It is the outermost of the four large Galilean moons of Jupiter,[3] which were discovered in 1610 with one of the first telescopes, and is today visible from Earth with common binoculars.

[12] It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts.

Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water[15] at depths greater than 100 km.

[19][20] Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide[9] and probably molecular oxygen,[10] as well as by a rather intense ionosphere.

Because of its low radiation levels, Callisto has long been considered the most suitable to base possible future crewed missions on to study the Jovian system.

Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell...

I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...[28][29]However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century.

[46] The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution.

[16][23] A salty ocean 150–200 km deep may lie beneath the crust,[16][23] indicated by studies of the magnetic fields around Jupiter and its moons.

[13][61] The cratered plains make up most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material.

[13] The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay[j] the surrounding terrain.

[63] Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean.

As mentioned above, small patches of pure water ice with an albedo as high as 80% are found on the surface of Callisto, surrounded by much darker material.

[7] High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs.

[7][13][14] Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features.

[7] In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.

Callisto's ionosphere was first detected during Galileo flybys;[21] its high electron density of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone.

Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements.

[68] The partial differentiation of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component.

[22] Such a prolonged accretion stage would allow cooling to largely keep up with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventing melting and fast differentiation.

[22] The further evolution of Callisto after accretion was determined by the balance of the radioactive heating, cooling through thermal conduction near the surface, and solid state or subsolidus convection in the interior.

[69] Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales.

[47] The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting differentiation that would have otherwise formed a large rocky core and icy mantle.

Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day.

[73][74][75] The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.

[77] As with Europa and Ganymede, the idea has been raised that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface.

The Galileo orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters of selected areas of Callisto.

[13] In 2000, the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra of the Galilean satellites including Callisto.

The European Space Agency's Jupiter Icy Moons Explorer (JUICE), which launched on 14 April 2023, will perform 21 close flybys of Callisto between 2031 and 2034.

[25][89] The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of the Solar System.