Mercury found to have liquid core

By looking at small changes or wobbles in the rotation of Mercury, scientists have discovered that the core (centre) of this tiny world moves at a different speed to its solid surface. Because the Mariner 10 spacecraft only found a weak magnetic field around Mercury, it was thought that the centre of the planet was mostly solid. However, this new discovery means that much of the material inside Mercury could be molten rock, similar to the stuff we see in volcanic lava flows on Earth.

Structure of Mercury
Artists impression of the internal structure of Mercury and the method of detection - © NSF

The discovery was made by beaming strong radio signals at Mercury's surface and measuring the time variation in the signals that were bounced back to detectors on Earth, such as the Arecibo radio telescope. In order to get an accurate result, astronomers had to measure Mercury's 58 day rotation to an accuracy of less than a minute. In the end, the strength of the planet's wobble was twice that expected for a solid planet, but in line with an object that has a solid exterior (i.e. a crust) and liquid core - similar to the structure of our own planet Earth.