molten iron core creates a magnetosphere that protects the planet from cancer-causing cosmic rays. The core also contains the majority of the planet’s phosphorus, which is an important nutrient for sustaining carbon-based life.

Using existing models of planetary formation, McDonough determined the speed at which gas and dust was pulled into the center of our solar system during its formation. He factored in the magnetic field that would have been generated by the sun as it burst into being and calculated how that magnetic field would draw iron through the dust and gas cloud.

As the early solar system began to cool, dust and gas that were not drawn into the sun began to clump together. The clumps closer to the sun would have been exposed to a stronger magnetic field and thus would contain more iron than those farther away from the sun. As the clumps coalesced and cooled into spinning planets, gravitational forces drew the iron into their core.

When McDonough incorporated this model into calculations of planetary formation, it revealed a gradient in metal content and density that corresponds perfectly with what scientists know about the planets in our solar system. Mercury has a metallic core that makes up about three-quarters of its mass. The cores of Earth and Venus are only about one-third of their mass, and Mars, the outermost of the rocky planets, has a small core that is only about one-quarter of its mass.

This new understanding of the role magnetism plays in planetary formation creates a kink in the study of exoplanets, because there is currently no method to determine the magnetic properties of a star from Earth-based observations. Scientists infer the composition of an exoplanet based on the spectrum of light radiated from its sun. Different elements in a star emit radiation in different wavelengths, so measuring those wavelengths reveals what the star, and presumably the planets around it, are made of.

“You can no longer just say, ‘Oh, the composition of a star looks like this, so the planets around it must look like this,’” McDonough said. “Now you have to say, ‘Each planet could have more or less iron based on the magnetic properties of the star in the early growth of the solar system.’”

The next steps in this work will be for scientists to find another planetary system like ours—one with rocky planets spread over wide distances from their central sun. If the density of the planets drops as they radiate out from the sun the way it does in our solar system, researchers could confirm this new theory and infer that a magnetic field influenced planetary formation.

Reference: “Terrestrial planet compositions controlled by accretion disk magnetic field” by William F. McDonough and Takashi Yoshizaki, 2 July 2021, Progress in Earth and Planetary Science.
DOI: 10.1186/s40645-021-00429-4

12 Comments on "Were Scientists Wrong About the Planet Mercury? Its Big Iron Core May Be Due to Magnetism!"

  1. iron at red hot temperatures is not magnetic so at liquid temperatures how would it be magnetic when the molecular activity is greater?
    also, the idea that a cooler center can exist in a hotter surrounding is questionable.
    did you ever see a cool center in a hot hard boiled egg?

  2. Your hard-boiled egg is a bad example.

  3. The elemental iron was not red-hot during accretion and was very much ferromagnetic material at the assembly stage in the proto-sun & post-ignition stages of the solar accretion disc that would one day become rocky planets. ONLY after it accreted & became compressed under incredible pressures did it become red-hot and lose it’s ferromagnetic properties. As yet we do not know for certain what magnetic properties large masses of ferromagnetic materials at very high temperature & compression actually display, other than we BELIEVE they facilitate the magnetosphere of the Earth and other planets.

    • Torbjörn Larsson | July 4, 2021 at 11:57 am | Reply

      What you say on accretion is consistent with the paper.

      The geodynamo of Earth, still poorly understood, is believed to be due to conductive material flows in the outer liquid core. Rock magnetism plays secondary fiddle to that, especially in a (slowly) convective mantle, plate tectonic planet.

  4. Isn’t it also true that iron is more dense than rock, regardless of magnetism? It seems like there could be other reasons why there would be more dense elements closer to the sun. Maybe gravity could be responsible. There would be more grativational attraction between more massive objects, whether there would be more attraction between heavier atoms, I’m not sure.

    I agree that the Big Bang theory is impossible, and contradicts all the evidence put forward by its proponents. Namely, that the red shifts & CMB are isotropic, meaning the Big Bang would have had to occur at the position of the oberver (Earth). However, this author doesn’t mention that and implies that the solar system was created from a cloud of dust that slowly came together because of gravity. The people who believe the universe was created from nothing in a Big Bang are essentially trying to prove their religious beliefs.

    • Torbjörn Larsson | July 4, 2021 at 11:59 am | Reply

      On the first I think you should read the paper, and on the second – irrelevant here – some basic cosmological texts – obviously the hot big bang is not impossible since it is an observational fact.

    • Torbjörn Larsson | July 4, 2021 at 12:04 pm | Reply

      Just for kicks, inflation has explained isotropy and homogeneity for the last 40 years (and ever better) and star formation has been the cosmological research edge since 2018.

      But the Starforge simulation do what you describe – makes stars from an originally spherical molecular cloud, from first principles – so planet formation as researched here is likely the new research edge. Watch the compelling video here: https://www.sciencenews.org/article/starforge-star-formation-simulation .

  5. Why these articles attract such comments makes me wonder about free time on some folks hands.
    My hard boiled eggs are very much hotter than their surroundings for quite some time before cooling to where I can eat them.
    As for hot and magnetic at the same time there are so many alloys that do that the count is beyond my ability. Locating pure iron with the attributes of pure iron has is actually a difficult procedure, it is a mismash of elements with no recipe in particular.
    Seems the more difficult the analysis the more adamant the laymen are that there is only one answer and it is theirs.

    Hard not to laugh, so I just let it loose. Dont really care if the religious aspect of the big bang fits or not as I do not believe in magic.
    I am waiting for more research before declaring my pet theory cast in stone.

    • Torbjörn Larsson | July 4, 2021 at 12:08 pm | Reply

      What “religious” nature!? It is completely observational science, and since it provides a natural model for 100 % of the universe as process and composition there is no place for “magic”.

      The observed 3D flat universe over sufficiently large volumes suggest the universe sums to zero energy, if that is what bothers you, and the inflation era before the hot big bang suggests we get the universe for free. (Since a quantum field vacuum such as the inflation era seems to constitute has an adiabatic free – spontaneous onset – expansion.)

  6. Torbjörn Larsson | July 4, 2021 at 11:52 am | Reply

    The paper discuss an interesting correlation and give mechanisms for its causes, but testing the correlation they do not as far as I can see.

    On the other hand, this has no contenders I think, and it places a lot of planetary compositions such as prominently Mercury in a model.

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