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How does a runaway greenhouse effect cause a planet to lose its water?
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Habitable zone
The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.
A runaway greenhouse effect occurs when a planet's atmosphere traps increasing amounts of heat, leading to a self-reinforcing cycle of rising temperatures that ultimately causes the planet to lose its water.1
The process unfolds as follows:
- Increased Heating: As a planet becomes hotter, more water evaporates from its surface into the atmosphere. Because water vapour is a potent greenhouse gas, its increased concentration traps more infrared radiation, further raising the surface temperature.231
- Boiling Away: This cycle continues, causing the planet to become progressively hotter until surface water boils away entirely.1
- Photodissociation and Escape: Once water vapour reaches the upper atmosphere, it is subjected to ultraviolet light. This energy causes photodissociation, the process by which water molecules are broken down into hydrogen and oxygen. Because hydrogen is a light gas, it can escape the planet's gravitational field into space, resulting in the permanent loss of the planet's water.24
This phenomenon defines the inner boundary of a star's habitable zone, beyond which a planet becomes too hot to maintain liquid water on its surface.1