The third planet from the Sun, having the following characteristics: mass 5·97 × 1024kg; orbital period 365·26 days; radius (equatorial) 6378 km/3963 mi; obliquity 23°27′; mean density 5·52 g/cm3; orbital eccentricity 0·017; equatorial gravity 978 cm/s2; mean distance from the Sun 149·6 × 106 km/93 × 106 mi; rotational period 23 h 56 min 4 s. It has one large natural satellite, the Moon. There is an oxygen/nitrogen-rich atmosphere, liquid water oceans filling lowland regions between continents, and permanent water ice caps at each pole. It is unique in the Solar System in being able to support life, for which there is fossil evidence in rocks dating from 3·5 thousand million years ago; human population over 6000 million (1999). The interior of the planet is differentiated into zones: an iron/nickel-rich molten core (radius c.3500 km/2175 mi) an iron–magnesium silicate mantle (c.85% by volume of the Earth); and a crust of lighter metal silicates (relatively thin, c.6 km/3½ mi thick under the oceans to c.50 km/30 mi under the continents). The boundary between core and mantle is called the Gutenberg discontinuity; that between mantle and crust is the Mohorovicic (or Moho) discontinuity. The interior is hot as a result of energy released through the continuing decay of a small proportion of long-lived radioactive isotopes of potassium, thorium, and uranium. Temperatures in the upper mantle cause partial melting at depths of c.100 km/60 mi. Large sections (plates) of the uppermost mantle and crust (the lithosphere) move slowly and horizontally relative to one another over the more fluid and partially molten zone (the asthenosphere), which extends to a depth of c.250 km/155 mi. The evolution of the Earth’s crust is dominated by such motions (plate tectonics): the spreading, subduction, and collision of lithospheric plates creates most of the major planetary scale features that have shaped Earth as a planet - volcanism, rifting, mountain building, and continental wandering. The effects of asteroidal impacts are erased/disguised by the rapid rate at which the Earth’s surface is reshaped and eroded. Such impacts would have been a dominant process in the earliest evolution of the Earth, and still occur periodically. A complex atmospheric circulation is driven primarily by the non-uniform solar heating of the planet and by its rapid rotation; and there is evidence of repeated climatic changes throughout the geological record.
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