Earth

Earth
Name	Earth
Caption	The Blue Marble photograph taken by Apollo 17 in 1972.
Adjectives	Terrestrial, Terran, Telluric
Discovery date	Prehistory

Earth. It is the third planet from the Sun and the only known astronomical object to harbor life. Approximately 29.2% of Earth's surface is land consisting of continents and islands, while the remaining 70.8% is covered with water, mostly by oceans but also by lakes, rivers, and other fresh water, which together constitute the hydrosphere. The vast majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic Ocean.

Physical characteristics

Earth is the largest and densest of the four terrestrial planets in the Solar System. Its shape is approximately an oblate spheroid, with a slight bulge around the equator due to its rotation. The planet's interior is divided into layers: a solid inner core primarily composed of iron and nickel, a liquid outer core that generates the Earth's magnetic field, a viscous mantle, and a solid crust. The crust is fragmented into several rigid tectonic plates that migrate across the surface over geological time scales, a process driven by mantle convection. Major surface features include continents like Africa and South America, vast ocean basins like the Pacific Ocean, and mountain ranges such as the Himalayas and the Andes.

Orbit and rotation

Earth orbits the Sun at an average distance of about 150 million kilometers, a measure defined as one astronomical unit. It completes one revolution every 365.256 days, a period known as a sidereal year. The planet's rotation relative to the Sun gives rise to the cycle of day and night, with one rotation taking approximately 24 hours. Earth's axis of rotation is tilted about 23.4 degrees relative to its orbital plane around the Sun, which is the primary cause of the seasonal variations experienced in most regions. This orbit is influenced by gravitational interactions with other bodies, notably the Moon and Jupiter.

Geological history

Earth formed over 4.54 billion years ago from accretion within the protoplanetary disk that surrounded the early Sun. The Hadean eon saw a period of heavy bombardment and the formation of the Moon, likely from a giant impact with a body named Theia. The first solid crust and oceans formed during the Archean, and the earliest evidence of life appears in rocks from this eon. The process of plate tectonics has continually reshaped the continents, from the ancient supercontinent Rodinia to the more recent Pangaea, whose breakup formed the current continental configuration. Major events like the Permian–Triassic extinction event and the Cretaceous–Paleogene extinction event have punctuated the Phanerozoic eon.

Atmosphere and climate

Earth's atmosphere is primarily composed of nitrogen and oxygen, with trace amounts of gases like carbon dioxide and argon. This atmospheric composition is largely a product of planetary outgassing and, crucially, the evolution of photosynthetic life. The atmosphere protects life by absorbing harmful ultraviolet solar radiation and reducing temperature extremes between day and night. The complex interaction between the atmosphere, oceans, and landmasses drives the global climate system and weather patterns, influenced by phenomena like the Gulf Stream and El Niño–Southern Oscillation. Human activities, particularly since the Industrial Revolution, have increased atmospheric greenhouse gas concentrations, leading to contemporary global warming.

Biosphere

The global sum of all ecosystems constitutes the biosphere, which has profoundly altered planetary conditions. Life is believed to have emerged in the oceans over 3.5 billion years ago. The subsequent development of oxygen-producing cyanobacteria led to the Great Oxidation Event, which dramatically changed the atmosphere and allowed for the evolution of complex, multicellular life. The biosphere is divided into biomes, ranging from tropical Amazon rainforest to arctic tundra, each hosting distinct flora and fauna. Biodiversity has been shaped by evolutionary processes like natural selection and events such as the Cambrian explosion.

Human geography

As the home to over eight billion humans, Earth's surface is extensively modified by human activity. Modern political organization is based on sovereign states, with major powers including the United States, China, and the European Union. Human societies are concentrated in major urban areas like Tokyo, Delhi, and São Paulo. Economic activity, from agriculture in the Indo-Gangetic Plain to finance in Wall Street, is supported by global infrastructure and trade networks. Cultural and technological development, from ancient civilizations like the Roman Empire to modern institutions like NASA, has given humans a dominant role in altering the planet's surface and systems.

Category:Planets of the Solar System Category:Terrestrial planets