Moons of Jupiter: Io, Europa, Ganymede and Callisto

Point a pair of binoculars at Jupiter and you may see a few tiny stars lined up beside it.

Watch for a few nights and those points of light shift position, dancing from one side of the planet to the other.

They are Jupiter's four largest moons, and they are some of the most fascinating worlds in the solar system.

Each is completely different: one is covered in erupting volcanoes, one hides an ocean beneath its ice,

one is bigger than the planet Mercury and one is among the most heavily cratered bodies known.

Here is a guide to all four and how you can follow them from your own backyard.

Four Moons That Changed Astronomy

In January 1610, Galileo Galilei turned his small homemade telescope toward Jupiter and noticed three, then four, faint points of light near it.

Over the following nights he realized they were moving with the planet and circling it.

They are still known as the Galilean moons in his honor.

The discovery mattered far beyond Jupiter. At the time, many people believed that everything in the heavens circled Earth.

Here was clear proof that other bodies had moons of their own,

which gave important support to the idea that Earth and the planets orbit the Sun.

Jupiter has more than 100 known moons in total, most of them small, irregular bodies only a few kilometers across.

The four Galilean moons are by far the largest, and together they hold nearly all of the mass orbiting the planet.

The Galilean Moons at a Glance

Io: The Most Volcanic World Known

Io is the most volcanically active body in the solar system.

It has hundreds of active volcanoes, some shooting plumes of gas and dust hundreds of kilometers above the surface,

and lava lakes and flows that constantly repaint the landscape.

Sulfur compounds color the surface in splashes of yellow, orange, red and white, making Io look a little like a pizza.

Io's volcanoes were discovered in 1979 in images from Voyager 1, a genuine surprise,

since a moon this size would normally have cooled and gone quiet long ago. The explanation is tidal heating.

Io's orbit is slightly stretched, kept that way by regular tugs from Europa and Ganymede.

As Io moves closer to and farther from Jupiter, the giant planet's gravity flexes it in and out,

and that constant kneading generates enormous heat inside.

Europa: An Ocean Beneath the Ice

Europa is slightly smaller than our Moon and covered in a bright shell of water ice, crisscrossed by long cracks and ridges.

There are very few craters, a sign that its surface is young and has been reshaped relatively recently in geological terms.

Beneath that ice, scientists are confident there is a global ocean of salty liquid water,

possibly holding more water than all of Earth's oceans combined. Key evidence came from the Galileo spacecraft,

which orbited Jupiter from 1995 to 2003 and detected a magnetic signal best explained by a salty ocean conducting electricity beneath the surface.

Tidal heating, milder than Io's, keeps the water from freezing solid.

Could Europa Host Life?

Europa is considered one of the most promising places to search for life beyond Earth.

It seems to have liquid water, and it may have the chemical ingredients and energy that living things need,

possibly from hot vents on its seafloor.

But no evidence of life has been found, and whether its ocean is truly habitable is unknown.

Answering that question is a major goal of planetary science, and NASA launched its Europa Clipper spacecraft in 2024 to study the moon in detail.

Ganymede: The Largest Moon in the Solar System

Ganymede is the biggest moon in the solar system, wider than the planet Mercury,

though only about half as massive because it contains so much ice.

Its surface is a mix of dark, ancient, cratered regions and lighter areas crossed by long grooves and ridges,

possibly formed as the icy crust stretched and cracked.

Ganymede is also the only moon known to generate its own magnetic field, discovered by the Galileo spacecraft.

That field produces auroras around its poles.

Careful observations of how those auroras shift suggest that Ganymede, too, hides a deep ocean of salty water, probably sandwiched between layers of ice.

The European Space Agency launched its JUICE mission in 2023 with Ganymede as a main target.

Callisto: A Battered, Ancient Surface

Callisto, the outermost of the four, is nearly as wide as Mercury and one of the most heavily cratered objects known.

Its dark surface has changed little for billions of years, preserving a record of impacts from the solar system's early history.

Its largest feature, a huge impact basin surrounded by rings of ridges, spans thousands of kilometers.

Because it orbits far from Jupiter, Callisto experiences little tidal heating and shows few signs of internal activity.

Even so, measurements of its magnetic environment hint that it may also have a salty ocean deep below the surface,

although the evidence is weaker than for Europa and Ganymede.

Callisto also lies outside Jupiter's most intense radiation, which is why it has been suggested as a possible site for a distant future base.

Why the Four Moons Are So Different

The Galilean moons formed around the young Jupiter, much like planets forming around a star, and their differences follow a clear pattern:

• Distance and heat. The closer a moon orbits to Jupiter, the more tidal heating it experiences. Io, the innermost of the four, is the hottest and most active, while distant Callisto is the quietest.
• Rock and ice. The inner moons are rockier and denser, while the outer ones contain more ice, possibly because the disk around the young Jupiter was warmer close to the planet.
• A linked rhythm. Io, Europa and Ganymede are locked in a steady beat: for every four orbits of Io, Europa completes two and Ganymede one. This resonance keeps their orbits slightly stretched and drives the heating that powers Io's volcanoes and helps keep Europa's ocean liquid.

See Galileo's Moons for Yourself

You can see the same moons Galileo discovered with ordinary binoculars.

Rest them on a fence, car roof or tripod to steady the view,

and look for tiny points of light arranged roughly in a line on either side of Jupiter.

You may see all four, or only two or three if some are passing in front of or behind the planet.

Check again an hour later and Io, the fastest, may have visibly moved. Many astronomy apps show which moon is which at any moment.

With a small telescope, you can sometimes watch a moon's tiny black shadow crossing Jupiter's cloud tops,

a sight that makes the system's motion wonderfully real.

A Miniature Solar System

Jupiter and its great moons are like a solar system in miniature, with worlds ranging from molten to frozen.

They also include some of the best places to search for life beyond Earth.

Next time Jupiter shines in the evening sky,

grab your binoculars and say hello to the moons that once helped rearrange our understanding of the heavens.