Planet:Prasandadr

Prasandadr is a life-bearing silicate planet, the fourth planet in the G-class Jaraccalt system and homeworld of the li. It has two moons, Jaratarna and Jaravarna.

Characteristics
Prasandadr has a mass of 1.23 Earth masses and a mean radius of 1.09 Earth radii, giving it a surface area of 6.07x108 km2 and an average surface gravity of 10 m/s2. It rotates eastwards on its axis once every 23.7 hours, and completes a full orbit of Jaraccalt every 370 local days. The axial tilt relative to the orbital plane varies over a 63,700-year cycle between approximately 22.66° and 25.34°, and is currently at 23.88°. It also precesses around in a complete circle over a 34,000-year cycle. Both of these cycles have lengthened over the planet's history as the orbits of its moons have increased in radius. Other cycles include the precession of Prasandadr's orbit and the variation of its inclination relative to the orbital plane; these both occur on scales of more than 100,000 years.

Prasandadr's atmosphere is primarily composed of nitrogen (74%) and oxygen (25%), with the remainder being primarily argon, carbon dioxide, and water vapour. The latter two of these, along with methane and ozone, act as greenhouse gases to raise the planet's average surface temperature from -20 °C to +10 °C. On the surface, the average pressure is 140 kPa and the scale height is 8.2 km. The scale height dictates the height of thermal columns in the atmosphere; it is smaller, and therefore large clouds are squatter, than on Earth. Having a higher density, clouds can also retain more water vapour. The difference in gravitational acceleration also magnifies imbalances in density, so winds can carry more force.

Interior
The average density of Prasandadr is 5.24 g/cm3, 95% of Earth's density, owing to the high proportion of silicates it aggregated. Prasandadr's composition is segregated into layers by the interplay of pressure and temperature on different minerals. The core is composed of iron and nickel (with about the same mass as Earth's) and is segregated into two layers: the inner core has a radius of just 644 km (due to stronger gravity, at a depth of 6300 km) and has a solid crystalline structure, while the outer core has a thickness of 2577 km and is liquid. The mantle and crust are dominated by silicates, and have a total thickness of 3773 km. The boundary that marks the lithosphere in the upper mantle is both chemical and mechanical, where rock begins to undergo brittle (rather than viscous) deformation, and varies dynamically at depth. Chemically, it consists of a scum of elements incompatible with crystalline structures lower in the mantle, and its solidity relates to heat flow.

As the planet rotates around its axis, kinetic energy is imparted to the liquid in the outer core, and it feels a force aligned with the rotation axis. This organises convecting liquid iron into columns, and because iron is electrically conductive, these columns circulate electric currents. This induces a magnetic field perpendicular to the direction of the electric current, as both are manifestations of the same field in different reference frames; and this magnetic field extends far beyond the atmosphere of the planet protecting it from solar wind and cosmic rays. It is slightly stronger than Earth’s, due to Prasandadr having a faster rotation and higher gravity.

All layers of the planet release heat, between 45 and 55 TW from the surface as a total budget. About half of this comes from the decay of radioactive isotopes (mostly in the mantle and crust), and the rest from the cooling of relic heat left over from formation of the planet and core. The mantle of Prasandadr is very thick and slightly warmer than Earth’s present day mantle, but is offset from being too warm because it formed in an earlier stage of the universe when radiogenic elements were less abundant. The deep mantle is under compression, and this slows convection. This sets the average thicknesses of the oceanic and continental plates, and they are slightly thinner than Earth’s.

Prasandadr has a supercontinent cycle, and continents are in recent times colliding together again.

Surface landforms
The lithosphere being thinner on Prasandadr than on Earth means that plate tectonics are more vigorous. New crust is created at a higher rate at mid-oceanic ridges; subduction of the oceanic plate also increases along with associated island arc volcanism. Topographic heights and lows are compensated for by regions of varying thickness and density directly beneath by reaching a gravitational equilibrium with the surface. For example, mountains may have equally sized roots beneath them, which rise as they erode. Likewise, basins filled with sediment thicken at the base. When the base of the crust sinks too far and begins to diffuse into the mantle, and this sets a limiting factor on the height of mountains. There are a number of factors why volcanoes and mountains are not as high as on Earth, partly due to the increase in gravity as well as being subject to greater wind erosion, but also because mountains have less time to form. Basins and inland seas are also shallower.

Geography
The geography of Prasandadr is subject to the effects of plate tectonics. As of the Assanocene epoch, the planet's surface is 65% covered in liquid water, the majority of which exists within the global ocean surrounding an emerging supercontinent. With a total water mass of 1.5x1021 kg, this gives the average depth of Prasandadr's hydrosphere as 3.8 km. The supercontinent is composed of the large northern landmass of Boreas and the smaller continents Arktos, Dusis, and Anatole; the plates containing the latter are being subducted under the Boreas Plate. Between Boreas and each of the other continents is a smaller sea, the product of convergent plate margins (continental and oceanic lithosphere) forming a basin via the subduction of the denser oceanic crust. These seas are much shallower than the global ocean, averaging 1.9 km deep compared to the ocean's 4.28 km.

Furthermore, the old, cold subducting crust is gradually heated, releasing water stored in the rocks and reducing the solidus of the (continental) mantle. This allows for the partial melt of magma (genesis of andesite rock) that rises to the surface with volatiles, creating explosive volcanoes. Importantly, this creates island arc volcanism, which contributes to the existence of a number of island chains within the seas and, to a lesser extent, in the global ocean where the Southern Oceanic Plate subducts under the Dusis Plate. While there are many mountain and volcanic ranges on Prasandadr, the strong winds caused by the dense atmosphere and large ocean cause many of them in the equatorial regions to be highly eroded, while those further north are worn down by glacial action.

Global climate and biomes
The Assanocene epoch saw Prasandadr enter an ice age. This glaciation is a consequence of changes in the planet's tilt, orbital eccentricity, and precession; and a long-term “icehouse” climate trend that links directly to the supercontinent cycle. The oceanic lithosphere on the far side of the planet spreads out over a wider area, allowing it to cool and sink. Deepening ocean basins cool and dissolve more CO2, while the glaciation event reduced global sea levels. Warm shallow seas near the equator proved productive to photosynthesising life and oxygen levels increased slightly.

Arktos and northern Boreas are now covered in a permanent ice sheet. South of this, the presence of shallow intercontinental seas results in drier climates, such as tundra and grassland steppe. In southern Boreas as well as Dusis, areas near the sea are warm and wet, featuring forest and grassland, but many of the inland areas range from dry grassland to arid desert due to moisture not moving far enough inland. Savannah and tropical grassland are the most widespread biomes. Anatole, being relatively small, is dominated by forest, woodland, and savannah; most of the tropical islands either are covered in rainforest, or were covered in rainforest before the assanli caused widespread deforestation during pre-industrial times.

The grassland environments are populated by some of the largest megafauna, which have generally slower metabolisms than more nimble animals of the forests. Increased oxygen levels could also contribute to the size and physiologies of various animals. Combustion is more intense with the heightened levels of oxygen, thus in semi-arid environments some trees have developed fire resistant bark.

Natural resources
Andesitic rocks are intermediate between "basic" and "acidic" rocks (which refers to SiO2 content), and despite being extrusive can have large crystals or phenocrysts (many extrusive rocks cool quickly so have smaller crystalline structure). These can form from the segregation of minerals crystallised in magma as temperature and pressure changes, leading to mineral deposits sometimes containing rare elements incompatible with the surrounding magma.

Both igneous and metamorphic rocks are equilibrated between pressure and temperature, and dependent on the bulk composition such regions are characterised by assemblages of minerals. A thin, warm lithosphere means that Prasandadr is tipped in the balance towards high temperature and low pressure zones. Anomalies can exist, particularly old, stiff regions of the crust known as cratons. These ancient regions were once intruded by komatiite magmas that formed when Prasandadr was even more active, drawing up diamonds from the upper mantle. As these cratons erode, diamond becomes exposed and often collects in riverbeds, along with various other magmatic ores of iron and copper, and rare-earths. The cratons could also retain a record of the great oxidation event 2-3 billion years prior, which created great quantities of iron oxide minerals.

Prasandadr’s shallow seas and river beds are highly fertile. The shallow seas of Prasandadr have accumulated salt deposits where they were subject to cyclical landlocking, as well as clay, silt and sand eroded from nearby mountains, while volcanic activity produces quantities of ash. Volcanic and hydrothermal regions are further associated with metal sulphide ores.

History
Prasandadr formed, along with Jaraccalt and the rest of its system, 3.8 billion years after the beginning of the universe, and cooled over the next half a billion years. Being the fourth planet, it lost out on some of the competition for iron and nickel (and some other iron-loving elements) during its planetesimal stage, such that it didn't form a large core. However, unlike its neighbors, Prasandadr accreted masterial in a region of the solar nebula where silicate-loving elements could condense (due to chemical differentiation caused by radiation pressure and heat), and did so without as much competition. It also happened to form with much of its water sourced from asteroids and comets with a high water to heavy water ratio (possibly disturbed inwards from the outer nebula), and as it cooled the water was released from outgassing and volcanic activity from a large silicate mantle, producing much of its atmosphere. Falling as rain, water began to fill the newly solidified basins and craters, collecting as early oceans. By this time, the core had already formed as denser elements like iron sank to the core.

Although Pransandadr originally formed as a double planet, a small but fast-moving planetoid collided with its smaller sister world early on. The remains of the original sister world reformed and became Jaravarna, while the debris that was launched to a higher orbit, along with some of the small planetoid, formed a planetary ring that later coalesced into the slightly larger Jaratarna.

Towards the end of this period, the first microorganisms appeared, with photosynthesis evolving a billion years after the formation of Prasandadr. It took a further billion years for atmospheric oxygen levels to become significant enough to trigger major ecological change, which involved the extinction of many anaerobic species and the rise of eukaryotes as a result of endosymbiosis. Multicellularity evolved many times over the next two billion years, but it was only towards the end of this period that complex animal life diversified into the numerous phyla that shaped the planet's later history.

The first terrestial life evolved 360 million years before the appearance of the assanli, but the direct ancestors of the assanli, placoderm-like aquatic creatures, did not become land-dwelling for a further 45 million years. One million years before the assanli became spacefaring, their genus evolved as a cooling climate led to the reduction of their forest homes along the southern Boreas coast, a trend that led to the Assanocene glaciation.

Recent
Tribes and early kingdoms of the assanli were afforded limitations and opportunities by their local environments. The Northern Steppe of Boreas was harsh, and inhabited by nomadic warriors. As cultures from the west removed forest for agriculture, this allowed them to move south. Meanwhile, cultures that inhabited the coasts enjoyed a diet of seaweed and shellfish.