Tellus

Tellus is the third planet from the Sun in the Tellus Solar System. It is the densest and fifth-largest of the eight planets in the Tellus Sector. It is also the largest of the Tellus Sector's four terrestrial planets. It is sometimes referred to as the world, but was most notably designated Earth prior to the Tellus Sector War.

The planet is home to millions of species of life, including Tellurians. Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global Tellurian population. These inhabitants were once grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Now, during the aftermath of the Tellus Sector War, Tellurians are migrating and regrouping on the continent of Atlantis.

Geographic features and structure
Tellus is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four terrestrial planets in size and mass. Of these four planets, Tellus also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation, and is probably the only one with active plate tectonics.

Tellus formed over 4.5 billion years ago, and life appeared on its surface within its first billion years. Tellus's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Tellus's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land. The physical properties of Tellus, as well as its geological history and orbit, have allowed life to persist.

Tellus's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface. Over 70% percent of Tellus's surface is covered with water, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

Tellus interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Tellus rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year. Tellus's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). The Moon is Tellus's only natural satellite. It began orbiting Tellus about 4.53 billion years ago. The Moon's gravitational interaction with Tellus stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.

Chemical composition
The mass of Tellus is approximately 5.98×1024kg. It 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. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

A geochemist once calculated that a little more than 47% of Tellus's natural crust consists of oxygen. The more common rock constituents of Tellus's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.

Heat
Tellus's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%). The major heat-producing isotopes in Tellus are potassium-40, uranium-238, uranium-235, and thorium-232. Much of the heat is provided by radioactive decay.

Tectonic plates
The mechanically rigid outer layer of the Tellus, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Tremors, volcanic activity,  mountain-building, and oceanic trench formation can occur along these plate boundaries. The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates, and their motion is strongly coupled with convection patterns inside the Tellus's mantle.

As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges.

Surface
Tellus's terrain varies greatly from place to place. A large portion of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus, and abyssal plains. The remaining portion not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies. After the war, given the devastation wrought upon the planet, continents shifted. Some land dropped below sea level while others rose.

The planetary surface also 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 from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts act to reshape the landscape in addition to any actions Tellurians have taken against their world's best interest.

Magnetic field
Tellus's magnetic field is shaped roughly as a magnetic dipole. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce Tellus's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes field reversals at irregular intervals averaging a few times every million years. The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of Tellus. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters Tellus's atmosphere at the magnetic poles, it forms the aurora.