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CHAPTER 22

Io: The Volcanic Moon

Rosaly M.C. Lopes

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

1. Introduction 5. Heat Flow and Interior


2. Io Exploration 6. Atmosphere, Torus, and the Jupiter Environment


3. Io’s Surface 7. Outstanding Questions and Future Exploration


4. Io’s Volcanic Eruptions Bibliography

I

o, the innermost of Jupiter’s four Galilean satellites, is the
only body outside the Earth so far known to have large-
scale active volcanism. Io’s heat flow is much higher than
the Earth’s, and at least one of its many active volcanoes
erupts lavas that are hotter than any erupted on the Earth
today.

1. Introduction

Io (Fig. 1) was discovered by the Italian scientist and
astronomer Galileo Galilei on January 8, 1610, and was
named after one of the ancient Roman god Jupiter’s illicit
lovers. The discovery of active volcanism was made by the
Voyager 1spacecraft, which flew close to Io in 1979. Im-
ages showed volcanic plumes up to 300 km in height and
a vividly colored surface dominated by largecaldera-like
and flowlike features. The study of Io’s remarkable volcan-
ism has continued since then using observations by tele-
scopes on Earth, by theHubble Space Telescope, and, from
1996–2002, by theGalileospacecraft.
Io’s unusual spectroscopic characteristics, due to its vol-
canic activity and widespread covering of sulfur dioxide,
were recognized in the 1970s. In 1979, just prior to the two
Voyagerflybys, Io’s 4:2:1 orbital resonance with Europa and
Ganymede was predicted to induce severe tidal heating and

subsequent active volcanism on Io. The twoVoyagerspace-

craft confirmed the prediction that Io is volcanically active.

Io’s size (Table 1) is similar to that of the Earth’s Moon

but its density is higher, indicating that there is more iron

in Io’s interior than in the Moon’s. Io’s mantle composition

is thought to be predominantlysilicates. However sulfur

compounds are abundant on the surface. After the discovery

of volcanism in 1979, a major question was the composition

of the erupting material: silicates or sulfur? Ground-based

observations and, later,Galileo’s results showed that Io’s

eruption temperatures were too high to be sulfur, but sul-

fur flows may also exist on the surface. Sulfur dioxide is

ubiquitous on Io’s surface, and sulfur and sulfur dioxide are

known to be present in Io’s volcanic plumes.

Io’sheat flow(Table 1) is very large compared with that

of the Earth and other planets. Io’s heat flow is about 200

times what could be expected from heating due to the decay

of radioactive elements, illustrating how crucial tidal heat-

ing is to driving Io’s active volcanism. The effect of Io’s vol-

canic eruptions extends well beyond the surface, and there

is considerable interaction of Io with the jovian magnetic

field. Io has both a patchy, very low density atmosphere and

an ionosphere. Sulfur dioxide is the main constituent of

the atmosphere, and it is thought to be supplied largely by

volcanic plumes, with a lesser amount coming from evapo-

ration of frost deposits on the surface. An important discov-

ery made during theGalileomission was Io’s aurora, caused

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