Future
Main article: Future of the Earth
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[24] 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.[55] After another billion years all surface water will have disappeared[25] and the mean global temperature will reach 70 °C[55] (158 °F). The Earth is expected to be effectively habitable for about another 500 million years from that point,[24] although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.[56] 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,[57] and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.[58]
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).[53][59] 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).[53] 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.[59]
Composition and structure
Main article: Earth science
Further information: Earth physical characteristics tables
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.[60] It also is the only terrestrial planet with active plate tectonics.[61]Shape
Main article: Figure of the Earth
The shape of the Earth is very close to that of an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[62] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the pole to pole diameter.[63] The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[64]
Local topography deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[65] The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.[66][67][68]
| Compound | Formula | Composition | |
|---|---|---|---|
| Continental | Oceanic | ||
| silica | SiO2 | 60.2% | 48.6% |
| alumina | Al2O3 | 15.2% | 16.5% |
| lime | CaO | 5.5% | 12.3% |
| magnesia | MgO | 3.1% | 6.8% |
| iron(II) oxide | FeO | 3.8% | 6.2% |
| sodium oxide | Na2O | 3.0% | 2.6% |
| potassium oxide | K2O | 2.8% | 0.4% |
| iron(III) oxide | Fe2O3 | 2.5% | 2.3% |
| water | H2O | 1.4% | 1.1% |
| carbon dioxide | CO2 | 1.2% | 1.4% |
| titanium dioxide | TiO2 | 0.7% | 1.4% |
| phosphorus pentoxide | P2O5 | 0.2% | 0.3% |
| Total | 99.6% | 99.9% | |
Chemical composition
See also: Abundance of elements on Earth
The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[70]The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right). All the other constituents occur only in very small quantities.[71]
Internal structure
Main article: Structure of the Earth
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 Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 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.[72] 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.[73]  Earth cutaway from core to exosphere. Not to scale. | Depth[75] km | Component Layer | Density g/cm3 |
|---|---|---|---|
| 0–60 | Lithosphere[note 9] | — | |
| 0–35 | Crust[note 10] | 2.2–2.9 | |
| 35–60 | Upper mantle | 3.4–4.4 | |
| 35–2890 | Mantle | 3.4–5.6 | |
| 100–700 | Asthenosphere | — | |
| 2890–5100 | Outer core | 9.9–12.2 | |
| 5100–6378 | Inner core | 12.8–13.1 |
Heat
Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[76] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[77] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[78] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3 billion years ago,[76] would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.[79]| Isotope | Heat release W/kg isotope | Half-life years | Mean mantle concentration kg isotope/kg mantle | Heat release W/kg mantle |
|---|---|---|---|---|
| 238U | 9.46 × 10−5 | 4.47 × 109 | 30.8 × 10−9 | 2.91 × 10−12 |
| 235U | 5.69 × 10−4 | 7.04 × 108 | 0.22 × 10−9 | 1.25 × 10−13 |
| 232Th | 2.64 × 10−5 | 1.40 × 1010 | 124 × 10−9 | 3.27 × 10−12 |
| 40K | 2.92 × 10−5 | 1.25 × 109 | 36.9 × 10−9 | 1.08 × 10−12 |
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