EarthSky.org's Top 5 Mercury mysteries that BepiColombo will solve says:
- Why does Mercury have a magnetic field?
Not too many planets have a magnetic field. Among the rocky planets of the inner solar system, only Mercury and Earth have one. Mars used to have a magnetic field in the past and lost it. Mercury appears too small to have one. Yet, it still does, even though it’s one hundred times weaker than the magnetic field of Earth. Scientists wonder what sustains this magnetic field despite the odds stacked against it.
Earth’s magnetic field is generated by the fast spinning of its liquid iron core. As for Mercury, scientists used to think that the core, due to the planet’s small size, must have cooled down and solidified since the planet’s formation. Is that really the case? Johannes Benkhoff explained:
Mercury’s core must be partially molten to explain this magnetism. We can also measure tides on the surface of Mercury, suggesting there must be liquid inside the planet. As Mercury orbits around the sun and interacts with its gravity, we expect a bulge to form and change its size while moving around the sun.
Question: Why must "Mercury’s core (be) partially molten to explain this magnetism"? If Mercury's core solidified, why couldn't have a frozen-out residual magnetization 100 times weaker than Earth's? It's core is huge compared to the diameter of the planet, so even a weak residual magnetization would be detectable in orbit near the surface.
Even iron that cools on Earth has residual magnetism, this is how we know it's field has reversed so many times.
Mercury’s magnetic field and the strong asymmetries introduced by its interaction with the solar wind. Image via ESA.