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  Why is Earth geologically active?
  Internal heat drives geological activity, and Earth retains plenty of internal heat because of its relatively large size for a terrestrial world. This heat causes mantle convection and keeps Earth’s lithosphere thin, ensuring active surface geology. It also keeps part of Earth’s core melted, and the circulation of this molten metal creates Earth’s magnetic field.
• What processes shape Earth’s surface?
  The four major geological processes are impact cratering, volcanism, tectonics, and erosion. Earth has experienced many impacts, but most craters have been erased by other processes. We owe the existence of our atmosphere and oceans to volcanic outgassing. A special brand of tectonics—plate tectonics—shapes much of Earth’s surface. Ice, water, and wind drive rampant erosion on our planet.
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  How does Earth’s atmosphere affect the planet?
  Two crucial effects are (1) protecting the surface from dangerous solar radiation—ultraviolet is absorbed by ozone and X rays are absorbed high in the atmosphere—and (2) the greenhouse effect, without which the surface temperature would be below freezing.

• Was there ever geological activity on the Moon or Mercury?
  Both the Moon and Mercury had some volcanism and tectonics when they were young. However, because of their small sizes, their interiors long ago cooled too much for ongoing geological activity.

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  What geological features tell us that water once flowed on Mars?
  Dry river channels, rock-strewn floodplains, and eroded craters all show that water once flowed on Mars, though any periods of rainfall seem to have ended at least 3 billion years ago. Mars today still has water ice underground and in its polar caps, and could possibly have pockets of underground liquid water.
• Why did Mars change?
  Mars’s atmosphere must once have been much thicker with a much stronger greenhouse effect, so change must have occurred due to loss of atmospheric gas. Much of the lost gas probably was stripped away by the solar wind, which was able to reach the atmosphere as Mars cooled and lost its magnetic field and protective magnetosphere. Water was probably also lost because ultraviolet light could break apart water molecules in the atmosphere, and the lightweight hydrogen then escaped to space.

• Is Venus geologically active?
  Venus almost certainly remains geologically active today. Its surface shows evidence of major volcanic or tectonic activity in the past billion years, and it should retain nearly as much internal heat as Earth. However, geological activity on Venus differs from that on Earth in at least two key ways: lack of erosion and lack of plate tectonics.
• Why is Venus so hot?
  Venus’s extreme surface heat is a result of its thick, carbon dioxide atmosphere, which creates a very strong greenhouse effect. The reason Venus has such a thick atmosphere is its distance from the Sun: It was too close to develop liquid oceans like those on Earth, where most of the outgassed carbon dioxide dissolved in water and became locked away in rock. Thus, the carbon dioxide remained in the atmosphere, creating the strong greenhouse effect.

• What unique features of Earth are important for life?
  Unique features of Earth on which we depend for survival are (1) surface liquid water, made possible by Earth’s moderate temperature; (2) atmospheric oxygen, a product of photosynthetic life; (3) plate tectonics, driven by internal heat; and (4) climate stability, a result of the carbon dioxide cycle, which in turn requires plate tectonics.
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  How might human activity change our planet?
  Ozone depletion can leave surface life more vulnerable to dangerous solar ultraviolet radiation, and the high rate of extinctions could have unknown consequences. The human release of greenhouse gases into the atmosphere may already be causing global warming and certainly would affect the climate if it continues.
• What makes a planet habitable?
  We can trace Earth’s habitability to its relatively large size and its distance from the Sun. Its size keeps the internal heat that allowed volcanic outgassing to lead to our oceans and atmosphere, and also drives the plate tectonics that helps regulate our climate through the carbon dioxide cycle. Its distance from the Sun is neither too close nor too far, thereby allowing liquid water to exist on Earth’s surface.

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