Internal structure of the inner planets
1 2016-05-17T10:45:05+00:00 Mariek Schmidt 3b678a5bd42eb8bf9a55fb761e5f17b11ce872c1 10 1 Mercury has an average density of 5430 kilograms per cubic meter, which is second only to Earth among all the planets. It is estimated that the planet Mercury, like Earth, has a ferrous core with a size equivalent to two-thirds to three-fourths that of the planet's overall radius. The core is believed to be composed of an iron-nickel alloy covered by a mantle and surface crust. It is believed that the composition of the planet Venus is similar to that of Earth. The planet crust extends to around 10-30 kilometers below the surface, under which the mantle reaches to a depth of some 3000 kilometers. The planet core comprises a liquid iron-nickel alloy. Average planet density is 5240 kilograms per cubic meter. The Earth comprises three separate layers: a crust, a mantle, and a core (in descending order from the surface). The crust thickness averages 30 kilometers for land masses and 5 kilometers for seabeds. The mantle extends from just below the crust to some 2900 kilometers deep. The core below the mantle begins at a depth of around 5100 kilometers, and comprises an outer core (liquid iron-nickel alloy) and inner core (solid iron-nickel alloy). The crust is composed mainly of granite in the case of land masses and basalt in the case of seabeds. The mantle is composed primarily of peridotite and high-pressure minerals. Average planet density is 5520 kilograms per cubic meter. Mars is roughly one-half the diameter of Earth. Due to its small size, it is believed that the martian center has cooled. Geological structure is mainly rock and metal. The mantle below the crust comprises iron-oxide-rich silicate. The core is made up of an iron-nickel alloy and iron sulfide. Average planet density is 3930 kilograms per cubic meter. Image credit: NASA plain 2016-05-17T10:45:05+00:00 Mariek Schmidt 3b678a5bd42eb8bf9a55fb761e5f17b11ce872c1This page is referenced by:
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Planetary differentiation
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Planets are all made from basically the same stuff – the solid chondritic materials that condensed and accreted from the solar nebula. So why then do planets have concentric layers like an onion?
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.
