8. Jupiter and Saturn

Overview

In this module, you’ll leave behind the inner planets of our Solar System and the stability of their solid surfaces — that one common feature that meant that, no matter how hot or airless a planet may be, you could at least imagine what it would be like if you were there. In contrast, if you tried to land a spacecraft on Jupiter or Saturn, you’d plunge into their atmospheres and encounter rapidly increasing pressures and temperatures. Deeper down, the gases behave like fluids, and still deeper the hydrogen is squeezed into a metallic state where it can conduct electricity. (You would not make it that far — you’d just become part of that fluid.)

Because the giant planets lack solid surfaces, and because several of their moons and ring particles may be habitable or even prebiotic, missions to Jupiter and Saturn are deliberately ended by sending the spacecraft into the planet, to avoid contaminating nearby moons such as Europa, Enceladus, or Titan. In some ways, Jupiter and Saturn are more like failed stars — massive, mostly hydrogen–helium worlds — than like the rocky planets we’ve studied so far. And the most intriguing “places to stand” in these systems are not the planets themselves but the many moons orbiting them.

Humans could, in principle, walk on the moons of these giant planets: stroll along the shores of Titan’s methane–ethane seas in dim twilight, or (with serious engineering) dive through the icy crusts of Europa or Enceladus and swim in global saltwater oceans beneath the ice.

In this module, you’ll investigate what spacecraft have revealed about Jupiter and Saturn — their interiors, their atmospheres, their ring systems, and (especially) their moons — and you’ll see why these systems are unlike any of the inner, terrestrial planets you’ve met so far. You’ll explore data from the Cassini–Huygens and Juno missions, trace the path paved by the Voyager and Galileo spacecraft, and look ahead to the next generation of explorers, including NASA’s Europa Clipper and Dragonfly missions and ESA’s JUICE mission to Jupiter’s icy moons.

Learning Objectives

When you have finished this module, you should be able to do the following:

  1. Compare Jovian planets with terrestrial planets in terms of bulk composition, physical characteristics, rings, and moons.
  2. Describe how internal pressure and composition lead to metallic hydrogen and magnetic fields in Jupiter and Saturn.
  3. Explain the mechanisms responsible for the formation and structure of planetary ring systems.
  4. Investigate the formation and evolution of Jupiter’s and Saturn’s moons and rings, including evidence that some icy moons are geologically active.
  5. Explore scientific results from spacecraft missions to the Jupiter–Saturn region (e.g., Cassini–Huygens, Juno, Voyager, Galileo) and preview upcoming missions such as Europa Clipper and Dragonfly.

Key Terms and Concepts

  • Terrestrial / Jovian planets
  • Planetary rings
  • Roche limit
  • Metallic hydrogen
  • Differential rotation
  • Tidal heating
  • Galilean moons / icy moons
  • Cassini–Huygens mission
  • Galileo mission
  • Juno mission
  • Voyager missions
  • Europa Clipper mission
  • Dragonfly mission