To'ul'hian worlds are an unusual variety of world named after the most famous of them, the homeworld of the To'ul'h. They might be regarded as a cool and wet version of the Cytherean type, or as very hot high-pressure Gaian worlds. Common imagination seems to prefer the first alternative, and in popular accounts To'ul'h worlds are often compared to the archetypical Cytherean world, Venus. Superficially they are indeed similar to true Cythereans. Their atmospheres consist primarily of carbon dioxide and their surfaces are concealed beneath layers of cloud and are subject to what most Terragens would regard as crushing pressures and intense heat (something in the region of 10 to100 bars, and 100 to 200 degrees Celsius). From a planetologist's or a xenobiologist's point of view, however, To'ul'hian worlds are much more similar to a standard Gaian world such as Old Earth. The fundamental similarity is liquid water at the planet's surface, and all that it implies — a carbon cycle, a geology involving plate tectonics and the possibility of life. As an early planetologist expressed it when the first To'ul'h type worlds were discovered, the result is not a hot dry oven world, but a moist and life-filled pressure cooker world. To'ul'h worlds represent a unique and meta-stable point in planetary development, and are perhaps best seen as being in a class of their own, though they are far outnumbered by standard Cytherean or Gaian worlds because of the relatively narrow set of initial conditions that allow them.
Like Cytherean or standard Gaian worlds, To'ul'hian worlds are geologically active over a sufficiently large span of time not to evolve into an Arean type. Like Cytherean worlds, and unlike typical Gaians, they typically receive strong insolation. Unlike the case for a Cytherean world such as Venus, however, they do not experience an early runaway greenhouse effect and a subsequent loss of their hydrosphere. On a typical Cytherean the heated oceans release volumes of water vapour into the air, increasing the greenhouse effect and resulting in the release of yet more water vapour. Water vapour in the upper atmosphere is split into its components by solar ultraviolet radiation and the hydrogen lost to interstellar space, so that over the course of a few hundred million years most of the planet's water is destroyed. Once there are no longer bodies of water on the surface the carbon cycle breaks down. Normally, as on a Gaian planet such as earth, carbon dioxide released by geological processes is incorporated into the crust again in the form of carbonates, but this process requires liquid water. Without water on the surface carbon dioxide is not recycled but instead builds up in the atmosphere. The result is an extremely dry and hot world, inimical to any Terragen-like biont life that might have evolved early in the planet's history.
To'ul'hian worlds avoid the fate of Cytherean worlds by refusing to dry out. A number of factors must combine to allow this, but the chief of them are a larger initial charge of volatiles in the planet's formation, a heavier gravity due to a large planet size, a star that is relatively poor in ultraviolet radiation and the early evolution or arrival of oxygen-producing life. All of these prevent or delay the progressive dehydration of a typical Cytherean world. A larger initial load of volatiles not only delays the eventual loss of oceans by providing a larger initial mass of water, but also slows the rate of water loss by diluting the water vapour in the atmosphere. Heavier gravity helps to retain some of the hydrogen so that its loss in the upper atmosphere is slowed. A lower flux of ultraviolet light means that water can survive for longer in the upper atmosphere before it is split into its component elements. The most important factor, however, is oxygen in the atmosphere. Oxygen leads to the formation of ozone, which has two important effects. Firstly, it absorbs ultraviolet light and therefore protects water vapour to a degree. Secondly, it is heated by its absorption of UV light, resulting in the formation of a thermosphere. This thermosphere helps form a "vapour trap" in the upper atmosphere and tends to prevent water from reaching such heights in the first place. Without such an effect no To'ul'hian worlds would be possible at all around any of the G type stars, and even those around the cooler and redder K stars would eventually become Cytherians. Of course all of these factors merely delay the inevitable. To'ul'hian worlds, just like standard Gaian worlds, are only meta-stable. Given sufficient time they become progressively drier and eventually evolve into the Cytherean state after all, or if they are small enough geological activity ceases, their thick carbon dioxide atmospheres are incorporated into carbonates, and they become warm Arean worlds.
The early development of oxygen is so critical to the development of To'ul'hian worlds that some have suggested that all are due to xenosophont interventions, either by early seeding of the world in its wet greenhouse stage or by later introduction of volatiles and To'ul'hian style life forms to an existing dry Cytherean. Seniis-2 and eir followers are the most ardent proponents of this idea. However, unlike the case of the Halogenics, in which a common biological origin and clear signs of sophont interference are evident, the evidence for such a hypothesis in the case of the To'ul'h worlds is rather weak. To'ul'h life forms are as diverse as more standard Gaian style life forms in their biochemistry and genetics, and like them seem to have several independent origins. There is some evidence that an extinct "seeder" xenosophont culture To'ul'hformed a dozen or more planets along the spinward marches of the Terragen expanse, but evidence of activity on the scale of the Halogenics is lacking.
Planetology, Geology, and Meteorology
The hydrosphere of To'ul'hian worlds varies considerably, from entire world-oceans to scattered seas barely sufficient to maintain a carbon cycle. Whatever their extent, such waters are acidic by Terragen standards. This is due to the high proportion of carbon dioxide in the atmosphere and the resulting formation of carbonic acid, as well as to small quantities of sulphuric acid. Wet or dry, the surface is poorly lit due to water clouds and to the "cloud forests" and "airplankton." Surface temperatures are high, usually in the 100° to 250° Celsius range, but the strong atmospheric pressure (10 to as many as 150 or more bars) is sufficient to keep water liquid. In the upper atmosphere there may be significant daily, seasonal or latitudinal differences, but the thick carbon dioxide atmosphere redistributes heat so efficiently that on the surface there is little temperature difference according to latitude, season or time of day. Altitude is far more important than latitude; local mountaintops are significantly colder than sea level temperatures, and cloud tops may be well below the freezing point of water. The atmosphere consists primarily of carbon dioxide and water vapour. Exclusive of its water content, which varies considerably, the remaining fraction consists of nitrogen and oxygen. The To'ul'h home world atmosphere, for instance, is 38% nitrogen and 9% oxygen by dry weight. Sulphur, tin and mercury compounds are all present in trace amounts due to the geochemistry of the surface, which concentrates these elements. Airborne dust, chiefly in the form of organic matter, is a large constituent. Weather patterns vary strongly according to the planet's spin and orbit; many such worlds that orbit close to a relatively dim star are either "one face" tidally locked worlds or else have some simple ratio of rotation to revolution that gives extremely long days. In such cases circulation in the upper atmosphere is strongly driven by heating of the subsolar point.
Winds in the depths of these worlds are extremely powerful due to atmospheric density. Even a moderate 10 kilometre per hour wind on a typical To'ul'h world is like a 100 to 1000 kilometre per hour gale on a Gaian world, so even the most sluggish surface wind can have tremendous effect. The infamous and misnamed "steam hurricanes" of the To'ul'h home world are one such example. Although the "steam hurricanes" do not cause the local pressure to drop below the local boiling point and cause all the local life forms to burst into steam, as sometimes misreported in the popular media, they do have tremendous power. Some of the air-dwelling life forms simply ride along in the storm as long as they have enough "sky room" and are not caught in a downdraft and cooked. Life forms adapted to the depths, if they are not battered beyond recognition as they are tumbled along the ground, may be cast into the upper regions by updrafts. There they may die of bitter cold or decompression effects (many have internal flotation bladders that explode if they rise too rapidly). On rare occasions, if they are raised very quickly and do not first die of the cold, their hot blood and other body fluids may indeed boil away just as depicted in the popular virchdramas.
Life on To'ul'h Worlds
To'ul'h life forms are broadly similar to those that have evolved on Gaian worlds, but the range of temperatures in the atmosphere between the surface and the cloud tops is such that an organism adapted to one of these environments is an "extremophile" by the standards of the other. The initial microbial life commonly evolves into several radically different macroscopic forms. Unlike the situation on a Gaian world, in which only one relatively narrow temperature regime prevails on the surface and all other life forms are necessarily subterranean and microscopic, there are ecological niches for complex macroscopic life at several levels in the To'ul'h biosphere. Thermophilic microbial life forms, freed from the constraints of a subterranean life, develop beyond the prokaryotic stage. There are several radically different eukaryote equivalents, each with a variety of multicellular forms. A typical To'ul'h biosphere contains not merely the five or so kingdoms of life typical of a Gaian world, but dozens. Although some rare types of To'ul'h life are able to travel between zones, or at least survive exposure to what are for them extraordinary conditions, most die quite quickly if removed to a deeper or shallower part of the biosphere. Some organisms near the cloud tops operate at temperatures and pressures that Terragens would find quite unexceptional, and aside from their adaptations to an atmosphere that is much richer in carbon dioxide and poorer in oxygen their biochemistry is not particularly unusual by Gaian standards. Surface life, on the other hand, is typically adapted to thrive in temperatures as high as 150° Celsius and to survive exposure to much higher temperatures in dormant or spore form.
All types of To'ul'h life incorporate elements such as tin or mercury that typical Gaian life would find toxic. The use of sulphur compounds is also much more common. These elements are more widely available than they would be on most Gaian worlds due to the high To'ul'hian surface temperatures. In other ways, however, To'ul'h biochemistry is fundamentally very similar to that of life found on Gaian worlds if one counts the full range of Gaian life forms such as the bacteria that live deep within the planet's crust, in the vicinity of black smokers or at hot springs.
Some To'ul'h worlds have a "dead zone" at the surface that is too hot even for the most thermophilic of water-based organisms, even though high pressure still maintains water in fluid form. At such temperatures too many key biomolecules are thermolabile and cellular processes cannot operate without becoming damaged, though inactive spores may persist until they reach cooler temperatures again. The archetypical To'ul'h homeworld is habitable over its entire surface and has no such dead regions, but there are some worlds in which life is found only on mountaintops and in the clouds, or only in the middle and upper parts of the atmosphere. Life on such worlds is not as rich as on those that permit a full To'ul'h biosphere because water and key nutrients tend to become trapped in the deep abiotic layers.
Because the nutrients and water are at the bottom of the atmosphere but the energy sources are at the top, To'ul'h ecosystems can be relatively unproductive unless there are strong vertical currents. Upwellings of nutrients due to atmospheric circulation patterns (most typically at the equator and at about 60 degrees poleward if the planet does not have a locked rotation) are very important, and there can be the equivalent of algal blooms after a strong storm. Airborne life forms of one sort or another are an important part of this nutrient cycling as well. This includes spores released by surface life and animal-like forms that forage on the surface but nest in the cloud forests. Most of the equivalents of algae and plants in the atmosphere must either descend to the depths for sustenance, receive nutrients from the droppings and bodies of surface foragers or somehow scavenge them from the dust, rain and mist in the atmosphere. The various "mat" symbionts as described below are an exception to the general rule.
The airborne life of a To'ul'h world is extensive. Plants usually make themselves neutrally buoyant by concentrating nitrogen in flotation bladders. The nitrogen is safely inert and less dense than the predominant carbon dioxide, so it is the most common method. Some other floating photosynthetic life forms simply retain the oxygen generated in photosynthesis, but this has the disadvantage of making them flammable. Lightning strikes in the cloud forests can be spectacular if the predominant species use oxygen. Others, usually fungus-like or animal-like organisms, generate methane gas with similar results. Many plants, especially the smaller species, simply use downy or sheet-like structures to remain aloft for the course of their lifetimes. Flying animals of various kinds are numerous, as in such dense atmospheres it is possible for extremely large creatures to remain aloft even with relatively small wings. Other animals are dirigible-like and either generate flotation gas themselves or pluck the gas bladders of the plant-like life forms to remain aloft. Some of these animals, especially the floating "skywhales" that browse on "airplankton" or on the cloud forests can reach tremendous size. The cloud forests, the various kinds of photosynthetic "airplankton" and their associated animal life extend from the thinnest parts of the upper atmosphere down to the surface, each adapted to a particular regime of temperature, pressure and light. Material from the upper layers constantly rains down on the surface below. The air of the lower atmosphere is dusty, and rain at those levels is typically muddy.
The foundation of the trophic pyramid at the surface consists of various fungus-like and bacteria-like decomposers, sessile animal-like filter feeders and various slow-moving detritivores. There are also many motile species, the equivalents of Old Earth animals, which feed on those or on each other. The omnivorous To'ul'h themselves are a typical example. Unlike the species of the atmosphere, many of which use vision and many of which may be brightly coloured, the creatures of the depths are black or colourless in ambient light. They may have huge eyes or none at all. In some cases parts of them are phosphorescent. Some depend primarily on sonar or on an electrical sense.
A common development in advanced To'ul'h ecosystems is a mat of two, three or even dozens or hundreds of symbiotic organisms with different temperature tolerances. The closest equivalents of such mats from Gaian worlds are lichen, corals with symbiotic algae and the multi-species colonies that form stromatolites. Such mats begin their life cycle on the surface where its hyperthermophilic life forms are active. At this stage they resemble a mass of fungal hyphae together with various sponge-like animals. These gather nutrients and generate flotation gases. Eventually the mat detaches from the surface, carrying with it a large volume of soil. As the mat rises species that are adapted to cooler temperatures come to the fore and the hot temperature surface life becomes dormant or produces spores. Decomposers and detritivores become less common, and various photosynthesizers become more common. Somewhere in the mid-to-upper layers photosynthesis becomes the primary source of energy and the mat becomes a variety of cloud forest with various plant-like species dominant. Filter feeders continue to gather nutrients from the dust in the atmosphere and saprophytes continue to recycle the available nutrients within the mass of the forest. The size and mass of the colony increases during this time as the plants fix more and more carbon dioxide and nitrogen, and the entire mat may fission as it grows. Each kind of association has a distinctive appearance and size (anything from a few square centimetres to many hectares in extent) beyond which the community has a tendency to fission. Finally, as micronutrients are gradually lost and the "soil" of the cloud forest becomes less and less fertile, the mat begins to break up. The pieces, carrying spores of all the various life forms, fall back to the surface to grow and sprout again in the detritus there. Such composite life forms are an important element in the nutrient recycling of To'ul'h world ecologies, and it is likely that the evolution of especially numerous and sophisticated mats of this kind accounts for the extraordinary fertility and diversity of the To'ul'h homeworld itself.
To'ul'hian worlds are most often brilliant white as seen from space, sometimes with tinges from the photosynthetic pigments of the cloud forests (green from a chlorophyll analogue is a common colour, though it is by no means universal). It is usually possible to distinguish the relative "deserts" where wind patterns tend towards the surface by their purer, whiter colour, while the regions of upwelling are stained with life. The cloud banks include entire islands and continents of growth adapted to the temperatures, light levels and nutrient availability of the different zones. As one proceeds downward the black of space gives way first to blue, then to various greenish hues, then to yellows and reds and finally to the dim reddish half-light of the depths, and the air becomes progressively smokier and dustier with organic and inorganic debris. The temperature gradually rises from a typical shirt-sleeve Terragen climate to that of a steam bath and finally to that of a pressure cooker. Finally, at the surface, the fungus-forests are crawling with benthic life. To human eyes the darkness is nearly complete except where the landscape is lit by lightning from some storm in the cloud decks above or by the natural phosphorescence of the fungi, the airplankton or local animals. Sounds, once high and thin, become increasingly and eerily loud as the pressure increases.