What's the Deal With Moons?What's the Deal With Moons?
Summary: The jovian moons range in size from only a few hundred kilometers in diameter to larger than the planet Mercury.
The Jovian Moons
The numerous moons orbiting the jovian planets exhibit an astonishing variety of features. Many are captured asteroids that have changed very little in the past 4.5 billion years, but others show extensive signs of past, recent, or ongoing geologic activity.
- Small moons: less than 300 km in diameter
- Medium-size moons: 300-1,500 km in diameter
- Large moons: more than 1,500 km in diameter
This figure shows all of the medium and large moons of the Jovian planets. These moons are most closely related to the terrestrial planets. They are large enough such that gravity has made them spherical, and their surfaces are solid. Most of the large and medium-size moons show evidence of current or past geologic activity. Some even have atmospheres and magnetic fields. However, unlike the terrestrial planets, these moons often contain a considerable amount of ice in addition to rock and metal.
Just as our Moon always has the same face pointed towards Earth, most jovian moons keep the same face turned towards their respective planet. This type of rotation is called synchronous rotation.
Click to learn more about synchronous rotation.
The small moons far outnumber the large moons. Many of the small moons are probably captured asteroids, which explains why so many of them have orbits with significant eccentricities, large orbital tilts, or even orbits that are opposite to the direction of the planet's rotation.
Close-up photos from spacecraft have revealed several jovian moons with spectacular past and present geologic activity. In the cases of Jupiter's moons Io and Europa, there is an ongoing source of internal heat quite different from that found on any terrestrial world. The jovian moons tend to be made from material with a high proportion of ice. Geological change occurs at much lower temperatures in ice than rock, which probably accounts for the geologic activity seen in many of the medium-size moons.