Earth

Earth is the third planet from the Sun and the only astronomical object known to harbor life. This is made possible by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its ocean, which covers 70.8% of Earth's crust. The remaining 29.2% of Earth's crust is land, which is predominantly located within Earth's land hemisphere in the form of continental landmasses. Most of Earth's land is at least somewhat humid and covered by vegetation, while large ice sheets at Earth's polar deserts retain more water than Earth's groundwater, lakes, rivers, and atmospheric water combined. Earth's crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth has a liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation. Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and ultraviolet light at entry. It is composed primarily of nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO 2 ), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun's light. This process maintains the current average surface temperature of , at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as carbon and nitrogen to cycle. Earth is rounded into an ellipsoid with a circumference of about . It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Overall, Earth is the sixth largest object in the Solar System after the Sun and the outer planets. Earth is about eight light-minutes (1 astronomical unit [AU]) away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at —1.28 light seconds—and is roughly a quarter as wide as Earth. The Moon's gravity helps stabilize Earth's axis, causes tides, and gradually slows Earth's rotation. Likewise, Earth's gravitational pull has already made the Moon's rotation tidally locked, keeping the same near side facing Earth. Earth, like most other bodies in the Solar System, formed about 4.5 billion years ago from gas and dust in the early Solar System. The formation of the ocean and the subsequent development of life occurred during the first billion years of Earth's history. Life spread globally and has been altering Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted the planet's environment. Humanity's current impact on Earth's climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.

Surface

image of Earth with an opaque cloud cover, making the different types of surface discernible: Earth's surface dominating Ocean (blue), Africa with lush (green) to dry (brown) land and Earth's polar ice in the form of Antarctic sea ice (grey) covering the Antarctic or Southern Ocean and the Antarctic ice sheet (white) covering Antarctica. Earth's surface is the boundary between the atmosphere and the solid Earth and oceans. The surface is continually being shaped by internal plate tectonic processes including earthquakes and volcanism; by weathering and erosion driven by ice, water, wind and temperature; and by biological processes including the growth and decomposition of biomass into soil. Earth's surface has an area of about and can be divided into two hemispheres: by latitude into the polar Northern and Southern hemispheres, by longitude into the continental Eastern and Western hemispheres, or roughly as Water and Land hemispheres. Most of Earth's surface is ocean water: 70.8% or . This vast pool of salty water is often called the world ocean, and makes Earth with its dynamic hydrosphere a water world or ocean world. Indeed, in Earth's early history, the ocean may have covered Earth completely. The world ocean is commonly divided into the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean, and Arctic Ocean, from largest to smallest. At Earth's polar regions, the ocean surface is covered by seasonally variable amounts of sea ice that often connects with polar land, permafrost, and ice sheets, forming polar ice caps. Earth's land covers 29.2%, or of Earth's surface. The land surface includes many islands around the globe, but most of the land surface is taken up by the four continental landmasses, which are (in descending order): Africa-Eurasia, America, Antarctica, and Australia. These landmasses are further broken down and grouped into the continents. Land can be covered by surface water, snow, ice, artificial structures, or vegetation. Most of Earth's land hosts vegetation, but considerable amounts of land are ice sheets (10%, not including the equally large area of land under permafrost) or deserts (33%). The pedosphere is the outermost layer of Earth's land surface and is composed of soil and subject to soil formation processes. Soil is crucial for land to be arable. Earth's total arable land is 10.7% of the land surface, with 1.3% being permanent cropland. Earth has an estimated of cropland and of pastureland.

Surface topography

Earth's surface topography comprises both the topography of the ocean surface and the shape of Earth's land surface. The terrain of the land surface varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from a low point of at the Dead Sea, to a maximum altitude of at the top of Mount Everest. The mean height of land above sea level is about .

Gravity and magnetic field

Moon and orbital space

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.

Atmosphere and climate

under moonlight with different layers of Earth's atmosphere visible: the troposphere with its clouds casting shadows, a band of stratospheric blue sky at the horizon, and a line of green airglow of the lower thermosphere around an altitude of 100 km, at the edge of space Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first of the surface; this lowest layer is called the troposphere. Energy from the Sun heats this layer and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy. The atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules. Water vapor content varies between 0.01% and 4% but averages about 1%. Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved , forming the primarily nitrogen–oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric into . The ozone layer blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature. This last phenomenon is the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be , in contrast to the current , and life on Earth probably would not exist in its current form.

Exosphere

The exosphere is where the atmosphere thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind. Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steady loss of the atmosphere into space. Because unfixed hydrogen has a low molecular mass, it can achieve escape velocity more readily, and it leaks into outer space at a greater rate than other gases. The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere. Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth. In the current oxygen-rich atmosphere, most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.

Troposphere

The troposphere lies below the stratosphere of the upper atmosphere, and immediately above Earth's surface (including the peplosphere). As such, it is the atmospheric layer of Earth's surface weather and climate. Its height varies with latitude, ranging between at the poles to at the equator, with some variation resulting from weather and seasonal factors. The atmospheric pressure at Earth's sea level averages , with a scale height of about .

Challenges for life on Earth

Extreme weather, such as tropical cyclones, occurs over most of Earth's surface and has a large impact on life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters. Human impact is felt in many areas due to pollution of the air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, and erosion. Human activities release greenhouse gases into the atmosphere, which cause global warming. This is driving changes such as the melting of glaciers and ice sheets, a global rise in average sea levels, increased risk of drought and wildfires, and migration of species to colder areas.

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