The Horizon System: Panthalassa, the Infinite Sea
1st A
Panthalassa (Temperate Oceanic Superterra, Planet)
System - Horizon-Actinophrys
Mass - 2.077 Earths
Radius - 8,278 kilometers (1.287 Earths)
Global Average Temperature - 33.48°C
Day Length - 8h 23m 37.5s
Year Length - ~2.908 years
Number of Satellites - 4
ESI - 0.941
Etymology - From the Panthalassic Ocean, a prehistoric ocean which occupied the basin of the Pacific during the Palaeozoic and Mesozoic eras. It once encompassed essentially all of the world’s seas, surrounding the supercontinent Pangaea during the Permian and Triassic periods.
Overview
Panthalassa is a world of moderate extremes. Though not the most massive, the hottest, the coldest, nor the most Earth-like, the unique properties of this super-Earth create an environment wholly unlike any other world in the Horizon System’s crown. Exceptionally water-saturated even among the ranks of the Resonant Worlds, Panthalassa’s crust is no more strong than wet tissue when confronted with the tremendous power of its geothermal dynamo, spurred on by its large mass and excessive concentrations of radioactive elements. Lubricated by ample surface water, Panthalassa does not undergo plate tectonics but rather a ‘distributed deformation’ tectonic mode in which huge belts of shifting, unstable crust bridge the gaps between stable interiors. Correspondingly, its surface is unlike that of any other planet, resembling the icy, crack-strewn surface of Uranus’s moon Miranda more than any terrestrial world of our Solar System.
The deformation belts that cover nearly half of Panthalassa’s surface manifest themselves as island archipelagos thousands of kilometers long mixed with scattered volcanic hotspots and huge, uplifted plateaus as large as Greenland, sometimes lying under only tens of meters of water. With the largest contiguous landmass on Panthalassa being only half the size of Australia despite the planet having 70% more surface than Earth, ocean currents are essentially free to flow unimpeded. Vast warm flows carry tropical heat straight to its poles, maintaining a globally warm and wet climate assisted by a carbon dioxide-dominated atmosphere several times thicker than Earth’s. Though surface conditions might be pleasantly tropical, abundant volcanic activity means that the atmosphere is unbreathable for Earth lifeforms, discounting even the crushing pressures.
Lifeforms
The low relative velocity between Horizon and Panthalassa means that material can be exchanged between them with comparatively little energy, allowing even relatively fragile organisms to survive the interplanetary journey. The spores of Horizonian flora and fauna have let many lineages establish a foothold on Panthalassa, to the point that some taxonomists have called it a ‘retirement home’ for extinct stem-lineages. Of course, this is not very fair, as the lineages in question have not remained primitive in their new home but rather taken advantage of ecological vacuities to evolve into all sorts of wonderful new forms. Though the two planets remain similar enough for the life of one to survive on the other, the much greater gravity of Panthalassa means that none of those who make their way here will ever see their erstwhile home world again.
Since natural selection usually favors the incumbents in competition for ecological space and because Panthalassa’s environmental conditions are different enough from Horizon’s that few lifeforms from the latter have a distinct advantage over its native biota, successful dispersal between planets is rare. Most of Panthalassa’s fauna trace their origins to one of two seeding events; one concurrent with the end-Caeliferan Mass Extinction 300 million years ago, which first brought complex life to Panthalassa, and one concurrent with the minor Hèxīng Event on Horizon and the major Late Erytholian Mass Extinction on Panthalassa 170 million years ago. By luck, this means that Panthalassa’s modern fauna is dominated by lineages from both the Archaeozoic and Plesiozoic Eras of Horizon, but not its present Hylozoic Era - this is where the ‘retirement home’ reputation comes from.
The flora of Panthalassa evolved more or less separately from that of Horizon, as the high tides raised by Panthalassa’s moon Mirovia provided an evolutionary impetus for the shallow-water flora that arrived at the first seeding to evolve desiccation-resistant adaptations. As such, it is more or less entirely separate from the Horizonian one. Where Horizon’s seas have violet meat-mosses and jet-black kelp forests Panthalassa has golden-yellow sponge moulds and fields of sea-silk nematophytes, while the few landmasses it has are blanketed in amphibian forests of fractal charniophytes and giant urchin-mosses, who crawled out of the sea some 70 million years before their counterparts on Horizon even considered leaving the water.
Panthalassan fauna, on the other hand, truly emulate a ‘lost world’. Though largely marine, animal life on Panthalassa is just as abundant and diverse as its counterparts on Horizon. The majority of the modern pelagic fauna belongs to the first wave of colonization and includes a great variety of bizarre stem lineages extirpated from Horizon in the Caeliferan, from piscine worms with myriad crystal teeth to carnivorous glass-curtain jellies, radiate, tail-brained crustaceans and bivalved, photosynthetic drifters. The benthic fauna is more mixed, as a large portion of it was taken out in the Erytholian extinctions, allowing newer lineages from Horizon to establish themselves. In some ways it resembles the modern marine fauna of Horizon, with forests of crinoid-like radiates, ponderous arthropodal flora and all-consuming columns of marching crab-squids, but these are only superficial convergences. Along with the seemingly modern forms are a whole assortment of now-extinct ones, from stilt-walking shells to titanic ceratopedes, and all the completely unique things that evolved in Panthalassa’s splendid isolation before them.
“You would brave the wrath of the Lightning’s Shadow for the sake of your broken cause? … I’m quite impressed your tenacity, youngster! But can you truly claim to have the people’s hearts on your side?
See, we don’t exactly take kindly to arrogant upstarts who think they know what other people want. Always going on and on about the ‘sanctity of life’ or ‘affronts to the works of God’, whatever that may mean. Not a single one I’ve seen ever stopped to consider whether the people they were ‘saving’ really wanted what they were offering, nor what their ‘God’ really thought of the whole ordeal.
Oh, you want to know what happened to the missionary delegation? Well, they’re currently serving as the Emperor’s biological testing department. Waste not, want not, you know my drift?“
- The Third Archon, “Shadows Before The Lightning’s Glow“, “Unofficial Response to the Manifesto of the Adeptus Revolt” (2247)
Past & Future
Panthalassa itself has changed relatively little since the days of its formation. Its atmosphere has always been thick, its climate has always been temperate, and it has always been effused with oxygen. Only a few times in the last 700 million years of biological history on this world has disaster come for its inhabitants, and every time they have recovered in full force. Like Horizon, Panthalassa’s atmosphere cleared of carbon dioxide clouds only some 100 million years ago. But while Horizon was dark and cold for much of that history, Panthalassa has always been warm. Even the sightless, sonar-pinging animals of that time never went anywhere, just retreating to the lightless depths of the abyssal seas when the haze cleared for the last time.
The co-orbital relationship between Horizon and Panthalassa is delicate, maintained only by the complex web of resonances between the other Resonant Worlds and even others further afield. But even they cannot preserve it forever; a catastrophic collision between the two is almost inevitable on a time scale of some ten billion years. Fortunately for the lifeforms of both worlds, by that point they will have nothing to worry about, having all dived into the fiery embrace of an aging, red-giant Actinophrys.
Civilization
Panthalassa appears as a pale blue dot, not unlike Chironex, Changxing, and Antipathes-Cerianthus. But unlike them, Panthalassa hangs near motionless in the night sky of Horizon, tracing out a long, looping path through the ecliptic over the course of a few centuries. At closest approach, Panthalassa grows from an infinitesimal point to a luminous disk large as our Moon in Horizon’s skies, then retreats the way it came and loops back across the sky. From the beginnings of Old Horizon to the modern day, the inhabitants of Horizon have always been fascinated with this world which comes just a few times every millennium. An approach that occurred during the middle of Horizon’s scientific revolution allowed early naturalists to make out its continents, its weather, and even blooms of plankton in its seas, definitively establishing the existence of complex life outside Horizon and triggering a technological race to the stars. As it is the only world in the Horizon System on which the Horizonians can breathe, there was even a colony there for a while, though the colonists seem to have lost their sapience in the 800,000 years since they last left the planet.
Today, though the Horizonian natives have long since forgotten the details of when they wandered between worlds, the oral game of telephone has not completely erased their awareness of Panthalassa. They now regard it as the closest of the cosmic realms, a plane populated with spirits far older than their oldest ancestors which drifts close to the mortal world once every few centuries, bringing great fortune or calamity depending on who you ask. Some communities even tell legends that long ago their ancestors built a ladder to the sky with which they settled this foremost among the cosmic realms, attesting to a dim cultural memory of their colonization attempts some million years ago.
For the spacefaring peoples of the Horizon System, Panthalassa is much less mysterious. As the most habitable natural world in the universe for Horizonian bioforms, they have wasted no time in shaping it to their own. In lieu of stable landmasses to drop a city on, the Denizens of Horizon have instead elected to construct their own. Masses of genetically modified flora and microbe-secreted bioplastics are bound together with carbon-nanotube cables into strong, flexible bases for seafaring cities sometimes tens of kilometers across. Though nomadic, these dense, drifting settlements are together home to some two billion people, making them the single biggest population center in the Horizon Moving Group, even trumping the capital world Horizon itself.
