 | This article may be too technical for most readers to understand.(January 2022) |
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.
Hydrodynamic escape occurs if there is a strong thermally driven atmospheric escape of light atoms which, through drag effects (collisions), also drive off heavier atoms. [1] The heaviest species of atom that can be removed in this manner is called the cross-over mass. [2]
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, momentum transfer from impacting meteoroids or asteroids, or the heat input from planetary accretion processes [3] may provide the requisite energy for hydrodynamic escape.
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 [4] 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 (, [ms]) 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 XUV absorption, and that (6) pressure decreases to zero as distance from the planet increases. [4]
where is the photon flux [J ms] over the wavelengths of interest, is the radius of the planet, is the gravitational constant, is the mass of the planet, and is the effective radius where the XUV absorption occurs. 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. [5] [6] [7]
However, as computational power has improved, increasingly sophisticated models have emerged, incorporating radiative transfer, photochemistry, and hydrodynamics that provide better estimates of hydrodynamic escape. [8]
The root mean square thermal velocity () of an atomic species is
where is the Boltzmann constant, is the temperature, and is the mass of the species. Lighter molecules or atoms will therefore be moving faster than heavier molecules or atoms at the same temperature. This is why atomic hydrogen escapes preferentially from an atmosphere and also explains 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. [10]
Exoplanets that are extremely close to their parent star, such as hot Jupiters can experience significant hydrodynamic escape [11] [12] 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. [13]
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. [14]
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large solar system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.
Ausonia is a stony Vestian asteroid from the inner region of the asteroid belt, approximately 100 kilometers in diameter. It was discovered by Italian astronomer Annibale de Gasparis on 10 February 1861, from the Astronomical Observatory of Capodimonte, in Naples, Italy. The initial choice of name for the asteroid was "Italia", after Italy, but this was modified to Ausonia, an ancient classical name for the Italian region.
144 Vibilia is a carbonaceous asteroid from the central region of the asteroid belt, approximately 140 kilometers in diameter. It was discovered on 3 June 1875, by German–American astronomer Christian Peters at Litchfield Observatory of the Hamilton College in Clinton, New York, United States. Peters named it after Vibilia, the Roman goddess of traveling, because he had recently returned from a journey across the world to observe the transit of Venus. Peters also discovered 145 Adeona on the same night. The official naming citation was published by Paul Herget in The Names of the Minor Planets in 1955.
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.
A biosignature is any substance – such as an element, isotope, or molecule – or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical or chemical structures and its use of free energy and the production of biomass and wastes. A biosignature can provide evidence for living organisms outside the Earth and can be directly or indirectly detected by searching for their unique byproducts.
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.
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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 that it is the only planet known to have oceans of liquid water on its surface. Liquid water, which is necessary for life as we know it, continues to exist on the surface of Earth because the planet is at a distance, known as the habitable zone, far enough 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 (2.8%), 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 than Earth's. The average surface pressure is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value. The currently thin Martian atmosphere prohibits the existence of liquid water on the surface of Mars, but many studies suggest that the Martian atmosphere was much thicker in the past. The higher density during spring and fall is reduced by 25% during the winter when carbon dioxide partly freezes at the pole caps. The highest atmospheric density on Mars is equal to the density found 35 km (22 mi) above the Earth's surface and is ≈0.020 kg/m3. The atmosphere of Mars has been losing mass to space since the planet's core slowed down, and the leakage of gases still continues today.
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An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surface with a hydrosphere, 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 like on Titan's surface.
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The atmosphere of Titan is the dense layer of gases surrounding Titan, the largest moon of Saturn. It is the only thick atmosphere of a natural satellite in the Solar System. 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 suggest 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.
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Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of underground oceans in Saturn's moon Titan and Jupiter's moon Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.
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.
TRAPPIST-1 is an ultra-cool red dwarf star in the constellation Aquarius with a planetary system of seven known planets. It has a mass about 9% of the Sun's, a radius slightly larger than the planet Jupiter, and a surface temperature of about 2,566 K (2,293 °C). The star is 40.7 light-years (12.5 pc) from the Sun and is estimated to be 7.6 billion years old, making it older than the Solar System.
The dwarf planet Pluto has an unusual set of climate zones, due to its atypical axial configuration. Five climate zones are assigned on the dwarf planet: tropics, arctic, tropical arctic, diurnal, and polar. These climate zones are delineated based on astronomically defined boundaries or sub-solar latitudes, which are not associated with the atmospheric circulations on the dwarf planet. Charon, the largest moon of Pluto, is tidally locked with it, and thus has the same climate zone structure as Pluto itself.
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.
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