Io: The Volcanic Moon 425
concentrated toward lower latitudes and follow a bimodal
distribution with longitude. According to P. Schenk and col-
leagues, the greatest frequency of mountains occurs in two
large antipodal regions near the equator at about 65◦and
265 ◦. In contrast, J. Radebaugh and colleagues studied the
distribution of Ionian patera, and although they found the
paterae to follow a similar bimodal distribution, the highest
concentrations are 90◦out of phase with that of the moun-
tains. When only the hot spots known to be currently or
recently active are studied, their distribution appears ran-
dom, though no active (or inactive) volcanic centers have
been detected at latitudes greater than 78◦. The distribution
pattern for volcanic centers is consistent with the pattern of
heat flow from tidal heating in Io’s asthenosphere predicted
from simulations. The anticorrelation in the distribution of
mountains and volcanic centers is further evidence that the
two are not related, but the reasons for the anticorrelation
are still unknown.
Because of the dynamic nature of Io’s volcanism, its sur-
face appearance can change in dramatic ways over time. De-
tectable changes occurred in the years between theVoyager
andGalileoobservations (1979–1995); however, many sur-
face changes at the timescale of months have also been de-
tected. One example is the change in the Pele plume deposit
between the twoVoyagerflybys, which were spaced about
4 months apart. Surface changes are mostly due to new vol-
canic eruptions, particularly sulfur and sulfur dioxide from
volcanic plumes and pyroclastic (ash andtephra) deposits.
Other changes include new lava flows, increases in the area
of flows, and changes in surface color. Volcanic materials
have been observed to fade or disappear due to burial, alter-
ation, radiation exposure, or erosion. Most surface changes
have been detected at visible wavelengths; however, within
individual volcanic centers, changes in temperature and sul-
fur dioxide coverage have been detected at infrared wave-
lengths. Most surface changes are localized and take place
inside dark volcanic paterae that cover only 1.4% of Io’s sur-
face, or are ephemeral volcanic plume deposits that fade or
change color on timescales of a few months to years. One
surprise from the firstGalileoobservations was that Io’s sur-
face appearance remained largely the same since the last
Voyagerflyby. Based on the changes observed between the
twoVoyagerflybys (4 months apart), major changes were
expected in the years betweenVoyagerandGalileo. In-
stead, more than 90% of Io’s surface remained unchanged
betweenVoyager(1979) and the end of the primeGalileo
mission (1999).
Localized changes from major eruptions, however, can
be dramatic. Two of these were particularly useful in the
study of surface changes fromGalileo. The eruption of the
Pillan volcanic center in 1997 left a conspicuous “black
eye” on Io’s surface (Fig. 6), covering an area of about
200,000 km^2 and reaching distances up to 260 km from
the source (Fig. 6). Later observations fromGalileo’s SSI
showed a spectral absorption at 0.9μm in these and other
FIGURE 6 Explosion-dominated or Pillanian eruptions on Io
occur in relatively brief (few months or less), intense outbursts
that produce very high (possibly ultramafic) temperatures,
plumes (top left), and rapidly emplaced lava flows (top right).
Plumes can reach great heights (several hundred kilometers),
and their deposits have produced black (Pillan), red (Pele,
Tvashtar), and white (Thor) rings. These compositions are
thought to be associated with silicate, sulfur, and SO 2 pyroclastic
materials, respectively. (Figure courtesy of David Williams.)dark materials on Io, suggesting silicate composition (most
likely magnesium-rich orthopyroxene). The dark deposit at
Pillan slowly faded between 1997 and 1999 as it was covered
by red sulfurous deposits from nearby Pele.
Surface colors are the most easily observed manifesta-
tions of surface change. Galileo results brought new insights
into the intriguing question of what causes the vivid colors
of Io’s surface. The global distribution of the different color
deposits gives some clues to their origin and Galileo’s re-
peated flybys allowed observations at different illumination
angles, which affect how colors appear in images. Io’s sur-
face has four primary color units: most of the surface is
yellow (about 40%), white-gray (about 27%), or red-orange
(about 30%), while black deposits are localized around vol-
canic centers. Red and orange materials are interpreted as
deposits of short-chain sulfur molecules (S 3 ,S 4 ). These are
concentrated at latitudes higher than 30◦north and south
and, where they are thought to result from the breakdown
of sulfur (cyclo-S 8 ) by charged particle irradiation. These
red deposits at high latitudes appear to last longer than
those at equatorial regions. At lower latitudes, patches of
red materials are associated with hot spots and plumes and
are thought to be formed by condensation from sulfur-rich
plumes. These red plume deposits are ephemeral, lasting
perhaps a few years if the deposit is not replenished.
The yellow materials that cover a lot of the surface are
interpreted to be sulfur (cyclo-S 8 ), with or without a cov-
ering of sulfur dioxide (SO 2 ) frosts deposited by plumes,
or alternatively polysulfur oxide and sulfur dioxide without
large quantities of elemental sulfur. White-gray materials