The Moons of Jupiter: Fire, Ice…and Life?

Moons of Jupiter - Io, Europa, Ganymede, Calisto

When NASA’s Voyager 1 swept by Jupiter in early 1979, scientists at NASA’s Jet Propulsion Laboratory were startled by pictures the spacecraft beamed back of the moon Io. The surface was covered with red and white patches, making it look exactly like a giant pizza.

Jupiter's Moon Io Erupting
The erupting surface of Io.

It isn’t tomatoes that cause these red blotches; it is sulfur from more than 400 active volcanoes. How volcanoes could exist in such a cold region so far from the Sun was a fascinating question for the amazed scientists, who had never seen volcanoes off the Earth before.

The answer? Tides. The immense pull of Jupiter’s gravity, coupled with the gravity of two nearby moons, produce tidal heating that causes sulfur from Io’s interior to rise high above the surface. As it turns out, Io isn’t the only moon of Jupiter affected by tides.

Io is one of 64 moons of Jupiter. The four largest satellites – Io, Europa, Ganymede and Callisto – are known as Galilean moons because they were discovered by famed Italian astronomer Galileo Galilei in 1610. All four moons are much more fascinating than Galileo could have imagined four centuries ago.

If Io is the closest approximation of Hell in the Solar System, then Europa is a good stand-in for Antarctica. It is literally an ice ball, a world covered in a sheet of ice that could be dozens or hundreds of miles deep.

Moons of Jupiter - Europa

Europa’s icy surface might conceal a vast ocean beneath it. During its eight year mission orbiting the Jupiter, NASA’s Galileo spacecraft sent back photographs showing areas where the ice crust has broken up, indicating that water has flowed up from below. It is possible that deep sea thermal vents, caused by the same tidal forces that created Io’s volcanoes, keep the water from freezing.

Jupiter's Moon Europa

Jupiter's Moon Europa

And where there is water, could there be life? Nobody knows, but scientists are planning future missions to find out. Proposals include orbiters, landers and even a submarine that would be placed under the moon’s icy surface.

Moons of Jupiter - Ganymede

Ganymede is also suspected of having a subsurface saltwater ocean sandwiched between layers of ice. Ganymede is the largest moon in our Solar System - bigger than Mercury.

Jupiter's Moon ganymede

Jupiter's Moon Ganymede

It is also the only satellite known to generate its own magnetic field. About a third of the moon is covered with impact craters that date back as far as 4 billion years ago. The rest is covered with lighter regions of grooves and ridge believed to have been caused by tidal heating.


Moons of Jupiter - Calisto

Callisto, which has a thin atmosphere composed of carbon dioxide and oxygen, might also possess a subsurface ocean at a depth of 62.5 to 93.75 miles (100-150 km). The heavily cratered moon is the second largest in the Jovian system and the third largest satellite in the Solar System, about 99 percent of the diameter of Mercury.

Jupiter's moon calisto

Jupiter's Moon Calisto

Jupiter’s four largest moons are fascinating worlds with many secrets still hidden beneath their surfaces. It’s unlikely that Galileo could have ever imagined how strange these satellites were when he pointed his telescope at them 400 years ago.

Links for more information about Jupiter's Moons

Jupiter's Moons
Voyager Missions
Galileo Mission


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