Satellites of Jupiter
ATLAS OF THE UNIVERSE
J
upiter has an extensive satellite family. Four are large,
and are bright enough to be seen with any small tele-
scope; even powerful binoculars will show them under
good conditions. They were observed in January 1610
by Galileo, using the first astronomical telescope, and
are therefore known collectively as the Galileans, though
they may have been seen slightly earlier by Simon Marius.
It was Marius who gave them their names: Io, Europa,
Ganymede and Callisto. Perhaps for this reason, the names
were not widely used before the onset of the Space Age.
Ganymede and Callisto are much larger than our
Moon, and Ganymede is actually larger than the planet
Mercury, though less massive; it may have been recorded
with the naked eye by the Chinese astronomer Gan De as
long ago as 364 BC. Io is slightly larger than the Moon,
and Europa only slightly smaller.
Callisto, outermost of the Galileans, is the faintest
of the four. There is an icy, cratered crust which may go
down to a depth of several hundred kilometres (two or
three hundred miles), below which may come a mantle of
water or soft ice surrounding a silicate core. There are no
signs of past tectonic activity, and certainly Callisto
seems totally inert. Ganymede is rather denser, and shows
more traces of past activity than in the case of Callisto; in
1996 the Galileo probe detected a weak but appreciable
magnetic field, indicating the presence of a metallic core.
Europa too has an icy surface, but there are almost no
craters, and the main features are unlike anything foundelsewhere; Europa has been likened to a cracked eggshell,
and is essentially smooth. According to one theory, the
crust lies above an ocean of liquid water, though it is also
possible that the mantle is composed of ‘slushy’ ice lying
over the core.
Io is a remarkable world. Its surface is sulphur-coated,
and during the Voyager 1 pass several active volcanoes
were seen, one of which, Pele, sent a plume up to a
height of 280 kilometres (175 miles). By the time of the
Voyager 2 pass Pele had ceased to erupt, but several of
the other volcanoes were more active than before, and
there is of course no reason to suppose that Pele is extinct.
The volcanoes can now be monitored by the Hubble
Space Telescope, and it is clear that eruptions are going
on all the time.
According to one theory, Io’s crust may be a ‘sea’
of sulphur and sulphur dioxide about 4 kilometres
(2.5 miles) deep, with only the uppermost kilometre
solid. Heat escapes from the interior in the form of lava,
erupting below the sulphur ocean, and the result is a
violent outrush of a mixture of sulphur, sulphur dioxide
gas, and sulphur dioxide ‘snow’. Some of the volcanic
vents may be as hot as 500 degrees C, though the general
surface is at a temperature of below 150 degrees C.
Jupiter and its satellite Io are connected by a powerful
electrical flux tube (which is why Io has a marked effect
upon the radio emissions from Jupiter itself), and material
from the Ionian volcanoes produces a torus round Jupiter▲ Amalthea, Jupiter’s fifth
satellite, as seen in
November 1999 from the
Galileo spacecraft. The
effective resolution is about
4 km (2.5 miles). The large
impact crater towards the
right is about 40 km
(25 miles) in diameter. The
linear bright streak towards
the left has been named Ida.
It is approximately 50 km
(31 miles) long.The four large Galilean
satellitesof Jupiter as
photographed by the
Galileo spacecraft. Io (top
left) was photographed on
3 July 1999, Europa (top
right) on 7 September 1996,
Ganymede (bottom left) on
26 June 1996, and Callisto
(bottom right) in May 2001.
The two brightest satellites,
Io and Europa, apparently
have surfaces of very
different composition. Io is
thought to be covered with
sulphur and salts, and its
surface shows evidence of
ongoing volcanic activity.
Europa’s surface is mainly
water ice with some rocky
areas; long fractures can
been seen in the crust.
Ganymede has both ice and
rock exposed on its surface,
and the bright spots are
relatively recent impact
craters. Callisto’s surface
is primarily rock-covered
ice, covered with craters.
These surface properties
contrast sharply with the
interiors of the satellites: Io
and Europa have rocky
interiors, which contain large
amounts of water or ice.
All the images were obtained
by the Solid State Imaging
(SSI) system on NASA’s
Galileo orbiter.C Atl of Univ Phil'03stp 2/4/03 3:09 pm Page 102