Source: American Geophysical Union Scientists have long known that Earth and Mercury have metallic cores. Like Earth, Mercury's outer core is composed of liquid metal, but there have only been hints that Mercury's innermost core is solid.
Now, in a new study, scientists report evidence that Mercury's inner core is indeed solid and that it is very nearly the same size as Earth's solid inner core. Some scientists compare Mercury to a cannonball because its metal core fills nearly 85 percent of the volume of the planet.
This large core, huge compared to the other rocky planets in our solar system, has long been one of the most intriguing mysteries about Mercury. Scientists had also wondered whether Mercury might have a solid inner core.
Mercury’s solid, iron core is about 1,260 miles (2,000 kilometers) wide and makes up about half of Mercury's entire core (about 2,440 miles, or nearly 4,000 kilometers, wide)