Atmospheric escape
Atmospheric escape is the loss of
Thermal escape mechanisms
Thermal escape occurs if the molecular velocity due to thermal energy is sufficiently high. Thermal escape happens at all scales, from the molecular level (Jeans escape) to bulk atmospheric outflow (hydrodynamic escape).
Jeans escape
One classical thermal escape mechanism is Jeans escape,
Three factors strongly contribute to the relative importance of Jeans escape: mass of the molecule, escape velocity of the planet, and heating of the upper atmosphere by radiation from the parent star. Heavier molecules are less likely to escape because they move slower than lighter molecules at the same temperature. This is why
Hydrodynamic escape
Non-thermal (suprathermal) escape
Escape can also occur due to non-thermal interactions. Most of these processes occur due to photochemistry or charged particle (ion) interactions.
Photochemical escape
In the upper atmosphere, high energy ultraviolet photons can react more readily with molecules. Photodissociation can break a molecule into smaller components and provide enough energy for those components to escape. Photoionization produces ions, which can get trapped in the planet's magnetosphere or undergo dissociative recombination. In the first case, these ions may undergo escape mechanisms described below. In the second case, the ion recombines with an electron, releases energy, and can escape.[5]
Sputtering escape
Excess kinetic energy from the solar wind can impart sufficient energy to eject atmospheric particles, similar to sputtering from a solid surface. This type of interaction is more pronounced in the absence of a planetary magnetosphere, as the electrically charged solar wind is deflected by magnetic fields, which mitigates the loss of atmosphere.[6]
Charge exchange escape
Ions in the solar wind or magnetosphere can charge exchange with molecules in the upper atmosphere. A fast-moving ion can capture the electron from a slow atmospheric neutral, creating a fast neutral and a slow ion. The slow ion is trapped on the magnetic field lines, but the fast neutral can escape.[5]
Polar wind escape
Atmospheric molecules can also escape from the polar regions on a planet with a magnetosphere, due to the polar wind. Near the poles of a magnetosphere, the magnetic field lines are open, allowing a pathway for ions in the atmosphere to exhaust into space.[8]
Impact erosion
The impact of a large meteoroid can lead to the loss of atmosphere. If a collision is sufficiently energetic, it is possible for ejecta, including atmospheric molecules, to reach escape velocity.[9]
In order to have a significant effect on atmospheric escape, the radius of the impacting body must be larger than the scale height. The projectile can impart momentum, and thereby facilitate escape of the atmosphere, in three main ways: (a) the meteoroid heats and accelerates the gas it encounters as it travels through the atmosphere, (b) solid ejecta from the impact crater heat atmospheric particles through drag as they are ejected, and (c) the impact creates vapor which expands away from the surface. In the first case, the heated gas can escape in a manner similar to hydrodynamic escape, albeit on a more localized scale. Most of the escape from impact erosion occurs due to the third case.[9] The maximum atmosphere that can be ejected is above a plane tangent to the impact site.
Dominant atmospheric escape and loss processes in the Solar System
Earth
Atmospheric escape of hydrogen on Earth is due to charge exchange escape (~60–90%), Jeans escape (~10–40%), and polar wind escape (~10–15%), currently losing about 3 kg/s of hydrogen.[1] The Earth additionally loses approximately 50 g/s of helium primarily through polar wind escape. Escape of other atmospheric constituents is much smaller.[1] A Japanese research team in 2017 found evidence of a small number of oxygen ions on the moon that came from the Earth.[10]
In 1 billion years, the Sun will be 10% brighter than it is now, making it hot enough for Earth to lose enough hydrogen to space to cause it to lose all of its water (See Future of Earth#Loss of oceans).
Venus
Recent models indicate that hydrogen escape on Venus is almost entirely due to suprathermal mechanisms, primarily photochemical reactions and charge exchange with the solar wind. Oxygen escape is dominated by charge exchange and sputtering escape.[11] Venus Express measured the effect of coronal mass ejections on the rate of atmospheric escape of Venus, and researchers found a factor of 1.9 increase in escape rate during periods of increased coronal mass ejections compared with calmer space weather.[12]
Mars
Primordial Mars also suffered from the cumulative effects of multiple small impact erosion events,[13] and recent observations with MAVEN suggest that 66% of the 36Ar in the Martian atmosphere has been lost over the last 4 billion years due to suprathermal escape, and the amount of CO2 lost over the same time period is around 0.5 bar or more.[14]
The MAVEN mission has also explored the current rate of atmospheric escape of Mars. Jeans escape plays an important role in the continued escape of hydrogen on Mars, contributing to a loss rate that varies between 160 - 1800 g/s.[15] Jeans escape of hydrogen can be significantly modulated by lower atmospheric processes, such as gravity waves, convection, and dust storms.[16] Oxygen loss is dominated by suprathermal methods: photochemical (~1300 g/s), charge exchange (~130 g/s), and sputtering (~80 g/s) escape combine for a total loss rate of ~1500 g/s. Other heavy atoms, such as carbon and nitrogen, are primarily lost due to photochemical reactions and interactions with the solar wind.[1][11]
Titan and Io
Saturn's moon Titan and Jupiter's moon Io have atmospheres and are subject to atmospheric loss processes. They have no magnetic fields of their own, but orbit planets with powerful magnetic fields, which protects a given moon from the solar wind when its orbit is within the bow shock. However Titan spends roughly half of its orbital period outside of the bow-shock, subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the orbit of Titan, causing neutral hydrogen to escape.[17] The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus around Saturn. Io, in its orbit around Jupiter, encounters a plasma cloud.[18] Interaction with the plasma cloud induces sputtering, kicking off sodium particles. The interaction produces a stationary banana-shaped charged sodium cloud along a part of the orbit of Io.
Observations of exoplanet atmospheric escape
Studies of exoplanets have measured atmospheric escape as a means of determining atmospheric composition and habitability. The most common method is
are experiencing significant atmospheric escape.In 2018 it was discovered with the Hubble Space Telescope that atmospheric escape can also be measured with the 1083 nm Helium triplet.[23] This wavelength is much more accessible from ground-based high-resolution spectrographs, when compared to the ultraviolet Lyman-alpha lines. The wavelength around the helium triplet has also the advantage that it is not severely affected by interstellar absorption, which is an issue for Lyman-alpha. Helium has on the other hand the disadvantage that it requires knowledge about the hydrogen-helium ratio to model the mass-loss of the atmosphere. Helium escape was measured around many giant exoplanets, including WASP-107b, WASP-69b and HD 189733b. It has also been detected around some mini-Neptunes, such as TOI-560 b[24] and HD 63433 c.[25]