The Jupiter System
Jupiter itself is blazing hot from its gravitational collapse, radiating far more heat into space than it receives from the sun. It serves to warm it’s mini-solar system of moons to a habitable degree, and it’s rich with water.
The Galilean Moons
All the Galilean moons are tide-locked, with the side facing Jupiter being warmer and more habitable. Along the longitude line facing Jupiter, there would be light either from the Sun or from Jupiter at all times except during solar eclipses which range in duration between 2 and 5 hours, shortest for Io and longest for Callisto. Points away from this line of longitude would have periods of darkness with both the Sun and the lit side of Jupiter not visible. The orbital periods (and thus the days) are 1.33 days for Io, 3.44 days for Europa, 7.15 days for Ganymede, and 16.68 days for Callisto.
The four Galilean moons are all inhabited by the Galileani, four closely related species of bats. The Galileani originated on Callisto, the furthest out of the Galilean Moons. Since then, Jupiter has collapsed further, and grown colder, so Callisto is drifting into an eternal ice age, and the Galileani have drifted Jupiterward, with the Callistans dwindling away in the cold.
Io is the hot-house moon, Tropical over most of it’s Jupiterward face, and completely habitable on the cooler, temperate outer face, Io is the youngest, smallest, densest, and heaviest of Jupiter’s moons, and the best source of heavy metals. The Jupiterward face is fairly flat, covered in lakes and rivers, with no real seas. The outer face is more rugged, with mountains and seas, and a fair-sized ocean.
Io is home to the famous Fruit Balloon vines, long vine-like plants bouyed up by hydrogen-filled bladders that undulate like streamers in the wind. Fruits of this plant are among the most favored treats for Ioans. Io’s gravity is 0.183G
Europa is currently habitable over its entire Jupitarward face. Fairly recently, it was habitable on it;s back face, and there are still some smaller cities there with local artificial sources of heat. On the more densely populated Jupiter-facing side, the planet ranges from tropical directly under Jupiter to sub-arctic at the Jovian Terminator.
Both faces have large seas over much of the surface, dotted with mats of Europan Sea Lilies, a favorite source of nectar for the Europans. There are tropical rainforests where enormous blossoms grow on the flower trees beloved by the Europans. Europa has a gravity of 0.134G
Ganymede is currently habitable over most of the surface facing Jupiterward. Climates range from subtropical directly under Jupiter to arctic about two thirds of the way to the Jovian Terminator. There are lakes and seas all over the surface, and some cities still living under artificial light out into the tundra. The anti-Jovian side is completely uninhabited, with nothing living. The gravity on Ganymede is 0.146G
Callisto is currently slowly icing over, with only the equatorial area directly opposite Jupiter receiving enough heat from the giant planet to remain habitable. Most of the moon is iced over, with many great cities lost under the ice.
The habitable area has a central sea filled with meltwater, and peppered with small, densely populated islands. Surrounding this are cool forest lands with a small, roving population of Callistans. These dwindle into tundra and permafrost, and finally into ice. The moon looks remarkably like an eyeball staring into the depths of Jupiter’s clouds. The gravity on Callisto is 0.126G