Why do the terrestrial planets lack hydrogen and helium in their atmospheres?

A. Hydrogen and helium were converted to other compounds as the planets formed.

B. Hydrogen and helium can't be retained because the planet masses are too small.

C. Hydrogen and helium were blown away by solar wind.

D. Hydrogen and helium are present in rings around the terrestrial planets.

B

Explanation:

Mostly, terrestrial planets (which are mainly made up of rocks) are usually smaller in size and therefore have lower mass, in comparison to gas planets. An example, in our solar system, is Saturn and Jupiter are bigger than Earth, Mercury and Venus. Due to their low mass, their gravitational pull is not strong enough to retain an atmosphere of very light gas such as hydrogen and helium. These terrestial planets can retain heavier gas such as carbon dioxide and oxygen.

The answer is B. Hydrogen and Helium can't be retained because the planet masses are too small.