By 4.54 Gya the primordial Terra had formed. The formation and evolution of the Solar System bodies occurred along with those of the Sun. The assembly of the primordial Terra proceeded for 10–20 Ma. The process that led to the formation of the Luna approximately 4.53 billion years ago was formed by accretion from material loosed from Terra after a Mars-sized object impacted with Terra. Some of this object merged with Terra, and the rest of it had formed Luna. The mass of Theia was 10% of that of Terra, it impacted Terra with a glancing blow, and some of its mass merged with Terra.
Terra's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, protoplanets, and comets. Atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Gya, Terra's magnetic field was established, which prevented the atmosphere from being stripped away by the solar wind. A crust formed when the molten outer layer of Terra cooled to form a solid as the accumulated water vapor began to act in the atmosphere. Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from Terra's interior. On time scales lasting hundreds of millions of years, the continents have formed supercontinents.
Terra's terrain varies greatly from place to place. About 70.8% of the surface is covered by water, with much of the continental shelf below sea level. The planetary surface undergoes reshaping over geological time periods due to tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering and erosion from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts also act to reshape the landscape.
The elevation of the land surface varies from the low point of −418 m to 8,848 m. The mean height of land above sea level is 840 m. Besides being divided logically into Northern and Southern hemispheres centered on the poles, Terra has been divided arbitrarily into Eastern and Western hemispheres. Terra has four noticeable continents which comprise about 29% of the surface of Terra.
Some places on Terra are descriptions of paradise. Conditions on the surface are habitable for most forms of life, with the average temperature being 288 K (15 °C; 59 °F). However, with most celestial bodies, Terra has strange and extreme weather. Tornados are able to reach speeds higher than 200 mph (322 kmh), able to damage buildings with a carbon nanotube skeleton. Hurricanes and typhoons can reach similiar speeds and are orders of magnitude larger, reaching diameters as high as 1,380 mi (2,220 km).
Terra's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical properties. The outer layer 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, 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 composed. 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 km 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. Terra's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%) from elements such as uranium.
The natural abundance of liquid, natural, flowing water on Terra's surface is a feature that distinguishes Terra from other planets in the Sol System. Terra's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is in the Pacific Ocean with a depth of 10.9 kilometers. About 97.5% of the water is saline; the remaining 2.5% is freshwater. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers, frozen.
The atmospheric pressure on Terra's surface is about 1 atmosphere, or 101.325 kPa, with a scale height of about 8.5 km. It has a composition of 78% nitrogen and 21% oxygen, 1% argon, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The outline shine of the atmosphere has a blue tint, matching the color of the water on Terra's surface. Terra's atmosphere is standard of planets, decreasing in density the further away from the surface.
Terra's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, 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 O2 into O3. 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 known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C, in contrast to the current 15 °C, and life would likely not exist as Terra would literally be a snowball planet.
Climate and weatherEdit
This lowest layer of Terra's atmosphere is called the troposphere. Energy from Sol 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 primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°. Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.
The amount of solar energy reaching Terra's surface decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at lower angles and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator. Terra's surface can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical, subtropical, temperate and polar climates. Terra also has several extreme isolated zones, with temperatures ranging from 56.7 °C (134.1 °F) to −89.2 °C (−128.6 °F).
Terra is very habitable for carbon-based organisms. Terra provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism. The distance of Terra from Sol, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climatic conditions at the surface.
Like all planets, the main part of Terra's magnetic field is generated in the core, the site of a dynamo process that converts kinetic energy of fluid convective motion into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Terra's surface. The magnetic poles is very close to the geographical poles. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. Terra's Magnetosphere sufficiently protects the atmosphere from intense solar wind from Sol. In turn, the atmosphere itself further protects life on Terra.
Orbit and rotationEdit
Terra rotates once in about 24 hours with respect to the sun and once every 23 hours, 56 minutes and 4 seconds with sidereal rotation. Terra's rotation is slowing slightly with time; decreasing 1.7 milliseconds each century on average due to the effects of tidal braking by Luna. This is accounted for by leap seconds.
Terra lies at an average distance of 149.60 million kilometers (92.96 million miles) from Sol and a complete orbit occurs every 365.256 days (1 sidereal year), during which time Terra travels 940 million kilometers (584 million miles). Terra also has an aphelion of 1.0167 AU and a perihelion 0.98329 AU. It's Longitude of the Ascending Node is 174.9° and Argument of Periapsis is 288.1.
The axial tilt of the Terra is approximately 23.439281°. Due to Terra's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward Sol, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and Sol climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. In northern temperate latitudes,Sol rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. Sol rises south of true east in the summer for the southern temperate zone, and sets south of true west.
Luna is Terra's only natural satellite. It is one of the largest natural satellites in the Sol system, and, among planetary satellites, the largest relative to the size of the planet it orbits. It is named after the Roman goddess of itself.
Like almost all natural satellites, Luna is tidally locked with it parent object. It always shows the same face to Terra, which is a third smooth dark land. It is the second brightest object in Terra's sky, and is visible during day. Despite this, Luna has a very low albedo, about black as asphalt. Luna's orbital distance is about thirty times the diameter of Terra, causing it to have an apparent size in the sky almost the same as that of Sol, with the result that the Luna covers Sol nearly precisely in total solar eclipse.
Prehistory and HistoryEdit
Main article: Timeline of human prehistory
Main article: Bellum Romanum
Economy and human geographyEdit
Terra is the prime example of a Terran economy: a diverse, self-sufficient economy. Terra does not focus on a specific export as it is self-sufficient itself. It has a high population of nearly 15 billion people, being the biggest employment center and prime object of the Sol system. As homeworld of humanity, it holds an extremely important cultural rank. But also, it is the capital of the Conglomerate, which brings extremely important economic and political importance on an interstellar scale. Due to both of these, Terra is highly guarded and most ships in orbit around Terra are government spacecraft. As the origin of all Terra life, it also serves an important place in science and culture. In addition to this, Terra is habitable to nearly all humans, except for those who have never been exposed to higher gravity, who need assistance when visiting.