Jupiter holds a special place in history because of the discovery by Galileo in 1610 of four large moons orbiting it. This observation gave Galileo strong evidence against the popular Earth-centered universe of his day. Like the Earth, Jupiter was a planet with moons orbiting it. Galileo took the Jupiter system to be like a miniature solar system. These four satellites are called the Galilean satellites in honor of their discoverer. In order of increasing distance from Jupiter they are Io, Europa, Ganymede and Callisto. Since then, fifty-nine other moons have been discovered orbiting Jupiter. The Galilean satellites are of particular interest here.
The
Galilean satellites (Jupiter's four largest moons) to the same scale.
Our Moon is also shown for reference.
Cutaway views of the possible internal structures of the Galilean satellites. Clockwise from top left: Io, Europa, Callisto, Ganymede. See the text below for information about the internal structure.
The Galilean moon closest to Jupiter is Io. Io has no impact craters even though it has a rocky, solid surface. The surface must be very young because something has erased the impact craters. Even though Io has nearly the same size and density as the Moon, Io is the most geologically active world in the solar system. Io has many volcanoes and all of its craters are volcanic in origin. It is so active despite its small size because of the enormous stresses it experiences from Jupiter.
Even though Io is about the same distance from Jupiter as the Moon is from the Earth, Io experiences much stronger tidal stretching because Jupiter is over 300 times more massive than the Earth. Io's orbit is kept from being exactly circular by the gravity of its Galilean neighbor Europa and the more distant Ganymede. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted. The tidal flexing heats Io's interior to the melting point just as kneading dough warms it up. The heat escapes through powerful eruptions spewing sulfur compounds in giant umbrella-shaped plumes up to almost 300 kilometers above the surface. The tidal heating from Jupiter has driven away much of the volatile materials like water, carbon dioxide, etc. Io's surface is a splotchy mixture of orange, yellow, black, red, and white. The colors are created by sulfur and sulfur dioxide at various temperatures in liquid and solid states.
The ocean of liquid water below Europa's icy surface may extend down serveral tens of kilometers (or more). Could life forms have developed in the warm waters below the icy surface? Recent discoveries of fish, albino crabs, and 10-foot-long tube worms huddled around active volcanic vents on the Earth's ocean floor far below where the sunlight energy can penetrate has bolstered the view that Europa could harbor life below its icy surface away from sunlight. Before the discovery of life around the geothermal vents, scientists thought that all life depended on sunlight. More recently, bacteria have been found to exist in rock a few kilometers below the sea floor and land surface. Clearly, life is more versatile than originally thought.
The second largest of the Galilean satellites and the farthest from Jupiter is the heavily-cratered moon called Callisto. It has a density of 1.8 times that of water, so it has proportionally more frozen water surrounding a smaller rocky core than Ganymede. Callisto's surface does not appear to have undergone any sort of geological activity. Callisto has a huge impact site called Valhalla that was produced about 4 billion years ago. When the asteroid hit Callisto, it exploded on impact. The explosion heated the ice to above the melting point and the shock waves produced a ripple pattern away from the impact site. The ripples later froze so Valhalla now looks like a big ``bull's eye''.
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(left) Boundary between an ancient, dark terrain (bottom) and younger, brighter fracture zones (top) on Ganymede as seen by Galileo. (right) The giant impact feature on Callisto called Valhalla is just above center right of this image taken by Voyager. The image has been enhanced to highlight the ripple pattern from the impact. Images courtesy of NASA/JPL |
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last updated: June 2, 2007
