Though we do not currently have the means to see directly inside the Earth (or any other planet), we can use clues to make inferences about what may be lying beneath their surfaces. On Earth and the Moon, our most helpful data stems from the analysis of seismic waves, or vibrations that travel along the world’s surface and through its interior after earthquakes. For other terrestrial worlds, we can use other measurements like average density and gravity to determine the distribution of mass in the world’s interior. In this blog post, I will be discussing the three major layers that are present inside the terrestrial worlds: the core, the mantle, and the crust.

The Core: The innermost layer of the terrestrial worlds also has the highest density. It is primarily composed of metals, including iron and nickel. Mercury has a very large core of iron that comprises around 85 percent of its interior. The cores of Earth and Venus are made up of a solid, inner core and a molten outer core. Tectonic activity is also caused by heat in the world’s core.

The Mantle: Thick, rocky, moderate-density mantles surround the cores of the terrestrial planets. They are composed of mostly minerals that contain oxygen, silicon, and other elements. With the exception of Mercury, the mantle makes up a large portion of a terrestrial world’s volume; Earth’s mantle makes up 84 percent of the planet’s total volume.

The Crust: The terrestrial planets have thin crusts composed of low-density rock that make up their outermost layer. The Earth’s crust contains a great assortment of metamorphic, sedimentary, and igneous rocks; however, it makes up less than 1 percent of the planet’s total volume. The crusts of the terrestrial planets were formed through various igneous processes, and they frequently change due to erosion, sedimentation, volcanism, and cratering.