Introduction

By comparison with the planets we’ve studied so far, Jupiter and Saturn are strange worlds indeed. Their swirling, multicoloured atmospheres host enormous storms — including Jupiter’s Great Red Spot, which has persisted for at least three centuries — and deep atmospheric bands that rotate at different speeds. Beneath those clouds there are no solid surfaces to stand on, only layers of gas and fluid that merge gradually into an interior of liquid and even metallic hydrogen. These are states of matter we rarely encounter on Earth. And orbiting around them are dozens of diverse moons — some of which, like Europa, Enceladus, and Titan, are more geologically active and potentially habitable than many of the rocky planets themselves.

Over the past few decades — and especially through the Voyager, Galileo, Cassini–Huygens, and Juno missions — our understanding of Jupiter, Saturn, and their moons has been transformed. These discoveries have reshaped how scientists think about habitability in the Solar System. The concept of a simple “habitable zone,” defined only by distance from the Sun, no longer captures the full picture. We now know that tidal heating and internal chemistry can sustain liquid water beneath thick ice shells, far beyond the traditional habitable zone.

In this module, you will explore what is currently known about Jupiter and Saturn. We will begin by comparing the terrestrial and Jovian planets in terms of size, mass, composition, and internal structure. Then we will turn to the spectacular ring systems and examine how they form and persist within the Roche limit. From there, you’ll explore the moons of Jupiter and Saturn, many of which reveal evidence of tidal heating and possible subsurface oceans. Finally, you’ll examine what spacecraft missions — from Voyager and Galileo to Cassini–Huygens and Juno — have taught us about these giant planets and why their exploration remains one of the most exciting frontiers in planetary science.