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# 6.3 The Terrestrial Planets

In our Solar System, the largest rocky planet is our own planet Earth, or "Terra" in Latin. In honor of our world, we call rocky planets "Terrestrial Planets." The only known terrestrial planets are: Mercury, Venus, Earth, and Mars. Like the Earth, each of these worlds are made of a mixture of rocky and metallic material. By contrast, the planets in the outer Solar System are composed of lower-density icy and gaseous material. Mercury, Venus, and Mars all rank between the Earth and the Moon in size.

At first glance, the four terrestrial planets in our Solar System seem to have few similarities. Mercury is the smallest; it is very hot, almost airless, its surface appears similar to the Earth's Moon, and it has no natural satellites. Venus, in contrast, has a very thick atmosphere of carbon dioxide and dense clouds that hide its surface; it also has no satellites. The blue and white Earth has an unusual atmosphere of nitrogen and oxygen, and the bulk of its surface is covered with oceans of liquid water. It has one relatively large satellite, the Moon. The red planet, Mars, has only a thin atmosphere of carbon dioxide, and has two very small satellites. Only the Earth has any known life forms. One planet with no atmosphere, two with carbon dioxide atmospheres, and one with nitrogen and oxygen. One planet with one large satellite, one with two small ones, two with none. These planets seem to have a random mix of properties. Yet they share a common history, and their evolution has been shaped by the same general events and scientific principles, so their current characteristics must be connected.

We have learned about the terrestrial planets from a sequence of space probes launched by the United States and the former Soviet Union since the 1960s. More recently, the European Space Agency and several other countries including Japan have started to send probes into space as well. The observations collected by these robotic probes have revealed many new facts about these worlds. Besides revealing more of the planets' individual characteristics, the new information also suggests some unifying principles. You can view it as a mystery story about our cosmic neighborhood. For example, what underlying principles control the geological features on the surfaces of these planets? What principles explain the different atmospheric state of each body? It has taken several decades of exploration to learn the answers to these questions, and many mysteries remain unsolved.

Several unifying principles can help us understand the planets. Gravity is the force that produces the layering of materials within a planet while it is still molten. Heavier materials sink towards the center in a process called differentiation. The pressure and heat from gravitational energy help drive geological activity (just as the collapsing solar nebula was heated from the conversion of gravitational potential energy into heat energy, a condensing planet will also heat up). The attractive force of gravity allows a planet to retain an atmosphere, and it also determines which gases will be retained.

Planets are also unified in their basic chemical compositions. The same basic elements and chemical processes exist throughout the universe, although in different proportions. Scientists once thought other planets might be made of totally unfamiliar materials. Instead, we've found more or less familiar minerals and rock types on the various planets, although the detailed chemistry is often affected by planets' differing temperatures and pressures. For example, basaltic lava exists on all four terrestrial planets.

A last unifying principle: heat inside a planet influences its geology. This internal heat is created from the gravitational potential energy of the entire planet and the radioactive decay of heavy elements in the interior. As long as a planet has a molten core, it experiences volcanic activity, and perhaps even plate tectonics. A planets' internally-driven geological activity combines with the external process of impact cratering to shape the surface features of the terrestrial planets.

Size comparison of the four terrestrial planets Mercury, Venus, Earth and Mars and the terrestrial dwarf planet Ceres. Click here for original source URL.