Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:
Once upon a time, there was a molecule on Mars that dreamed of going to space. In fact, once upon a time there must have been a whole lot of molecules in the Martian atmosphere that wanted to go to space, and they apparently succeeded because today Mars’s atmosphere is mostly all gone.
Several factors must have contributed to the success of this molecule-scaled space program. One factor was temperature. The temperature of a gas is really a measure of the average velocity of the molecules in that gas. But remember, that’s the average velocity meaning some individual molecules may be considerably faster or slower than average.
As gas molecules bounce off each other, some of them may also gain or lose momentum, and in some cases a molecule might gain enough momentum to achieve escape velocity (11 kilometers per second on Earth, or 5 kilometers per second on Mars).
At that point, that molecule could achieve its dream and fly off into space (assuming it doesn’t collide with any other molecules on the way out). This can happen with virtually any gas on any planet, but it works best for light-weight molecules (like hydrogen or helium) on low gravity worlds (like Mars).
This process is sometimes called thermal escape, but in the scientific literature I’ve read it seems to be more commonly referred to as Jeans escape.
Sir James Hopwood Jeans was a British mathematician and astronomer. In the early 20th Century, he published prolifically on subjects ranging from star formation to blackbody radiation to the thermal properties of planetary atmospheres. It was this planetary atmospheres work that first led to the idea that a planet could gradually lose its atmosphere to space.
Or at least it was the first time we humans knew anything about it. The atmospheric gas molecules of Mars figured it out a long, long time before that.