Galileo's Moons
In 1610 Galileo pointed his newly invented telescope at Jupiter. He discovered four new worlds orbiting Jupiter. They are now known as Jupiter's Galilean moons. In order of their distance from Jupiter these moons are: Io, Europa, Ganymede, and Callisto.
These moons are comparable in size to Earth's moon. They are mixtures of rock and ice. The more distant moons from Jupiter have a lower density. This observation tells us that the more distant moons contain a greater percentage of ice and a lower percentage of rock.
Geological Activity
The two more distant moons, Ganymede and Callisto, are covered with craters formed by meteorite impacts. The closer moons, Io and Europa, are not. This fact tells us that Io and Europa are more geologically active. Geological activity destroys impact craters. As a general rule in the solar system, worlds that have more geological activity have fewer impact craters on their surfaces.
For a planet or moon to have geological activity, the interior must be hot. Interior heat energy provides the energy needed to drive geological activity such as volcanoes, earthquakes, and other tectonic activity.
Small objects cool more quickly than large objects. Hence moons and smaller planets are usually not geological active. Io and Europa are geologically active, and Io is among the most geologically active worlds in the solar system. Why? The key is tidal heating from Jupiter. The same difference in gravitational force that causes tides on Earth causes tidal bulges on the Jupiter's closer moons. Friction from the moving tidal bulges heats the interior. The closer the moon is to Jupiter, the greater the tidal heating effect.
Io
Io is the closest of the Galilean moons to Jupiter. Tidal forces make it the most geologically active. Io is the only world in the solar system other than Earth that has known active volcanoes. These volcanoes spew ionized gas into a donut shaped ring around Jupiter along Io's orbit.
Europa
Europa's surface looks like a smooth icy surface covered with small cracks. Europa is a mixture of rock and ice with a small metallic core. The denser materials sunk to the interior. The frozen icy surface has many cracks because tidal heating from Jupiter warmed and melted the interior. As the interior ice melted, it shrunk (Remember that water expands when it freezes.) and caused the surface ice to crack. The oceans of water underneath Europa's frozen surface lead's many scientists to speculate about the possibility of life on Europa.
Ganymede
Ganymede is also a mixture of about half rock and half ice with a small metallic core. It is far enough from Jupiter that there is insufficient tidal heating to melt the ice. Much of the terrain on Ganymede is covered with impact craters indicating that these areas are geologically inactive. Ganymede does however have areas of grooved terrain, which indicate some geologic activity in the distant past.
Callisto
Callisto is about half ice and half rock. Callisto's surface is saturated with impact craters, telling us that Callisto is geologically inactive. The largest impact feature on Callisto is Valhalla. It is a crater about 600 kilometers (370 miles) in diameter surrounded by a couple dozen concentric rings similar to the ripples that form when a small rock impacts a still pond. They apparently melted from a meteorite impact and refroze too quickly after the impact for the ripples to subside.
Further Reading
Zeilik, M., Astronomy: The Evolving Universe, 9th ed., Cambridge, 2002.
Galileo Galilei, Sidereus nuncius, 1610; reprinted in Discoveries and Opinions of Galileo, translated by Stillman Drake, Anchor, 1957.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
