Io: The Volcanic Moon 423
These measures, along with changes made in the Deep
Space Network (DSN), maximized the amount of data that
could be returned.Galileoobservations were so successful
and spectacular that two mission extensions to gather ad-
ditional data were approved. TheGalileo Europa Mission
(GEM) lasted from 1998–1999 and theGalileo Millennium
Mission(GMM) from 2000–2002. These extensions were
particularly important for Io because all the high-resolution
remote sensing observations obtained by Galileo were col-
lected during these mission extensions. The close Io flybys
during which high spatial resolution remote sensing obser-
vations were collected happened in October and Novem-
ber 1999, February 2000, and August and October 2001.
The main remote sensing instruments observing Io were
the Solid State Imaging System (SSI), the Near-Infrared
Mapping Spectrometer (NIMS), and the Photopolarime-
ter Radiometer (PPR).
3. Io’s Surface
Io’s phenomenal volcanic activity makes it the most geo-
logically active object in the solar system. Remote sensing
observations fromGalileorevealed its surface in unprece-
dented detail and substantially changed our understanding
of Io’s geology and geophysics. Galileo’s remote sensing in-
struments (visible and infrared) were used to study the sur-
face features and volcanic activity, while the tracking of the
spacecraft itself provided new constraints on the interior.
Gravity measurements from tracking indicated that Io has
a large iron/iron sulfide core and a silicate mantle. Galileo’s
close flybys of Io failed to reveal an intrinsic magnetic field,
suggesting that little core convection is taking place.
The surface of Io contains three primary types of fea-
tures (Fig. 4): (1) broad, flat, layered plains, which are
partially covered with visible, diffusepyroclastic mate-
rials; (2) volcanic structures includingpaterae(caldera-
like depressions),flucti(lava flow fields), andtholi(shield
volcanoes and other positive-relief structures); and (3)
mountains of volcano-tectonic origin. The complementary
imaging coverage ofGalileoandVoyagerhas allowed these
features to be mapped in a global scale, thus giving us a
window into not only local but also global processes.
Between the paterae, mountains, and other major geo-
logic features, Io’s surface appears smooth except for scarps
that cut across the plains. Some scarps are linear and occur
in parallel groups, which suggest a tectonic origin. Other
scarps, however, are irregular and appear to be erosional,
sometimes forming a series of mesas or large plateaus. The
presence of these features on Io is somewhat puzzling be-
cause of the lack of a significant atmosphere or flow of liquid
water. Sulfur and sulfur dioxide, possibly escaping explo-
sively from a subterranean “aquifer,” have been suggested
as the main eroding agent on Io’s surface.
Volcanic features dominate Io’s surface, and the volca-
noes cover a wide range of sizes and present varying charac-
FIGURE 4 Mosaics of images acquired byGalileo’s camera of
three regions that accentuate the different types of geologic
materials and terrains on Io. (Top) The Chaac-Camaxtli region
shows paterae in various sizes, shapes, and colors, indicating
varying volcanic and tectonic influences on their formation (from
Williams et al., 2002,J. Geophys. Res. 107 , 5068). (Middle) The
Culann-Tohil region, which contains paterae, fluctii (lava flow
fields), a mountain (Tohil Mons), and a volcanic construct (Tsui
Goab Tholus). (From Williams et al., 2004,Icarus 169 , 80–97).
(Bottom) The Zamama-Thor region, dominated by two eruptive
centers, Zamama and Thor. Zamama has a long lava flow field
and Thor was the site of the tallest eruptive plume seen on Io.
(From Williams et al., 2005,Icarus 177 , 69–88). (Figure courtesy
of David Williams, Alfred McEwen, and Moses Milazzo.)teristics such as power output, persistency of activity, and as-
sociation with plumes. Interestingly, most of Io’s volcanoes
manifest themselves as caldera-like depressions, referred to
as paterae. Unlike terrestrial volcanoes, those on Io rarely
build large topographic structures such as tall shields (like
Mauna Loa) or stratovolcanoes (like Mount St. Helens).
There are only a few tholi scattered across Io.
Io’s surface shows a few remarkably large flucti. The lava
flow field from the Amirani volcano is∼300 km long, the