Science

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Jupiter's Moons: The Galilean Satellites

March 24, 2010
by findingDulcinea Staff
When Galileo first saw Jupiter's four largest moons, he realized that Earth was not the center of the universe. Since then, we've discovered many exciting things about Jupiter's moons.

Jupiter's Moons End Geocentric View of Universe

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Jupiter is the largest planet in our solar system and contains more matter than all of the other planets combined. Jupiter has a composition much like the Sun, and the planet has Auroral emissions much like Earth's northern lights. Jupiter also has a ring system, but the rings are invisible from Earth.

On January 7, 1610 when Galileo observed Jupiter through his telescope, he thought he saw three stars near the planet, but later the stars appeared to move. Galileo observed four "stars" total, and later realized that the stars were actually moons. This was proof that everything in the uinverse did not revolve around the Earth. 

According to NASA, German astronomer Simon Marius may have observed Jupiter's moons before Galileo, but did not publish his discovery. Galileo referred to the moons as the "Medicean planets" in homage to the Medici family. Marius had his own names for the moons, suggested to him by fellow German astronomer Johannes Kepler, and those are the ones we use today: Io, Europa, Callisto and Ganymede, three maidens and a boy beloved by the god Jupiter. These four moons are now known as the Galilean moons.

Io, Europa, Callisto and Ganymede

Jupiter has 62 known moons, but the Galilean moons, which are the four largest, are the most well known. Io, Euorpa, Ganymede, and Callisto may share a common host planet, but they are very distinct orbs.
 
Io is the most volcanically active body in the solar system. Jupiter affects the surface of Io very powerfully; according to NASA, the surface bulges inward and outward by as much as 330 feet. The incredible pressures that Jupiter brings to bear on Io is the primary cause of its volcanic activity.  Io also cuts across Jupiter's magnetic field; the resulting radiation contributes to Jupiter's auroras. 

Europa is thought to be covered by an ocean of salt water. Because Europa is so far from the Sun, the surface of the planet is completely frozen. NASA recently announced that it has plans to send a spacecraft to Europa to explore the possibility of life beneath the ice.

Ganymede is the largest moon in the solar system.  In 1995 the Hubble telescope discovered that Ganymede had a very thin ozone layer.

Callisto is the most heavily cratered moon in the solar system, it is thought to have almost no surface activity. Callisto's surface is dark compared with the other large moons of Jupiter, but it is brighter than the Earth's moon.

How to View Jupiter's Moons Yourself

In December of 2000, the Cassini spacecraft took the highest quality photo of Jupiter ever. In this image you can see the planet's famous "red spot" as well as the distinct colors and lines that cover the gasous planet.  The Hubble Telescope has captured many images of moons within our solar system; browse a few of those images on HubbleSite.

See the moons for yourself with help from Sky and Telescope magazine, which offers tips for tracking Jupiter's moons in orbit.
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