Why does Mercury have a large iron core? Now, new research from the University of Maryland suggests that proximity to the sun's magnetic field determines the planet's interior, which may shed light on the mystery of Mercury's iron core.
The study contradicts the common assumption that Mercury has a large core, with a size relative to its mantle (the layer between the planet's core and crust). For decades, scientists have assumed that a "hit-and-run" collision between Mercury and other celestial bodies during the formation of the solar system blew away much of mercury's rocky mantle, leaving behind a huge, dense metal core inside, but new research suggests the sun's magnetic field was not the culprit.
New research suggests that after the formation of the inner planets, the sun's magnetic field gradually drew iron to the center of the solar system, which explains why Mercury, the closest planet to the Sun, has a larger, denser iron core than the outer layers of other rocky planets like Earth and Mars.
William MacDonald, a geology professor at the University of Maryland, and Takashi Yoshizaki of Tohoku University, Japan, built a model showing how the density, mass and iron content of a rocky planet's core are affected by its distance from the sun's magnetic field.
McDonald said: "4 of the solar system's innermost planet, mercury, Venus, earth and Mars, is composed of different ratio of metal and rock, along with more and more distant from the planet from the sun, the metal content of the core will decline, this is a change of gradient, by showing the early solar system formation phase distribution of the raw materials are controlled by the sun's magnetic field, Our paper explains how this process happens."
McDonald developed a model of the composition of the Earth, which is commonly used by planetary scientists to determine the composition of exoplanets. His latest model shows that during the early formation of the solar system, the young sun was surrounded by swirling clouds of dust and gas, and iron particles were drawn to the central region of the solar system by the sun's magnetic field. When solar planets are born from swirling clouds of dust and gas, the cores of planets closer to the Sun absorb more iron than those farther away.
Researchers have found that the density and ratio of iron in a rocky planet's core is related to the strength of the magnetic field around the Sun during the planet's formation, and their latest study suggests that magnetic fields should be taken into account in future attempts to describe the composition of rocky planets, including extrasolar planets.
The composition of a planet's core is important for the possibility of supporting life, for example: Earth's molten iron creates a magnetosphere that protects the planet from cancer-causing cosmic rays. The core contains most of The planet's phosphorus, an important nutrient that supports carbon-based life.
Using existing models of planet formation, McDonald determined the rate at which gas and dust were sucked into the center of the solar system during its formation, taking into account the magnetic field created when the Sun formed and figuring out how the magnetic field pulled iron through the dust and gas clouds.
When the early solar system began to cool, dust and gas that hadn't been sucked into the center of the solar system began to clard together. Dust gas closer to the sun was exposed to a stronger magnetic field, and thus contained more iron than dust gas farther away. As the dust and gas coalesced and cooled to form spinning planets, gravity pulled iron into their cores.
When MacDonald incorporated the model into planetary formation calculations, it revealed gradients in metal content and density that matched exactly what scientists knew about planets in our solar system. Mercury's metallic core makes up about three-quarters of its mass, the cores of Earth and Venus are only about a third of their mass, and the core of Mars is only about a quarter of its mass.
The new understanding of the role of magnetism in planet formation spells trouble for exoplanet exploration, since scientists have no way to observe the magnetism of stars from earth. Scientists infer the composition of an exoplanet from the spectrum of solar radiation. Different elements in a star emit radiation at different wavelengths, so by measuring those wavelengths, we can tell what the star and its surrounding planets are made of.
"A star is made in this way, so the planets around it must be made in this way," McDonald said. "Each planet in the solar system contains iron to varying degrees, based on the early magnetism of the sun."
The next step in the work is for scientists to find another planetary system similar to our solar system -- one made of rocky planets farther from the star. If the density of these exoplanets drops as they radiate outward from the Sun, as our solar system does, then researchers can confirm the new theory and infer that stellar magnetic fields influence planet formation.