In this module, our primary focus was planetary habitability, and specifically the conditions required for liquid water to exist on a planet’s surface. Everything we know about life on Earth indicates that water is essential to its existence.
You learned that sustaining liquid surface water requires four essential, interconnected ingredients:
- Geological activity — a molten metal core and an outward temperature gradient;
- A global magnetic field generated by that core, which shields the atmosphere from the solar wind;
- An atmosphere thick enough to trap heat and maintain surface temperature and pressure above water’s freezing point; and
- Liquid surface water itself, which maintains tectonic activity and helps preserve the internal temperature gradient needed for an active magnetic dynamo.
Together, these form what might be called the planetary circle of life—though perhaps that’s a bit corny.
Earth is the only planet in our Solar System with persistent surface water, but the conditions on both Venus and Mars are thought to have once been far more Earth-like. By comparing these three worlds, we gain valuable insight into the fragile balance that sustains Earth’s habitability. We saw that Earth maintains liquid water through a stable greenhouse effect, while Venus became uninhabitable when a runaway greenhouse boiled its oceans away. Mars, being smaller, likely lost its internal heat and magnetic field early on, allowing the solar wind to strip away its atmosphere and surface water. Our models suggest that Earth, too, will eventually become uninhabitable within roughly 1–3 billion years, when solar brightening triggers its own runaway greenhouse.
Size, then, is a critical variable: had Earth been as small as Mars, its magnetic field and atmosphere might not have endured.
But other variables remain to be explored. What if a planet were much larger than Earth? What about moons orbiting such planets? The first question is hard to answer from our Solar System alone, since no giant planets orbit within the Sun’s habitable zone. Yet by studying the outer giants (Jupiter, Saturn, Uranus, and Neptune) in the next two modules, and comparing them with the growing population of exoplanets (in Module 11), we can infer how large planets might evolve closer to their stars.
The second question—about the moons of the outer planets—is equally compelling. As you’ll learn next, several of these moons, including Europa, Enceladus, and Titan, show strong evidence of vast subsurface oceans that may contain more liquid water than all of Earth’s combined. These worlds extend our search for habitability far beyond the traditional boundaries of the Sun’s habitable zone.
cosmiCave.org 