Earth facts for kids

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Earth Earth symbol.svg
Nasa blue marble.jpg
Earth seen from a satellite
Names
Other names Terra,
the World
Orbit
Reference date J2000.0
Longest distance from the Sun 152,097,701 km
1.0167103335 AU
Shortest distance from the Sun 147,098,074 km
0.9832898912 AU
Longest distance from the center of its orbital path
("semi-major axis")
149,597,887.5 km
1.0000001124 AU
How long it takes to complete an orbit 365.256366 days
1.0000175 yr
Average speed 29.783 km/s
107,218 km/h
Angle above the reference plane
("inclination")
1°34'43.3"
to the invariable plane
Size and other qualities
Average radius 6,371.0 km (3,958.8 mi)
Surface area 510,072,000 km²
Volume 1.08321 × 10¹² km³
Mass 5.9736 × 1024 kg
Average density 5.515 g/cm³
Surface gravity 0.99732 g
Escape velocity 11.186 km/s
Avg. surface temp. 14°C

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon (which is Earth's only natural satellite). Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates around its axis about 366.26 times.

Earth's axis of rotation is tilted, producing seasons on Earth. The gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, and gradually slows its rotation. Earth is the densest planet in the Solar System and the largest of the four terrestrial planets. Earth is situated in the Milky Way and orbits about 28,000 light-years from its center.

Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water, mostly by oceans. Earth is the only planet in our solar system that has a large amount of liquid water. About 71% of the surface of Earth is covered by oceans. Because of this, people some times called it "blue planet".

The remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, and a convecting (currents carrying heat from the interior to the planet's surface) mantle that drives plate tectonics.

Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the rapid increase of aerobic and anaerobic organisms, these are organisms that do not require oxygen for growth. Some geological evidence indicates that life may have arisen as much as 4.1 billion years ago.

Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that ever lived on Earth are extinct. Estimates of the number of species on Earth today vary widely; most species have not been described. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans have developed diverse societies and cultures.

Jupiter, Earth size comparison
Jupiter, Earth size comparison
Neptune Earth Comparison
Neptune, Earth size comparison

Geological history

Earth's atmosphere and oceans were formed by volcanic activity and outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.

A crust formed when the molten outer layer of Earth cooled to form a solid. Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Over the period of hundreds of millions of years, the supercontinents have assembled and broken apart. Roughly 750 million years ago, one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, then finally Pangaea, which also broke apart.

Pangaea colors
Pangaea with present-day continent boundaries

Origin of life and evolution

Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose. The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant oxygen accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer in the upper atmosphere.

Among the earliest fossil evidence for life are fossils found in Western Australia. The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of micro-organisms.

Human Evolution Panorama Under Construction - Science Exploration Hall - Science City - Kolkata 2015-12-04 6858
Human Evolution - Science City, India

During the Neoproterozoic, much of Earth might have been covered in ice. This has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion. Following the Cambrian explosion, there have been five mass extinctions. The most recent such event was when an asteroid impact triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared some small animals such as mammals.

Mammalian life expanded rapidly and branched out, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. This led to tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the evolution of humans.

Future

The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819
The Sun

Earth's expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%. The Earth's increasing surface temperature will accelerate the inorganic carbon cycle, reducing Carbon dioxide concentration to levels lethally low for plant photosynthesis in approximately 500–900 million years.

The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible. After another billion years all surface water will have disappeared and the mean global temperature will reach 70 °C (158 °F).

The Sun will evolve to become a red giant in about 5 billion years. Models predict that the Sun will expand to roughly about 250 times its present radius. Most, if not all, remaining life will be destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level). A 2008 simulation indicates that Earth's orbit will eventually decay due to tidal effects and drag, causing it to enter the Sun's atmosphere and be vaporized.

Physical characteristics

Iron Ore
Iron Ore

Chemical composition

Earth's mass 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. The core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia, potash and soda.

Internal structure

Earth-cutaway-schematic-english
Cutaway diagram of Earth's internal structure

Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical properties. The outer layer is a silicate solid crust, which is underlain by a highly viscous (sticky consistency between solid and liquid) solid mantle. The thickness of the crust varies from about 6 kilometres (3.7 mi) under the oceans to 30–50 km (19–31 mi) for 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 composed.

The structure of Earth changes from the inside to the outside. The center of earth (Earth's core) is mostly iron (88.8%), nickel (5.8%), sulfur (4.5%), and less than 1% other elements. The Earth's crust is largely oxygen (47%). Oxygen is normally a gas but it can join with other chemicals to make compounds like water and rocks, 99.22% of rocks have oxygen in them. The most common oxygen-having rocks are silica, alumina, rust (made with iron), lime (made with calcium), magnesia, potash (made with potassium), and sodium oxide, and others. The radius of the inner core is about one fifth of that of Earth.

Tectonic plates

Tectonic plates
Tectonic plates

Earth's rigid outer layer is divided into tectonic plates. These plates are rigid segments that move relative to each other at one of three boundaries types:

Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 million years old. By comparison, the oldest dated continental crust is 4030 million years old.

The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American.

Surface

Pacific Ring of Fire
Volcanic arcs and oceanic trenches partly encircling the Pacific Basin form the Pacific Ring of fire, a zone of frequent earthquakes and volcanic eruptions

The total surface area of Earth is about 510 million km2 (197 million sq mi). Of this, 70.8%, or 361.13 million km2 (139.43 million sq mi), is below sea level and covered by ocean water. Below the ocean's surface the continental shelf, mountains, volcanoes, oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system.

The remaining 29.2%, or 148.94 million km2 (57.51 million sq mi), not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms.

Tectonics and erosion, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape the Earth's surface over geological time.

The most abundant silicate minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine. Common carbonate minerals include calcite (found in limestone) and dolomite.

The elevation of the land surface varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest.

Marianatrenchmap
Mariana trench map

Hydrosphere

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from other planets in the Solar System. Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of 10,911.4 m (35,799 ft).

About 97.5% of the water is saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir. Shifts in the oceanic temperature distribution can cause major weather changes, such as the El Niño–Southern Oscillation.

Atmosphere

MODIS Map
Satellite image of Earth cloud cover

The air animals and plants use to live is only the first level of the air around the Earth (the troposphere). The day to day changes in this level of air are named weather; the changes between places far away from each other and from year to year are named the climate. Rain and storms are both in this level. Above this first level, there are four other levels.

The air gets colder as it goes up in the first level; in the second level (the stratosphere), the air gets warmer as it goes up. This level has a special kind of oxygen called ozone. The ozone in this air keeps living things safe from damaging rays from the Sun. Some gasses – especially methane and carbon dioxidework like a blanket to keep things warm.

The middle level (the mesosphere) gets colder and colder with height; the fourth level (the thermosphere) gets warmer and warmer; and the last level (the exosphere) is almost outer space and has very little air at all. The three outer levels have a lot of electric power moving through them; this is called the ionosphere and is important for radio and other electric waves in the air. It is also where the Northern Lights are.

Northern lights in Tromsoe
Northern lights in Tromsoe

Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism. The distance of Earth from the Sun, as well as its orbit, rate of rotation, axial tilt, geological history, sustaining atmosphere, and magnetic field all contribute to the current climatic conditions at the surface.

Natural resources and land use

Komatsu bulldozer pushing coal in Power plant Ljubljana (winter 2017)
Bulldozer pushing coal in a power plant for use in central heating and electricity)

Earth has resources that have been exploited by humans. Those termed non-renewable resources, such as fossil fuels, only renew over geological timescales.

Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas. These deposits are used by humans both for energy production and for chemical production. Mineral ore bodies have also been formed within the crust, these bodies form concentrated sources for many metals and other useful elements.

Earth's biosphere produces many useful biological products for humans, including food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.

Natural and environmental hazards

Pavlof2014iss
A volcano injecting hot ash into the atmosphere

Large areas of Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.

There is a scientific agreement linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.

Asteroids and artificial satellites

Tracy Caldwell Dyson in Cupola ISS
Tracy Caldwell Dyson viewing Earth from the ISS 2010

Earth has at least five co-orbital asteroids. The tiny near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.

As of April 2018, there are 1,886 operational, human-made satellites orbiting Earth. There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris. Earth's largest artificial satellite is the International Space Station.

Cultural and historical viewpoint

NASA-Apollo8-Dec24-Earthrise
Earthrise, taken by William Anders, an astronaut on board Apollo 8
ISS March 2009
ISS photo taken from shuttle Discovery 2005

Human cultures have developed many views of the planet. Earth is sometimes personified as a deity. Scientific investigation has resulted in several shifts in people's view of the planet. Initial belief in a flat Earth was gradually changed in the Greek colonies of southern Italy during the late 6th century BC. By the end of the 5th century BC, the sphericity of Earth was universally accepted among Greek intellectuals.

Earth was generally believed to be the center of the universe until the 16th century. Westerners before the 19th century generally believed Earth to be only a few thousand years old at most. It was only during the 19th century that geologists realized Earth's age was at least many millions of years, when radioactivity and radioactive dating were discovered. The perception of Earth shifted again in the 20th century when humans first viewed it from orbit, and especially with photographs of Earth returned by the Apollo program.

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Earth Facts for Kids. Homework Help - Kiddle Encyclopedia.