420 Encyclopedia of the Solar System
FIGURE 1 Io imaged byGalileo’s Solid State Imaging System
on September 7, 1996, at a range of about 487,000 km. The
image is centered on the side of Io that always faces away from
Jupiter. The black and bright red materials correspond to the
most recent volcanic deposits. The near-infrared filter makes
Jupiter’s atmosphere (in the background) look blue. The active
volcano Prometheus is seen as a dark sinuous feature near the
right-center of the disk.
TABLE 1 Io’s Basic Orbital and Physical PropertiesMean radius: 1821.6±0.5 km
Bulk density: 3528±3kgm−^3
Orbital period: 1.769 days
Orbital eccentricity: 0.0041
Orbital inclination: 0.037
Orbital distancea: 421,800 km
Rotational period: synchronous with orbit
Maximum moment of inertia: 0.3769±0.0004
Potential Love numberk^2 : 1.292±0.003
Mass: (8.9320±0.0013)× 1022 kg
Surface gravity: 1.80 m s−^2
Global average heat flow:>2.5Wm−^2
Radius of core: 656 km (if pure iron)
947 km (iron and iron sulfide mixture)
Surface equatorial magnetic field strength:<50 nT
Geometric albedo: 0.62
Local topographic relief: up to∼17 km
Active volcanic centers: at least 166
Typical surface temperature (away from hot spots): 85 K
(night) to 140 K (day)
Atmospheric pressure:< 10 −^9 bar, higher at locations
of plumesSource: Lopes and Williams (2005).by collisions between Io’s atmospheric gases and energetic
charged particles trapped in Jupiter’s magnetic field.
Io has an ionosphere and a thin atmosphere. Materials
escaping from Io form a cloud of neutrals along Io’s or-
bital path. Escaping materials also populate the Io torus, a
doughnut-shaped region along Io’s path, made up of ion-
ized particles of sulfur and sulfur dioxide held by Jupiter’s
powerful magnetic field.
Io is therefore a wonderful natural laboratory for the
study of geological and geophysical processes, and its loca-
tion within Jupiter’s magnetic field makes it a rich source
for studies of the fields and particles environment in space.2. Io Exploration
Since its discovery in 1610, Io has been important to our
understanding of the solar system, along with the other
Galilean satellites, Europa, Ganymede, and Callisto. After
a few observations, Galileo Galilei concluded that the four
objects were not stars as he originally thought, but satel-
lites in orbit around Jupiter. Galileo’s studies of the motion
of the newly discovered satellites had a profound effect on
human history becoming, along with Galileo’s discovery of
the phases of Venus, key evidence in favor of the Copernican
theory of the universe. Another major step for science came
in 1675, when Danish astronomer Olaus Romer noted that
the times of the eclipses and occultations of the four moons
by Jupiter showed a phase shift with a periodicity of about
6.5 months. He concluded that, when Jupiter is at opposi-
tion (when Jupiter and Earth are closest, on the same side as
the Sun), light from the jovian system must travel a distance
of approximately 4 astronomical units (AU) to reach Earth.
However, when Jupiter is at conjunction (when Earth and
Jupiter are farthest apart, on opposite sides of the Sun),
light traveled about 6 AU on its journey to Earth. Romer
concluded that this phase shift in the arrival time of jovi-
centric events meant that light has a finite velocity, and he
used the motions of the Galilean satellites to determine the
speed of light.
In 1805, Laplace demonstrated that the Galilean satel-
lites have an orbital configuration (known as the Laplace
resonance), which suggested a special dynamical relation-
ship among Io, Europa, and Ganymede. For each time
that Ganymede orbits Jupiter, Europa orbits almost exactly
twice, and Io orbits four times. Later studies would reveal
that this resonance plays a key role in the existence of active
volcanism on Io.
Even before the first close-up images of Io were returned
byVoyagerin 1979, there were indications that Io was a re-
markably different world from our Moon and other moons
in the solar system. Telescopic observations showed that Io’s
brightness varied according to its position in its orbit, sug-
gesting that the moon always keeps one face toward Jupiter
(now referred to as the subjovian hemisphere). During the