Io: The Volcanic Moon 429
magnetic field. Io’s atmospheric density is low (about 10−^9
bar), equivalent to good laboratory vacuums on Earth, but
the density is greater at the locations of active volcanic
plumes. The main constituent of the atmosphere is SO 2 ,
which is supplied largely by volcanic plumes, with a lesser
amount coming from evaporation of the SO 2 frost deposits
on the surface. Io’s low gravity allows some of the atmo-
sphere to escape, but it is continuously replenished by vol-
canic outgassing.
Since the time of theVoyagerflybys, Io has been known
to produce volcanic plumes hundreds of kilometers high,
which serve as an efficient delivery mechanism for gas and
dust particles into the magnetosphere and the space sur-
rounding Io, although only a relatively minor amount of at-
mospheric gas is lost to space. The dynamics of Io’s plumes
are very complex, particularly because models of plume
emplacement have to take into account the very low atmo-
spheric pressure on Io.
One of the last surprises from Galileo observations was
the detection of four large plumes at high northern lati-
tudes. Prior to 2000, there was no detection of high latitude
plumes by Galileo, though deposits on the surface indicated
that plume activity had occurred in the past. The largest
plume known on Io (500 km high) was detected from images
obtained in August 2001, shortly after theGalileospacecraft
had flown through it. Observations by the plasma science
experiment indicated the presence of SO 2 molecules in the
plume. This in situ measurement is consistent with others
that show the presence of SO 2 in plumes and SO 2 frost in
plume deposits. Sulfur (S 3 and S 4 ), in addition to SO 2 , was
detected in the Pele plume from measurements made from
theHubble Space Telescopeby J. Spencer and colleagues.
The temperatures of the frost deposits on Io’s surface
are sufficiently low that cold-trapping of SO 2 by condensa-
tion is a very important process. Some material does escape
Io, forming a corona and neutral clouds, and the Io torus
further away. The corona refers to the region within Io’s
gravitational pull, where bound and escaping atoms and
molecules populate a low-density shell. The neutral clouds
of sodium, oxygen, and sulfur extend from the corona to
distances of many times the radius of Jupiter.
An important discovery made during theGalileomis-
sion was Io’s aurora. The aurora (Fig. 9) was detected
through color eclipse imaging with the camera while Io was
in Jupiter’s shadow. The vivid colors detected (red, green,
and blue) are caused by collisions between Io’s atmospheric
gases and energetic charged particles trapped in Jupiter’s
magnetic field. The green and red emissions are probably
produced by mechanisms similar to those in Earth’s polar
regions that produce terrestrial aurorae. The green (actu-
ally yellow) glow comes from emission from sodium ions,
whereas the red glow is associated with oxygen ions. The
bright blue glows mark the sites of dense plumes of volcanic
vapor and may represent the locations where Io is electri-
cally connected to Jupiter via a flux tube.
FIGURE 9 Io aurora. Bright blue glows represent sulfur dioxide
excited by electrical currents flowing between Io and Jupiter in
the flux tube. The blue glow on the right is over the Acala hot
spot, which is thought to be the site of a “stealth plume”
(composed of mostly gas and therefore hard to detect from
images). Red and green (actually yellow) glows represent atomic
oxygen and sodium, respectively.Observations from thePioneerspacecraft were the first
to reveal a cloud of neutrals along Io’s orbital path. The
most easily observed of these neutral clouds around Io is
the sodium cloud. The cloud is populated by sodium atoms
escaping Io at about 2.6 km s−^1. It appears as a diffuse yel-
lowish emission produced by scattered light from volcanic
plumes and Io’s lit crescent. This emission comes from neu-
tral sodium atoms within Io’s extensive material halo that
scatter sunlight at the yellow wavelength of about 589 nm.
Although neutral sodium atoms are most easily detectable
by spectroscopy, it has been determined through extensive
Earth-based telescopic studies that sodium is a minor com-
ponent of the neutral material escaping from Io. The pri-
mary neutral elements in the cloud are oxygen and sulfur,
which are thought to have dissociated from sulfur dioxide
gas (SO 2 ) expelled from many of Io’s active volcanoes at a
rate of∼1 ton s−^1. So far, sodium has not been detected on
Io’s surface or plumes, but its existence is inferred because
of its detection in the cloud.
The Io torus is a doughnut-shaped trail about 143,000 km
wide along Io’s orbital path. The torus is made up almost
exclusively of various charged states of sulfur and oxygen,
thought to be derived from the break-up of volcanic SO 2
and S 2. The ionized particles are held within the torus by
Jupiter’s magnetic field, in much the same way that charged
particles are held in the Van Allen radiation belts around
the Earth. Measurements made by theGalileospacecraft
during its close flybys showed that the plasma in the torus
is slowed by Io’s ionosphere, redirected around Io and then
reaccelerated in Io’s wake. OtherGalileomeasurements
showed that Io strongly perturbs Jupiter’s magnetic field.
These perturbations vary with time, suggesting that Io’s vari-
able volcanic activity influences the density of the plasma
torus and the strength of its interactions with the jovian
magnetic field. [SeePlanetaryMagnetospheres.]