The four planets closest to the Sun -- Mercury, Venus, Earth, and Mars -- are called the "terrestrial" planets because of their similarities to Earth. They are smaller and denser than the gas-giant planets of the outer solar system.
The structures of the terrestrial planets are similar. Each planet has a layered structure that developed through a process called differentiation. After the small bodies (called planetesimals) clumped together and melted, the gravity of the newborn planets pulled heavier elements to their centers, while lighter elements floated toward the surface.
Although we see only the top of the crust, by studying mountain ranges, volcanoes, basins, canyons, and fault zones, scientists can infer the structure of the rest of the crust, mantle, and even the core of a planet. And by measuring the motions of spacecraft near the planets, scientists have determined their densities and, in some cases, the thicknesses of their outer layers.
Detection of magnetic fields by spacecraft has made it possible to infer the deeper structures of planets, too. The cores of the terrestrial planets are all about the same size in relation to their total diameters, but the mantle of Mercury is proportionally smaller than the mantles of the other three planets.
Volcanic activity was part of the early geological history of all the terrestrial planets plus the Moon. The Moon and Mercury have been geologically "dead" for at least a billion years. Spacecraft discovered indirect evidence of recent volcanic activity on Venus, but have not observed any active volcanic eruptions. Giant volcanoes on Mars may have erupted in the last few million years, and a few may be dormant.
The atmospheres of the terrestrial planets were created by three processes: outgassing (carbon dioxide and water vapor released by volcanoes), evaporation from ices at the surface, and bombardment by icy comets. The outer layers of each planet contained the water vapor, carbon dioxide, and other gases that have become the present-day atmospheres of the terrestrial planets. Since these planets are not very massive, their gravity could not hold onto most of the lighter gases, such as hydrogen and helium, or even to the heavier gases in their original warm atmospheres. Molecules of oxygen, nitrogen, carbon dioxide, and water vapor are heavy enough to be retained in their present atmospheres, which are much cooler.
Because the mass and distance from the Sun is different for each world, their atmospheres have evolved quite differently during the solar system's history. The Moon and Mercury have only thin wisps of atmosphere. The Martian atmosphere is less than one percent as thick as Earth's, while Venus' atmosphere is 90 times thicker than Earth's.
Weather and climatic processes have significantly changed the geological record we see on the planets. Wind and water erosion on Earth have wiped out most of our planet's impact craters, for example. On Mars, we see evidence of ancient rivers and perhaps even a global ocean, but no liquid water.
The different factors that have shaped each of the terrestrial planets not only make each world unique, they help scientists learn more about Earth by studying how its evolution has varied from that of the other planets.
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