Moons of Jupiter

Why This Matters

Jupiter's moons aren't just a list of names to memorize—they're a laboratory for understanding how tidal heating, differentiation, and orbital dynamics shape planetary bodies. When you study the Galilean moons, you're seeing the same processes that govern geology and potential habitability across the solar system. Europa's subsurface ocean, Io's volcanic fury, and Ganymede's magnetic field all connect to core astronomy concepts about energy transfer, planetary interiors, and the conditions necessary for life.

You're being tested on your ability to explain why these moons differ so dramatically despite orbiting the same planet. The key is their distance from Jupiter and how gravitational interactions generate internal heat. Don't just memorize that Io has volcanoes—know that tidal heating from orbital resonance with Europa and Ganymede drives that volcanism. Understanding the mechanism will serve you far better on exams than rote facts alone.

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Tidal Heating and Geological Activity

The closer a moon orbits Jupiter, the stronger the gravitational flexing it experiences. This tidal heating—where gravitational tugs stretch and compress a moon's interior—generates enormous amounts of heat, driving volcanism and resurfacing.

Io

• Most volcanically active body in the solar system—over 400 active volcanoes constantly resurface this moon, erasing impact craters
• Tidal heating from gravitational interactions with Jupiter, Europa, and Ganymede generates the internal heat; Io is locked in a 1:2:4 orbital resonance
• Sulfur-rich surface gives Io its distinctive yellow, red, and orange coloring; the thin atmosphere is primarily sulfur dioxide vented from volcanoes

Europa

• Subsurface ocean beneath a 10–30 km thick ice shell makes Europa a prime candidate for astrobiology research
• Tidal heating keeps the interior warm enough to maintain liquid water; surface ridges and cracks indicate ongoing tectonic activity
• Thin oxygen atmosphere is produced by radiation splitting water ice molecules—not by biological processes

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Internal Structure and Differentiation

Larger moons have enough mass and internal heat to undergo differentiation—the separation of materials by density into distinct layers (core, mantle, crust). This process determines whether a moon can generate a magnetic field or sustain a subsurface ocean.

Ganymede

• Largest moon in the solar system—bigger than Mercury, though less massive due to its ice-rock composition
• Only moon with its own magnetic field, generated by convection in a liquid iron or iron-sulfide core; evidence of a differentiated interior
• Two terrain types visible on the surface: bright, grooved icy regions and older, dark, heavily cratered areas suggesting past geological activity

Callisto

• Most heavily cratered object in the solar system—ancient surface indicates minimal geological activity over billions of years
• Undifferentiated or only partially differentiated interior; lacks the internal heat needed to separate into distinct layers
• Possible subsurface ocean exists, but evidence is weaker than for Europa or Ganymede; thin carbon dioxide atmosphere

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Inner Moons and Ring Dynamics

Jupiter's small inner moons orbit within or near the planet's ring system. These bodies are too small for differentiation and instead serve as sources of ring material through micrometeorite impacts.

Metis

• Closest moon to Jupiter with an orbital period of just 7.1 hours—faster than Jupiter's rotation
• Irregularly shaped and heavily cratered; low mass means no geological activity or atmosphere retention
• Primary dust source for Jupiter's Main Ring; impacts eject material that feeds the ring system

Thebe

• Small, irregular moon orbiting just outside Metis, also contributing to ring dynamics
• Low density suggests a porous composition of ice and rock, possibly a rubble pile structure
• Supplies the Gossamer Ring—one of Jupiter's faint outer rings composed of dust from Thebe and Amalthea

Amalthea

• Largest inner moon with a reddish color, possibly from sulfur ejected by Io's volcanoes
• Extremely low density (less than water) indicates a porous, primitive composition—possibly a captured asteroid or rubble pile
• Feeds the Gossamer Ring along with Thebe; its weak gravity cannot retain any significant atmosphere

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Irregular Moons and Capture

Jupiter's outer irregular moons have eccentric, inclined, or retrograde orbits—strong evidence they were captured from the asteroid belt or Kuiper Belt rather than forming in place around Jupiter.

Himalia

• Largest irregular moon of Jupiter, roughly 170 km in diameter with a dark, carbonaceous surface
• Prograde but inclined orbit suggests capture from a heliocentric orbit; likely a primitive asteroid
• Non-spherical shape due to insufficient gravity for hydrostatic equilibrium; part of the Himalia group of similarly orbiting moons

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Quick Reference Table

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Self-Check Questions

1. Which two Galilean moons share evidence for subsurface oceans, and what mechanism keeps those oceans liquid?

2. Io and Europa both experience tidal heating—why does this produce volcanoes on one moon but a subsurface ocean on the other?

3. What evidence supports the conclusion that Himalia was captured rather than formed around Jupiter?

4. Compare and contrast Ganymede and Callisto: both are large, icy moons, so why does only Ganymede have a magnetic field?

5. If an FRQ asked you to explain how Jupiter's inner moons contribute to its ring system, which moons would you discuss and what process would you describe?