The Galilean Moons of Jupiter: Dead Callisto, Cracked Ganymede, Ocean Europa, Volcanic Io

Spacecraft have shown how different Jupiter's four large moons, the Galilean moons, are. From 1996 to 1999 the Galileo spacecraft looped through the Jovian system, passing close to each, and the Juno orbiter has since taken close looks at Ganymede and Europa. Going inward from Jupiter's outermost large moon to its innermost, the moons tell a story of steadily rising geological activity.

Callisto: dead since the beginning

Callisto orbits about 2 million kilometres from Jupiter every 17 days, always turning the same face to the planet. At noon its surface is only 130 K, about 140 °C below freezing, so ice never evaporates. It is almost as wide as Mercury but only a third as massive, so it must be largely ice inside. Surprisingly, measurements of its gravity show it never fully separated into layers — it froze before a dense core could form.

Its surface is crowded with craters, and nothing inside has ever driven geological change: Callisto has been dead for more than 4 billion years. At these temperatures ice is nearly as hard as rock and does not flow like glacier ice on Earth.

Callisto: a heavily cratered surface (a), and icy spires 80 to 100 metres tall eroding into darker dust (b). Credit: modification of work by NASA/JPL/DLR and NASA/JPL/Arizona State University, via OpenStax, CC BY 4.0.

Ganymede: the largest moon

Ganymede is the largest moon in the solar system. About a quarter of its surface is as old and cratered as Callisto's; the rest is younger, perhaps 2 to 3 billion years old, and some features may be only a few hundred million. Unlike Callisto, Ganymede did separate into a rocky core about the size of our Moon beneath a mantle and crust of ice. Galileo found that it has a magnetic field, the sign of a partly molten interior, and there is very likely liquid water inside.

Its younger ground was shaped by tectonic and volcanic forces: flooded craters, long parallel ridges and valleys, and old craters split and pulled apart, hints of something like plate tectonics. The likeliest reason Ganymede stayed active while Callisto did not is Jupiter's gravity.

Ganymede (a), and an old crater in Nicholson Regio split apart by tectonic forces (b). Credit: modification of work by NASA/JPL/DLR and NASA/JPL/Brown University, via OpenStax, CC BY 4.0.

Tidal heating

A tidal force comes from the unequal pull of gravity on the two sides of a body. Jupiter's large moons are caught between the giant planet and each other, and the constant kneading heats their interiors — tidal heating. The closer a moon is to Jupiter, the stronger the effect.

Europa: an ocean under the ice

Europa and Io, the inner two moons, are mostly rock, not ice: in Jupiter's first few million years, the young planet radiated enough heat to drive the ice away from the material that formed them. Even so, Europa is covered in ice, and so few craters mark it that the surface can be no more than a few million years old. By its power to erase craters, Europa is more geologically active than Earth.

Its smooth crust is criss-crossed by cracks and ridges thousands of kilometres long, mostly double or multiple, like a vast freeway system. Straight lines like these can form if the ice is floating, almost without friction, on liquid water. Close-up images show ice broken into giant floes and refrozen, like sea ice in the Arctic, and Europa's magnetic signature is that of a liquid water ocean. The crust may be from about a kilometre to 20 kilometres thick.

Conamara Chaos on Europa, 70 km wide: blocks of crust that seem to have slid and rotated on a slushy or liquid layer below (a), and ridges crossing ridges (b). Credit: modification of work by NASA/JPL, via OpenStax, CC BY 4.0.

To stay liquid, the ocean must be warmed from inside, perhaps by hot springs like those on Earth's deep sea floor, where whole ecosystems live without sunlight. Many scientists think Europa is the likeliest place beyond Earth to find life in the solar system. NASA's Europa Clipper, planned for launch in 2024, is designed to swoop past in brief flybys, since Jupiter's radiation would quickly destroy electronics in orbit, to study the ocean and ice and look for material risen from inside.

Io: the most volcanic world

Io is almost a twin of our Moon in size and density, yet instead of a dead, cratered world it has the highest level of volcanism in the solar system. Voyager 1 saw eight volcanoes erupting in March 1979; Galileo counted more than 50 eruptions in 1997 alone, some throwing plumes hundreds of kilometres into space.

Two sides of Io: orange sulfur snow and white sulfur dioxide. Credit: modification of work by NASA/JPL/USGS, via OpenStax, CC BY 4.0.

Two eruptions on Io: a bluish plume about 140 km high, and the Prometheus plume, about 75 km. Credit: modification of work by NASA/JPL, via OpenStax, CC BY 4.0.

The reason is tides. Io is about as far from Jupiter as our Moon is from Earth, but Jupiter is more than 300 times as massive, stretching Io into an egg shape with a bulge several kilometres high. Tugs from Europa and Ganymede keep Io's orbit slightly off-circular, so it twists and flexes on every orbit, heating up like a wire coat hanger bent back and forth. Over billions of years this has driven off water and carbon dioxide, leaving sulfur as the most volatile material; Io's interior is entirely melted, and volcanoes constantly recycle its crust.

Europa, Ganymede and Callisto at the same scale: craters, and so surface age, increase outward from Jupiter. Credit: modification of work by NASA/JPL/DLR, via OpenStax, CC BY 4.0.

Just as the planets differ with their distance from the Sun, Jupiter's moons differ with their distance from Jupiter.

Sources

• Andrew Fraknoi, David Morrison and Sidney C. Wolff, Astronomy, section 12.2, "The Galilean Moons of Jupiter", OpenStax (Rice University), licensed under CC BY 4.0. Changed: rewritten in hubnx's own words and shortened, Titan and reflectivity left out of the table; the figures and their credits are the book's. This page is shared under the same licence.