Earth

• 1. Etymology And Naming Conventions

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  The Modern English word Earth developed from an Old English noun most often spelled ertha. This term has cognates in every Germanic language, tracing back to a reconstructed root that meant the ground or soil. In its earliest attestation, the word translated many senses of Latin terra and Greek ge, covering meanings like dry land, the human world, and the globe itself. As with Roman Terra and Greek Gaia, Earth may have been personified as a goddess in Germanic paganism. Late Norse mythology included Jörð, a giantess often given as the mother of Thor. Historically, Earth was written in lowercase until the Early Middle English period when the definite sense began using the phrase the earth. By the time of Early Modern English, capitalization became common, particularly when referencing other heavenly bodies. House styles now vary between Oxford spelling recognizing the lowercase form as more common and capitalized forms as acceptable variants. The name Terra sees use in scientific writing while Tellus appears in poetry to denote personification. Romance languages evolved names like Italian Terra and Spanish Tierra from the Latin root. The Latinate form Gaia has become common due to the Gaia hypothesis, though its pronunciation differs from traditional English usage.

• 2. Formation And Early History

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  The oldest material found in the Solar System is dated to 4.568 billion years ago. By 4.5 billion years ago, the primordial Earth had formed from gas and dust in the early Solar System. A solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk. Planets grow out of that disk with the Sun according to nebular theory. Planetesimals formed by accretion, with the primordial Earth taking anywhere from 70 to 100 million years to form. Estimates of the age of the Moon range from 4.5 billion years to significantly younger. A leading hypothesis states it was formed by accretion from material loosed from Earth after a Mars-sized object named Theia collided with Earth. This collision hit Earth with a glancing blow and some of its mass merged with Earth. Between approximately 4.0 and 3.8 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment. Earth's atmosphere and oceans were formed by volcanic activity and outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets. Sufficient water to fill the oceans may have been on Earth since it formed. Atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 4.4 billion years ago, Earth's magnetic field was established, helping prevent the atmosphere from being stripped away by the solar wind.

• 3. Geological Dynamics And Tectonics

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  As the molten outer layer of Earth cooled, it formed the first solid crust thought to be mafic in composition. The first continental crust, more felsic in composition, formed by partial melting of this mafic crust. Grains of the mineral zircon of Hadean age found in Eoarchean sedimentary rocks suggest that at least some felsic crust existed as early as 4.0 billion years ago. New continental crust forms as a result of plate tectonics, a process driven by continuous loss of heat from Earth's interior. Over hundreds of millions of years, tectonic forces caused areas of continental crust to group together to form supercontinents that subsequently broke apart. At approximately 1.1 billion years ago, one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia at 600 million years ago, then finally Pangaea, which also began to break apart at 200 million years ago. The most recent pattern of ice ages began about 2.58 million years ago and intensified during the Pleistocene about 2.58 million years ago. High- and middle-latitude regions have since undergone repeated cycles of glaciation and thawing repeating every 21,000, 41,000, and 100,000 years. The Last Glacial Period ended about 11,700 years ago covering large parts of continents to middle latitudes in ice.

• 4. Origin Of Life And Evolutionary Milestones

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  Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose. The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. Resultant molecular oxygen accumulated in the atmosphere and formed a protective ozone layer in the upper atmosphere through interaction with ultraviolet solar radiation. Incorporation of smaller cells within larger ones resulted in development of complex cells called eukaryotes. True multicellular organisms formed as cells within colonies became increasingly specialized. Among earliest fossil evidence for life are microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia. Biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland provides additional early evidence. Remains of biotic material found in 4.1 billion-year-old rocks in Western Australia suggest even earlier biological activity. During the Neoproterozoic between 750 and 635 million years ago, much of Earth might have been covered in ice under the Snowball Earth hypothesis. This period preceded the Cambrian explosion when multicellular life forms significantly increased in complexity. Following the Cambrian explosion there have been at least five major mass extinctions including one 66 million years ago when an asteroid impact triggered extinction of non-avian dinosaurs.

• 5. Physical Characteristics And Orbital Mechanics

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  Earth has a rounded shape through hydrostatic equilibrium with an average diameter of about 12,742 kilometers making it fifth largest planetary sized object of Solar System. Due to rotation it has ellipsoid shape bulging at equator where diameter is 43 kilometers longer than at poles. Largest local variations include Mariana Trench below sea level shortening radius by 0.17% and Mount Everest above sea level lengthening it by 0.14%. Summit of volcano Chimborazo in Ecuador is farthest point out from center of mass. Earth's surface area covers about 510 million square kilometers divided into Northern and Southern hemispheres or Eastern and Western hemispheres. Most surface is ocean water covering 70.8% or 361 million square kilometers forming world ocean. Land covers 29.2% or 149 million square kilometers taken by four continental landmasses: Africa-Eurasia, America, Antarctica, and Australia. Elevation varies from Dead Sea at minus 430 meters to Mount Everest at 8,848 meters. Mean height of land above sea level is about 840 meters. Earth rotates around its own axis in slightly less than day taking 23 hours and 56 minutes for one complete revolution. Orbit takes year about 365.25 days completing one revolution around Sun located eight light-minutes away.

• 6. Human Impact And Environmental Crisis

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  Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Human population has grown exponentially to eight billion in the 2020s projected to peak at around ten billion in second half of 21st century. Since 19th century majority living in urban areas with most growth expected in sub-Saharan Africa. From 1980 to 2000 extreme weather events like tropical cyclones caused average of 11,800 human deaths per year. Human activities release greenhouse gases into atmosphere causing global warming driving changes such as melting glaciers ice sheets rising sea levels increased drought risk wildfires species migration. Global temperatures in year 2020 were warmer than preindustrial baseline by approximately 1.2 degrees Celsius. Five planetary boundaries crossed include biosphere integrity climate change chemical pollution destruction wild habitats nitrogen cycle. As of 2018 no country meets basic needs without transgressing planetary boundaries. Large deposits of fossil fuels obtained from crust consist coal petroleum natural gas used for energy production and chemical feedstock. In 2019 forest woodlands covered 40 million square kilometers shrub grassland 56 million square kilometers animal feed grazing 34 million square kilometers cultivated croplands 15 million square kilometers.