Ganymede
Ganymede is the biggest moon of Jupiter and is also
the biggest moon in the Solar system. It
has the greatest diameter as well as the greatest mass. Ganymede even has a larger diameter than the
planet
Mercury
, but its mass is lower than Mercury’s.
Ganymede also has a greater mass than any of the dwarf
planets so far discovered.
Ganymede has a lower mass than Mercury because of its
high proportion of light elements and compounds of these elements, especially
water. It is estimated that about half
of Ganymede’s mass is water.
Discovery
Ganymede was discovered in 1610, probably
on January
th
e 11
th
. Its discovery is
usually credited to Galileo Galilei (on the Right), although Simon Marius (on the left) also discovered this
moon at about the same time. Galileo had
a tendency to claim all the early telescopic discoveries.
Naming
Ganymede was a young Trojan Prince.
He was the most attractive of all the
mortals. He was abducted by Zeus, the
king of the Gods, in the form of an
eagle
, and became the cupbearer to the Gods on Mount Olympus.
The name the Romans gave to Zeus was
Jupiter. The four moons of Jupiter that
Galileo discovered are all named after people that Jupiter abducted. Simon Marius gave the name Ganymede to Jupiter's moon.
Heavy Core
Detailed measurements made by the Galileo probe
suggest that Ganymede has a small, probably iron rich, core with the next layer
being silicate based rock. This is
surrounded by a lighter layer consisting mainly of water ice, but with many
other things in it as well.
Magnetic
Field
Ganymede is the first moon to be found to have its own
magnetic field. This field is not as
strong as that of the Earth, and is dwarfed by Jupiter’s magnetic field.
Liquid
Ocean
?
It is speculated that there is a layer of liquid salty
water between a very thick icy crust and the solid core. This is still very
uncertain and more measurements will be needed to see if this liquid layer
exists. Data from the Galileo Probe suggests this liquid ocean does exist.
There is considerable question about whether or not
there is enough heat to keep part of the water of Ganymede in a liquid form.
The tidal heat from the interactions with Jupiter and
the other moons would appear to be insufficient by itself to keep some of
Ganymede’s water liquid. Other sources
of heat in Ganymede are possible like the decay of radioactive elements, but
the quantities of these elements are unknown.
The surface of Ganymede gets about one thirtieth of
the concentration of solar energy as is received on the Earth.
Atmosphere
Traces of Ozone (O
3
)
have been detected and the presence of a very thin Oxygen atmosphere is
inferred.
Life on
Ganymede?
The only place that we are sure has life is the planet
Earth. We can only speculate about other
places. If there is liquid water in
Ganymede, it would be a possible place for life. It will probably be quite a long time before
we are reasonably sure one way or the other.
Observing
Ganymede
Ganymede is the only moon of our solar system, except
Earth’s own moon, that can be seen from the surface of the Earth with the naked
eye. However, this is very difficult
even under perfect conditions. The main
problem with seeing it is the brightness of Jupiter.
Galileo and many other astronomers have seen it
through telescopes, and more recently better pictures have been taken by the
Hubble Telescope.
Galileo
Probe
The Galileo Probe was launched on October the 18
th
1989 (my son’s birt
hday). After a
complicated fight path to gain velocity by fly pasts of Venus and the Earth
(Twice) it arrived in the Vicinity of Jupiter; its Jupiter atmospheric probe
was released on July the 13
th
1995 and entered the Jovian atmosphere
on December the 7
th
1995.
Although the Galileo was the sixth probe to visit the
vicinity of Jupiter it added enormously to our knowledge of the Jovian system
as well as making many other important discoveries on its long journey.