How Does a Planet Lose Its Air? Molecules Leaving One at a Time

Molecules in a gas move at a range of speeds around an average set by the temperature, and the average speed depends on the mass of the molecule, with lighter ones moving faster at the same temperature. At the top of an atmosphere, where collisions become rare enough that a molecule can travel a long way without hitting anything, any molecule moving upward faster than the escape velocity leaves. Because there is always a tail of unusually fast molecules, some escape continuously even when the average speed is far below the threshold, and the rate depends extremely sensitively on the ratio between the two. That sensitivity means a small difference in temperature or in planetary mass produces an enormous difference in how much is retained.

Several processes remove gas and they operate differently:

• •Thermal escape, where fast-moving molecules simply leave, which affects hydrogen and helium most
• •Hydrodynamic escape, where the upper atmosphere flows outward as a wind, which can carry heavier species with it
• •Charged particles from the stellar wind stripping gas directly where no magnetic field deflects them
• •Photochemical escape, where a molecule split by ultraviolet light leaves its fragments with enough energy to go
• •Impact erosion, where a large collision blows part of the atmosphere into space
• •Sequestration into rock and ice, which removes gas without it leaving the planet

The comparison between the inner planets shows the factors acting together. Mars has about a tenth of Earth's mass, so its escape velocity is far lower, and it lost its global magnetic field early, so the stellar wind has been stripping its upper atmosphere directly for billions of years, which spacecraft have measured in progress. The result is an atmosphere less than a hundredth as dense as Earth's, and the geological evidence for ancient flowing water indicates it was once far thicker. Venus has nearly Earth's mass and no magnetic field either, and retains an enormously thick atmosphere, having lost its water instead, since ultraviolet light split water vapour high in the atmosphere and the hydrogen escaped, leaving the oxygen to react with the surface.

Retaining an atmosphere for billions of years requires several things together and Earth has all of them. Its mass gives an escape velocity high enough that the important molecules move far too slowly to leave in quantity, with nitrogen, oxygen, carbon dioxide and water vapour all safely retained while hydrogen and helium escape steadily and are not replenished. A global magnetic field deflects the stellar wind around the planet, which prevents the direct stripping that Mars suffers. The distance from the sun keeps the upper atmosphere cool enough that the speed distribution does not extend far past the threshold. And geological cycling returns carbon dioxide from rock, replacing what is removed. Losing any one of those over a long enough period would change the outcome substantially.

Escape is observed rather than only calculated, by several methods. Spacecraft in orbit measure the composition and speed of gas in the upper atmosphere and detect it flowing away, which the mission orbiting Mars since 2014 was designed specifically to do and which has quantified the current loss rate. Isotope ratios record the history, since lighter isotopes escape preferentially and an atmosphere enriched in the heavier one has plainly lost a great deal, which is how the scale of Martian and Venusian loss was established before anything was measured directly. Observations of planets orbiting other stars detect atmospheres escaping as enormous clouds trailing behind them, visible when the planet passes in front of its star.

Molecules in the tail of the speed distribution exceed escape velocity and leave continuously, at a rate extremely sensitive to temperature and planetary mass. Stellar wind stripping, ultraviolet splitting and impacts remove gas by other routes. Mars lost most of its atmosphere through low gravity and the early loss of its magnetic field, while Venus lost its water and kept everything else.