A Discovery That Changed Astronomy

In January 1610, Galileo Galilei pointed his telescope at Jupiter and noticed four small points of light that moved from night to night. Within weeks he had established that they orbited the planet — the first clear demonstration that not everything in the universe revolved around Earth. These four bodies, now known as the Galilean moons, remain among the most scientifically fascinating objects in the solar system.

Io: The Volcanic Powerhouse

Io is the innermost of the four and the most volcanically active body in the entire solar system. Its surface is in constant flux, painted vivid yellows, reds, and whites by sulphur compounds deposited by hundreds of active volcanoes. Some plumes reach heights of over 300 kilometres.

The energy driving this activity comes not from radioactive decay, as on Earth, but from tidal heating. Jupiter's immense gravity and the gravitational tugs of Europa and Ganymede flex Io's interior continuously, generating enormous frictional heat. As a result, Io has virtually no impact craters — its surface is resurfaced so rapidly that old craters are buried almost as soon as they form.

  • Diameter: ~3,643 km (slightly larger than Earth's Moon)
  • Orbital period: ~1.77 Earth days
  • Key feature: Hundreds of active volcanoes; sulphur-rich surface

Europa: Ocean World and Astrobiological Priority

Europa is perhaps the most scientifically compelling moon in the solar system from an astrobiology standpoint. Beneath its smooth, cracked, icy shell lies a global subsurface ocean of liquid water — kept liquid by the same tidal heating that torments Io, albeit more gently.

The surface is crisscrossed by a network of reddish-brown lineae — fractures filled with material welling up from below — and relatively few impact craters, suggesting the ice shell is geologically young and active. Europa's ocean is estimated to contain roughly twice the volume of all Earth's oceans combined.

NASA's Europa Clipper mission, launched in 2024, is en route to conduct detailed reconnaissance of the moon's habitability potential.

  • Diameter: ~3,122 km (slightly smaller than Earth's Moon)
  • Orbital period: ~3.55 Earth days
  • Key feature: Global subsurface liquid water ocean

Ganymede: The Solar System's Largest Moon

Ganymede holds the record as the largest moon in the solar system — at 5,268 km in diameter, it is bigger than the planet Mercury. It is also the only moon known to generate its own magnetic field, producing a small magnetosphere embedded within Jupiter's much larger one.

Its surface shows two distinct terrain types: ancient, heavily cratered dark regions and younger, grooved bright terrain created by tectonic activity. Like Europa, Ganymede likely harbours a subsurface ocean of liquid water sandwiched between layers of ice.

ESA's JUICE (Jupiter Icy Moons Explorer) spacecraft, launched in 2023, will eventually enter orbit around Ganymede — the first mission to orbit a moon other than our own.

  • Diameter: ~5,268 km (larger than Mercury)
  • Orbital period: ~7.15 Earth days
  • Key feature: Own magnetic field; likely subsurface ocean

Callisto: Ancient and Undisturbed

Callisto is the outermost of the four Galilean moons and one of the most heavily cratered objects in the solar system. Unlike the other three, it appears largely undifferentiated and shows little sign of geological activity. Its ancient, battered surface is a record of billions of years of impacts.

Despite its quiet exterior, Callisto may also host a subsurface ocean — suggested by magnetic field measurements from the Galileo spacecraft. However, unlike Europa and Ganymede, Callisto lacks the internal heat sources that might support more dynamic processes.

  • Diameter: ~4,821 km
  • Orbital period: ~16.69 Earth days
  • Key feature: Most heavily cratered surface; possible subsurface ocean

The Laplace Resonance

Io, Europa, and Ganymede are locked in a precise orbital resonance: for every one orbit Ganymede completes, Europa completes exactly two and Io completes exactly four. This 1:2:4 Laplace resonance is what maintains the orbital eccentricities that drive tidal heating on the inner moons — and it has persisted for billions of years.

Together, the Galilean moons represent a miniature planetary system and a natural laboratory for understanding ocean worlds, volcanism, and the conditions for habitability far from the Sun.