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Solar System Moons Series

Callisto

The oldest world in sight: four billion years of unerased craters.

Author: Juan Carlos Euyoque Aguilera · October 7, 2026 · CC BY 4.0

Image of Callisto (NASA/JPL, public domain)
Callisto (Galileo spacecraft, NASA/JPL, public domain). Not to scale.
4,820.6 km
Diameter
1,883,000 km
Distance from Jupiter
16.689 days
Orbital period
~134 K
Temperature

Callisto, the outermost Galilean, orbits Jupiter at 1,883,000 km. Far from the tidal heating that stirs its sisters, its surface froze in time: it is the most heavily cratered in the solar system, with an estimated age of ~4 billion years.

That stillness makes it a time capsule: every crater is an impact that geology would have erased on other worlds. And beneath the ice, it too may hide an ocean.

Measured data [DATO]

DataValueStatusSource
Diameter4,820.6 km[DATO] observadoNASA Planetary Fact Sheets
Mass1.0759×10²³ kg[DATO] observadoNASA Planetary Fact Sheets
Mean density1,830 kg/m³[DATO] observadoNASA Planetary Fact Sheets
Surface gravity1.235 m/s²[DATO] observadoNASA Planetary Fact Sheets
Orbital period16.689 days[DATO] observadoNASA Planetary Fact Sheets
Rotation periodsynchronous (16.689 days)[DATO] observadoNASA Planetary Fact Sheets
Distance from Jupiter1,883,000 km[DATO] observadoNASA Planetary Fact Sheets
Temperature~134 K[DATO] observadoNASA Science
AtmosphereTenuous: carbon dioxide[DATO] observadoNASA Science
Notable featureSurface ~4 billion years old (crater counting)[MODELO] estimadoNASA Science

Truths [DATO]

  • [DATO] Callisto has the oldest surface in the solar system: ~4 billion years by crater counting, an intact record of the primordial bombardment.
  • [DATO] Unlike its sisters, Callisto is not fully differentiated: the ice and rock inside never completely separated.
  • [DATO] Galileo detected a tenuous carbon-dioxide atmosphere and an induced magnetic field hinting at an inner salty ocean.
  • [DATO] Valhalla, its largest impact structure, spans rings nearly 4,000 km across: one of the biggest scars in the solar system.

Labeled myths [MITO]

[MITO] Callisto is a dead moon of no scientific interest.

False: its surface is the oldest archive of the outer solar system, and its possible inner ocean keeps it on the worlds-to-study list.

Fuente: NASA Science

[MITO] Callisto has no atmosphere.

False: Galileo detected a tenuous carbon-dioxide atmosphere; tenuous is not the same as nonexistent.

Fuente: NASA Science

Open questions [PREGUNTA]

  • [PREGUNTA] Does Callisto have a subsurface ocean? Magnetic induction hints at it, unconfirmed.
  • [PREGUNTA] Why did its interior never fully differentiate?
  • [PREGUNTA] What does its crater record tell us about the early bombardment?

The chain in brief

  1. [DATA] 1. Far from Jupiter's kneading: The outermost Galilean, 1,883,000 km from Jupiter: far from the tidal heating that stirs its sisters, its surface froze in time.
  2. [DATA] 2. The oldest surface: It is the most heavily cratered in the solar system: ~4 billion years old by crater counting, an intact record of the primordial bombardment.
  3. [DATA] 3. A scar nearly 4,000 km wide: Valhalla, its largest impact structure, spreads rings nearly 4,000 km across: one of the biggest scars in the solar system.
  4. [DATA] 4. Never fully differentiated: Unlike its sisters, the ice and rock in its interior never completely separated.
  5. [DISPUTED] 5. An ocean beneath the ice here too?: Galileo detected a tenuous carbon-dioxide atmosphere and an induced magnetic field hinting at an inner salty ocean, unconfirmed; why its interior never fully differentiated remains open.