Mercury's internal layers, artwork. The cutaway section shows that the metallic core is larger than previously thought and is molten rather than solid. This result was published in 2007, following ground-based telescope observations that measured the amount by which Mercury wobbled as it spun about its axis. The observations showed that the interior is decoupled from the exterior, providing strong evidence of a non-solid core. Above the core is a thick layer of silicate rocks and a cratered surface. Mercury is the closest planet to the Sun. Its diameter (4880 kilometres) is less than half that of Earth and it has a surface area that is about the same as that of the Atlantic Ocean.