More of Earth's fresh water sits in polar ice than in rivers, lakes and sky
The great ice sheets of the polar deserts lock away more water than all of Earth's groundwater, lakes, rivers and atmospheric moisture put together. It is one of many quirks of the only world known to carry life, and the only one in the Solar System with liquid water on its surface.
Ocean covers 70.8 percent of Earth's crust; the remaining 29.2 percent is land, clustered mostly in one land hemisphere. The crust is broken into slowly drifting tectonic plates whose collisions raise mountains and trigger volcanoes and earthquakes. Deep inside, a liquid outer core generates a magnetic field that turns away most of the harmful solar wind and cosmic radiation.
Above the surface, an atmosphere mostly of nitrogen and oxygen stops most meteoroids and filters ultraviolet light. Water vapour forms clouds over most of the planet and, together with carbon dioxide and other greenhouse gases, traps sunlight's energy to keep the average surface temperature at 14.76 degrees Celsius, warm enough for liquid water. Because the equator receives more sunlight than the poles, the uneven heating drives winds and ocean currents that shape climate zones, weather and the cycling of carbon and nitrogen.
The planet is a slightly flattened sphere about 40,000 kilometres around, the densest in the Solar System and the biggest and heaviest of the four rocky worlds. It sits about eight light-minutes from the Sun, circles it in roughly 365.25 days and spins once in about 23 hours 56 minutes, with a tilted axis that gives us seasons. The Moon, about a quarter of Earth's width and 384,400 kilometres away, steadies that tilt, raises tides and gradually brakes Earth's spin, while Earth's pull has long since locked the Moon so it shows us one face.
Earth formed about 4.5 billion years ago from the gas and dust of the young Solar System, and oceans and life both appeared within its first billion years. Living things later reshaped the air itself, bringing on the Great Oxidation Event two billion years ago. Humans arose in Africa 300,000 years ago and now alter the climate and biosphere at a pace that is driving widespread extinctions. The name comes from the Old English eorthe, which covered ground, soil, dry land and the whole world, and in late Norse myth a giantess named Jord, Earth, was often called Thor's mother.
Source: Earth
