Earth

Earth is the 3rd planet from the Sun, as well as the largest rocky planet in the Solar System. It is also the first planet to have a moon, which is unsurprisingly called "The Moon", as it was the first moon discovered and named. Earth is also the most studied, well known, loved, taken care of and understood object in our Universe and has been since our very first primates firstly observed the landscapes and world around them.
Earth is one of the smallest planets in the Solar System. it is 12.756 km in equatorial diameter. It's actually 318 times less massive than Jupiter. It's nearly 10 times smaller in diameter than the mighty giant planet Saturn. Yet, Earth is more massive than all the rocky planets combined. The masses of Mercury, Venus and Mars combined, make up only 98% of the mass of Earth. Earth is just the largest rocky planet.
Earth is also a planet that has a great abundance of chemicals. It is the most dense of all the planets in the entire Solar System. Its density is 5.51 g/cm³. As it is the most abundant planet in chemicals, including oxygen, nitrogen, carbon, phosphorus, calcium, iron, magnessium etc. etc., Earth has life. Over 99% of all species that ever lived on Earth, since the very beginning of life are thought extinct. Approximately 8.1 billion people currently live on Earth, but we will likely pass that number and reach 11 billion until 2065 or so.
== Origins ==
Earth fromed nearly 4.543.000.000 years ago. It was formed a llitle while after the Sun formed. Earth was formed because there was a lot of materilas left after the extremely violent formation of the Sun. Many of these rocks and metals were clumped together. The big planet, Jupiter, entered the Solar System, breaking up some of the materials that formed the iiner rocky planets. 
Many rocks and metal created the inner rocky planets, Mercury, Venus, Earth, Mars, while Jupiter was moved away from the Sun, by the formation of Saturn. The asteroid belt exists between Jupiter and Mars, since Jupiter prevents asteroids from forming into another planet. Thankfully, Earth was in the habitable zone, where it was safe and sound with liquid water that came from meteors, and Earth could have life. 
== Composition ==
Earth is made out of the crust, mantle, the outer core, and the inner core. https://courses.lumenlearning.com/geophysical/chapter/the-composition-and-structure-of-earth/
=== The Crust ===
The crust layer contains oceanic crust and continental crust, which together make up the tectonic plates. Lithospheric processes, such as earthquakes, also happen here, thanks to immense siesmic energy being released from the tectonic plates.https://www.intechopen.com/books/earth-crust/introductory-chapter-earth-crust-origin-structure-composition-and-evolutionhttp://www.geo.mtu.edu/UPSeis/why.htmlEarth's crust contains a lot of chemicals that are vital for life and survival. Earth's oceanic crust makes up 70,8% while the rest 29,2% is made up of the crust of land. 
The crust's width ranges from 5 to 70 km. The mantle and crust are seperated by the Mohorovicic discontinuity, where earthquakes sharply increase in speed (1 km/s +).https://www.universetoday.com/40229/what-is-the-earths-mantle-made-of/Volcanic eruptions also happen, along with earthquakes, spitting materials out of the crust to the surface.
=== The Mantle ===
The mantle is divided into the upper and lower mantle, here, 100 to 200 km from the surface, rock reaches its melting point, upon which, it is turned into magma and later ejected out as lava. The outermost layer of the mantle, the Asthenosphere, is the molten matrix into which colliding tectonic plates, also known as Subduction zones, strike deep into. https://en.wikipedia.org/wiki/Mohorovi%C4%8Di%C4%87_discontinuity
Olivine, pyroxenes, garnets, spinels, perovskite and ferropericlase minerals are all found and created in the mantle. https://www.researchgate.net/figure/Simplified-cross-section-of-the-Earth-according-to-Hirose-and-Lay-2008-The-main_fig2_260672254
Separating the mantle and core is the Gutenberg discontinuity, otherwise known as the D" layer. This is where siesmic waves completely disappear, as the core below is liquid.https://sciencing.com/gutenberg-discontinuity-8747365.html
=== The Core ===
The core of earth is primarily made from an iron-nickel alloy, as well as other heavy elements, such as lead and uranium. https://en.wikipedia.org/wiki/Structure_of_the_Earth#Core In the past, Earth was mostly radioactive, but as temperature increased, iron and nickel were allowed to move more freely, creating the core. This caused gravitational potential energy to release, melting the entire core and covering it in silicate magma. https://en.wikipedia.org/wiki/Iron_catastrophe
This event, dubbed the iron catastrophe, gave the Earth the magnetosphere, shielding it from most of the Sun's radioactivity. 
The outer core is a low-viscosity, molten, nickel-iron encasing of the inner core. Inside of the outer core, eddy currents happen, which are electrical currents that are thought to give the Earth its magnetic field.
Unlike the outer core, the inner core is a completely solid, possibly being a single, giant iron crystal with temperatures of 5,400 to 5,700 Kelvin. The inner core is thought to be slowly growing, due to the inner-outer core boundary cooling 100 °C per a billion years. It is also theorized that the inner core rotates very slightly faster than the earth, about 0.3 - 0.5 degrees per year more than Earth. https://en.wikipedia.org/wiki/Earth%27s_inner_core
== The Atmosphere ==
The atmosphere of Earth is mostly comprised of N2 (78%), O2 (21%), argon (>1%), CO2 (>0.03%) and more. Earth's atmosphere also contains water vapor. The clouds that exist in the atmosphere are actually water that evaporates, because of the Sun. The further one goes, the thinner and lighter the atmosphere becomes, with the first 11 km on the surface making up 3/4 of all of its weight.https://en.wikipedia.org/wiki/Atmosphere_of_Earth
The atmosphere is divided into 5 main layers, the troposphere, stratosphere, mesosphere, thermosphere and exosphere.
=== Troposphere ===
This is the first and shortest atmospheric layer, going from the surface and thinning out at 12 km at the tropopause. It is the heaviest and warmest of atmospheres. Most of Earth's water vapor ends up here (99%), as its cooling point is too high to reach the freezing temperatures of the stratosphere. https://web.physics.ucsb.edu/~lgrace/chem123/troposphere.htm
Due to this, the water cycles completely happens here, with all clouds, from the Stratus to the Cirrus, being in the troposphere.
=== Stratosphere ===
The second layer, beginning from the tropopause and ending at 50 km above Earth. Here, the higher one goes, the warmer it gets, as the ozone layer absorbs the Sun's rays. https://en.wikipedia.org/wiki/Stratosphere
Here, many radioactive and dynamic interactions cause processes and circulations occur, dubbed the Brewer-Dubson circulations.
=== Mesosphere ===
The mesosphere goes from 60 km to 100 km. Altitudes of the scales of the mesosphere are usually called "near space". This term doesn't have a formal definition, but it usually refers to the region between the Armstrong limit, where water boils due atmospheric pressure (18-19 km), and the Kármán line, where astrophysics is used instead of aerodynamics.
Here, alike to an inverted troposphere, temperatures slowly decrease as one goes higher due to CO2 radiation emissions. https://en.wikipedia.org/wiki/Mesosphere
=== Thermosphere ===
The thermosphere is the last atmospheric layer with an accepted boundary, that being from 100 km to around 600 km.
Contrary to the mesosphere, the thin gases in the thermosphere can reach temperatures of 2,500 °C during the day, however, and observer wouldn't experience these temperatures, only coldness.
Just like oceanic tides on the surface, atmospheric tides of larger proportions occur at this level.
Interestingly, at the edge of the thermosphere, a 10 km thick band of elemental sodium occurs naturally. This band has a very low concentration, analogous to gas, however, it is used by scientists to create guided stars
=== Exosphere ===
This is the last official atmospheric layer of Earth, ranging from 600 km to 10,000 km (roughly). Mercury and several large moons only contain the exosphere without any lower atmosphere layers, referred to as a surface boundary exosphere.
The upper limit of the exosphere may not have a well-defined limit, it does have a lower limit, the exobase. Calculated using a full formula, it is the distance form Earth where barometric functions no longer apply.
== Habitability ==
Earth's habitability depends on multiple factors, which Earth abides by, as suggested by the copious amount of life present.https://learn.genetics.utah.edu/content/astrobiology/conditions/A habitable planet must be located within the goldilocks zone, Earth is located within the first half closest to the Sun, even with strict limits, Earth is clearly hospitable.http://depts.washington.edu/naivpl/sites/default/files/hz.shtmlA life-bearing planet must be made out of a solid substance, gas planets like Jupiter could theoretically only hold life on their cores, on which conditions are too extreme for life to form. Thankfully, Earth is primarily made out of rock and soil.A planet with life must have a molten core, as mentioned above, a molten core gives the planet a magnetosphere, which protects the planet from radioactivity emitted from the Sun.Hospitable planets must be good candidates for an atmosphere, atmospheres allow for comfortable temperatures for the organisms present and obtains our oxygen.https://www.windows2universe.org/earth/Atmosphere/overview.htmlThe chance for a hospitable planet is very low, much less with intelligent life, however, microorganisms can be found as close as the Moon, as they are much more common and durable.https://en.wikipedia.org/wiki/Reports_of_Streptococcus_mitis_on_the_Moon
==References==
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