Introduction
Jupiter is the most massive planet in
the solar system. Its mass is 318 times
that of Earth,
while only about 1/1000 that of the
Sun. Jupiter has
been well known from ancient times
because of its brightness in the sky.
The Greeks associated it with
Zeus, the king of the
gods. At the times of the year when the
planet is visible, it appears as bright
as the brightest stars in the sky.
Jupiter's distance from the Sun is about
five times the Earth's, or
5.2 AUs. The planet completes a full
orbit in 11.86 years.
Jupiter is the closest to the Sun of the
so-called 'gas giants,' i.e. the outer
planets of the solar system made up
primarily of gas, rather than rock. Its
radius is 142,984 km at the equator, or
about 11 times the Earth's radius. Its
composition is similar to that of the
Sun: about 71% of its mass is hydrogen,
24% is helium, and 5% of all the other
elements. Though Jupiter has a large
rocky core, about 10 times more
massive than the whole Earth, and made
of similar rocks, the core is truly a
small percentage of the planet's total
mass.
Jupiter's composition is thought to be
correlated with its mass. Water ice was
once abundant at the distance from the
Sun where Jupiter formed. It initially
constituted a good fraction of Jupiter's
mass. The massive core of the young
planet was able to gravitationally
capture the hydrogen and helium from the
nebula that originated our solar
system, further increasing the planet's
mass and making its composition similar
to the average of the Sun and the
primeval nebula. In contrast, Earth did
not have enough mass to do so and it
remained primarily a rocky planet.
Jupiter has seventeen moons, four of
which are large enough to be seen even
with binoculars. The four largest
moons
Io,
Europa, Ganimede and
Callisto, are among the most
interesting moons in the solar system.
Io and Europa are geologically
active, and Io is the most
volcanically active body anywhere in the
solar system. The orbiting probe
Galileo actually witnessed volcanic
eruptions, and took images of plumes of
gas hovering well above Io's surface.
Europa, completely covered by white ice,
seems to harbor a vast water ocean
underneath, and it has become a prime
target for planetary exploration in
search for life.
