Callisto

Callisto is the Solar System's third-largest moon.

Callisto is the outermost of the four Galilean moons.

Valhalla is a huge multi-ring impact structure on Callisto.

Callisto's densely cratered surface preserves ancient history.

Callisto's diameter is close to Mercury's, but its mass is much lower.

A natural satellite forms naturally and orbits a larger body; an artificial satellite is built by people.

Tidal locking matches rotation to orbital motion; it does not mean no rotation.

A moon's orbit need not lie in the same plane as its planet's orbit around the Sun.

Differences in gravity across a moon can deform it through tides.

Repeated tidal deformation can turn orbital energy into internal heat.

Orbital resonance means periods near simple integer ratios, allowing repeated pulls between moons to build up.

Some moons formed in discs around planets; others may have been captured later.

Small moons are often irregular because their gravity cannot overcome the strength of their materials.

Large moons tend to be rounder because gravity shapes them.

Icy moons may have liquid oceans under their shells; a frozen surface does not mean a fully frozen interior.

Finding water or organic molecules is not the same as finding life.

A planet and moon both orbit their common center of mass; the heavier body's wobble is usually smaller.

Other bodies perturb a moon's orbit; a fixed ellipse is only an approximation.

Moons can be larger than some planets; classification is not based on diameter alone.

The farthest of the four Galilean moons from Jupiter. Dense craters preserve a long history, and an ocean may exist inside.

A moon's visible brightness usually comes from reflected sunlight.

Moons have sunlit and dark sides too; the side facing away from the planet is not permanently dark.

Crater counts help estimate surface ages, but resurfacing and the impact environment also matter.

A moon hiding a background star causes an occultation, useful for measuring size or probing an atmosphere.

A moon entering its planet's shadow experiences an eclipse, depending on the orbits' spatial alignment.

Tiny changes in a spacecraft's speed can reveal a moon's mass and gravity field.

Composition, grain size, and lighting all affect surface color; enhanced-color images differ from naked-eye views.

The day–night boundary is the terminator; long shadows nearby bring out terrain.

Surfaces with little or no atmosphere lack strong weather erosion and preserve old impact scars more easily.

A moon's observed phases come from changing angles between the Sun, moon, and observer.