READING: Chapter 8
Unlike the Earth and its single moon, the Jovian planets have many moons, including several very large moons. Below is a list of the properties of some of the largest of these objects.
JOVIAN MOONS:
- Four largest are Io, Europa, Ganymede, and Callisto
- Jupiter's moons appear to have formed as a miniature version of the Solar System.
- The moons are differentiated, with the inner moons being denser and with different internal structure.
- Io is the most volcanic object in the Solar System with almost 100 active volcanoes.
- Volcanism on Io is powered by the TIDAL FORCE of Jupiter.
- The synchronous orbit of Io is disrupted by Europa, which causes the moon to wobble as it orbits Jupiter.
- Europa's surface has no craters, indicating that it is a new surface erased by recent activity.
- Surface features look like water ice features on Earth's polar regions.
- Thin crust of ice on Europa likely covers a deep saltwater ocean that may contain more water than on Earth.
- Europa has a weak magnetic field that changes strength and direction that appears to originate in the saltwater layer.
- The moon Ganymede is larger than Pluto and Mercury.
- Cratered, but with lowlands that were filled with liquid water, like the Maria on the Moon were filled with molten lava.
- Callisto is in some ways very similar to Ganymede, but is an undifferentiated mix of ice and rock.
- Callisto has a set of concentric rings that are evidence of a major impact.
OTHER MOONS:
- Titan is the largest moon of Saturn, and it is unique because it has a thick atmosphere.
- Titan is able to retain an atmosphere (unlike Ganymede & Callisto), because of its low surface temperature.
- Triton is the largest moon of Neptune, and it orbits Neptune in a RETROGRADE orbit.
- Triton is spiraling inwards closer to Neptune, and will eventually be broken up to add to the planet's ring system.
- Triton does not orbit around Neptune's equator, suggesting it may have been captured from the Kuiper Belt.
The ROCHE LIMIT is the distance from a planet where its gravitational tidal force is stronger than the internal forces holding a moon together. If a hypothetical moon were to move inside of this limit, it would be broken up, and its individual chunks would orbit the parent planet. See Figure 8.17 from C&M:
All of the Jovian planets have rings, with Saturn showing the most spectacular system of rings. However, even in the case of Saturn the material that makes up its rings, if put back together, would only be enough to create a very small moon, much smaller than Ganymede or Titan. The ring systems of each of the Jovian planets is inside or near the Roche limit of the planet.
The ring system of Saturn shows a few interesting features; it contains GAPS, it is very THIN, and has some odd features like BRAIDS. These features can be explained by a few possible mechanisms:
The ring systems of all four Jovian planets were probably created recently, and it is likely they are transient features. That is, unless they are replenished, they might disappear. It may happen that Saturn's rings will eventually disappear, while Triton will create a significant new ring system around Neptune.
Is Pluto a planet? Well, it depends on your definition of planet. It appears that Pluto (like Triton), is just one of the largest objects from the KUIPER BELT. The Kuiper Belt is the home of short period comets like Halley's Comet, and like we mentioned in class on April 12, there are a number of objects similar in size to Pluto's moon, Charon (Ixion, Quaoar, Varuna, and others), that are also part of the Kuiper Belt. Several of them have similar orbits to Pluto, because they appear to be in synchronous orbits with Neptune, in a 3:2 resonance (they orbit the Sun twice in the same amount of time it takes Neptune to orbit three times). Pluto is considered a planet for historical reasons, but it may be more proper to consider it a MINOR PLANET like the asteroids in the asteroid belt and the objects in the Kuiper Belt and Oort Cloud.