 | This article may be too technical for most readers to understand.(January 2022) |
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, typically hydrogen. This mechanism may explain why some planetary atmospheres are depleted in oxygen, nitrogen, and heavier noble gases, such as xenon. [1]
Particles in the atmosphere need to achieve sufficiently high velocity (higher than the escape velocity) to escape from the planetary gravity field. There are different ways to achieve this velocity. Those processes in which the high velocity is related to the temperature are called thermal escape. The root mean square thermal velocity (vth) of an atomic species is
where k is the Boltzmann constant, T is the temperature, and m is the mass of the species. Lighter molecules or atoms will therefore be moving faster than heavier molecules or atoms at the same temperature. Thus they are easier to escape from planetary gravity field. This is why atomic hydrogen escapes preferentially from an atmosphere.
If there is a strong thermally driven atmospheric escape of light atoms, heavier atoms can achieve the escape velocity through viscous drag by those escaping lighter atoms. [2] This is another way of thermal escape, called hydrodynamic escape. The heaviest species of atom that can be removed in this manner is called the cross-over mass. [3]
In order to maintain a significant hydrodynamic escape, a large source of energy at a certain altitude is required. Soft X-ray or extreme ultraviolet radiation (solar EUV heating), momentum transfer from impacting meteoroids or asteroids, or the heat input from planetary accretion processes [4] may provide the requisite energy for hydrodynamic escape. Such conditions may have been reached in H- or He-rich thermospheres heated by the strong extreme ultraviolet radiation flux of the young Sun. [5] Thus hydrodynamic escape is more likely to occur in the early atmosphere of planets.
Estimating the rate of hydrodynamic escape is important in analyzing both the history and current state of a planet's atmosphere. In 1981, Watson et al. published [6] calculations that describe energy-limited escape, where all incoming energy is balanced by escape to space. Recent numerical simulations on exoplanets have suggested that this calculation overestimates the hydrodynamic flux by 20 - 100 times.[30] However, as a special case and upper limit approximation on the atmospheric escape, it is worth noting here.
Hydrodynamic escape flux (Φ, [m-2s-1]) in an energy-limited escape can be calculated, assuming (1) an atmosphere composed of non-viscous, (2) constant-molecular-weight gas, with (3) isotropic pressure, (4) fixed temperature, (5) perfect extreme ultraviolet (XUV) absorption, and that (6) pressure decreases to zero as distance from the planet increases. [6]
Hydrodynamic escape flux of hydrogen can be expressed as:
where (in SI units):
Corrections to this model have been proposed over the years to account for the Roche lobe of a planet and efficiency in absorbing photon flux. [7] [8] [9]
However, as computational power has improved, increasingly sophisticated models have emerged, incorporating radiative transfer, photochemistry, and hydrodynamics that provide better estimates of hydrodynamic escape. [10]
On the other hand, the hydrodynamic escape flux of heavier species can be expressed as: [11]
where
It can be observed from this formula that the hydrodynamic escape flux of heavier species is higher for less heavier atoms, which is discussed in detail in the next section.
Hydrodynamic escape is a mass fractionating process since all isotopes are dragged by protons with the same force but heavy isotopes are more gravitationally bound compared to light ones. [11] Therefore, hydrogen preferentially drags lighter isotopes to space, leaving the residual atmosphere enriched in heavier isotopes. [12] This is why the ratio of lighter to heavier isotopes of atmospheric particles can indicate hydrodynamic escape.
Specifically, the ratio of different noble gas isotopes (20 Ne/22Ne, 36 Ar/38Ar, 78,80,82,83,86 Kr/84Kr, 124,126,128,129,131,132,134,136 Xe/130Xe) or hydrogen isotopes (D/H) can be compared to solar levels to indicate likelihood of hydrodynamic escape in the atmospheric evolution. Ratios larger or smaller than compared with that in the sun or CI chondrites, which are used as proxy for the sun, indicate that significant hydrodynamic escape has occurred since the formation of the planet. Since lighter atoms preferentially escape, we expect smaller ratios for the noble gas isotopes (or a larger D/H) correspond to a greater likelihood of hydrodynamic escape, as indicated in the table.
| Source | 36Ar/38Ar | 20Ne/22Ne | 82Kr/84Kr | 128Xe/130Xe |
|---|---|---|---|---|
| Sun | 5.8 | 13.7 | 20.501 | 50.873 |
| CI chondrites | 5.3±0.05 | 8.9±1.3 | 20.149±0.080 | 50.73±0.38 |
| Venus | 5.56±0.62 | 11.8±0.7 | -- | -- |
| Earth | 5.320±0.002 | 9.800±0.08 | 20.217±0.021 | 47.146±0.047 |
| Mars | 4.1±0.2 | 10.1±0.7 | 20.54±0.20 | 47.67±1.03 |
Matching these ratios can also be used to validate or verify computational models seeking to describe atmospheric evolution. This method has also been used to determine the escape of oxygen relative to hydrogen in early atmospheres. [14]
Exoplanets that are extremely close to their parent star, such as hot Jupiters can experience significant hydrodynamic escape [15] [16] to the point where the star "burns off" their atmosphere upon which they cease to be gas giants and are left with just the core, at which point they would be called Chthonian planets. Hydrodynamic escape has been observed for exoplanets close to their host star, including the hot Jupiters HD 209458b. [17]
Within a stellar lifetime, the solar flux may change. Younger stars produce more EUV, and the early protoatmospheres of Earth, Mars, and Venus likely underwent hydrodynamic escape, which accounts for the noble gas isotope fractionation present in their atmospheres. [18]
It can be observed from the above table that atmospheric Xe experiences more fractionation than Kr, which seems unreasonable since Xe is heavier than Kr and should be less influenced by hydrodynamic escape than Kr. Actually, according to the formula of hydrodynamic escape flux above, it requires extreme high , which can only be achieved during the first 100 Ma of Earth’s history when the EUV flux from the young Sun was sufficiently strong. [19] However, from the analysis of ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks, it has been observed that the fractionation of atmospheric Xe was still ongoing at about 2.1 Ga before.
One possible explanation is that Xe may be the only noble gas which escapes as an ion as it is the only noble gas more easily ionized than hydrogen. [20] Ionized Xe+ can interact with H+ protons via the strong Coulomb force, which effectively decreases the binary diffusion coefficient b(Xe+, H+) by several orders of magnitude compared to the case of neutral Xe. [11] That means it needs lower hydrogen escape fluxes compared with neutral Xe. Actually, its requisite is lower enough to be met during Archean eon [21] , which means the mass-fractionated hydrodynamic escape of Xe can persist during Archean.
The noble gases are the members of group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some cases, oganesson (Og). Under standard conditions, the first six of these elements are odorless, colorless, monatomic gases with very low chemical reactivity and cryogenic boiling points. The properties of the seventh, unstable, element, Og, are uncertain.
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to transmitted radiant power through a material. Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through the material. Spectral optical depth or spectral optical thickness is the natural logarithm of the ratio of incident to transmitted spectral radiant power through a material. Optical depth is dimensionless, and in particular is not a length, though it is a monotonically increasing function of optical path length, and approaches zero as the path length approaches zero. The use of the term "optical density" for optical depth is discouraged.
The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with outer space. It is located directly above the thermosphere. Very little is known about it due to a lack of research. Mercury, the Moon, Ceres, Europa, and Ganymede have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The Earth's exosphere is mostly hydrogen and helium, with some heavier atoms and molecules near the base.
An atmosphere is a layer of gases 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.
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.
Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) and the lithosphere (the Earth’s crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. The oxygen cycle is the biogeochemical cycle of oxygen atoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. The word oxygen in the literature typically refers to the most common oxygen allotrope, elemental/diatomic oxygen (O2), as it is a common product or reactant of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O2 production) or sink (O2 consumption).
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 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.
The Great Oxidation Event (GOE) or Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Revolution, Oxygen Crisis or Oxygen Holocaust, was a time interval during the Earth's Paleoproterozoic era when the Earth's atmosphere and shallow seas first experienced a rise in the concentration of free oxygen. This began approximately 2.460–2.426 Ga (billion years) ago during the Siderian period and ended approximately 2.060 Ga ago during the Rhyacian. Geological, isotopic and chemical evidence suggests that biologically produced molecular oxygen (dioxygen or O2) started to accumulate in the Archean prebiotic atmosphere due to microbial photosynthesis, and eventually changed it from a weakly reducing atmosphere practically devoid of oxygen into an oxidizing one containing abundant free oxygen, with oxygen levels being as high as 10% of modern atmospheric level by the end of the GOE.
A runaway greenhouse effect will occur when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A runaway version of the greenhouse effect can be defined by a limit on a planet's outgoing longwave radiation which is asymptotically reached due to higher surface temperatures evaporating water into the atmosphere, increasing its optical depth. This positive feedback means the planet cannot cool down through longwave radiation and continues to heat up until it can radiate outside of the absorption bands of the water vapour.
The origin of water on Earth is the subject of a body of research in the fields of planetary science, astronomy, and astrobiology. Earth is unique among the rocky planets in the Solar System in having oceans of liquid water on its surface. Liquid water, which is necessary for all known forms of life, continues to exist on the surface of Earth because the planet is at a far enough distance from the Sun that it does not lose its water, but not so far that low temperatures cause all water on the planet to freeze.
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 0.6% of the Earth's value.
The Bond albedo, named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space.
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.
The atmosphere of Titan is the dense layer of gases surrounding Titan, the largest moon of Saturn. Titan is the only natural satellite of a planet 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.
A Mini-Neptune is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans.
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.
Chemical cycling describes systems of repeated circulation of chemicals between other compounds, states and materials, and back to their original state, that occurs in space, and on many objects in space including the Earth. Active chemical cycling is known to occur in stars, many planets and natural satellites.
Diffusion-limited escape occurs when the rate of atmospheric escape to space is limited by the upward diffusion of escaping gases through the upper atmosphere, and not by escape mechanisms at the top of the atmosphere. The escape of any atmospheric gas can be diffusion-limited, but only diffusion-limited escape of hydrogen has been observed in our solar system, on Earth, Mars, Venus and Titan. Diffusion-limited hydrogen escape was likely important for the rise of oxygen in Earth's atmosphere and can be used to estimate the oxygen and hydrogen content of Earth's prebiotic atmosphere.
The prebiotic atmosphere is the second atmosphere present on Earth before today's biotic, oxygen-rich third atmosphere, and after the first atmosphere of Earth's formation. The formation of the Earth, roughly 4.5 billion years ago, involved multiple collisions and coalescence of planetary embryos. This was followed by a <100 million year period on Earth where a magma ocean was present, the atmosphere was mainly steam, and surface temperatures reached up to 8,000 K (14,000 °F). Earth's surface then cooled and the atmosphere stabilized, establishing the prebiotic atmosphere. The environmental conditions during this time period were quite different from today: the Sun was ~30% dimmer overall yet brighter at ultraviolet and x-ray wavelengths, there was a liquid ocean, it is unknown if there were continents but oceanic islands were likely, Earth's interior chemistry was different, and there was a larger flux of impactors hitting Earth's surface.
Xenon isotope geochemistry uses the abundance of xenon (Xe) isotopes and total xenon to investigate how Xe has been generated, transported, fractionated, and distributed in planetary systems. Xe has nine stable or very long-lived isotopes. Radiogenic 129Xe and fissiogenic 131,132,134,136Xe isotopes are of special interest in geochemical research. The radiogenic and fissiogenic properties can be used in deciphering the early chronology of Earth. Elemental Xe in the atmosphere is depleted and isotopically enriched in heavier isotopes relative to estimated solar abundances. The depletion and heavy isotopic enrichment can be explained by hydrodynamic escape to space that occurred in Earth's early atmosphere. Differences in the Xe isotope distribution between the deep mantle, shallower Mid-ocean Ridge Basalts (MORBs), and the atmosphere can be used to deduce Earth's history of formation and differentiation of the solid Earth into layers.
