422 Encyclopedia of the Solar System
FIGURE 2 Io’s active volcanoes were discovered from this
image, taken byVoyager 1on March 8, 1979, looking back
4.5 million km. The Pele plume is seen on the lower right, rising
nearly 300 km above the surface. The bright spot near the
terminator (shadow between day and night) is the top of the Loki
plume, illuminated by the Sun.
emission from parts of Io’s surface—some areas had tem-
peratures of about 400 K, much higher than the rest of
the surface, which has noontime equatorial temperatures
of about 107–124 K. When one of the hot areas was found
to coincide with one of the plumes, there was no doubt that
active volcanism was taking place.
Eighteen weeks afterVoyager 1’s dramatic discovery, the
companion spacecraft,Voyager 2, flew close to Io. Intense
activity was still taking place, but significant changes had
occurred between the two flybys, including the cessation of
the largest plume, Pele (Fig. 3), and the altered shape of
the deposits associated with this plume. An area of about
10,000 km^2 had been filled in, presumably by fresh mate-
rial falling down from the plume. It became evident that
dramatic changes of Io’s surface could occur over short
timescales.
Initial analysis of the Voyager observations showed
9 plumes and 9 hot spots, though not all plumes coincided
with hot spots and vice versa. “Hot spot” is a term used by
Io researchers to define a region of enhanced thermal emis-
sion, a sign of active volcanism. TheVoyagerIRIS experi-
ment did not observe the whole surface, so it was suspected
that other hot spots existed. The surface showed many
features with morphologies similar to volcanic landforms
on other planets, such as calderas (volcanic craters) and
flows.
After the twoVoyagerspacecraft left the Jupiter sys-
tem on their way to Saturn and beyond, the study of Io’s
FIGURE 3 Voyager 1image showing the Loki plume on the
limb and the heart-shaped Pele plume deposit in the lower part
of the image. WhenVoyager 2arrived 18 weeks later, the “heart”
had become an oval, as material from the plume had filled out
the area.volcanism was continued from Earth by astronomers using
infrared detectors mounted on telescopes. These observa-
tions showed that brightenings and fadings of hot spots oc-
cur, indicating variations in the level of volcanic activity. Ob-
servations by W. Sinton, R. Howell, J. Spencer, J. Rathbun,
and other astronomers have shown that Io’s most powerful
hot spot, Loki, has brightenings that switch on in 1 month
or less and last several months before fading. Telescopic ob-
servations were also used to analyze the reflected light from
Io’s surface to determine surface composition, confirming
that it was dominated by sulfur dioxide (SO 2 ). Io was also
observed by theInternational Ultraviolet Explorersatellite
and by theHubble Space Telescope.
The first spacecraft to orbit Jupiter wasGalileo, which
was able to image Io and monitor its volcanic activity from
1996 through early 2002. Galileo was designed to orbit the
planet Jupiter for 2 years (1996–1997) and collect data on
the planet’s atmosphere, its moons, rings, and magnetic
field. However, the failure ofGalileo’s high-gain antenna to
deploy (a problem discovered while the spacecraft was on
its way to Jupiter) drastically reduced the quantity of images
and data that could be returned to Earth. However, the mis-
sion still accomplished its objectives, thanks to the success-
ful reconfiguration of the spacecraft’s software to utilize a
lower gain antenna and perform data compression on board.