Io: The Volcanic Moon 427
critical question about Io’s volcanism—the composition of
the erupting magma and crust—remains open.
Not all Ionian eruptions are vigorous like Pillan. On
Earth, volcanic eruptions are often classified depending
on their character—effusive, explosive, very explosive—and
these eruptions are often named after volcanoes or locations
where they have been studied (e.g., Hawaiian, Icelandic,
Strombolian, Pelean). High-resolution observations and
temporal data on Io’s hot spots showed that some broad gen-
eralizations can also be made for Ionian eruptions. The ma-
jority of Ionian eruptions can be placed in three classes ini-
tially designated “Promethean,” “Pillanian,” and “Lokian,”
though a single hot spot can exhibit more than one eruption
style over time. Explosion-dominated (Pillanian) eruptions
(Fig. 6) have an intense, short-lived phase that may corre-
spond to the outbursts detected from Earth. These erup-
tions originate from either paterae or fissures and produce
extensive dark lava flow fields and dark pyroclastic deposits
through short-lived, high effusion rate, vigorous activity.
These events may or may not include eruption episodes with
large (>200 km high) explosive plumes, which can produce
large plume deposits such as that around Pillan itself.
Less intense but more persistent flow-dominated erup-
tions (Promethean) are named after the Prometheus hot
spot (Fig. 7), which has a persistent plume about 100 km
high, active during bothVoyagerencounters in 1979 and
throughout theGalileomission. Surprisingly, distant images
obtained by Galileo in 1996 showed that the Prometheus
plume site had moved about 80 km west since 1979, but
its size and appearance had not changed. A new lava flow
linked the old and new plume sites. Images and infrared
observations obtained in 1999 showed that the main vent
FIGURE 7 Flow-dominated or Promethean eruptions are
relatively long-lived (months to years) and are associated with
long-lived plumes and flow fields. In this montage,GalileoSSI
views of the Prometheus plume (center) are surrounded by
increasingly higher resolution views of the Prometheus flow
field. Note also the bright plume deposit forming an annulus in
the lower right image.
FIGURE 8 Intra-patera or Lokian eruptions are confined to
paterae (caldera-like depressions) and are thought to represent
the resurfacing of paterae floors by lava flows or overturning lava
lakes. The style of eruption is exemplified by Loki (inset upper
left,Voyagerimage). In this montage,GalileoNIMS images
track the temperature changes across the floor of Loki. The top
right image is at 2μm, showing the hottest areas, while the
bottom image, at 4μm, shows the distribution of cooler areas.
Note that the center of Loki (which appears white in the SSI
image) is cold at infrared wavelengths.of this volcano was near theVoyagerplume site and that
the plume, not the volcano, had moved west. The plume’s
movement was modeled in terms of the interaction between
the advancing hot lava and the underlying sulfur dioxide
snowfield by Susan Kieffer and colleagues. The movement
of lava flows on Earth over marshy ground can give rise
to small, short-lived explosive activity, but nothing on the
scale of the Prometheus plume has ever been observed.
This type of eruption may be common on Io and, once the
flow stops moving, the plume eventually shuts off, as has
been observed at the Amirani volcano. The lava flows asso-
ciated with these eruptions can be quite extensive and are
thought to be emplaced through repeated small breakouts
of lava, similar to the slowly emplaced flow fields at Kilauea
in Hawaii.
Intrapatera (Lokian) eruptions (Fig. 8) are confined
within the caldera-like paterae. These eruptions are thought
to be lava lakes, some of which are possibly overturning.
Observations fromGalileoflybys showed that lava lakes are
abundant on Io, and they may be a significant mechanism
for heat loss from the interior. Io’s most powerful hot spot,
Loki, is thought to be a giant lava lake that perhaps un-
dergoes periodic overturning, leading to brightenings that
have been observed from Earth for decades. Many other
hot spots on Io appear to be persistent lava lakes.5. Heat Flow and Interior
Observations of Io have also provided knowledge about
the satellite’s interior, where the tidal heat is being dissi-
pated, driving the volcanic eruptions. Observations from