Planetary Volcanism 835
FIGURE 7 The upper part of the figure shows the
chain of calderas called Tvashtar Catena on Io,
showing a fissure eruption in progress. The high
temperature of the lava overloaded the spacecraft
imaging system causing “bleeding” of data values
down vertical lines of the image. Using later
images, the appearance of the eruption as it would
have been seen by human eyes was reconstructed
as shown in the lower part of the figure. (NASA
Galileoimage.)the history of the Moon, but this does not in itself guarantee
that these materials on Mercury were emplaced volcanically
after the era of early intense bombardment that created the
craters. [SeeMercury.]
1.6 Io
The bulk density of Io is about the same as that of Earth’s
Moon, suggesting that it has a silicate composition, similar
to that of the inner, Earth-like planets. Io and the Moon also
have similar sizes and masses, and it might therefore be ex-
pected by analogy with the Moon’s thermal history that any
volcanic activity on Io would have been confined to the first
one or two billion years of its life. However, as the innermost
satellite of the gas-giant Jupiter, Io is subjected to strong
tidal forces. An orbital period resonance driven by the mu-
tual gravitational interactions of Io, Europa, and Ganymede
causes the orbit of Io to be slightly elliptical. This, coupled
with the fact that it rotates synchronously (i.e., the orbital
period is the same as that of the axial rotation), means that
the interior of Io is subjected to a periodic tidal flexing. The
inelastic part of this deformation generates heat in the inte-
rior on a scale that far outweighs any remaining heat source
due to the decay of naturally radioactive elements. As a
result, Io is currently the most volcanically active body in
the solar system. At any one time, there are likely to be up to
a dozen erupting vents. Roughly half of these produce lava
flows, generally erupted from fissure vents (Fig. 7) associ-
ated with calderas located at the centers of very low shield-
like features, and half produce umbrella-shaped eruption
clouds into which gases and small pyroclasts are ejected at
speeds of up to 1000 m/s to reach heights up to 300 km
(Fig. 8). [SeeIo:TheVolcanicMoon.]
The main gases detected in the eruption clouds are sul-
fur and sulfur dioxide, and much of the surface is coated
with highly colored deposits of sulfur and sulfur compounds
that have been degassed from the interior over solar system
history and are now concentrated in the near-surface layers.
However, it seems very likely, based on the fluid dynamic
and thermodynamic analysis of the eruption clouds, that the
underlying cause of the activity is the ascent of very hot basic
magmas from the interior of Io. Temperatures up to∼1700–
1900 K were initially derived fromGalileospacecraft data,
suggesting that the magmas might be ultra-basic, similar to
the komatiites that erupted on Earth earlier in its history.
However, recent reappraisals of the early analyses suggest
somewhat lower temperatures, and models of magma as-
cent on Io show that basalts, made unusually hot by friction
effects as they rise through the crust, are more likely can-
didates. When these magmas, which may themselves have
very low volatile contents, reach the surface in places with
few volatile deposits, they produce lava flows. However,
when they encounter copious deposits of sulfur compounds,
they melt and then vaporize the deposits, providing the very
high volatile contents needed to drive the violently explosive
eruptions. Most of these volatiles condense as they expandFIGURE 8 An explosive eruption plume on Io. The great height
of the plume, more than 100 km, implies that magma is mixing
with and evaporating volatile materials (sulfur or sulfur dioxide)
on the surface as it erupts. (NASAVoyagerimage.)