Chronology

Formation

Main article: History of the Earth

Artist's impression of the birth of the Solar System

The earliest material found in the Solar System is dated to 4.5672±0.0006 bya;^[34] therefore, it is inferred that the Earth must have been formed by accretion around this time. By 4.54±0.04 bya^[23] 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.^[35] The Moon formed shortly thereafter, about 4.53 bya.^[36]
The Moon's formation remains debated. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object dubbed Theia impacted with Earth.^[37] The model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of the Earth's mass,^[38] it impacts with the Earth in a glancing blow,^[39] 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.

Geological history

Main article: Geological history of 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.^[40] In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity.^[41] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.^[42]
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^[43] that explain land mass propose either a steady growth to the present-day forms^[44] or, more likely, a rapid growth^[45] early in Earth history^[46] followed by a long-term steady continental area.^[47][48][49] 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.^[50]
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.^[51]

Evolution of life

Main article: Evolutionary history of life

Stratocumulus clouds over the Pacific, viewed from orbit. Over 70% percent of Earth's surface is covered with water, which contains about half of the planet's species.^[52]

Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 bya and half a billion years later the last common ancestor of all life existed.^[53] 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.^[54] 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.^[55]
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.^[56]
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.^[57] The most recent such event was 66 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 66 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.^[58] 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,^[59] affecting both the nature and quantity of other life forms.

Future

Main article: Future of the Earth

See also: Risks to civilization, humans, and planet Earth

The life cycle of the Sun

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 byr and by 40% over the next 3.5 byr.^[60] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.^[61]
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.^[25] 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.^[62] After another billion years all surface water will have disappeared^[26] and the mean global temperature will reach 70 °C^[62] (158 °F). The Earth is expected to be effectively habitable for about another 500 myr from that point,^[25] although this may be extended up to 2.3 byr if the nitrogen is removed from the atmosphere.^[27] 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.^[63]
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).^[60][64] 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 its 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 5,000 times its present level).^[60] A 2008 simulation indicates that the Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.^[64] 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 may one day become incorporated into a new generation of planets and other celestial bodies.