Io, the Volcanic Moon of Jupiter

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley Ph.D., Johns Hopkins University

Io, the moon that never stops roaring, is one of the four main moons of Jupiter. Each moon has interesting characteristics, but Io stands out by far when it comes to volcanoes. Io’s volcanic plumes stretch into space more than 30 times higher than Earth’s volcanoes can. At the same time, it is the most volcanically active body in the solar system! Such records must have strong reasons and leave strong impacts.

Illustration of Jupiter and its volcanic moon, Io.
Io, the moon covered with volcanoes and sulfur, is one of the four main moons of Jupiter. (Image: Nicholas Hylton/Shutterstock)

Jupiter is the gas giant with about 79 moons orbiting it. That was enough reason for some commentators to call Jupiter and its moons a mini solar system. Initially, only four moons were discovered, but that was in the 1600s. Io, the moon named after a nymph, was among the first four discovered. To know Io, one must first know how it was discovered.

The Story of the Discovery of Jupiter’s Moons

In 1610, Galileo discovered four big moons around Jupiter. Hence, the moons were called Galilean satellites. He named them by numbers, one to four. At the same time, Simon Marius was also studying Jupiter and discovered the four moons.

Marius was an independent scientist and discovered the moons on his own, but he went through a dispute with Galileo as to who discovered the moons first. In the end, it was revealed that Marius started taking notes about the moons only one day later than Galileo. Today, Marius’s names are used for the moons, even though Galileo was credited with discovering the moons.

Io’s Orbit and Orbital Resonance

A rendered size comparison of the Jupiter moons, Ganymede, Callisto, Io, and Europa on a clean black background.
The orbital resonance Io has with Europa and Ganymede creates many unique characteristics. (Image: Tristan3D/Shutterstock)

The orbital resonances around Io are important to understand why Io works as it does. The two innermost moons of Jupiter are Io and Europa. When Europa completes one orbit, Io completes two, that is, they have a 2:1 orbital resonance. Next comes Ganymede, which completes one orbit when Europa completes two. Thus, the three moons have a 4:2:1 orbital resonance, or a Laplace resonance. The resonance is a result of the moons’ gravitational effects on each other. How does Io’s orbit affect it?

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

Io, the Moon With Volcanoes

Io looks like a yellowish ball with numerous black dots. Each black spot is a volcano, and the surrounding yellow is the result of sulfur-rich magma. There are about 400 volcanoes discovered on Io, making it the most volcanic body in the solar system.

The New Horizons spacecraft witnessed one eruption in 2007, and the Galileo mission witnessed an active volcanic eruption in 2000, in the Tvashtar Catena—a chain of giant volcanic calderas in the northern hemisphere of Io.

The Galileo mission was there to study Jupiter and its surrounding for a few years, so it kept going close to Io to observe the changes. New volcanoes were formed and erupted at each visit, changing the surface significantly.

Learn more about exploring the Earth-Moon system.

The Strong Volcanoes of Io

On Earth, a volcano’s plume reaches around 10 kilometers high, while on Io, the height is up to 330 kilometers. That is around 20% of Io’s radius—1800 kilometers. Io’s gravity is much weaker than Earth’s, and its atmosphere is very thin. Sometimes particles from eruptions become a part of Io’s atmosphere, and sometimes they escape it due to Jupiter’s giant magnetosphere.

Io loses about one metric ton per second of gases and dust, creating a ring of hot plasma—a plasma torus—around the orbit. It creates a bright auroral footprint on Jupiter, as it connects Jupiter and Io near their poles. Why are there so many active volcanoes on Io?

Io’s Orbit and Its Volcanic Eruptions

The moon Io, satellite of the planet Jupiter, in front of the Milky Way galaxy.
The volcanic plume on Io reaches a height of almost 20% of the moon’s entire radius. (Image: Dotted Yeti/Shutterstock)

Being so close to Jupiter, Io experiences massive conflicts from the gravity of Jupiter and the other moons, specifically Europa. Thus, Io’s interior is continuously bent and shifted by tidal interactions, and a small but important ellipticity forms in its orbit. Thus, Io is constantly changing shape, and extensive heat is created due to frictional forces.

The heat melts the interior of Io, and to cool the temperature, the numerous eruptions occur. They erase all the craters and evidence of older transformations.

Learn more about mighty Jupiter, the ruling gas giant.

What is Io Made of?

Io has an average density of 3530 kilograms per cubic meter, with a typical terrestrial structure—a rocky silicate crust and mantle, and an iron-rich core. However, its mantle is hot enough to be partly molten and create a global magma ocean 50 kilometers under Io’s surface. Any water or ice on its surface would evaporate immediately.

All the differences Io, the volcanic moon, has with other moons root in its unique orbital resonance.

Common Questions about Io, Jupiter’s Moon

Q: Is the moon Io habitable?

Io, a moon around Jupiter, is far away from being habitable. It is more volcanic than any moon or planet in the solar system, with 400 volcanoes discovered so far.

Q: Why is Io so called?

Simon Marius was the scientist who discovered Jupiter’s moons only one day later than Galileo. Today, Marius’s Greek mythology names are used for the moons.

Q: Is Io Jupiter’s largest moon?

Io, the moon with the most active volcanoes in the whole solar system, is not comparable in size to Ganymede. It is not the largest, but has the most active volcanoes on its surface.

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