The Sun's light only strikes one side of Earth at a time. The dark side of Earth stays warm because the Sun's heat A) is stored in the ice caps. B) is trapped by the atmosphere. C) is blown there by the wind. D) is conducted through the Earth.

Answer: is trapped by the atmosphere.

During the day, the atmosphere lets through solar radiation that heats the Earth's surface. Once the surface of the planet has warmed up, this heat is returned to space in the form of infrared radiation.

However, not all infrared radiation is returned to space, because the same atmosphere (specifically carbon dioxide and water vapor present in it) prevents the release of much of this thermal radiation, reflecting part of it and returning it to the surface; thus maintaining the heat and allowing life on our planet. This phenomenon is known as the greenhouse effect.

Thus, of the solar energy that reaches the Earth by radiation, only a small percentage is reflected back into space by the surface and the atmosphere.

In fact, if the atmosphere did not exist, all that heat would completely escape into space and the Earth would cool rapidly during the night.

Therefore, the correct option is B:

The dark side of Earth stays warm because the Sun's heat is trapped by the atmosphere.