Earth's equatorial radius is 6,378.1 km, mean radius is 6,371.0 km, and polar radius is 6,356.8 km. In the first billion years of Earth, life appeared on its surface.
Earth's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle. Earth orbits at 1 astronomical unit from the Sun.