What determines a planet's atmospheric pressure?

Question

Specifically, why is atmospheric pressure greater on Venus, yet less on Mars than it is on Earth?

Answer 1

The atmospheric pressure is simply the weight of the atmosphere, distributed over the surface of the planet. So it depends on the total mass of the atmosphere and the gravitational acceleration at the planet's surface. Venus has much higher pressure simply because there is a lot more stuff in its atmosphere. Higher temperature causes more stuff to evaporate into the atmosphere, resulting in higher pressure. If Earth were as hot as Venus, our oceans would boil off into the atmosphere and we'd have much higher atmospheric pressure too.

Mars has lower gravity than Earth or Venus and therefore both less ability to hang onto an atmosphere and lower pressure from a given amount of atmosphere. In addition, much of Mars's atmosphere appears to have been lost over time, possibly due in part to extreme climate variations causes by changes in its axial tilt.

Answer 2

We have standardized atms based on the pressure of the Earth's atmosphere. Venus has a much thicker (and denser) atmosphere than the Earth.

This is one of the reasons Venus has no water and underwent a "runaway greenhouse effect". Not only does Venus receive more solar radiation because of its closer proximity to the sun, but it has a dense and thick atmosphere composed of about 95% CO2; this contributes to greenhouse warming, because CO2 is a greenhouse gas. Our atmosphere is mostly innert nitrogen (N2).

Answer 3

The weight of the atmosphere, density and chemical composition is the determining factor in a planet's atmospheric pressure. On earth it is 14.7 lbs per sq. inch at sea level.