Europa

Europa is one of the colonized moons of Jupiter.

Geography
Like its neighbor Ganymede, Europa is a cold ice moon, with a surface made up of thick ice sheets that have a maximum thickness of 40 kilometers. It has only a trace atmosphere, primarily made up of oxygen compounds, with temperatures averaging -160oC (−260°F) at the equator and dropping to −220°C (−370°F) at the poles. The moon has unique geological activity where the ice sheet is broken up into distinctive tectonic plates, creating distinctive dark lines called linae where warmer ice spreads and cools on the surface. This process is driven by powerful gravitational interactions from Jupiter, which has resulted in Europa being tidally locked to the body. This has also led to damaging cryovolcanic activity and moonquakes, which most commonly occur on the leading face of the moon.

A subsurface ocean lies between 70 to 100 kilometers from the surface, and is heated by a combination of tidal activity and internal sources. There are some forms of native life that live here, but to the disappointment of the first colonists, these largely consist of bacteria and archaeans that are usually clustered around radioactive sources or volcanic vents. Still, this has led to the establishment of a thriving xenobiology research sector.

Due to intense radiation from Jupiter's magnetosphere, most permanent settlements on Europa were built deep within the ice sheet from early on. In the 24th century, artificially-maintained magnetospheres were built around Europa and the other close-orbiting Jovian moon-colonies, allowing the construction of surface-based (albeit still enclosed) communities.

Most traditional Europan cities are built around a wide central shaft, or a network thereof, with smaller sub-warrens of varying size crisscrossing between them. Though most of Europa's towns and cities are subterranean, most still reside within one kilometer from the surface due to cost and safety concerns. However, there have been attempts to establish settlements deep within its crust, and some have even reached the ocean layer. Human activity in the subsurface ocean is largely scientific in nature, though it also acts as Europa's primary tourist attraction. Of the hundreds of thousands of towns and cities catalogued, only a small fraction of these are actually inhabited, as Europa never recovered from the mass-exodus of the Domus Diaspora. It is only after the Human-Covenant War that the United Earth Government and Phoenix Initiative have looked into finally refurnishing these to alleviate the refugee crisis.

Europa's capital city, Katreus, is built around a complex network of elevators that run right through the moon's crust to reach the subsurface ocean, which are used for transferring goods between ships and submarines below. Katreus started as a mining town for ice and volatile substances and gradually expanded into a massive vertical urban sprawl.

Culture
Europa is home to Scuttlers, an umbrella term for subterranean human populations living in the ice sheets of Europa, Ganymede and a small handful of other Solar worlds; however, the Europan and Ganymedian Scuttler populations are by far the best known and most enduring. The Scuttlers descend from some of the earliest colonists of Europa and have a very particular cultural identity, distinct to the point of being regarded an indigenous population. However, they have long been a culture in decline. By the 26th century, most of the Scuttler populations had died out either due to offworld migration or assimilation to the newer communities which live in closer contact with the rest of the system. Most of the better-known Europan cities today share little of the Scuttlers' traditional culture and societal organization, though some vestiges of these characteristics have remained. The Scuttlers' famously independence-minded nature, particularly early on, drove some of their communities to extreme isolationism, which has caused some communities to simply disappear off the map over the centuries. A handful of these communities would be rediscovered only centuries later, subsequently becoming anthropological sensations due to their long-isolated nature.

A persistent conspiracy theory claims that Europa's subsurface ocean is home to a population of extremophilic bioengineered humans variously called "Merfolk" or "Phibians", supposedly created by corporate experiments or transhumanist splinter groups in the 23rd or 24th centuries. Purported "evidence" of these creatures includes blurry photos as well as various equipment malfunctions and disappearances in the moon's ocean. The ocean is fairly well-charted by the 26th century, and though small "Scuttler" communities have sometimes been found in the ice sheet, no evidence of the supposed ocean-adapted humanoids has surfaced, and most of the actual Europans regard the theory as offworlder exoticism. Proposals for radical human bioforming did occur in public discourse in the early centuries of human spaceflight, but nothing as drastic as the Merfolk is known to have ever been attempted. However, the idea has been the subject of various works of speculative fiction, cementing its presence in the public's collective imagination.