Planet Earth

Earth (or the Earth) is the third planet from the Sun and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System’s four terrestrial planets. It is sometimes referred to as the World, the Blue Planet, or by its Latin name, Terra.

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation. It also is the only terrestrial planet with active plate tectonics.

Home to millions of species including humans, Earth is currently the only astronomical body where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years.

Earth’s biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks harmful solar radiation, permitting life on land.

The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years.

Earth’s outer surface 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 with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing to the hydrosphere.

Liquid water, necessary for all known life, is not known to exist in equilibrium on any other planet’s surface. Earth’s poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet’s interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26 solar days, or one sidereal year. 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).

Earth’s only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet’s rotation. Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.

Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. 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 personification as a deity, a belief in a flat Earth or in the Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.

The Moon was new on July 16. Its familiar nearside facing the surface of planet Earth was in shadow. But on that date a million miles away, the Deep Space Climate Observatory (DSCOVR) spacecraft’s Earth Polychromatic Imaging Camera (EPIC) captured this view of an apparently Full Moon crossing in front of a Full Earth. In fact, seen from the spacecraft’s position beyond the Moon’s orbit and between Earth and Sun, the fully illuminated lunar hemisphere is the less familiar farside. Only known since the dawn of the space age, the farside is mostly devoid of dark lunar maria that sprawl across the Moon’s perpetual Earth-facing hemisphere. Only the small dark spot of the farside’s Mare Moscoviense (Sea of Moscow) is clear, at the upper left. Planet Earth’s north pole is near 11 o’clock, with the North America visited by Hurricane Dolores near center. Slight color shifts are visible around the lunar edge, an artifact of the Moon’s motion through the field caused by combining the camera’s separate exposures taken in quick succession through different color filters. While monitoring the Earth and solar wind for space weather forcasts, about twice a year DSCOVR can capture similar images of Moon and Earth together as its crosses the orbital plane of the Moon.

From a Million Miles Away, NASA Camera Shows Moon Crossing Face of Earth  NASA – August 6, 2015
These images were taken between 3:50 p.m. and 8:45 p.m. EDT on July 16, showing the moon moving over the Pacific Ocean near North America. The North Pole is in the upper left corner of the image, reflecting the orbital tilt of Earth from the vantage point of the spacecraft. This animation features actual satellite images of the far side of the moon, illuminated by the sun, as it crosses between the DSCOVR spacecraft’s Earth Polychromatic Imaging Camera (EPIC) and telescope, and the Earth – one million miles away. EPIC maintains a constant view of the fully illuminated Earth as it rotates, providing scientific observations of ozone, vegetation, cloud height and aerosols in the atmosphere. Once EPIC begins regular observations next month, the camera will provide a series of Earth images allowing study of daily variations over the entire globe. About twice a year the camera will capture the moon and Earth together as the orbit of DSCOVR crosses the orbital plane of the moon.

History

Scientists have been able to reconstruct detailed information about the planet’s past. The earliest dated Solar System material was formed 4.5672 ± 0.0006 billion years ago, and by 4.54 billion years ago (within an uncertainty of 1%) the Earth and the other planets in the Solar System had formed out of the solar nebula – a disk-shaped mass of dust and gas left over from the formation of the Sun.

This assembly of the Earth through accretion was thus largely completed within 10Ð20 million years. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4.53 billion years ago.

The current consensus model for the formation of the Moon is the giant impact hypothesis, in which the Moon was created when a Mars-sized object (sometimes called Theia) with about 10% of the Earth’s mass impacted the Earth in a glancing blow. In this model, some of this object’s mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to coalesce into the Moon.

Outgassing and volcanic activity produced the primordial atmosphere of the Earth. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans. The newly formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid – a contradiction dubbed the faint young Sun paradox.

A combination of greenhouse gases and higher levels of solar activity served to raise the Earth’s surface temperature, preventing the oceans from freezing over. By 3.5 billion years ago, the Earth’s magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.

Two major models have been proposed for the rate of continental growth: steady growth to the present-day and rapid growth early in Earth history. Current research shows that the second option is most likely, with rapid initial growth of continental crust followed by a long-term steady continental area.

On time scales lasting hundreds of millions of years, the surface continually reshaped as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (Ma), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600Ð540 Ma, then finally Pangaea, which broke apart 180 Ma.

Snowball Earth

The Snowball Earth hypothesis proposes that Earth surface’s became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a “slushball” with a thin equatorial band of open (or seasonally open) water. 

The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the Huronian glaciation, which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the “Great Oxygenation Event”.

Geoscientists suggest ‘snowball Earth’ resulted from plate tectonics  PhysOrg – May 7, 2018
About 700 million years ago, the Earth experienced unusual episodes of global cooling that geologists refer to as “Snowball Earth.” Several theories have been proposed to explain what triggered this dramatic cool down, which occurred during a geological era called the Neoproterozoic. Now geologists suggest that those major climate changes can be linked to one thing: the advent of plate tectonics. Plate tectonics is a theory formulated in the late 1960s that states the Earth’s crust and upper mantle – a layer called the lithosphere – is broken into moving pieces, or plates. These plates move very slowly – about as fast as your fingernails and hair grow – causing earthquakes, mountain ranges and volcanoes.

Study debunks theory on end of ‘Snowball Earth’ ice age  PhysOrg – May 25, 2011
A team of scientists led by researchers from Caltech report in this week’s issue of the journal Nature that the rocks on which much of a theory on how the “Snowball Earth” ice age ended was based were formed millions of years after the ice age ended, and were formed at temperatures so high there could have been no living creatures associated with them.

Ice Once Covered the Equator  Live Science – March 5, 2010

Ancient Rocks Show How Young Earth Avoided Becoming Giant Snowball Science Daily – February 6, 2007
A greenhouse gas that has become the bane of modern society may have saved Earth from completely freezing over early in the planet’s history, according to the first detailed laboratory analysis of the world’s oldest sedimentary rocks.

Great Oxygenation Event

The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen (O2) in Earth’s atmosphere. Geological, isotopic, and chemical evidence suggest this major environmental change happened around 2.4 billion years ago (2.4 Ga).

Cyanobacteria, which appeared about 200 million years before the GOE, began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE the excess free oxygen started to accumulate in the atmosphere.

Free oxygen is toxic to obligate anaerobic organisms and the rising concentrations may have wiped out most of the Earth’s anaerobic inhabitants at the time. It was a catastrophe for these organisms. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth’s history. Additionally the free oxygen reacted with the atmospheric methane, a greenhouse gas, reducing its concentration and thereby triggering the Huronian glaciation, possibly the longest snowball Earth episode. Free oxygen has been an important constituent of the atmosphere ever since.

Ancient soils provide early whiff of oxygen   BBC – September 25, 2013
Oxygen may have been accumulating in Earth’s atmosphere hundreds of millions of years earlier than we thought. An international team has made the claim in Nature magazine after studying the oldest soils on Earth. The researchers say elements in the three-billion-year-old material show evidence for oxidative weathering. This is some 700 million years before the Great Oxidation Event when other geological data points to a dramatic rise in free O2 in the atmosphere.

Evolution of Life

At present, Earth provides the only example of an environment that has given rise to the evolution of life. Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago and half a billion years later the last common ancestor of all life existed.

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. 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.

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, 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.

The mystery of how Earth’s primordial soup came to life NBC – February 20, 2012
Just as species are believed to have evolved over time, the individual molecules that form the basis of life also likely developed in response to natural selection, scientists say. Life on Earth first bloomed around 3.7 billion years ago, when chemical compounds in a “primordial soup ” somehow sparked into life, scientists suspect. But what turned sterile molecules into living, changing organisms? That’s the ultimate mystery. By studying the evolution of not just life, but life’s building blocks as well, researchers hope to come closer to the answer.

Humble moss helps to cool Earth and spurred on life BBC – February 2, 2012
Primitive moss-like plants could have triggered the cooling of the Earth some 470 million years ago, say researchers. A study published in Nature Geoscience may help explain why temperatures gradually began to fall, culminating in a series of “mini ice ages”. Until now it had been thought that the process of global cooling began 100 million years later, when larger plants and trees emerged. The simple plants’ interactions with rocks are believed to be the cause. The humble moss has created the climate which we enjoy today.

Snowball Earth: Deep Freeze May Have Spawned Complex Life Live Science – October 27, 2010
The “Snowball Earth” idea that the planet was coated in ice for millions of years might help explain the emergence of complex animals, some scientists say. The Snowball Earth hypothesis Suggests the planet was covered from pole to pole with a thick sheet of ice, perhaps more than once, for millions of years at a time. Supporters of this proposal suggest the glaciations, which would have been the most severe in Earth history, occurred between roughly 750 million and 635 million years ago.

Life may have survived ‘Snowball Earth’ in ocean pockets BBC – December 14, 2010
Life may have survived a cataclysmic global freeze some 700 million years ago in pockets of open ocean. Researchers claim to have found evidence in Australia that turbulent seas still raged during the period, where microorganisms may have clung on for life. Conditions on what is dubbed “Snowball Earth” were so harsh that most life is thought to have perished. Following the Cambrian explosion, about 535 Ma, there have been five major mass extinctions. The most recent such event was 65 Ma, 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 million years, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. 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, affecting both the nature and quantity of other life forms. The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40Ð100,000 years. The last continental glaciation ended 10,000 years ago.

Internal Structure

Superdeep diamonds confirm ancient reservoir deep under Earth’s surface  PhysOrg – August 15, 2019
Analyses show that gases found in microscopic inclusions in diamonds come from a stable subterranean reservoir at least as old as the Moon, hidden more than 410 km below sea level in the Earth’s mantle.

Ancient crystals offer evidence of the start of Earth’s core solidifying

Planetary scientists have found strong evidence that suggests the Earth has an inner and an outer core. The inner core is believed to be solid, while the outer core is made up of molten material. Prior evidence has also indicated that the entire core was once liquid, but as the interior cooled, the innermost part began to crystallize. It is at this point that scientists disagree – some suggest the start of solidification began as far back as 2.5 billion years ago. Others believe it was much more recent – perhaps as recent as just 500 million years ago. In this new effort, the researchers have found evidence that supports the latter theory.

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle.

The crust is separated from the mantle by the Mohorovicic discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30Ð50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised.

Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1-0.5° per year.

Researchers confirm Earth’s inner core is solid but softer than previously thought   PhysOrg – October 19, 2018

New candidate for ‘missing element’ in Earth’s core   BBC – January 10, 2017
Japanese scientists believe they have established the identity of a “missing element” within the Earth’s core. They have been searching for the element for decades, believing it makes up a significant proportion of our planet’s centre, after iron and nickel. Now by recreating the high temperatures and pressures found in the deep interior, experiments suggest the most likely candidate is silicon. The discovery could help us to better understand how our world formed.

Iron ‘jet stream’ detected in Earth’s outer core  BBC – December 19, 2016
Scientists say they have identified a remarkable new feature in Earth’s molten outer core. They describe it as a kind of “jet stream” – a fast-flowing river of liquid iron that is surging westwards under Alaska and Siberia. The moving mass of metal has been inferred from measurements made by Europe’s Swarm satellites. This trio of spacecraft are currently mapping Earth’s magnetic field to try to understand its fundamental workings. The scientists say the jet is the best explanation for the patches of concentrated field strength that the satellites observe in the northern hemisphere.

Molten ‘Jet Stream’ Discovered Deep Inside Earth  Live Science – December 19, 2016
A band of molten iron is churning slowly deep inside Earth, much in the same way as a jet stream, a new study finds. Scientists discovered the so-called molten jet stream while analyzing data from a trio of European satellites, called Swarm. The satellites launched in 2013 with the goal of studying Earth’s magnetic field. In this case, Swarm’s observations helped create a view akin to an X-ray of the planet, the researchers said.

Future of the Earth

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 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr. 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.

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 million to 900 million years. 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. After another billion years all surface water will have disappeared and the mean global temperature will reach 70 °C (158 ¡F).

The Earth is expected to be effectively habitable for about another 500 million years from that point, although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere. Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism, and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.

The Sun, as part of its evolution, will become a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km). 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). However, 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.