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Module 1 - The Solar System

Terrestrial Planets

The inner or terrestrial planets consist of Mercury, Venus, Earth, Mars.

All four are relatively small high-density bodies rich in silicate rocks and iron. Their densities vary from 3933 – 5514 kg/m³. If you are more familiar with a density measurement of g/cm³, those densities translate to 3.933 – 5.514 g/cm³. Please recall that the density of water is 1 g/cm³. When you pick up the next pebble on the beach to skip across that lake it will likely have a density of just under 3 g/cm³. Because these planets are denser than most rocks we find on the surface, we know that they must contain denser material in their interior. We know that the Earth has an iron/nickel core. In a later module, we will discuss later how the core formed, and why we are certain that the other terrestrial planets have similar interiors.

In contrast to the giant outer planets, terrestrial planets have a solid surface. The surface of terrestrial planets can have canyons, craters, mountains, and volcanoes.

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Terrestrial Planet Interiors *Mercury* 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. *Venus* 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. *Earth* 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* 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. *Pluto* The structure of Pluto is not very well understood at present. Nevertheless, spectroscopic observation from Earth in the 1970s has revealed that the planet surface is covered with methane ice. Surface temperature is -230?C (-382?F), and the frozen methane exhibits a bright coloration. However, with the exception of the polar caps, the frozen methane surface is seen to change to a dark red when eclipsed by its moon Charon. Average planet density is 2060 kilograms per cubic meter. The low average density requires that the planet must be a mix of ice and rock. *Image Credit*: Lunar and Planetary Institute