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What are the Galilean Moons?

Updated on May 13, 2013
The Galilean Satellites and Jupiter's Great Red Spot
The Galilean Satellites and Jupiter's Great Red Spot | Source

Jupiter and its Moons

There are over 170 known moons in our Solar System, with more than 60 of these orbiting Jupiter alone. The four largest of these are known as the 'Galilean Moons,' named after their discoverer, Galileo Galilei. On 7 January 1610, Galileo turned his telescope towards Jupiter where he saw three 'little stars.' Over the next few nights he realised that these stars appeared and disappeared around the planet and so must be objects orbiting Jupiter.

Jupiter's four largest moons are named after the lovers of the Greek God, Zeus:

  • Callisto
  • Ganymede
  • Europa
  • Io

We probably know more about each of the Galilean Moons than we do about Mercury. These moons are as different from each other as one planet is from the next - they are worlds in their own rights. In fact, many scientists now believe that it is a moon, not a planet, that is most likely to harbour extraterrestrial life.

The Galilean Satellites

 
Io
Europa
Ganymede
Callisto
Equatorial radius (km)
1821
1565
2631
2403
Mass (relative to Earth)
0.0149
0.00803
0.0248
0.0181
Density (g/cm3)
3.53
3.01
1.94
1.83
Surface Gravity (relative to Earth)
0.18
0.13
0.14
0.13
Composition of surface
rocky
salty ice
dirty ice
dirty ice
Average Surface Temperature
-150C
-150C
-160C
-160C

Galileo and the Moons of Jupiter

Missions to Jupiter

  1. Pioneer 10 (Dec. 1973)
  2. Pioneer 11 (Dec. 1974)
  3. Voyager 1 (Mar. 1979)
  4. Voyager 2 (July 1979)
  5. Ulysses (Feb. 1992)
  6. Galileo (Dec 1995 - 2003)
  7. Cassini (Oct. 2000 - Mar. 2001)
  8. Europer Orbiter (2010-2012, now cancelled)

Images of Io

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Io transiting JupiterThe Prometheus Plume on IoPele erupting on Io
Io transiting Jupiter
Io transiting Jupiter | Source
Source
The Prometheus Plume on Io
The Prometheus Plume on Io | Source
Pele erupting on Io
Pele erupting on Io | Source

Io

For over 400 years scientists had assumed that Io was a cold, dead world. In 1979 this was proved to be spectacularly incorrect - Io is the most geologically active body in the solar system. There are often more than a dozen volcanoes erupting on Io at any one time. Over 500 volcanoes have been identified on the moon, and around 100 have been seen to erupt.

Due to these frequent eruptions, Io even has an atmosphere of sulphur dioxide, atomic oxygen, soidum and potassium. This atmosphere is constantly 'leaking away' into space and so must be continually repleneshed through volcanic activity

Io's surface is dominated by the results of its volcanic activity. There are lava flows several hundred kilomsters in length and very few impact craters. The eruptions are so frequent that fresh materials are deposited across the moon at an average rate of 1cm thickness per year. Volcanic calederas (craters formed after the roof of a magma chamber collapses) up to 200km across are clearly visible on the surface of Io.

Europa

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The 'fractured' sufrace of Europa. Europa has surprisingly few impact craters on its surface, suggesting processes are still modifying the surface.Close up of Europa, the smallest of the Galilean moons of Jupiter.Close up on the complex ridge fractures on the surface of Europa
The 'fractured' sufrace of Europa. Europa has surprisingly few impact craters on its surface, suggesting processes are still modifying the surface.
The 'fractured' sufrace of Europa. Europa has surprisingly few impact craters on its surface, suggesting processes are still modifying the surface. | Source
Close up of Europa, the smallest of the Galilean moons of Jupiter.
Close up of Europa, the smallest of the Galilean moons of Jupiter. | Source
Close up on the complex ridge fractures on the surface of Europa
Close up on the complex ridge fractures on the surface of Europa | Source

Europa

Of all the Jovian moons, Europa is probably the world that excites the most scientific interest. Not only is there the prospect of ice-lava, but also an ocean far more substantial than any other in the Solar System.

Europa is the smoothest body in the solar system; its surface a shell of ice frozen at -160°C, where ice is as hard as steel. Look closely and you can see the surface is crissed cross with deep cracks and fissures. There are even regions that are analogous to areas of the poles of our own planet, where sea ice has broken into icebergs and been jumbled up before refreezing. It is thought that ice volcanoes could be present on the surface, spewing ice-lava onto the surface of the moon.

By analysing the magnetic field of the planet, scientists believe that any ocean may be salt water in composition and may be up to 60miles (100km) deep. This would give Europa twice as much water than all of Earth's oceans combined. It is important to remember, however, that there is no proof that Europa's ocean exists, although it does seem to be the simplest way to explain all of the observations made of Europa's surface and magnetic field.

On our own planet, wherever there is liquid water we find life. It is highly likely that, if a liquid ocean is confirmed on Europa, we may well find truly extraterrestrial life. So interested are scientists in the potential for life on Europa, that training missions to Lake Vostok (a lake 10,000km2 beneath 4km of ice in Antarctica) are being used as trial runs for when we eventually send probes to the Ice-moon.

Images of Ganymede

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Ganymede, the largest moon in the solar system. The dark areas are older, more heavily cratered regions whereas the lighter areas are younger.Close up of the terrain on Ganymede
Ganymede, the largest moon in the solar system. The dark areas are older, more heavily cratered regions whereas the lighter areas are younger.
Ganymede, the largest moon in the solar system. The dark areas are older, more heavily cratered regions whereas the lighter areas are younger. | Source
Close up of the terrain on Ganymede
Close up of the terrain on Ganymede | Source

Ganymede

Ganymede is the largest moon in the Solar System and is bigger than the planet Mercury. Despite being made of mainly ice, you can see in the images above, the ice is frozen so solid it behaves like rock. Many of the impact craters look that same as those on rockier worlds.

The presence of impact craters on Ganymede (something missing from the surfaces of Europa and Io) suggests that the surface of Ganymede is much older than that of the inner Galilean moons. This older surface also explains why Ganymede is much darker than Europa - icy surfaces darken as they age.

Callisto

Callisto, the third largest moon in the Solar System.
Callisto, the third largest moon in the Solar System. | Source

Callisto

Callisto is the most distant of the Galilean Satellites from Jupiter, orbiting around 2 million km (1.2million miles) from the planet. It is also the least reflective of the moons - half as reflective as Ganymede and three times less reflective than Europa.

Callisto is a giant moon, its the third largest satellite in the Solar System and is made up of around 50% water ice. The great abundance of craters on the surface of Callisto suggests that it is the most ancient surface of the Galilean moons. There is also a lack of fractures in the surface which suggests that the ice shell of the planet must be very thick.

This is at odds, however, with data gathered by the Galileo probe which measured the magnetic field of the moon. The strength of the field suggests that a salty ocean should exist around 100km below the surface of the moon. We shall have to wait for more data from further trips to the outer reaches of our solar system to clear up this mystery. Sadly, with budget cuts abound at all of Earth's space agencies, this is likely to be far in the future.

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    • John Sarkis profile image

      John Sarkis 4 years ago from Los Angeles, CA

      Great hub! I just got back from lunch and took Hawking's book "The Grand Design" and was reading about Galileo and so on...your hub caught my eye.

      Nicely done - voted up

      John

    • Patty Kenyon profile image

      Patty Kenyon 4 years ago from Ledyard, Connecticut

      AWESOME Hub with Amazing pictures!!!!

    • ponder profile image

      Irma Cowthern 3 years ago from Los Angeles,CA

      Great article!!!

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