Mercury has a molten core

Mercury, it seems, could have an electromagnetic dynamo—a phenomenon that occurs as Earth's molten, metallic outer core rotates around a solid inner core, spawning currents that generate a magnetic field.

The new study also indicates that an unknown element that formed beyond Mercury's orbit somehow got mixed into the planet's "batter" as it was forming billions of years ago.

"The prevailing notion is that a planet as small as Mercury would have cooled off and solidified a long time ago," said Jean-Luc Margot, the Cornell University planetary scientist who led the new study.

To stay liquid, Mercury's core must contain significant amounts of a light element—probably sulfur—that would lower the core's melting temperature, scientists say.

But during the solar system's birth, light elements condensed relatively far from the sun.

Small proto-planets moving through the system might have transported sulfur into Mercury's path while the planet was taking shape, Margot said.