Earth (Alternis)

INDEX / LEXICA / TIMELINE

The fifth planet in the Solar System, Earth is the second largest terrestrial world orbiting Sol and one of the few to harbor life. As the homeworld of Humans, Earth is the most populous world in the Solar System by far, and holds great importance across history. Most of the planetary surface is covered in liquid water, a feature found elsewhere only on the sister planet Theia, and it is one of the few planets known to have an active plate tectonics system. Earth's dynamic atmosphere sustains its unique living conditions, as well as protecting its surface from all manner of meteoroids and harmful radiation.

Earth is the homeworld of the Human species, one of two sapient species in the known universe. As such, it has been inhabited many orders of magnitude longer than any other world except Theia, homeworld of the Loriset. Its surface is populated by a great number of nations, most of which have been unified into the United Nations of Humanity, its capital set at the ancient city of Serra just north of the equator. Over seventeen billion humans populate the planet across every continent and even the oceans. Earth and its sister Theia also make up the only known binary planet in the universe.

Natural History

Formation

Earth, like the rest of the Solar System, is dated to approximately 4.56 billion years ago. By 4.54 Ga, the primordial Earth had coalesced out of a great number of planetesimals and as much dust as it could gulp down out of the protoplanetary disk. Surrounded as it was with dust and gases, it was inevitable for a collection of moons to form around the nascent world.

As the five moons moons of Lilith, Nestria, Waltemath, Pickering, and Petit formed, a fellow protoplanet came far too close to Earth, almost colliding with the planet. If it did, Earth and its moons would have likely been destroyed or at least severely scarred. Luckily for everyone, it didn't, and it became Theia. It captured the two far outer moons and entered a binary configuration with Earth, affecting both planets ever after.

Neither of these worlds were habitable yet by any stretch of the imagination. Between 4.1 and 3.8 Ga, numerous asteroid impacts in what is known as the Late Heavy Bombardment caused drastic changes to the surface geology of the largest moon, Waltemath, and by extension to that of Earth and Theia.

The early atmosphere of Earth, after the primordial hydrogen and helium was stripped by Sol's ignition, was formed alongside the first oceans by a spate of volcanic activity resulting in mantle outgassing. Water vapor condensed into the oceans as the planet cooled enough to allow that, presumably helped along by water and ice from unlucky asteroids and comets.

During this time, Sol had less than 80% of its current luminosity. The huge volumes of greenhouse gases expelled from the volcanic arcs of ancient Earth prevented the oceans from freezing completely, although they were likely covered in a crust of ice similar to modern Europa.

Origin of Life and Evolution

The first self-replicating molecules were created in random chemical reactions around four billion years ago, alongside similar events on Theia. The last common ancestor to all current Earthly life arose less than 500 million years later. Around 1.5 billion years after this, the evolution of photosynthesis caused an explosion in biodiversity as life became able to tap the energy of the sun. The process created an enormous amount of molecular oxygen, which accumulated in the atmosphere, formed the ozone layer, and finally made complex life on the surface possible.

Around this time, the development of eukaryotes occurred when smaller cells entered symbiotic relationships with larger ones, gradually merging over generations into a single organism. This paved the way to specialization, which itself made multicellular life possible as colonies became increasingly specialized. With the ozone layer shielding the planet from harmful radiation and these new advances making existence easier for everyone, life proliferated across the surface of Earth.

Within the last billion years, there have been periods when much of Earth was covered in ice. This era, known as the Cryogenian period, forced life to adapt to hardship, which gradually mounted into the Great Proliferation 535 million years ago after the ice finally cleared. In this very short period, multicellular life forms suddenly went from the occasional sessile disk glued to a rock to an enormous variety of fish species, increasing the number of species by at least two orders of magnitude.

Following the Great Proliferation, there have been six major mass extinctions and a great number of minor ones with miscellaneous causes. The most recent, caused directly by Human influences, ended less than four hundred years ago. The others were caused by all manner of things, ranging from the Nakaran Supernova 443 million years ago to the impact of a string of small asteroids sent towards the planet by Savis around 115 Mya to multiple occurrences of enormous volcanic activity.

When most large reptiles were killed off by that string of small asteroids, mammalian life had the chance to diversify away from the niche of "small rodent that will get eaten" to an enormous variety of species. Several million years ago, one of these new species gained the ability to stand upright, facilitating tool use and encouraging communication. This provided the evolutionary need and ability to create a larger brain, creating the first sapience on Earth. The first Humans had arrived.

Future

Without intelligent intervention, the future of Earth will be tied to that of Sol. Solar luminosity will continue to increase as it always has, reaching 110% its current value within 1 billion years. By 3.5 Gy from now, it will have increased by 40%. This increasing insolation will raise the surface temperature, accelerating the inorganic carbon cycle and leaving plants bereft of CO2 within 900 million years at best. The death of plants means that oxygen will stop being replenished, making animal life impossible as well. After this point, Earth's mean temperature may reach the level at which ocean water will begin to evaporate, leaving the planet as dry as a bone within 1.6-3 billion years and possibly starting a runaway greenhouse effect far worse than Venus. At least you can see that Venus has a surface!

In about 5 billion years, barring some insane solar engineering, Sol will leave the main sequence. It will begin to evolve to a red giant, consuming the other planets beneath Earth/Theia's orbit and expanding to around 1 AU. This process will cause it to shed over thirty percent of its mass, leaving Earth's eventual fate unclear. Depending on the extent of tidal effects, it may either move out to ~1.7 AU and join Mars in the former cold zone club, or it may enter the solar atmosphere. Even this has an unclear ending. Sol may collapse entirely as this is happening, ejecting much of the atmosphere, letting everything else collapse onto the inert carbon-oxygen core, and leaving Earth alive. Alternatively, the slow braking caused by the rareified atmospheric plasma will send the planet plunging into the denser layers of the star, destroying it altogether.

Human History

Human Evolution

The Human species evolved on the continent of <PLACEHOLDER> from the lineage of primates, separating from the rest of them roughly 7-5 million years ago. The ability to walk on two legs emerged a few million years later after the split from the rest of the great apes. The humanoid lineage began to use primitive stone tools ~3.3 million years ago defining the beginning of the Paleolithic era.

A 2.8 million year old specimen found in the desert outside Serra marks the earliest record of the human genus Homo, which branched into many species of archaic humans including the Neanderthals, Denisovans, and many other less populous examples. Hybridization occurred often between substantially diverged lineages, with genes of Denisovan origin present among the vast majority of the Human population.

Anatomically modern Homo sapiens emerged as its own species in Nakaa, the first specimens a few hundred miles outside the modern metropolitan sprawls of Ile-Ife and Serra. They continued to develop culturally and mentally, developing jewelry, funerary practices, projectile weapons, and seafaring technology by 50,000 years ago. Early artistic expression dates to this period as well.

Humans began to migrate from Nakai in multiple waves around this time. Many of the early waves of migration likely died out, but all modern non-Nakaians are descended from three groups that left Nakai at different times starting 177,000 years ago. This creates great differences in appearance between populations still detectable today. Humans reached the continent of Auster in the far north around 15,000 years ago, mostly completing the Human colonization of Earth. The warming climate forced the Neanderthals to migrate north, and they were gone before Humans even arrived, but the Denisovans likely interbred with humans to the point that a distinction no longer made sense. Human expansion, by "sheer coincidence", happened concurrently with the extinction of many animals and plants across the world.

Recorded History

Around 12,000 years ago, many groups of people began to independently develop the practice of agriculture, ending millions of years of hunter-gatherer dominance. At least nine separate centers of origin exist, including the Ilidari River valley in 9000 BFE, the Blue Rivers in 8500 BFE, the terrace farming of the Orocarni Mountains around the same time, and a great many others. Many of these developments were spurred by the fact that it was easy to undertake in that area, but others were forced by the preexisting difficulty of hunting and gathering in a given area, like the thousands of potato varieties engineered in the Orocarni.

After these largely planetwide improvements in food supply, population boomed. Early proto-cities appeared at Urumqi and Pretoria, possibly as early as the 9th and 8th millennia BFE. Concurrent with the farming revolution, herding societies also developed on lands too infertile to farm plants. These pastoral societies would evolve into a number of nomadic nations. Conflict between pastoral and sedentary farmers would recur for millennia across history.

The first civilizations arose around 6000 BFE, first in the marshland cities of the Ilidari Delta and the jagged, terraced peaks of the Orocarni range, followed by Kemet along the Saru River, the early Blue Rivers dynasties, and many more.

All of these societies developed a number of commonalities. These include central governments, recordkeeping systems, and distinct cultures and religions disparate from their predecessors and neighbors. These cultures also produced a great number of inventions, trading them with each other across great distances.

Writing, independently developed in at least five ancient civilizations and spread to their neighbors, facilitated everything that a civilization realistically requires to thrive. Most early writing systems were pictorial in nature, with the symbols gradually becoming more simplified and abstract as time went on and people realized that they were in a hurry.

Transport across civilizations were facilitated by a variety of mechanisms, ranging from the trusty walking-to-a-place-and-spending-the-next-decade-waiting-for-the-soreness-to-go-away to newfangled things like boats and other water transport. Bison-based cavalry and chariots allowed armies to move faster as well. Trade, facilitated by these mechanisms, became the backbone of civilization as the various urban societies grew rich off of exchanging manufactured goods for raw materials or other manufactured goods, creating impressively widespread commercial networks.

Cities are followed by the next level up: states and empires. In the 45th century BFE, the empires of Valakis and Menkar arose among the peaks of the Orocarni mountains. Along the Saru river, a variety of warring city states were finally unified by Emperor Hakoda of Khartoum, creating the first dynasty of Kemet around 5100 BFE. Around 4600, the Corat Valley civilization created great cities at Lahore and Karachi before unifying into what was likely the largest state of the time.

Miscellaneous cultures arose elsewhere, including the Wessley culture in the Degran archipelago c. 3500 BFE and the Kahl culture in southern Nakai prior to 24th century BFE. The Kahl culture was among the first to develop ironworking, completely skipping the bronze age, and the Wessley culture expanded across many uninhabited islands of remote Oceania, narrowly avoiding the early discovery of the west coast of Elizar by 2700.

In Elizar, the Lima-Sucre civilization rose on the coast of the northern lobe of the continent, to the west of the monumental Iron Mountains. They built great architectural achievements including the palace city of Lima, which began habitation c. 4627 BFE. They were likely destroyed by a combination of catastrophic earthquakes and flooding, as well as possible religious infighting. The later state of Acre avoided these issues by becoming the first state within the Iron Mountains. While the Orocarni across the ocean were inhabited by civilization just as much if not more, they were nowhere near as skyscrapingly high, with permanent snow beginning to appear at less than a third of their height. Other cultures gestating within the high valleys of the Iron Mountains include the Winge and the Loushine cultures, the latter of which carved images into the desert which appeared from the air to be perfect representations of human bodies in varying states. To the south of Lima-Sucre, Acre, and the Winge and Loushine cultures laid the Varlac people, who coalesced prior to the 31st century BFE in the lowlands to the east of the mountains, known for their impressive funerary practices, including great mausoleums and underground architectural projects. Southern Elizar was generally more easily hospitable for hunter-gatherers, removing some of the societal pressures which pushed the rising population to coalesce into city-states elsewhere, so the region was primarily populated by hunter-gatherer groups. These people did have a selection of permanent sites which they often used as meeting places, including a variety of great earthworks.

Physical Characteristics

Surface

Earth's surface is composed of a silicate crust of material, ranging from six to fifty kilometers thick. Most of the surface is covered in ocean water, with roughly 79% of it making up the world ocean. Indeed, in the planet's early history it may have covered the planet completely. The world ocean is generally divided into the Oyun, Materica, and Kugaya oceans, from smallest to largest, along with a great number of smaller seas as well as hydrologically separate freshwater lakes.

The ocean flows atop Earth's oceanic crust, the much thinner basaltic portion of the planetary crust, peppered with a number of features including vast abyssal plains, submarine mountains, oceanic trenches, and an impressive mid-ocean ridge system which creates new crust and drives tectonic activity. At some points in history, the oceans have been frozen, but there has not been a natural ice age for over a hundred million years. At the height of the Homeworld Wars, there were rumors of a supposed Loriset attempt to freeze the planet by blocking the sun, but such a thing would be so impractical and easily defeated that no military would actually do such a thing.

Earth's land covers <PLACEHOLDER>% of the planetary surface. Most of this is taken by the <PLACEHOLDER> continental landmasses, which are: Nakaa, Auster, <PLACEHOLDER>. The terrain of the surface varies just as much as it does beneath the sea, including vast mountains, deserts, lush plains, not-lush plains, and other landforms. The elevation varies from a low point of -502 m at Lake Kuei to the high of 9,072 m at the peak of Mount Kilane.

Land may be covered by surface water in the case of lakes, transient snow in the polar regions, ice, artificial structures, or the abundant vegetation of the temperate and tropical zones. Most of the planet's land is covered in lush vegetatation, including a variety of forests on every continent, but several areas of land are overtaken by deserts.

Tectonic Activity

Like most terrestrial planets, Earth's crust is topped by a mechanically rigid outer layer known as the lithosphere. A somewhat rarer feature is that the lithosphere is divided into several tectonic plates, rigid segments that move relative to each other over the liquid mantle.

As these plates move, oceanic crust is subducted beneath the leading edges of the plates they are impacting at convergent boundaries, and upwelling of mantle material creates replacement crust at divergent boundaries. Due to this recycling, most of the ocean floor is less than 100 million years old, with the oldest only barely breaking 200 in the eastern Oyun. This marks a distinction from continental crust, with the oldest dating back to over four billion years old.

Internal Structure

Like all planets, Earth's interior is divided into stratified layers by chemical or physical properties. The outermost layer is a very thin but chemically distinct silicate crust, sitting atop a the least viscous layer of the technically-not-solid but very slow-moving mantle. Beneath the mantle lies a completely liquified molten metallic outer core, atop a solid inner core. This core is in fact rotating slightly faster than the rest of the planet, advancing by 0.9 degrees per year. The inner core is so dense and compressed that matter experiences just as much downward gravity at the surface of the inner core as it does on the surface, despite most of the planet's mass being above it.

Chemical Composition

Earth is composed primarily of iron, oxygen, silicon, and magnesium, with the remainder consisting of traces of every other natural element. Gravitational separation led to most of the denser elements, like iron and nickel as well as those elements that liked to bond with them, to be pulled into the core. The vast majority of the crust is composed of oxide compounds of a variety of elements, most commonly silicon.

37.1% iron
26.1% oxygen
16.1% silicon
11.9% magnesium
2.9% sulfur
1.8% nickel
1.5% calcium
1.4% aluminum
1.2% trace elements

Internal Heat

Earth has a subtantial budget of internal heat, produced primarily by radiogenic means. Common heat-producing isotopes within Earth's structure include potassium-40, which produces the atmosphere's Argon in its decay, uranium-238, and thorium-232. The center may be up to 6000ºC, hotter than the surface of the sun, and with a pressure of up to 360 billion pascals. Early in Earth's history before short-half-life isotopes decayed away, the internal heat production was likely substantially higher, keeping oceans liquid under the dimmer sun, increasing the rates of mantle convection and plate tectonics, and helping create uncommon forms of igneous rock that rarely form today.

Magnetic Field

Unlike Venus, whose bare magnetic field is stimulated by solar interaction with the atmosphere, Earth's magnetic field is enormously powerful and generated by the molten outer core of the planet. This is the site of an enormous dynamo process, taking the convectional energy inherent to a temperature gradient and converting it into electrical and magnetic fields. This field is a large dipole, extending far past the Earth's physical structure and far enough into space to protect the sister planet Theia for much of its orbit about the barycenter. The poles of the dipole are highly inclined and mostly static, with the south magnetic pole currently wobbling in a field less than 400 kilometers from Serra.

Convection in the core is often unpredictable, causing the magnetic poles to drift and periodically flip entirely, averaging a few such flips every million years or so. The most recent one occurred in recorded history, showering early civilization with stunning planet-wide aurorae as the field reinitialized after the flip. The Sunrise Gallery outside Lutetia, among other museums, plays host to a number of impressive paintings using this event as a backdrop.

Solar wind pressure compresses the day side of Earth's magnetosphere to roughly 20 Earth radii, elongating the night side into a long tail. Due to the solar wind's "speed of sound" being slower than the wind's velocity, a supersonic bow shock precedes the field within the solar wind. Charged particles are also often swiped from the solar wind and trapped within the field, creating the plasmasphere, defined by low-energy particles following magnetic field lines, and the Mandragora radiation belts, filled with high-energy particles with functionally random motion.

Charged particles can often be deflected from the outer magnetosphere or even directly from the solar wind itself during particularly strong solar storms, and pulled along field lines into the atmosphere over the magnetic poles. This excites upper atmospheric atoms into a frenzy, creating stunningly bright aurora over the magnetic poles. During particularly strong solar storms, this happens over fully thirty percent of the planet. During the magnetic realignment period ending roughly 7,000 years ago, the magnetic field was of low enough strength after a pole flip that low-intensity aurorae were detectable across the majority of the planet.

Orbit and Rotation

Earth's orbit and rotation are inextricably tied to the planet Theia as it is with much of its history. Earth exists in a binary configuration with the planet orbiting 415,000 km from it, with a similar mass ratio to Pluto and Charon in the Kuiper Belt, and as such is heavily gravitationally affected by it. The two planets are tidally locked to each other, giving both a day of roughly 9 standard days or 213 hours and fixing them in each other's skies. The tidal lock also fixes Earth's axial tilt relative to the ecliptic at 7.9º, equal the inclination of the Earth-Theia binary orbit.

The binary pair orbits Sol, making the combined pair the fifth and sixth planets of the Solar System and a part of the Middle Solar System extending from Earth/Theia out to Phaeton. Their average distance to the Sun has been measured as 149,597,871 km, defining the Astronomical Unit (AU) and equalling ~8.3 light minutes. Earth and Theia orbit Sol every 365.2564 Standard Days, or 41.1584 Earth Days, defining the Standard Year.

The Earth-Theia pair has a sphere of influence extending roughly 1.6 million km away from the barycenter, marking the maximum distance at which an object is more gravitationally affected by the two planets than they are by the Sun or any other object.

Moons

Earth as three small to medium-sized moons in its orbit, all orbiting below 100,000 km due to gravitational interference from Theia.

Lilith

Lilith, the smallest moon of Earth, is a miniscule asteroid orbiting less than three earth radii away from the planet. Whipping around the planet in just over nine hours, orbiting twenty-three times in a single Earth day, Lilith was always going to be the easiest celestial body to reach. A memorial to this first landing remains today. It is likely a leftover from the original disk of material that coalesced around Earth as it formed, just like its larger companions.

In a few million years, tidal drag will cause Lilith's breakup into a thin ring system, which will slowly rain down on the planet after everyone manages to reinforce their ships to survive the constant dust bombardment, giving future Humans a light show for the ages.

Lilith was not accessible to public travel for most of the Homeworld Wars, with travel only being reopined after the recent armistice with the Loriset Intendancy. During this time, it was used as a top-secret military base, testing out the latest and greatest in destructive technology. This was an open secret, with one test coming close to destroying the entire moon in a very visible way.

Nestria

Nestria is the second and largest moon of Earth. It is the only moon in the Earth-Theia system that is indisputably in hydrostatic equilibrium, and has the largest population. Orbiting in a 1:2 resonance with Lilith, Nestria likely is why Lilith remains where it is. Its surface is pocked with craters and canyons, and its radius of 530 kilometers means that it more than covers the sun in the fairly common instances when its shadow crosses Earth's disk.

Nestria's surface supports a population of several hundred million in domed cities across the lunar surface, including the city of Verne amidst the shadowed icy craters of the southern pole. The moon orbits Earth once in just eighteen hours, visibly moving roughly 20 degrees across the sky every hour. Its tidal lock, close proximity, and stark features lead to some impressive views from its surface.

The moon spent the entire war under the control of the United Nations of Humanity, as its close proximity to Earth meant that its protection was a very high priority. With the armistice, Nestria is beginning to return to a peacetime state as it spent much of the war under martial law.

Waltemath

Waltemath orbits far beyond the other moons of Earth, at almost three times the distance of Nestria and three and a half times the orbital period. Waltemath is not quite as large as its sister moon, similar in size to Mimas. Again like Mimas, Waltemath's status of hydrostatic equilibrium is unclear, with the potential for an undifferentiated interior. Waltemath is a geologically inert body, pocked with craters left over from the origin of the solar system and with very few other surface features to speak of.

Waltemath's surface is quite bright for a silicate body, but nowhere near as white as any of the outer moons. The largest city, Wiggins, can be found on the near side of the moon near the equator in an impressively sized crater clearly visible from the surface of Earth under the correct conditions. While it is smaller than Nestria, it still maintains a roughly spherical shape.

Every moon that orbits just Earth is doomed eventually to be tidally pulled down, breaking up into a ring system after tens of millions of years, due to Earth's agonizingly slow rotation period. But of all the moons, Waltemath will last the longest at over a billion years after its companions are destroyed.

Theia

Earth is unique in the solar system for its enormous traveling companion, its sister planet Theia. The two planets are close enough in mass to prevent the stabilization of the lagrange points and pull their center of mass well out of Earth's surface. They are both habitable by the same kind of life, and it is likely that one or both planets were populated by panspermia. Nationalists on both worlds argue that their world is the source of all life, but serious scientists agree that there's a decent chance it was actually from Venus. After all, Venus had oceans less than 20,000 years ago.

Theia was captured in the early solar system after it narrowly missed a collision with Earth, which would have fragmented and possibly permanently destroyed the planet. It captured some of the material from Earth's budding debris disk, forming the moons of Petit and Pickering and forcing the rest to coalesce as Earth's moons before it had the chance to fall to the planet.

Theia is home to the Loriset, the only other known sapient species in the universe. Unlike in the case of Earth, more than one branch of genus Senecus remained alive by the end of the migration period, with a second Loriset-related species surviving reclusively in very small numbers in the mountains of Sekramar.

Both worlds can see the other hanging motionless in the sky, inspiring both civilizations to reach for the stars, for better or for worse.

Atmosphere

Earth is one of two worlds in the Solar System to possess a habitable, oxygenated, and hydrated atmosphere. The atmospheric pressure at its sea level is approximately 101 kPa, with a scale height of just under nine kilometers. The atmosphere is composed primarily of nitrogen, oxygen, and argon, with everything else that isn't the chaotically fluctuating proportion of water vapor making up less than a tenth of a percent of the gaseous composition. The aforementioned water vapor is varies between 0 and 4%, with an average of ~1%. This creates clouds covering ~3/5 of the planet's surface, with oceans being particularly adept at generating them.

Like Theia and unlike all other known worlds, Earth's atmosphere has been enormously affected by its biosphere. The photosynthesis which spread between cyanobacteria species between one and two billion years ago spurred the replacement of the original, rarified volcanic gas atmosphere, and enabled the proliferation of aerobic organisms and indirectly the formation of the ozone layer. This layer blocks all ultraviolet radiation, simultaneously making land life possible and making ultraviolet vision completely useless.

Earth's atmosphere also possesses a greenhouse effect of decent strength, where trace molecules capture thermal energy emitted from the surface and prevent it from escaping into space. This heat retention effect, caused by a combination of water vapor, carbon dioxide, methane, and other gases, raises the average surface temperature from a frigid -18º C to the current 15. Were this not the case, there would be life on neither planet.

Weather and Climate

While Earth's atmosphere, like all atmospheres, has no definite boundary, three quarters of its mass can be found within 11 kilometers of the surface. This layer, the troposphere, is the home of atmospheric circulation and wind. Earth's extremely slow rotation period causes poleward air at the top of the troposphere to traverse the entire distance between the equator and the poles before the coriolis effect can pull the air currents off course, negating the presence of circulation bands which exist on worlds like Mars and the gas giants.

Sol sends Earth ~1361 W/m² of irradiance on average, which decreases as latitude increases. This means that average sea level air temperature goes down by 0.4º C each degree of latitude away from the equator, on average. As a result, the planetary surface can be subdivided latitudinally into roughly homogeneous climate, into the tropical (equatorial), subtropical, temperate, and polar climates.

Proximity to oceans and the temperature circulation of the ocean and atmosphere can also affect a location's climate, as coastal regions tend to have less temperature variation due to the high specific heat capacity and enormous volume of water nearby. Atmospheric circulation also plays a role, with land-to-sea winds having different effects than sea-to-land winds. Finally, temperature decreases with height as there is less atmosphere to hold in the heat, resulting in very cold mountainous regions compared to their latitudinal companions.

Atmospheric circulation also facilitates precipitation of various varieties. Water condenses from the atmosphere and falls to the surface as either rain, snow or some other ice related form, before flowing downwards via river systems and entering a lake or ocean. This cycle is vital for the continuation of life on land and also facilitates erosion of the surface.

Upper Atmosphere

Above the troposphere, there are four universally accepted layers: the stratosphere, mesosphere, and thermosphere and exosphere. Each layer boundary is defined by the change in rate of temperature change with height, with the exosphere having virtually none as it merges with the magnetosphere and space as a whole. Due to this lack of hard boundaries, the edge of space is generally defined as residing exactly 100 km above the surface, where any object must achieve orbital velocity to gain any atmospheric lift.

A slow but steady atmospheric loss into space is caused by the boost that solar thermal energy provides to upper atmospheric molecules. Some of them can increase their velocity this way to escape Earth's sphere of influence entirely, particularly the very low massed molecular hydrogen. The loss of these reducing agents is widely assumed to have been a requirement for the continued advancement of life and oxygen's accumulation in the atmosphere. At the moment, most hydrogen is tied up in water before it can escape, and most of the loss comes instead from bonds in methane being broken.

Hydrosphere

The hydrosphere is defined as the sum total of Earth's water environment, composed of the global ocean, inland seas, lakes, rivers, groundwater, and atmospheric water vapor.

The mass of Earth's global ocean makes up approximately 1/4000 of the planet's total mass, and they cover roughly 79 percent of the planet (~402,956,880 km²) with a mean depth of ~2.8 kilometers resulting in a total volume of 1.128 billion cubic kilometers. Approximately 94.1% of Earth's water is saline, with the remaining 5.9% existing as "fresh" water. Most of this, over two thirds in fact, can be found in glaciers in the high mountains. The remaining is a fairly even split between ground water and surface water, with most of the surface freshwater being found in the Inland Seas of Rhan and Ringil on the continent of Elizar.

In the very coldest regions of Earth, found exclusively in the peaks of mountains like the Iron Mountains, the Orocarni, and the Spine of Nakai, snow survives the summer and gradually transforms into ice, creating glaciers over thousands of years. These bodies of ice are heavy enough to flow under their own gravity. Once, Earth was cold enough to form great ice sheets over Auster and the southern ocean. These glaciers erode the surface grievously, creating many permanent signs of their presence even after they finally melt away.

Earth's oceans have a salinity of approximately 2.9% salt of various types, most of which was released from cool igneous rocks or active volcanic activity. There are also a number of dissolved atmospheric gases, which are necessary for most aquatic life forms to continue to survive. Earth's oceans also act as a large heat reservoir, preventing wild temperature swings over the course of the long day.

Ecosphere

Civilization

Nations

Urban Areas

Human Environmental Effects