Because Venus has no oceans or rain to dissolve the carbon dioxide, large amounts of this greenhouse gas has remained in the atmosphere. The result is a dense atmosphere about 80 times the pressure of Earth's atmosphere. The planet's lack of a magnetic field and closer proximity to the Sun resulted in the loss of its hydrogen (in the form of water) after two billion years.

Because Mars is small, cold, and lacks a magnetic field, it has retained only a sparse atmosphere. The surface air pressure