Mercury found to have liquid core
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.