Earth formed approximately 4.54 billion years ago, and life appeared on its surface within one billion years.[22] Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land.[23] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will to be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).
Earth's crust 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 interacts with other objects in space, especially the Sun and the Moon. During one orbit around the sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year.[note 7] The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[27] The Moon is Earth's only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
The planet is home to millions of species, including humans.[28] Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population.[29] These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.
Name and etymology
The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), ultimately deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, German Erde, and Swedish, Norwegian, and Danish jord.[30] The Earth is personified as a goddess in Germanic paganism (appearing as Jörð in Norse mythology, mother of the god Thor).[31]In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. "Earth", "the Earth", "earth", or "the earth"). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. "It does not cost the earth.", "What on earth are you doing?").[32]
Chronology
Formation
The earliest material found in the Solar System is dated to 4.5666-4.5678 bya;[33] therefore, it is inferred that the Earth, must have formed around this time. By 4.50-4.58 bya[22] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.[34] The Moon formed shortly thereafter, about 4.53 bya.
The Moon's formation remains a mystery. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object, dubbed Theia, had a giant impact with Earth,[36] but the model is not self-consistent. In this scenario the mass of Theia is 10% of the Earth's mass,[37] it impacts with the Earth in a glancing blow,[38] and some of its mass merges with the Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth.
Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets.[39] In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity.[40] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[41]
A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models[42] that explain land mass propose either a steady growth to the present-day forms[43] or, more likely, a rapid growth[44] early in Earth history[45] followed by a long-term steady continental area.[46][47][48] Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.
Evolution of life
Main article: Evolutionary history of life
For more details on the current eon, see Geological history of Earth.
Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 bya and half a billion years later the last common ancestor of all life existed.[50] The development of photosynthesis
allowed the Sun's energy to be harvested directly by life forms; the
resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[51] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[52]Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[54] The most recent such event was 65 mya, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 65 myr, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[55] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[56] affecting both the nature and quantity of other life forms.
The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.
Future
The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm) for C4 photosynthesis) in approximately 500-900 myr.[24] The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[60] After another billion years all surface water will have disappeared[25] and the mean global temperature will reach 70 °C[60] (158 °F). The Earth is expected to be effectively habitable for about another 500 myr from that point,[24] although this may be extended up to 2.3 byr if the nitrogen is removed from the atmosphere.[26] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years due to reduced steam venting from mid-ocean ridges.[61]
The Sun, as part of its evolution, will become a red giant in about 5 byr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).[58][62] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level). A 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized. After that, the Sun's core will collapse into a white dwarf, as its outer layers are ejected into space as a planetary nebula. The matter that once made up the Earth will be released into interstellar space, where it will one day become incorporated into a new generation of planets and other celestial bodies.
Cultural viewpoint
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.
Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century. Earth has often been personified as a deity, in particular a goddess. In many cultures a mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism or Islam, assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life. Such assertions are opposed by the scientific community and by other religious groups.[A prominent example is the creation-evolution controversy.
In the past there were varying levels of belief in a flat Earth, but this was displaced by the concept of a spherical Earth due to observation and circumnavigation. The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective. This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet.
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