Mercury is hot stuff. That's the conclusion of a new radar study demonstrating that the core of the solar system's innermost planet is at least partially molten. The finding settles a long-simmering debate about the least studied of the planets. It may also provide insight about how the solar system created its planets.
With Mercury averaging just a third as far from the sun as Earth is, it might seem clear-cut that Mercury's core would be molten. But sunlight has a negligible effect compared with the heat left over from the planet's formation. And considering Mercury's small size, only 40 percent the diameter of Earth, astronomers calculate that the planet ought to have cooled and solidified long ago.