Atmospheric escape

Atmospheric escape is the process where a planet loses its atmospheric gases to outer space. This phenomenon is critical for scientists to understand because it determines if a planet can maintain an atmosphere over time. The presence of an atmosphere is a key factor in determining a planet's habitability and its potential to support life. Escape occurs when the kinetic energy of a gas molecule overcomes the gravitational energy of the planet. Essentially, a molecule can only escape if its velocity exceeds the planet's specific escape velocity. This escape velocity depends on the mass of the planet, meaning larger planets have stronger gravity and higher escape velocities.

Thermal escape is one primary mechanism for atmospheric loss. This occurs when the thermal energy of molecules provides them with enough velocity to break free. One specific type is Jeans escape, named after British astronomer Sir James Jeans. In a gas, temperature represents the average velocity of molecules, but individual molecules constantly collide and exchange kinetic energy. This variation is described by the Maxwell distribution. In the exosphere, where collisions are rare, individual molecules in the high-speed tail of this distribution may reach escape velocity and leave. The rate of Jeans escape depends on the mass of the molecule, the planet's escape velocity, and how much the upper atmosphere is heated by its star. For example, lighter hydrogen escapes much more easily than heavier carbon dioxide.

Another thermal process is hydrodynamic escape. This happens when an atmosphere absorbs massive amounts of thermal energy, often from extreme ultraviolet radiation. As the gas heats up, it expands upward and accelerates. In this bulk outflow, the escaping gas can act like a wind. As the lighter, faster molecules rush outward, they can actually drag heavier, slower molecules along with them through collisions. This large-scale escape has been observed on exoplanets located very close to their stars, such as the hot Jupiter HD 209458 b.

Non-thermal, or suprathermal, escape involves interactions like photochemistry or charged particle interactions. Photochemical escape occurs when high-energy ultraviolet photons strike molecules in the upper atmosphere. This can cause photodissociation, which breaks a molecule into smaller parts that have enough energy to escape. It can also cause photoionization, creating ions. Some ions may undergo dissociative recombination, where an ion recombines with an electron and releases enough energy to escape. Another non-thermal method is sputtering escape. This happens when the solar wind imparts excess kinetic energy to atmospheric particles, ejecting them like particles being chipped off a solid surface. This is more common on planets without a protective magnetosphere.

Charge exchange escape is another specific non-thermal process. This occurs when a fast-moving ion from the solar wind or magnetosphere captures an electron from a slow atmospheric neutral molecule. This interaction creates a new, fast-moving neutral molecule and a new, slow ion. While the slow ion remains trapped by the planet's magnetic field lines, the newly energized fast neutral is free to escape into space. Additionally, planets with magnetospheres can experience polar wind escape. Near the poles, magnetic field lines are open, creating a pathway for ions to be accelerated out of the ionosphere into space by an ambipolar electric field.

Impact erosion provides a third way for atmospheres to be lost. When a large meteoroid strikes a planet, the collision can be energetic enough to eject atmospheric molecules into space. This happens in three ways: the meteoroid heats the gas as it travels through the air, solid debris from the crater heats particles through drag, or the impact creates a vapor that expands rapidly. Most impact erosion occurs through the expansion of vapor. For an impact to significantly affect the atmosphere, the radius of the impacting body must be larger than the atmospheric scale height.

We can see these processes clearly within our own solar system. Earth currently loses about 3 kg of hydrogen every second, mostly through charge exchange and Jeans escape. However, in 1 billion years, the Sun will be 10% brighter. This increased heat could dissociate water vapor in our atmosphere, allowing hydrogen to escape until the oceans dry up. Mars provides another example; MAVEN mission data suggests that 66% of the 36Ar in the Martian atmosphere was lost over the last 4 billion years due to suprathermal escape. Even moons like Titan and Io are affected. Titan, lacking its own magnetic field, loses hydrogen to space, creating a neutral hydrogen torus around Saturn.

Astronomers study these processes on distant exoplanets to understand their composition. One common method is observing Lyman-alpha line absorption, which helps detect the amount of hydrogen surrounding a planet. This has shown significant escape on planets like Gliese 436 b. In 2018, scientists also discovered they could use the 1083 nm Helium triplet to measure escape. This method is useful because it is less affected by interstellar absorption than ultraviolet lines and can be studied using ground-based telescopes. By measuring these escapes, we gain a better understanding of whether distant worlds might be habitable.