Jupiter has more than 90 known moons, but four of them stand out as planet-sized worlds: Io, Europa, Ganymede, and Callisto. These are called the Galilean moons because Galileo Galilei observed them in 1610, giving strong evidence that not everything orbits Earth. They are bright enough to be seen with small telescopes and they form a natural laboratory for gravity, geology, ice, and possible habitability.
Studying them helps scientists understand how moons and planets form around giant planets.
Key Facts
- The Galilean moons are Io, Europa, Ganymede, and Callisto.
- Orbital period: Io = 1.77 days, Europa = 3.55 days, Ganymede = 7.15 days, Callisto = 16.69 days.
- Mean distance from Jupiter: Io = 421,700 km, Europa = 671,100 km, Ganymede = 1,070,400 km, Callisto = 1,882,700 km.
- Ganymede is the largest moon in the Solar System, with diameter about 5,268 km.
- Kepler-style orbital relation for moons around Jupiter: T^2 is proportional to r^3.
- Tidal heating is strongest for Io because it orbits closest to Jupiter and is pulled by both Jupiter and nearby moons.
Vocabulary
- Galilean moons
- The four largest moons of Jupiter, Io, Europa, Ganymede, and Callisto, first observed by Galileo Galilei in 1610.
- Orbital period
- The time an object takes to complete one full orbit around another object.
- Tidal heating
- Internal heating caused when gravity stretches and squeezes a moon as it moves through its orbit.
- Resonance
- A repeating orbital pattern in which moons line up their periods in simple ratios that strengthen gravitational effects.
- Subsurface ocean
- A layer of liquid water hidden beneath a moon's icy surface.
Common Mistakes to Avoid
- Assuming all four Galilean moons are similar, which is wrong because Io is volcanic, Europa is icy and ocean-bearing, Ganymede has its own magnetic field, and Callisto is heavily cratered.
- Thinking the closest moon always moves slowest, which is wrong because objects in smaller orbits around the same planet usually have shorter orbital periods.
- Confusing size with distance from Jupiter, which is wrong because Ganymede is the largest moon but it is not the farthest Galilean moon.
- Saying Europa has confirmed surface oceans, which is wrong because the strongest evidence points to a subsurface ocean beneath an ice shell.
Practice Questions
- 1 Io orbits Jupiter in 1.77 days and Europa orbits in 3.55 days. About how many Io orbits occur during one Europa orbit?
- 2 Ganymede orbits about 1,070,400 km from Jupiter, while Callisto orbits about 1,882,700 km from Jupiter. How much farther from Jupiter is Callisto than Ganymede?
- 3 Io is the most volcanically active world in the Solar System, while Callisto is much less geologically active. Explain how distance from Jupiter and tidal heating help account for this difference.