Planetary differentiation
Planetary differentiation is the separating out of different constituents with different physical and chemical properties, where the body develops into compositionally distinct layers (metallic core, stony mantle, and crust).
Earth is an example of a differentiated body, meaning that it has a solid metallic inner core, a liquid metallic outer core, and stony mantle and outermost crust. Earth also has an atmosphere composed of gasses held in by gravity. Other differentiated bodies include other terrestrial planets and dwarf planets (Pluto, Ceres, and Vesta).
Differentiation into concentric layers occurs within molten portions of a rocky body. This requires energy and heat. Some of that energy and heat is provided by impacts, but that is generally localized at the site of the impact and short-lived. Other sources of heat include the initial energy of the solar nebula and radioactive decay. Also, increasing gravitational pressure as the body grows outward melts a portion of the body.
A planetary body differentiates into chemically distinct constituents (iron metal vs. rock), which have very different physical properties.
- The physical properties of iron metal, including its malleability (ability to be hammered or pressed permanently into shape without breaking) and electrical conductivity (ability to transport an electric current) are a product of its metallic bonds holding the metal atoms together. Metallic bonding is the force of attraction between positively charged ions and shared detached electrons that act as a glue holding the substance together.
- In contrast to metal, rock is brittle when it is struck by a hammer. Rock is an insulator and does not transmit an electric charge. This is because rock is mostly made up of silicate (silicon-bearing) minerals that are held together by ionic and covalent bonds. In ionic bonds, electrons are completely transferred from one atom to another, creating two oppositely charged ions that are attracted to one another. In covalent bonds, electrons are shared between pairs of ions.
Pallasite meteorites well demonstrate the immiscibility of rock and metal. These are stony-iron meteorites that may originate from the core-mantle boundary of differentiated asteroids. Pallasites are beautiful, but rare meteorites (only 93 known specimens, sells for $5 to $20 per gram on ebay).
Iron metal and rock also have very different densities. And it is this contrast in density that causes the layering in bodies. Density is defined in the next section and in Exercise 2, you will calculate density and learn more about its very important role in planetary differentiation.
