Why do some planets and moons have atmospheres while others do not?
Quick Answer
Atmospheres depend on gravity, temperature, and whether a world has been able to keep or replenish its gases. Small or hot worlds lose gas more easily than large, cold ones.
The Full Story
A planet or moon needs enough gravity to hold onto gas molecules, and cooler temperatures help prevent them from escaping. Solar wind, impacts, and volcanic activity also affect whether an atmosphere is lost or maintained. Mercury has only a tenuous exosphere because it is small and close to the Sun, while Titan has a thick atmosphere because it is cold enough to retain gases. Mars lost much of its atmosphere over time as it cooled and its magnetic shielding weakened.
Key Facts
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"Why can some worlds breathe and others can’t?"
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