Mars has a sufficient mass and a low enough temperature that water molecules could exist in its atmosphere as vapor. One reason Mars atmosphere does not contain a significant amount of water vapor is that

Question

Quizplus · Accepted Answer

Ultraviolet radiation from the sun breaks apart water molecules into their constituent atoms (hydrogen and oxygen), which can then escape the planet's low gravity and be lost to space. This is a major reason why Mars' atmosphere does not contain a significant amount of water vapor. Ultraviolet radiation can break the water molecule (H2O) into separate hydrogen (H) and oxygen (O) atoms, which have less mass and can escape the planet's weak gravitational pull. This process, known as photodissociation, is one of the main reasons Mars atmosphere does not contain a significant amount of water vapor.

Mars Has a Sufficient Mass and a Low Enough Temperature