Atmospheric escape

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Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and impact erosion. The relative importance of each loss process depends on the planet's escape velocity, its atmosphere composition, and its distance from its star. Escape occurs when molecular kinetic energy overcomes gravitational energy; in other words, a molecule can escape when it is moving faster than the escape velocity of its planet. Categorizing the rate of atmospheric escape in exoplanets is necessary to determining whether an atmosphere persists, and so the exoplanet's habitability and likelihood of life.

Contents

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).

A visualization of Jeans escape. Temperature defines a range of molecular energy. Above the exobase, molecules with enough energy escape, while in the lower atmosphere, molecules are trapped by collisions with other molecules. Jeans escape.png
A visualization of Jeans escape. Temperature defines a range of molecular energy. Above the exobase, molecules with enough energy escape, while in the lower atmosphere, molecules are trapped by collisions with other molecules.

Jeans escape

One classical thermal escape mechanism is Jeans escape, [1] named after British astronomer Sir James Jeans, who first described this process of atmospheric loss. [2] In a quantity of gas, the average velocity of any one molecule is measured by the gas's temperature, but the velocities of individual molecules change as they collide with one another, gaining and losing kinetic energy. The variation in kinetic energy among the molecules is described by the Maxwell distribution. The kinetic energy (), mass (), and velocity () of a molecule are related by . Individual molecules in the high tail of the distribution (where a few particles have much higher speeds than the average) may reach escape velocity and leave the atmosphere, provided they can escape before undergoing another collision; this happens predominantly in the exosphere, where the mean free path is comparable in length to the pressure scale height. The number of particles able to escape depends on the molecular concentration at the exobase, which is limited by diffusion through the thermosphere.

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 hydrogen escapes from an atmosphere more easily than carbon dioxide. Second, a planet with a larger mass tends to have more gravity, so the escape velocity tends to be greater, and fewer particles will gain the energy required to escape. This is why the gas giant planets still retain significant amounts of hydrogen, which escape more readily from Earth's atmosphere. Finally, the distance a planet orbits from a star also plays a part; a close planet has a hotter atmosphere, with higher velocities and hence, a greater likelihood of escape. A distant body has a cooler atmosphere, with lower velocities, and less chance of escape.

A visualization of hydrodynamic escape. At some level in the atmosphere, the bulk gas will be heated and begin to expand. As the gas expands, it accelerates and escapes the atmosphere. In this process, lighter, faster molecules drag heavier, slower molecules out of the atmosphere. Hydrodynamic escape.png
A visualization of hydrodynamic escape. At some level in the atmosphere, the bulk gas will be heated and begin to expand. As the gas expands, it accelerates and escapes the atmosphere. In this process, lighter, faster molecules drag heavier, slower molecules out of the atmosphere.

Hydrodynamic escape

An atmosphere with high pressure and temperature can also undergo hydrodynamic escape. In this case, a large amount of thermal energy, usually through extreme ultraviolet radiation, is absorbed by the atmosphere. As molecules are heated, they expand upwards and are further accelerated until they reach escape velocity. In this process, lighter molecules can drag heavier molecules with them through collisions as a larger quantity of gas escapes. [3] Hydrodynamic escape has been observed for exoplanets close to their host star, including the hot Jupiter HD 209458b. [4]

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]

The fast ion captures an electron from a slow neutral in a charge exchange collision. The new, fast neutral can escape the atmosphere, and the new, slow ion is trapped on magnetic field lines. Charge exchange.png
The fast ion captures an electron from a slow neutral in a charge exchange collision. The new, fast neutral can escape the atmosphere, and the new, slow ion is trapped on magnetic field lines.

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]

Atmospheric escape from impact erosion is concentrated in a cone (red dash-dotted line) centered at the impact site. The angle of this cone increases with impact energy to eject a maximum of all the atmosphere above a tangent plane (orange dotted line). Impact erosion.png
Atmospheric escape from impact erosion is concentrated in a cone (red dash-dotted line) centered at the impact site. The angle of this cone increases with impact energy to eject a maximum of all the atmosphere above a tangent plane (orange dotted line).

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 Lyman-alpha line absorption. Much as exoplanets are discovered using the dimming of a distant star's brightness (transit), looking specifically at wavelengths corresponding to hydrogen absorption describes the amount of hydrogen present in a sphere around the exoplanet. [19] This method indicates that the hot Jupiters HD209458b [20] and HD189733b [21] and Hot Neptune GJ436b [22] 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-69 b and HD 189733b. It has also been detected around some mini-Neptunes, such as TOI-560 b [24] and HD 63433 c. [25]

Other atmospheric loss mechanisms

Sequestration is not a form of escape from the planet, but a loss of molecules from the atmosphere and into the planet. It occurs on Earth when water vapor condenses to form rain or glacial ice, when carbon dioxide is sequestered in sediments or cycled through the oceans, or when rocks are oxidized (for example, by increasing the oxidation states of ferric rocks from Fe2+ to Fe3+). Gases can also be sequestered by adsorption, where fine particles in the regolith capture gas which adheres to the surface particles.

Related Research Articles

<span class="mw-page-title-main">Solar wind</span> Stream of charged particles from the Sun

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of materials found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as C, N, O, Ne, Mg, Si, S, and Fe. There are also rarer traces of some other nuclei and isotopes such as P, Ti, Cr, and 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.

<span class="mw-page-title-main">Atmosphere</span> Layer of gases surrounding an astronomical body held by gravity

An atmosphere is a layer of gasses that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere; stars of low temperature might have outer atmospheres containing compound molecules.

<span class="mw-page-title-main">HD 209458 b</span> Gas giant exoplanet orbiting HD 209458

HD 209458 b is an exoplanet that orbits the solar analog HD 209458 in the constellation Pegasus, some 157 light-years from the Solar System. The radius of the planet's orbit is 0.047 AU, or one-eighth the radius of Mercury's orbit. This small radius results in a year that is 3.5 Earth-days long and an estimated surface temperature of about 1,000 °C. Its mass is 220 times that of Earth and its volume is some 2.5 times greater than that of Jupiter. The high mass and volume of HD 209458 b indicate that it is a gas giant.

<span class="mw-page-title-main">Chthonian planet</span> Gas giants with their atmospheric layers stripped

Chthonian planets are a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.

<span class="mw-page-title-main">Ice giant</span> Giant planet primarily consisting of compounds with freezing points exceeding 100°K

An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.

<span class="mw-page-title-main">Atmosphere of Mars</span> Layer of gases surrounding the planet Mars

The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.85%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and noble gases. The atmosphere of Mars is much thinner and colder than Earth's having a max density 20g/m3 with a temperature generally below zero down to -60 Celsius. The average surface pressure is about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.

<span class="mw-page-title-main">55 Cancri e</span> Hot Super-Earth orbiting 55 Cancri A

55 Cancri e is an exoplanet in the orbit of its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth, thus making it the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes fewer than 18 hours to complete an orbit and is the innermost-known planet in its planetary system. 55 Cancri e was discovered on 30 August 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star. In October 2012, it was announced that 55 Cancri e could be a carbon planet.

<span class="mw-page-title-main">Atmosphere of the Moon</span> Very scant presence of gases around the Moon

The atmosphere of the Moon is a very sparse layer of gases surrounding the Moon. For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular particles in its vicinity compared to interplanetary medium, referred to as "lunar atmosphere" for scientific objectives, is negligible in comparison with the gaseous envelopes surrounding Earth and most planets of the Solar System. The pressure of this small mass is around 3×10−15 atm (0.3 nPa), varying throughout the day, and in total mass less than 10 metric tonnes. Otherwise, the Moon is considered not to have an atmosphere because it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment due to the high rate at which its gases are lost into space.

<span class="mw-page-title-main">Ocean world</span> Planet containing a significant amount of water or other liquid

An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.

<span class="mw-page-title-main">Gliese 436 b</span> Hot Neptune exoplanet orbiting Gliese 436

Gliese 436 b is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436. It was the first hot Neptune discovered with certainty and was among the smallest-known transiting planets in mass and radius, until the much smaller Kepler exoplanet discoveries began circa 2010.

<span class="mw-page-title-main">Extraterrestrial atmosphere</span> Area of astronomical research

The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus and Titan. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.

<span class="mw-page-title-main">Atmosphere of Mercury</span> Composition and properties of the atmosphere of the innermost planet of the Solar System

Mercury, being the closest to the Sun, with a weak magnetic field and the smallest mass of the recognized terrestrial planets, has a very tenuous and highly variable atmosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10−14 bar. The exospheric species originate either from the Solar wind or from the planetary crust. Solar light pushes the atmospheric gases away from the Sun, creating a comet-like tail behind the planet.

<span class="mw-page-title-main">Atmosphere of Titan</span> Thick atmospheric layers of Saturns moon Titan

The atmosphere of Titan is the dense layer of gases surrounding Titan, the largest moon of Saturn. Titan is the only natural satellite in the Solar System with an atmosphere that is denser than the atmosphere of Earth and is one of two moons with an atmosphere significant enough to drive weather. Titan's lower atmosphere is primarily composed of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, propane, PAHs and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, acetonitrile, argon and helium. The isotopic study of nitrogen isotopes ratio also suggests acetonitrile may be present in quantities exceeding hydrogen cyanide and cyanoacetylene. The surface pressure is about 50% higher than on Earth at 1.5 bars which is near the triple point of methane and allows there to be gaseous methane in the atmosphere and liquid methane on the surface. The orange color as seen from space is produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.

This page describes exoplanet orbital and physical parameters.

<span class="mw-page-title-main">MAVEN</span> NASA Mars orbiter

MAVEN is a NASA spacecraft orbiting Mars to study the loss of that planet's atmospheric gases to space, providing insight into the history of the planet's climate and water. The name is an acronym for "Mars Atmosphere and Volatile Evolution" while the word maven also denotes "a person who has special knowledge or experience; an expert". MAVEN was launched on an Atlas V rocket from Cape Canaveral Air Force Station, Florida, on 18 November 2013 UTC and went into orbit around Mars on 22 September 2014 UTC. The mission is the first by NASA to study the Mars atmosphere. The probe is analyzing the planet's upper atmosphere and ionosphere to examine how and at what rate the solar wind is stripping away volatile compounds.

<span class="mw-page-title-main">Energetic neutral atom</span> Technology to create global images of otherwise invisible phenomena

Energetic Neutral Atom (ENA) imaging is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere.

<span class="mw-page-title-main">GJ 1214 b</span> Super-Earth orbiting GJ 1214

GJ 1214 b is an exoplanet that orbits the star GJ 1214, and was discovered in December 2009. Its parent star is 48 light-years from the Sun, in the constellation Ophiuchus. As of 2017, GJ 1214 b is the most likely known candidate for being an ocean planet. For that reason, scientists often call the planet a "waterworld".

<span class="mw-page-title-main">Hydrodynamic escape</span> Method by which atoms of a planets atmosphere are ejected into space

In atmospheric science, hydrodynamic escape refers to a thermal atmospheric escape mechanism that can lead to the escape of heavier atoms of a planetary atmosphere through numerous collisions with lighter atoms.

<span class="mw-page-title-main">Helium planet</span> Planet with helium-dominated atmosphere

A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways. Gliese 436 b is a possible helium planet.

<span class="mw-page-title-main">Gas giant</span> Giant planet mainly composed of light elements

A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranus and Neptune are really a distinct class of giant planets, being composed mainly of heavier volatile substances. For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.

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  24. ↑ Zhang, Michael; Knutson, Heather A.; Dai, Fei; Wang, Lile; Ricker, George R.; Schwarz, Richard P.; Mann, Christopher; Collins, Karen (2022-07-01). "Detection of Atmospheric Escape from Four Young Mini Neptunes". The Astronomical Journal. 165 (2): 62. arXiv: 2207.13099 . Bibcode:2023AJ....165...62Z. doi: 10.3847/1538-3881/aca75b . S2CID   251104690.
  25. ↑ Zhang, Michael; Knutson, Heather A.; Wang, Lile; Dai, Fei; dos Santos, Leonardo A.; Fossati, Luca; Henry, Gregory W.; Ehrenreich, David; Alibert, Yann; Hoyer, Sergio; Wilson, Thomas G.; Bonfanti, Andrea (2022-01-17). "Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune". The Astronomical Journal. 163 (2): 68. arXiv: 2106.05273 . Bibcode:2022AJ....163...68Z. doi: 10.3847/1538-3881/ac3f3b . ISSN   0004-6256.

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