Callisto

Callisto is the bigger of the two female Galilean moons of Jupiter, the other one is Europa, she is the second largest moon of Jupiter and the third largest moon in the system, she has 99% of Mercury's size but only a third of his mass, the fourth Galilean moon, and is the eighth moon in the Jovian system.

Callisto is the most heavily cratered person in the system. she is made out of water ice, carbon dioxide, silicates, and organic compounds, she has a small probability of hosting alien life on her water ocean.

she has an extremely thin atmosphere and an intense ionosphere, like her brother Ganymede, she was formed by slow accretion from Jupiter's disk during her birth, it took 100,000 years for hear to be born.

she has low radiation levels, because of this, she is the most suitable place for human bases during future exploration of the Jovian system

she loves to play video games, similar to what Ganymede does

Orbit and rotation
like most of her friends, she is tidally locked, so she keeps facing Jupiter, her rotational day is about 16.7 days.

her dynamical isolation means that she never experienced tidal heating in her life before

Composition and Internal Structure
her battered surface lies on top of a cold, stiff, and icy lithosphere that is between 80 and 100 kilometers thick. a salty ocean lies 150-200 kilometers deep beneath the crust, under that is a monoclinic layer of Ice 5, under that is a layer of rock and tetragonal Ice 6, under that is a layer of rock and cubical Ice 7, and under that is a rock and metal core.

Surface features
Callisto has the most heavily cratered skin of any solid object in the system, any new crater will erase an older one. her craters are shallower than that of the Moon.

two of her large craters are enormous, the Valhalla crater and the Asgard crater, these 2 craters formed when something hit her and concentrically fracturing and breaking her lithosphere, this resulted in massive bleeding, a historic example of a disruption was from the deceased comet Shoemaker-Levy 9.

Atmosphere and Ionosphere
Callisto has a tenuous atmosphere composed of carbon dioxide

Origin and Evolution
Callisto was born from accretion by Jupiter's subnebula

Life
like Europa and Ganymede, Saturn's moons Enceladus, Dione, and Titan, and Neptune's moon Triton, she could have a saltwater ocean, which can support extraterrestrial microbial life on her saltwater ocean, though Europa has the greatest chance of supporting microbial life

In Fiction
Callisto has never had as much appearances in fiction as the other 3 big moons do, though she appears in many works of fiction, examples include
 * in The Callistan Menace, Callisto has a way thicker atmosphere than she had and is inhabited by large slug-like creatures who use Jupiter's magnetic field to stun their prey
 * in Farmer in the Sky, efforts to introduce an atmosphere began on Callisto
 * the TV show Jupiter Moon took place in a space university on Callisto

Trivia

 * she was originally going to be male, but it was left out and she is female