The structure of Earth. (A) A solid section of Earth from the center to the surface. (B) A 200-m-deep section through the crust and upper mantle showing a subduction zone where a plate of continental crust is advancing over an oceanic plate. A tongue of oceanic crust and lithosphere plunges down into the asthenosphere, remaining identifiable down to depths of about 700 km.
Earth is the third planet of the Solar System and the only world in space on which it is certain there is life. Liquid water covers 70.8% of its surface to an average depth of 3,900 meters and played a crucial role in the origin and evolution of terrestrial organisms (see life, origin). Volcanism and impact cratering (see Earth impact craters) are also in evidence, although the dominant geological process is erosion and deposition by water or ice.
Earth has a relatively thick atmosphere composed of 76% nitrogen, 21% oxygen, 1% argon, plus traces of other gases including carbon dioxide and water vapor (see Earth, atmosphere). The interior of Earth is, like that of the other terrestrial planets, divided into an outer silicate-rich solid crust, with a highly viscous mantle, an outer liquid core that is less viscous than the mantle, and a solid inner nickel-iron core. Convection currents in the material of the outer core give rise to a weak magnetic field. Earth is unusual in the solar system in having a terrestrial planet-like satellite, the Moon, that is about one-quarter of Earth's diameter. By coincidence, the Moon is just far enough away to have, when seen from Earth, the same apparent angular size as the Sun, enabling total solar eclipses to be seen.
|distance from Sun|
|mean||149.6 million km (93.0 million miles, 1.00 AU)|
|min.||147.0 million km (91.4 million miles, 0.98 AU)|
|max.||152.2 million km (94.6 million miles, 1.02 AU)|
|equatorial diameter||12,756 km (7,928 miles)|
|axial period||23.93 hours|
|orbital period||365.26 days|
|orbital inclination||0° (by definition)|
|atmospheric composition||78% nitrogen, 21% oxygen, 0.9% argon|
|max. temperature||50°C; (122°F;)|
|min. temperature||-70°C; (-94°F;)|
|mean temperature||15°C; (59°F;)|
|escape velocity||11.18 km/s (40,248 km/h, 25,014 mph)|