Io: The Volcanic Moon 421
mid-20th century, photometric and color data showed that
Io is the reddest object in the solar system and has a marked
color variation with orbital phase angle. These observations
also showed Io to be very different from the other Galilean
satellites (and most other satellites in the outer solar system)
because of the absence of water bands in its spectra.
The peculiar nature of Io’s surface became more evi-
dent in 1964, when astronomers A. P. Binder and D. P.
Cruikshank reported an anomalous brightening of Io’s sur-
face as it emerged from eclipse. This first report of “post-
eclipse brightening” and the suggestion of a possible at-
mosphere spurred more telescopic observations but, even
though the presence of an atmosphere was confirmed, post-
eclipse brightening has remained controversial and has not
been confirmed to this day.
The first evidence of an electromagnetic link between
Io and the jovian magnetosphere was put forward in 1964
by E. K. Bigg, who found that bursts of decametric ra-
dio emission by Jupiter were apparently controlled by Io’s
orbital position. Models of electrodynamic interaction be-
tween Jupiter and Io addressed the coupling mechanism
between Io and Jupiter’s inner magnetosphere.
The first spacecraft to flyby the Jupiter system was
Pioneer 10in 1973. These observations revealed that Io
has an ionosphere and thin atmosphere.Pioneermeasure-
ments also showed a cloud of neutrals along Io’s orbital
path. Ground-based measurements in the mid-1970s re-
vealed ionized sulfur emission in the inner jovian magne-
tosphere, but on the opposite side of Jupiter from the po-
sition of Io at the time. Subsequent studies revealed this to
be a plasma torus. The Io torus is a doughnut-shaped trail
along Io’s orbital path, made up almost exclusively of various
charged states of sulfur and oxygen, thought to be derived
from the break-up of volcanic sulfurous compounds (SO 2
and S 2 ). The ionized particles are held within the torus by
Jupiter’s magnetic field, in a similar way to the mechanism
that holds charged particles in the Van Allen radiation belts
around the Earth.
The first clues to Io’s bulk composition came from mea-
surements of Io’s mean radius using a stellar occultation
and from mass derived from thePioneerflyby in 1973. The
bulk composition of 3.54 g cm−^3 indicated silicates were
dominant on Io, but the surface’s high albedo and cold
temperatures indicated frosts. Telescopic observations us-
ing improved spectral reflectance techniques were used to
attempt to determine Io’s surface composition, and poly-
sulfides were suggested as a possible coloring agent for the
surface. The idea of sulfur on Io was strongly supported by
laboratory experiments by W. Wamsteker, which showed
that sulfur and its compounds matched the strong UV ab-
sorption and reflectance spectrum of Io, suggesting that
these compounds might be abundant on Io’s surface. How-
ever, the discovery of a strong absorption band near 4μm
could not be explained by sulfur. It was later found to be
due to sulfur dioxide (SO 2 ), which is now known to be the
dominant compound covering Io’s surface. Other key dis-
coveries during the 1970s were those of the Io sodium cloud
in 1973 by R. Brown and in 1975 of a potassium cloud by
L. Trafton.
The first indications of volcanic activity were given by
infrared photometry and radiometry that showed higher
brightness temperatures at 10 μm than at 20μm, but
the thinking at the time was that Io was a cold and dead
world, and these observations remained puzzling. How-
ever, shortly beforeVoyager 1arrived at the Jupiter sys-
tem in March 1979, several scientists published works that,
in retrospect, are suggestive of active volcanism. In 1978,
R. Nelson and B. Hapke reported a spectral edge at 0.33μm
and proposed that sulfur was the major contributor to this
spectral feature. They suggested that the presence of al-
lotropes of sulfur explain this and several other spectral
features, and that these allotropes could be produced by
melting yellow sulfur and subsequently quenching it, pos-
sibly “in the vicinity of a volcanic fumarole or hot spring.”
Astronomers F. Witteborn and colleagues reported a tele-
scopic observation of an intense temporary brightening of
Io in the infrared wavelengths from 2 to 5μm. They ex-
plained it, although with some skepticism, as thermal emis-
sion caused by part of Io’s surface being at a temperature
of about 600 K, much hotter than the average expected
daytime temperature of about 130 K. A few days before
theVoyager 1flyby of Io, a seminal theoretical paper by
Stan Peale and colleagues was published. They had stud-
ied the tidal stresses generated within Io as a result of the
gravitational “tugs” from Jupiter and Europa. Their calcu-
lations showed that the possible heat generated by tidal
stresses was in the order of 10^13 W, much greater than heat
that could be released from normal radioactive decay. Their
prediction—that Io might have “widespread and recurrent
volcanism”—was spectacularly confirmed byVoyager 1.
Active volcanoes were not immediately obvious in the
first images returned byVoyager 1.The most striking aspect
of Io shown in the first images was its colorful surface, with
yellows, oranges, reds, and blacks. Scientists on the imag-
ing team nicknamed Io the “pizza moon” and suggested the
colors were likely due to large quantities of sulfur on the sur-
face. Another surprising aspect was the absence of impact
craters. The obvious conclusion was that Io’s surface was
very young and the craters must have been obliterated—
but how? The answer came soon after, when a navigation
engineer at the Jet Propulsion Laboratory, Linda Morabito,
noticed a peculiar umbrella-shaped feature emanating from
Io’s limb in one of the images that was taken to aid naviga-
tion of the spacecraft (Fig. 2). The pattern turned out to be
an eruption plume rising about 260 km above the surface.
A second plume was found on the same image, and more
plumes were seen upon close examination of various other
images. Additional evidence for active volcanism came from
another ofVoyager’s instruments, the infrared interferome-
ter spectrometer (IRIS), which detected enhanced thermal