MCQ Detailed View

The ability of a planet to retain an atmosphere depends mainly on two physical factors: gravity and temperature.

A planet with high gravity has a stronger pull on gas molecules, keeping them bound to its surface. For example, Earth and Jupiter have strong gravitational fields, which allow them to hold onto thick atmospheres. On the other hand, small bodies like the Moon and Mercury have weak gravity, so their gases escape easily into space, leaving them without significant atmospheres.

Temperature is equally important. When temperature is high, gas molecules move faster. If the average speed of these molecules becomes close to or greater than the planet’s escape velocity, they will leave the planet. This is why lighter gases such as hydrogen and helium are often lost from small, warm planets. By contrast, low temperature slows down the motion of gas molecules, making it easier for gravity to retain them. This is why distant, cold planets and moons like Titan or Neptune can keep nitrogen or even hydrogen-based atmospheres.

The correct condition for long-term atmospheric retention is therefore high gravity and low temperature. This combination ensures that gas particles remain below the escape speed.

In summary:

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  High gravity → stronger hold on gases.

  
  


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  Low temperature → slower gas motion, less escape.

  
  


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  Planets close to the Sun with low gravity struggle to keep atmospheres, while massive or distant cold planets retain thick atmospheres.

  
  

Hence, a planet can retain its atmosphere when gravity is high and temperature is low.