Europa
Overview
Europa is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 80 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Slightly smaller than Luna, Europa is primarily made of silicate rock and has a water-ice crust. It has a very thin atmosphere, composed primarily of oxygen. Its white-beige surface is striated by light tan cracks and streaks, but craters are relatively few. Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface is primarily due to the liquid water ocean that exists beneath the surface. The heat from tidal flexing allows the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below. Europa is by far the most populated body in the Solar System. A massive urban sprawl covers almost the entire surface, and several spines connect to the orbital ring station Damocles that encircles the moon. With a population density of approximately 30,000 persons per square kilometer, there are almost one trillion people who call Europa home. The moon serves as the Sol System's cultural, financial, and political capital. At the center of Capital City is the Concentric, the Consensus headquarters where representatives from across the Solar System meet to debate legislation. Surrounding the Consensus are the headquarters of countless agencies, institutions, and corporations that run the Solar System.Geography
Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter appear to hang directly overhead. Europa's prime meridian is a line passing through this point. Research suggests that the tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins slightly faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution due to the subsurface ocean. If this discrepancy does exist it affects Europa's rotation so slightly to be unnoticeable over hundreds of years.
The slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives it a source of heat, allowing its ocean to stay liquid while driving subsurface geological processes.
Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters. Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called lineae. The larger bands are more than 20 km across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material. These prominent markings crisscrossing Europa's surface are not geological features but merely albedo patterns that result from the heating and cooling of Europa's icy crust. At night Europan city lights mirror the moon's natural lineae, giving fluorescence to the surface features.
Climate
Europa has no oceans except for the layer of liquid water that exists beneath the surface, heated by tidal flexing. Europa's surface temperature is frigid which keeps Europa's icy crust as hard as granite. This subsurface ocean has many effects on Europan life. The most dramatic example is "chaos terrain", a feature on Europa's surface where the subsurface ocean has melted through the icy crust to form rivers, lakes, and seas. However, most areas have "thick ice", in which the ocean rarely, if ever, directly interacts with the present surface. It is estimated that "thick ice" is approximately 10–30 km deep, reaching 100km at its thickest.
In the areas known as "chaos terrain" the subsurface ocean has a direct impact on life above the surface. Over the course of the 3-day tidal cycle, liquid water can melt through the surface and flood the surface above. The water level in such rivers and lakes can change by dozens of meters over the course of the tidal cycle, requiring significant considerations for any infrastructure nearby. The largest of these features are the sister lakes Thera Maculo and Thera Macula, massive freshwater lakes nearly 50km across.
History
Europa's human history spans almost eight thousand years. While there have been discoveries of archaeological evidence of human occupation in the 22nd century, the dense layer of much younger ice obscures pre-Wipe and First Age sites. Several excavations support the view that Europan cities grew from subglacial enclaves built around the area of the future city of Conamara. During the First Age, most caves in the Conamara Chaos were populated with small enclaves. However, none of them yet had an urban quality or any standing structures above the ice. Nowadays, there is a wide consensus that the city developed gradually through the aggregation of several enclaves around the largest one through manmade subglacier tunnels.
Traditional stories handed down by the ancient Europans themselves explain the earliest history of their city in terms of legend and myth. The most familiar of these myths, and perhaps the most famous of all Europan myths, is the story of the Mother's Tears. The story goes that the original Europan colonists landed on the moon by accident, originally aiming for what they believed to be the warm, grassy green moon of Io. Upon realizing their mistake the colonists decided to stay in their ship as long as they could, preserving its warmth and air. The bravest young men would brave the cold, searching for any shelter from the brutal cold. As mothers wept for their dying sons the tears would leak from the ship onto the ground below. Due to their saltiness, the tears didn't freeze and eventually melted the ice below the ship, cracking it open to reveal safety in the caverns below.
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