Final answer:
Mercury's high density, large metallic core, and weak magnetic field all provide evidence for its partially molten, metallic core. Giant impacts likely stripped away much of its silicate mantle, and historic solar heating may have evaporated some silicates, leaving a metal-rich core.
Step-by-step explanation:
The strong evidence for a partially molten, metallic core in Mercury stems from a few key observations. First, Mercury's high density suggests that it is composed largely of heavy metals, specifically, a metallic iron-nickel core constituting about 60% of its total mass. Furthermore, the core has a significant diameter of around 3500 kilometers, reaching up to within 700 kilometers of the planet's surface. The rocky crust surrounding this metallic core is approximately 700 kilometers thick. Additional evidence comes from the existence of Mercury's weak magnetic field, which aligns with the hypothesis of a large, metallic, and partially liquid core, necessary to generate a magnetic field. Historical giant impacts during the planet's formation are believed to have stripped away much of the mantle and crust, resulting in a body dominated by its iron core.
Moreover, the silicate loss can also be explained by early solar heat, which might have evaporated lighter silicates, leaving behind a planet rich in metals. This alternative explanation also aligns with our understanding that terrestrial planets, like Mercury, would have originally formed with similar metal to silicate ratios as Earth or Venus.
Lastly, the internal structures of terrestrial planets indicate that originally, they were hot enough to melt, which allowed denser metals to sink and form a central core with lighter silicates near the surface. This understanding provides a logical framework for Mercury's current composition and internal structure.