Mercury is the smallest and innermost planet in the Solar System. But what type of planet is Mercury?
Mercury is classified as a terrestrial planet. The terrestrial planets include the 4 rocky worlds in the inner Solar System: Mercury, Venus, Earth and Mars. The term “terrestrial” comes from the latin term “terra”, meaning Earth. So the terrestrial planets are similar to Earth.
For example, the average density of Mercury is 5.427 g/cm3. Compare this to Earth’s density of 5.515 g/cm3. Planets with this kind of density are made up of mostly rock and metal. Compare this to the density of Saturn, at 0.687 g/cm3. Saturn is so light that it would float on water is you had a pool large enough.
Mercury is made up of distinct layers, similar to Earth. It has a large metallic core, made of solid or liquid iron. This is surrounded by a mantle of hot rock, and this is surrounded by a thin crust of rock. There are no active volcanoes on Mercury, but astronomers think the planet might still be releasing volcanic gasses through vents.
The terrestrial planets are also unusual because they have few or no moons. Mercury has no moons, and neither does Venus. Mars has two tiny asteroid-like moons, and only Earth has a moon of any significant size.
We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.