The atomspheric pressure measures the amount of material contained in the atmosphere of a planet. The higher the pressure the larger the amount of mass in the atmosphere. Why is this so? Well, imagine you are standing on the surface of a planet.

This weight determines the pressure you feel due to the atmosphere.

• On Earth at sea level, there is around 14.7 pounds per square inch pushing on you because of the weight of the overlying atmosphere. This is a huge pressure (for example, see youtube video of crushing a coke can)

  Why don't we feel this pressure?

• On Venus the atmospheric pressure is 90 bars or roughly 90 times that of the Earth. On Venus you would feel a pressure of 1,300 pounds per square inch! This is like living at a depth of around 1 kilometer in the ocean.

The mass of Venus's atmosphere is thus much greater than ours. Why is this so and where did Venus' large atmosperic mass come from or, it turns out, a better question is Why is the Earth's atmospheric mass so small?