But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. The temperature on Venus is slightly hotter but very stable. Mercury is the second densest major body in the solar system, after Earth . This indicates that Mercury's dense iron core is relatively larger than Earth's, probably comprising the majority of the planet.
Mercury therefore has only a relatively thin silicate mantle and crust.
Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. At least some of the core is probably molten. Some cut thru the rings of craters and other features in such a way as to indicate that they were formed by compression. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system. In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains.