Pedagoguery

Planetary atmospheres come in a tremendous variety. From the dense, deep atmospheres of gas giants like Jupiter to the wispy, tenuous atmospheres of rocky planets like Mercury, they run the spectrum of possibilities. Even worlds of comparable size, such as Earth and Venus, have tremendously different atmospheres. Some of the difference is accounted for by the source of the atmosphere, but the vast majority is probably a result of the loss of gasses over time.

There are several different mechanisms involved in atmospheric loss. They can be broadly categorized into thermal, non-thermal, and impact mechanisms. In thermal processes, heat causes some molecules of the atmosphere to reach escape velocity. In non-thermal mechanisms, atoms or molecules reach escape velocity through processes such as chemical or charged particle reactions. Finally, impact from comets or asteroids blast away the air.

Thermal escape is probably the most common and straightforward process. All bodies in the solar system are heated by the sun. That heat is shed through the emission of infrared radiation, or through the ejection of particles. In bodies like the planets, the former process predominates, while in bodies like comets, the latter does. Thermal processes are a tug-of-war between solar or other heating on the one hand, and gravity on the other. Bodies with a higher gravity hold on to their atmospheres more tightly than those with a lower gravity. On the other hand, atmospheres that are more strongly heated tend to expand, lessening the effective gravity in the upper reaches, and the particles tend to move faster, thus having a higher chance of reaching escape velocity.

There are two main thermal processes. The first is called Jeans escape, after the English astronomer who first described the process in the early 20th century. In this process, molecules evaporate off molecule by molecule. In the lower atmosphere, particles collide with each other before they can go very far. However, above a level called the exobase, which on Earth is about 500 km high, collisions are very rare, so if a particle reaches escape velocity, it leaves. The temperature on average at Earth's exobase is about 1000 K. Temperature is a statistical measure of the kinetic energy of the particles in a substance, so lighter particles move faster. Hydrogen, being the lightest element, tends to move the fastest, and thus is the most easily lost. Jeans escape accounts for between 10 to 40 percent of Earth's hydrogen loss.

The second thermal escape mechanism is far more dramatic. When an atmosphere is strongly heated, it expands, and in extreme cases, this can give rise to a “planetary wind”, which is analogous to the solar wind. Once again, lighter particles are more strongly affected, but they also tend to drag along heavier gasses. This process is called hydrodynamic escape and there is evidence that it once took place on Earth. The abundance of noble gasses on Earth such as neon and argon is lower than the abundance of such gases in the sun, indicating that Earth has lost some of those elements. Hydrodynamic escape is the only process which could account for this loss. In addition, hydrodynamic escape explains the condition of Venus's atmosphere today. The early atmosphere of Venus would have been hydrogen rich, and as the young sun heated up, hydrodynamic escape would have driven away much of this hydrogen, and the hydrogen would have carried off much of the oxygen and nitrogen along with it, leaving behind primarily carbon dioxide.

On some planets, including the Earth today, non-thermal processes predominate. Most of these processes involve ions. Charge exchange is one such. Ions, for the most part, are kept caged by magnetic fields, such as Earth's. An ion will spiral around magnetic field lines and be unable to escape. However, if an ion meets up with a neutral particle, it will sometimes steal an electron from it. The now neutral particle is now no longer bound by the magnetic field and can then escape. This process accounts for between 60 and 90 percent of Earth's hydrogen loss.

A second non-thermal process involves a loophole in the magnetic capture. Most magnetic field lines loop around from one pole to another, but widely looping lines get pulled by the solar wind and can open. This is called the polar wind (not to be confused with a planetary wind). Ions can follow these open magnetic field lines and escape. This process accounts for between 10 to 15 percent of Earth's hydrogen loss and nearly all of its helium loss.

A third process is called photochemical escape and is not common on Earth, but is common on Mars and Saturn's moon Titan. In this process, oxygen, nitrogen, and carbon monoxide drift high into the atmosphere, where the sun's ultraviolet light ionizes them. When the ionized molecules recombine with free electrons or collide with one another, the energy released can propel one of the particles to escape velocity.

The final non-thermal process operates on worlds without global magnetic fields such as Mars, Venus, and Titan. This exposes the atmosphere to the full brunt of the solar wind. Solar wind particles can collide with atmospheric molecules, undergo charge exchange, and escape. This process is called sputtering. Observations of Mars's atmosphere indicate that between photochemical escape and sputtering, Mars has probably lost as much as 90 percent of its original atmosphere.

Impact is the final, and most dramatic, means of atmosphere loss. When a large impactor its an atmosphere, it creates an enormous explosion that propels large amounts of gasses away from the planet. Mars probably lost much of its early atmosphere to impacts, given its proximity to the asteroid belt.

Next time, improbable planetary systems.