Air pressure on rocky planets 

There are four rocky planets in the solar system: Mercury, Venus, Earth, and Mars. The amount of atmosphere on each planet is very different. The air pressure on Venus is 93 times of that on Earth. The air pressure on Mars is about 0.6% of that on Earth. The air pressure on Mercury is essentially zero. What cause the huge differences in air pressure?

We first look at the sizes of the planets. Larger planets have stronger gravitational pull. Earth is the largest. Venus is slightly smaller. The radius of Mars is about half of Earth. The radius of Mercury is about a third of Earth. Venus is slightly smaller than Earth. Yet the air pressure on Venus is 93 times of that on Earth.

Solar wind plays a role. Solar wind can blow away air molecules. Mercury is closest to the sun. It has little air left. But Venus has a lot more air than Earth, which is further away from the sun than Venus.

Air composition is important. Carbon dioxide is heavier than nitrogen gas, the dominant gas on Earth. More than 95% of Venus’ air is carbon dioxide. We naturally expect there are more on  Venus than on Earth. More than 95% of Mars’ air is carbon dioxide as well. But air pressure on Mars is only about 0.6% of that on Earth.

Earth rotates while Venus barely. Rotation reduces Earth’s gravitational pull. But even at equator, the centrifugal force is about 0.5% of the gravitational force. This can’t be an important factor.

Earth has a magnetic field while Venus doesn’t. Magnetic field shields much of the bombardment by cosmic rays. This should help preserve atmosphere. Yet, Earth’s atmosphere is far less abundant than Venus.

I had thought the explanation for the air pressures on rocky planets is straightforward. But it doesn’t seem to be the case.

Next, we will discuss briefly on the compositions of atmosphere. On Venus and Mars, more than 95% of air is carbon dioxide. Carbon dioxide, with molecular weight of 44, is the heaviest among common gases. Lighter gases have higher velocities and are easier to escape gravitational pull. If Earth has no life, its atmosphere will be dominated by carbon dioxide as well. 

Finally, we may mention the history of Earth’s atmosphere. It seems that atmosphere pressure of the early Earth was about one hundred times higher. It is roughly equal to today’s atmosphere pressure on Venus. This makes sense to me. Over time, air pressure declines, due to the absorption of carbon dioxide by life. The released oxygen can’t fill up the same amount. Oxygen is very reactive. Oxygen’s molecular weight, 32, is also smaller than that of carbon dioxide.

The proportion of oxygen reached highest at Carboniferous period, at 30% of the atmosphere. It was the same period when plant making was most ferocious. Huge amount of carbon was separated from carbon dioxide, leaving oxygen in the atmosphere.