Photo of Earth taken by the Apollo 17 crew in 1972. Credit: NASA/ Apollo 17 crew
Earth is the third closest planet to the Sun with an average distance of 1.0 AU or 1.496 x 108 km. It takes 365.256 Earth days to orbit the Sun, and rotates at a rate of 1 rotation every 23.9345 hours. Its mass is 35.974 x 1024 kg. It has one moon, and an atmosphere composed of 78% nitrogen, 21% oxygen, 0.035% carbon dioxide, and 1% water vapor. The surface temperature ranges from 183 to 333 K (−90 °C to 60 °C, −130 °F to 140 °F).
The Earth is the only planet in the solar system with life on it as far as we know. About 70% of the Earth's surface is covered with water. 97% of Earth's water is salty and is contained in oceans, seas, and some lakes and rivers. 80% of the Earth's fresh water is frozen at the north and south poles. 99.5% of the remaining fresh water is unavailable for human use because it is either too far underground, trapped in soil, polluted, etc., so only about 0.003% of Earth's water is potable.
Like Venus and Mars, Earth has volcanos, but in addition Earth has tectonic activity. The surface layer of Earth is called the crust. On the continents, this crust is about 35 km thick, while the crust under the oceans is about 6 km thick. The crust and the upper layer of the mantle (together called the lithosphere) ride on top of the viscous athenosphere, which is very hot and has convection currents flowing throughout causing the plates on top to move. When the plates colide, this leads to earthquakes and mountains forming. When the plates pull apart, molten rock known as lava, seeps upward in the space created and cools. Scientists believe that originally there was one large continent, which has since split into the seven we see today.
