Q&A on Pluto’s Atmosphere 

Question

Hi Leila,

[…]

I am reading the book Chasing New Horizons: Inside the Epic First Mission to Pluto by Alan Stern & David Grinspoon. It's a really interesting read and written for the general public, so you would think I would be able to understand it all, but no. I came upon a statement made in the book about atmospheric pressure and temperature that confused me. Here is the passage that appears on pages 70-71.

"But unlike Earth, Pluto's nitrogen atmosphere is created by the sublimation of its snowy surface. In this process, the atmospheric pressure is very strongly, in fact exponentially, dependent on the surface temperature. As a result, each few degrees of surface cooling cuts the atmospheric pressure in half. So as Pluto traveled outward in its orbit, and the Sun's warming of Pluto's surface naturally decreased, the atmospheric temperature was expected to decrease, and as a result the atmospheric pressure would begin to fall, steeply, perhaps to levels hundreds or thousands of times lower than its perihelion pressure." ... "Atmospheric models were predicting that this pressure collapse might well happen sometime between 2010 and 2020"

[Further discussion of pressure on Earth.]

Answer

It sounds like the book is saying the atmosphere on Pluto consists entirely of nitrogen evaporating (sublimating) from solid to gas. So when the temperature of the surface warms, nitrogen is released into the atmosphere. The more particles there are in the atmosphere, the higher the pressure. So the higher the temperature, the more particles, and the more pressure.

As the temperature of the surface cools, the nitrogen freezes and deposits itself back on the surface. As the atmosphere thins out (fewer particles) the pressure drops.

The basic science affecting the atmosphere is the ideal gas law, PV = nRT where P is pressure, V is volume, n is the number of moles of gas (essentially, the number of particles), R is a constant, and T is temperature. In this equation, higher temperature means higher pressure, IF the volume is kept constant (as in an enclosed container) and the number of particles is constant.

In the case of Pluto, the number of particles is NOT constant. On the right hand side we have both T and n. The higher the temperature, the higher the number of particles, because of the sublimation effect. So if you increase T and therefore increase n, the right hand side of the equation multiplies fast, and therefore the left hand side of the equation (the pressure, assuming a constant volume) is going to rise quickly. And if you decrease T and therefore decrease n, the left hand side, the pressure, will decrease quickly.

The complications with Earth’s atmosphere include the fact that varying amounts of water vapor can be mixed in (and displace other molecules). Water vapor is less dense than dry air. That is why moist weather systems are low pressure systems, and sunny dry weather is a high pressure system. I used to get confused about this because I was thinking of water vapor as being added in, and if you add something in, the pressure should increase. But the water vapor is displacing other molecules like Nitrogen, and the water vapor is less dense than the Nitrogen so it creates less pressure.

Another complication in atmospheres is the fact that air that is warmed tends to become less dense, and rise. In a closed container, if you warmed the air, the pressure would increase. But we’re not talking about a closed container, so if the warmed air has a chance to expand, it becomes less dense, and then the pressure is less than it was before (for that parcel of air).

 I’m appending a link to the text of a really good article about humid versus dry air. I hope at least some of this helps...

Leila