424 Encyclopedia of the Solar System
largest active flow field known in the solar system. Io’s large
lava flows are possibly analogs of the continental flood basalt
lavas on Earth, such as the Columbia River Basalts in the
United States. These ancient terrestrial flows were never di-
rectly observed, but they are suspected of producing major
climatic effects.
A major question about Ionian volcanism afterVoyager
was the nature of volcanism—whether sulfur or silicates
were predominant. Although temperature measurements
from Galileo clearly showed that many hot spots have tem-
peratures far too high for sulfur, the possibility that some
sulfur flows occur on the surface cannot be ruled out. At
the time of theVoyagerflybys, Carl Sagan argued that the
colorful flows around Io’s Ra Patera volcano were sulfur
(Fig. 5). Unfortunately, the flows could not be studied by
Galileoas the area had been covered over by new erup-
tions beforeGalileo’s first observations in 1996. However,
other locations may have sulfur flows. Most Ionian flows
appear dark, but a few location show pale yellow or white
flows that may well be molten sulfur. D. Williams and col-
leagues proposed that flows radiating from Emakong Pat-
era may be sulfur and that low-temperature liquid sulfur
(∼450 K) could explain many of the morphological fea-
tures seen around Emakong Patera, such as a meandering
FIGURE 5 This image of Ra Patera volcano was taken by
Voyager 1at a range of 128,500 km (77,100 miles). The width of
the picture is about 1000 km. Ra Patera is the dark spot with the
irregular radiating pattern of flows, which were interpreted as
being sulfur.
channel 105 km in length that appears to feed a gray-white
flow some 270 km in length. Infrared measurements us-
ingGalileoNIMS indicated temperatures less than 400 K
inside Emakong caldera, and much cooler (below the in-
strument’s detection capabilities) over the flows. However,
Galileo’s instruments could not distinguish between sulfur
flows or cooled silicates coated by bright sulfurous materials
after erupting. One possibility, suggested by R. Greeley and
colleagues, based on studies of a sulfur flow at Mauna Loa
in 1984, is that rising silicate magma may melt sulfur-rich
country rock as it nears the surface, producing “secondary”
sulfur flows (as opposed to “primary” flows that originate
from molten magmas at depth). Sulfur dioxide is ubiqui-
tous on Io, and the colorful surface is thought to be the
result ofsulfur allotropedeposits, making the possibility
of secondary sulfur flows likely.Galileodata of the volca-
noes Balder and Tohil Paterae suggest that sulfur dioxide
could be mobilized as “flows” in very cold regions. However,
the presence of sulfur and SO 2 , flows on Io have not been
confirmed and whether these flows exist on Io remains an
open question.
The most common type of volcanic feature on Io is the
patera. Although the origin of paterae is still somewhat un-
certain, they are thought to be similar to terrestrial volcanic
calderas, formed by collapse over shallow magma chambers
following partial removal of magma. Some paterae show
angular shapes that suggest some structural control, indi-
cating that they may be structural depressions that were
later used by magma to travel to the surface. At least 400
Ionian paterae have been mapped. Their average diameter
is∼40 km, but Loki, the largest patera known in the solar
system, is over 200 km in diameter. In contrast, the largest
caldera on Earth, Yellowstone, is∼80 km by 50 km in size.
The larger sizes of the Ionian features probably reflect the
much larger sizes of magma chambers.
Mountains are major structural landforms on Io and
tower over the surrounding plains. Ionian mountains are
defined as steep-sided landforms rising more than∼1km
over the plains. At least 115 mountains have now been iden-
tified and mapped. Io’s mountains rise, on average, about
6 km high, with the highest rising 17 km above the sur-
rounding plains.Galileoimages revealed that many moun-
tains are partly or completely surrounded by debris aprons,
plateaus, and layered plains. Mountains appear to be un-
stable and are thought to be relatively short-lived features.
They are not active volcanoes, but their origin is still un-
certain. Various models have been proposed to explain the
origin of the mountains. Their asymmetrical shapes sug-
gest the uplift and rotation of crustal blocks, implying that
compressional uplift is probably the dominant mechanism.
Neither the volcanic features nor the mountains ap-
pear to follow a distinct global pattern such as seen on the
Earth, suggesting that, on Io, surface expressions of internal
dynamics are subtle. However, the distribution of moun-
tains and paterae is not totally random; both features are