Jupiter's moons hold a special place in the history of human knowledge.
Galileo Galilei's discovery in 1610 of a "solar system" with Jupiter
at its hub helped prove Earth
is not the center of the universe.
Galileo observed Jupiter's
four largest moons - Ganymede, Io, Europa, and Callisto - making them the
first celestial objects discovered with a telescope. The twin Voyager 1
and Voyager 2
spacecraft and the one named for Galileo
gave us close-up looks at these "Galilean" moons and discovered many more. There are currently 63 known moons orbiting Jupiter.
Ganymede, Callisto, and Europa are all suspected of having oceans of liquid
water beneath their icy crusts. If true, Ganymede and Callisto can probably
thank the natural radioactivity of their rocky interiors for keeping their
slushy water from freezing solid.
As the closest of the three to giant Jupiter, Europa
has an additional heat source. Jupiter and the other large moons subject
Europa to a gravitational tug-of-war that stretches and squeezes the satellite
in much the same way that Earth's moon raises tides in our oceans. This
generates enormous heat inside the moon, which may keep its ocean (if any)
in a liquid state. That and the ocean's suspected location only a few kilometers
beneath the smooth, icy surface may make Europa a prime target in the search
for extraterrestrial life.
, innermost of the Galilean satellites and slightly larger that Earth's moon
goes through even greater gravitational flexing, with "tides" of as much
as 100 meters (328 feet) in its solid rock surface. The resulting frictional
heat keeps Io's subsurface rock layer molten and pressurized, constantly
ready to feed the moon's many volcanoes.
Covered in sulfurous lava
that makes it look a little like a giant pizza, Io is the most volcanically
active body in the solar system. Voyagers 1 and 2 saw nine eruptions and
the Galileo spacecraft has since spotted hundreds of smaller eruptions.
Plumes of red and yellow sulfur dust shoot 300 km (186 miles) into the sky.
Some escapes Io and paints a bright red coat on a small, neighboring moon
Io contributes to fireworks on Jupiter, too, giving rise to powerful radio
transmissions and contributing to auroras as the rapidly spinning planet
sweeps its powerful magnetic field across the natural satellite.
is the biggest moon in the solar system and, in fact, is larger than the planets Mercury
It has two distinct surface types - one dark and rugged, the other bright
and smooth, with parallel grooves hundreds of meters deep and thousands of
kilometers long. The bright areas show fewer impact craters, indicating
a much newer terrain, possibly resurfaced by ice volcanoes.
, about the size of Mercury, is the third largest moon in the solar system (Saturn's Titan
is #2) and is the outermost of the Galilean satellites. It is completely
covered in craters, more heavily cratered than any other moon or planet in
the solar system. Unlike the other large bodies, which have recoated at
least parts of their surfaces, Callisto remains as it was when it formed
four billion years ago, during the period of intense meteoroid bombardment
that the entire solar system experienced.
Four "ring moons" circle Jupiter inside Io's orbit: Metis
, and the red-coated Amalthea. As meteoroids
strike them and explode, they blow off the dust and debris that form Jupiter's rings
Many more small moons circle Jupiter outside the orbits of the four Galilean satellites.