Which planet does not have a well developed atmosphere because of its small size?

Which planet does not have a well developed atmosphere because of its small size?

• A. Mercury
• B. Earth
• C. Mars
• D. Venus

Answer: (A)

A planet’s ability to keep an atmosphere depends on gravity and temperature: gases stay bound only if the planet’s gravity is strong enough to prevent them from escaping into space, which is harder to do when the molecules move faster at higher temperatures. Mercury, being the smallest planet, has the weakest gravity, so it can’t hold onto gases well. Its close proximity to the Sun also heats its surface, speeding up gas molecules and making escape more likely. Add in the lack of a strong magnetic field to shield against solar wind, and any trace gases are continually stripped away. That’s why Mercury has almost no atmosphere. Earth and Venus have strong, substantial atmospheres, while Mars has a thinner one, but their larger sizes and other protective factors help them retain gases much more effectively than Mercury.

A planet’s ability to keep an atmosphere depends on gravity and temperature: gases stay bound only if the planet’s gravity is strong enough to prevent them from escaping into space, which is harder to do when the molecules move faster at higher temperatures. Mercury, being the smallest planet, has the weakest gravity, so it can’t hold onto gases well. Its close proximity to the Sun also heats its surface, speeding up gas molecules and making escape more likely. Add in the lack of a strong magnetic field to shield against solar wind, and any trace gases are continually stripped away. That’s why Mercury has almost no atmosphere.

Earth and Venus have strong, substantial atmospheres, while Mars has a thinner one, but their larger sizes and other protective factors help them retain gases much more effectively than Mercury.