The Galilean Satellites – from Galileo
ATLAS OF THE UNIVERSE
Changes on Io (right
and bottom right images):
the right-hand picture
was taken from Voyager 1
on 4 March 1979, from
862,000 km (500,000 miles).
The bottom-right picture
was taken from Galileo on
7 September 1996, from
487,000 km (302,000 miles);
the image is centred on
the face of Io which is
always turned away from
Jupiter. The active volcano
Prometheus appears near
the right centre of the disk.
The black and bright red
materials correspond to
the most recent volcanic
deposits, probably no more
than a few years old. Three views of Io:
Galileo, June 1996. Some
areas are truly red, while
others are yellow or light
greenish. The major red
areas are associated with
very recent pyroclastics
erupted in the form of
volcanic plumes. The
most prominent red oval
surrounds the volcano Pele
(far right); an intense red
spot lies near the active
plume Marduk, east of
Pele. Loki and Amirani
are inactive, whereas
Volund is active.T
he orbiting section of the Galileo probe has sent
back superb images of Jupiter’s Galilean satellites.
Ganymede and Callisto are icy and cratered; the discovery
of a magnetic field on Ganymede was a great surprise.
However, the most spectacular images are those of Io and
Europa.
The Ionian scene has changed markedly since the
Voyager passes, while the existence of an underground
ocean inside Europa is a real possibility. Some surface
details on the satellites can be seen with the Hubble Space
Telescope, and the events on Io can be monitored, but of
course the clarity of the Galileo pictures is unrivalled.Io is the most volcanically active world in the Solar
System, and eruptions are going on all the time. It is
strange that there is so great a difference between Io and
the inert, ice-coated Europa. Io is also the only satellite to
show activity apart from Triton, in Neptune’s system, with
its ice geysers.C Atl of Univ Phil'03stp 2/4/03 3:10 pm Page 104