The Moons of Jupiter
• Most people think that Jupiter has four moons. They are
wrong! There are in fact 67 recognised moons orbiting in
the Jovian system. However there are four that are large
and easily visible from Earth using a small telescope.
• Galileo is credited with discovering the first four main moons
hence them being called the Galilean Moons. This initial discovery was made in 1610. The latest Jovian moon
to have been discovered was not long ago in 2011.
• The moons of Jupiter are split into two main categories: those with regular orbits and those with irregular orbits. The moons with regular orbits travel round Jupiter in
an almost circular orbit aligned with the planet’s equatorial plane. The irregular moons have elliptical orbits, high
inclinations and are much further away from Jupiter.
• The moons vary dramatically in size from 1km in diameter to over 5,200km. Ganymede is the largest and
there are 48 moons with a diameter of 5km or less.
The Galilean Moons
• Of the four moons Galileo discovered, Ganymede
is the largest, followed by Callisto, Io and Europa.
Ganymede is in fact the ninth largest body in the
solar system and is bigger than Pluto and Mercury.
• The Galilean Moons are the fifth through eighth closest of the known moons orbiting Jupiter. Io is the closest with Europa, Ganymede, and Callisto outside it.
• Io, the closest of the Galilean moons, is the
most dense of the moons and is highly volcanic. Its surface is therefore very young.
Io’s core is thought to be made up of iron
and iron sulphide, there is then a mantle of
igneous rocks, the viscous upper layer of the
mantle then the outer crust of the moon. This
composition is similar to the inner planets.
Formed at about the same time as Jupiter it
has an orbital radius of around 260,000 miles.
• Europa, the smallest of the Galilean moons,
orbits outside Io. Europa is thought to be the
ocean planet, with a vast watery ocean lying under a crust of ice. It is speculated that
the gravitational effects of Jupiter create
tides that in turn warm the water. The composition of Europa presents the highest probability for the discovery of life as we know it
in the Solar System. The Hubble Space Telescope has recorded huge spouts of water
vapour jetting from the surface. The moon
orbits at just over 415,000 miles from Jupiter.
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Credits: NASA/JPL
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