A new study challenges the prevailing hypothesis as to why Mercury has a large core relative to its mantle (the layer between a planet’s core and crust). For decades, scientists have argued that collisions with other bodies during the formation of our solar system washed away much of Mercury’s rocky mantle and left the large, dense metallic core inside. But new research shows that the collisions aren’t to blame – the sun’s magnetism is.
William McDonough, professor of geology at the University of Maryland, and Takashi Yoshizaki of the University of Tohoku developed a model showing that the density, mass and iron content of a rocky planet’s core are influenced by its distance relative to the sun’s magnetic field. The article describing the model was published on July 2, 2021 in the journal Advances in Earth and Planetary Sciences.
âThe four inner planets of our solar system – Mercury, Venus, Earth and Mars – are made up of different proportions of metal and rock,â McDonough said. “There is a gradient in which the metal content of the core decreases as the planets move away from the sun. Our article 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. field. “
McDonough previously developed a model for the composition of the Earth which is commonly used by planetologists to determine the composition of exoplanets. (His seminal article on this work has been cited over 8,000 times.)
McDonough’s new model shows that at the start of our solar system’s formation, when the young sun was surrounded by a swirling cloud of dust and gas, grains of iron were drawn to the center by the sun’s magnetic field. . When planets began to form from clusters of this dust and gas, planets closer to the sun incorporated more iron into their cores than those farther away.
Researchers have found that the density and proportion of iron in the core of a rocky planet correlates with the strength of the magnetic field around the sun during planetary formation. Their new study suggests that magnetism should be taken into account in future attempts to describe the makeup 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 example, a molten iron core creates a magnetosphere that protects the planet from carcinogenic cosmic rays. The nucleus 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 rate at which gas and dust were drawn to the center of our solar system during its formation. He took into account the magnetic field that would have been generated by the sun when it appeared and calculated how this magnetic field would attract iron through the cloud of dust and gas.
As the early solar system began to cool, dust and gas that were not attracted to the sun began to clump together. Tufts closest to the sun would have been exposed to a stronger magnetic field and therefore contain more iron than those farther from the sun. As the clusters merged and cooled into rotating planets, gravitational forces pulled iron into their core.
When McDonough incorporated this model into the calculations of planetary formation, he revealed a gradient of metal content and density that matches perfectly with what scientists know about the planets in our solar system. Mercury has a metallic core which makes up about three-quarters of its mass. The cores of Earth and Venus are only about a third of their mass, and Mars, the most distant of the rocky planets, has a small core that’s only about a quarter of its mass.
This new understanding of the role that magnetism plays in planetary formation creates a problem in the study of exoplanets, as there is currently no method to determine the magnetic properties of a star from Earth observations. Scientists deduce the composition of an exoplanet based on the spectrum of light radiated by its sun. Different elements of a star emit radiation in different wavelengths, so measuring those wavelengths reveals what the star, and possibly the planets around it, are made of.
âYou can’t just say, ‘Oh, a star’s makeup looks like this anymore, so the planets around it have to look like this,â McDonough said. “Now you have to say, ‘Each planet might have more or less iron depending on the star’s magnetic properties when the solar system starts growing.'”
The next steps in this work will be for scientists to find another planetary system like ours – one with rocky planets spread out over great distances from their central sun. If the density of the planets decreases when they radiate from the sun as in our solar system, researchers could confirm this new theory and deduce that a magnetic field influenced planetary formation.