Researchers led by the University of Maryland have found that the sun's magnetic field is the reason behind the large size of Mercury’s iron core. The study has been published in Progress in Earth and Planetary Science.
The metal content of the four planets closest to the Sun- Mercury, Venus, Earth, and Mars- drops off as the planets get farther from the Sun. This means that Mercury has a relatively large amount of metal in comparison to more distant planets.
Previously, scientists had speculated that collisions were the reason behind Mercury’s large iron core. They said that hit-and-run collisions with other bodies, while our solar system was forming, blew away much of Mercury’s rocky mantle and left a proportionally larger iron core behind. This new research however provides another explanation.
Using a new model, the researchers were able to show that during the early years of our solar system, the sun’s magnetic field drew iron particles towards its center. When planets began to form from surrounding dust and gas, those that formed closer to the sun incorporated more iron into their cores than those further away.
By incorporating this model into calculations of planetary formation, the researchers were able to find a gradient in metal content and density that perfectly matched what we currently know about planets in our solar system. Using these calculations, they were able to confirm that Mercury’s iron core makes up around three-quarters of its mass, the iron cores of Earth and Venus make up around a third of their mass, and that Mars's iron core makes up around a quarter of its mass.
The researchers say that their new findings will influence how we study the composition of exoplanets. Now, rather than just looking at the composition of their stars, scientists may also consider the magnetic properties of the star during its early growth in its solar system.