Volcanoes produce almost half of the atmosphere of Jupiter’s moon Io
New discoveries

Volcanoes produce almost half of the atmosphere of Jupiter’s moon Io

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Astronomers using ALMA have discovered that the thin atmosphere of Io is largely due to its active

Io (pronounced EYE-oh) is a large moon of the giant planet Jupiter. Thanks to tidal forces from the
planet, it has a hot interior. The surface of Io is dotted with active volcanoes. They spew ash and
sulfur into space. Most of the sulfur falls back to the surface. As a result, the moon has a striking
yellow color.

Io also has a very thin atmosphere. It’s a billion times more tenuous than the atmosphere of the
Earth. Astronomers already knew that the atmosphere contains a lot of sulfur dioxide – molecules
that consist of one sulfur atom and two oxygen atoms. But they weren’t sure how the sulfur dioxide
ended up in Io’s atmosphere.

There are two possible ways. Either the sulfur dioxide is directly brought into the atmosphere by
volcanic eruptions. Or it is due to slow evaporation of volcanic deposits on Io’s surface.

ALMA has now observed Io when it passed through the shadow of Jupiter. During such an eclipse, no
direct sunlight falls on the moon. Sulfur dioxide that has been in the atmosphere for a while will
freeze and fall down as snow.

Still, ALMA detected quite a lot of sulfur dioxide gas in Io’s atmosphere, blown into space by active
volcanoes. From the observations, the astronomers could deduce that some 30 to 50 percent of the
moon’s atmosphere is the direct result of volcanic activity.

The ALMA measurements also revealed that some volcanoes do not spew sulfur into space, but
another gas: potassium chloride. Apparently, the magma (molten rock) below Io’s surface doesn’t
have the same composition everywhere.


Io is the innermost of the four largest moons of Jupiter. It was discovered in 1610 by Italian astronomer Galileo Galilei. Io is slightly larger than our own Moon. It has more than 400 active volcanoes on its surface. Some, like Loki Patera, are more than 200 kilometers in diameter. Io has more volcanic activity than the Earth!


The ALMA observations of Io were carried out by Imke de Pater. Imke is a professor of planetary
sciences at the University of California in Berkeley. She also works at Delft University of Technology in
The Netherlands. Imke worked together with five other astronomers, from the United States and
Chile. The team studied Jupiter’s moon Io during eclipses, when the moon passes through the
shadow of the planet. The observations were carried out in March and September 2018. The results
will be published in The Planetary Science Journal, a professional magazine on solar system research.