There are a lot of planetary atmospheres in the solar system. They vary a lot and this can be seen by comparing Earth and Venus which are of the same size and mass roughly, The surface of Venus is extremely hot which is caused by an atmosphere of carbon dioxide that has the weight of a kilometre of water. Compared toVenus the Earth’s atmosphere is a good friend to us supportting life although it can be a little wet at times.
Callisto and Titan can be compared as they are nearly the same size. They are both moons of Jupiter and Saturn respectively but titan has a nitrogen rich atmosphere which is actually thicker than our own whereas Callisto is mostly airless. So why are there such extreme differences in planetary atmospheres when the size and mass of planets and moons can be the same?
So how does a planet or a moon acquire an atmosphere? Well, there is not one answer to that question but a few. Materials from comets and asteroids can be captured or gravity can pull in the gases from space. This is not the only reason why planetary atmospheres vary so much as it is also due to gases that escape the gravity of the planet. All atmospheres leak into space even the Earth’s. The loss is very small but over time the loss can add up. The atmosphere’s that we see today are merely shadows of much larger grander atmospheres in the distant past.
An atmosphere loses some of its molecules when the molecules reach escape velocity. There are three ways that they can reach escape velocity-
- By thermal escape when the gasses get too hot and they cannot be retained.
- In non thermal processes when chemical or charged particle reactions release atoms and molecules.
- When asteroid and comet impacts blast the air away.
Atmospheres that have a lot of hydrogen are the most vulnerable to hydrodynamic escape. Heavier molecules can be picked up and dragged along as hydrogen flows outwards. This is a bit like when the desert wind blows dust across an ocean and sand grains from dune to dune. This would leave large rocks behind. From this we can workout the present make up of the atmosphere when this process has occurred. This has been seen not just in the solar system but outside on a Jupiter like exoplanet HD 209458b. This is about as close to its parent star as an atmosphere can survive as planets within 3,000,000 km of their stars lose their atmosphere by hydrodynamic escape.
On some planets such as the Earth thermal escape is less important than non-thermal escape. In nonthermal escape chemical reactions or particle on particle collisions throw atoms away from the planet at the escape velocity. Lots of nonthermal escapes involve ions. One type of ion escape is known as a charge exchange where a fast hydrogen ion collides with a neutral hydrogen atom and then catches its electron. This process accounts for the loss of 60 to 90% of the Earth’s hydrogen and almost all the hydrogen from Venus.
So what about Earth’s atmosphere, will it last forever? At the moment hydrogen escape is only a trickle because water vapour which bears the hydrogen gas condenses in the lower atmosphere and then rains back to the surface. But unfortunately for future generations the sun is slowly brightening by about 10% every billion years (that is if future generations exist in billions of years of course). As the sun brightens and the atmosphere warms the atmosphere will get wetter and hydrogen will flow rapidly away from the Earth. This is expected to get important in 1 million years and after that it will take another billion years to evaporate the oceans. Hopefully we will have migrated by then!