Mercury's Molten Core
This diagram shows the interior structure of Mercury. The metallic core extends from the center to a large fraction of the planetary radius.
Researchers working with high-precision planetary radars, including the Goldstone Solar System Radar of NASA's Jet Propulsion Laboratory, Pasadena, Calif., have discovered strong evidence that the planet Mercury has a molten core. The finding explains a more than three-decade old planetary mystery that began with the flight of JPL's Mariner 10 spacecraft. The research appears in this week's issue of the journal Science.
Scientists theorized that Mercury consisted of a silicate mantle surrounding a solid iron core. This iron was considered solid - or so the theory went - because small planets like Mercury cool off rapidly after their formation. If Mercury followed this pattern, then its core should have frozen long ago.
Many believed the Mercury mystery would only be resolved if and when a spacecraft landed on its aggressively toasty surface. Then, in 2002, scientists began pointing some of the most powerful antennas on our planet at Mercury in an attempt to find the answer.
Measuring the echo of particular surface patterns from the surface of Mercury and how long they took to reproduce at both Goldstone and the Robert C. Byrd Green Bank Telescope in West Virginia allowed scientists to calculate Mercury's spin rate to an accuracy of one-thousandth of a percent.
With these data the science team was able to detect tiny twists in Mercury's spin as it orbited the sun. These small variations were double what would be expected for a completely solid body. This finding ruled out a solid core, so the only logical explanation remaining was that the core - or at the very least the outer core - is molten and not forced to rotate along with its shell.