# Atmospheric escape

Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape, operating at different time scales; the most prominent is Jeans Escape, named after British astronomer Sir James Jeans, who described the process of atmospheric loss to the molecular kinetic energy.[1] The relative importance of each loss process is a function of the planet's mass, its atmosphere composition, and its distance from its sun.

Graphs of escape velocity against surface temperature of some Solar System objects showing which gases are retained. The objects are drawn to scale, and their data points are at the black dots in the middle.

## Thermal escape mechanisms

One classical thermal escape mechanism is Jeans escape.[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 and mass of a molecule determine its velocity by ${\displaystyle E_{\mathit {kin}}={\frac {1}{2}}mv^{2}}$.

Individual molecules in the high tail of the distribution may reach escape velocity, at a level in the atmosphere where the mean free path is comparable to the scale height, and leave the atmosphere.

The more massive the molecule of a gas is, the lower the average velocity of molecules of that gas at a given temperature, and the less likely it is that any of them reach escape velocity.

This is why hydrogen escapes from an atmosphere more easily than carbon dioxide. Also, if the planet has a higher mass, the escape velocity is greater, and fewer particles will escape. This is why the gas giant planets still retain significant amounts of hydrogen and helium, which have largely escaped from Earth's atmosphere. The distance a planet orbits from a star also plays a part; a close planet has a hotter atmosphere, with a range of velocities shifted into the higher end of the distribution, hence, a greater likelihood of escape. A distant body has a cooler atmosphere, with a range of lower velocities, and less chance of escape. This helps Titan, which is small compared to Earth but further from the Sun, retain its atmosphere.

An atmosphere with a high enough pressure and temperature can undergo a different escape mechanism - "hydrodynamic escape". In this situation the atmosphere simply flows off like a wind into space, due to pressure gradients initiated by thermal energy deposition. Here it is possible to lose heavier molecules that would not normally be lost. Hydrodynamic escape has been observed for exoplanets close-to their host star, including several hot Jupiters (HD 209458b, HD 189733b) and a hot Neptune (GJ 436b).

## Non-thermal escape

### Significance of solar winds

One escape mechanism is atmospheric heating by a solar wind in the absence of a planetary magnetosphere. Excess kinetic energy from solar winds can impart sufficient energy to the atmospheric particles to allow them to reach escape velocity, causing atmospheric escape. The solar wind, composed of ions, is deflected by magnetic fields because the charged particles within the wind flow along magnetic field lines. The presence of a magnetic field thus deflects solar winds, preventing the loss of atmosphere. On Earth, for instance, the interaction between the solar wind and earth's magnetic field deflects the solar wind around the planet, with near total deflection at a distance of 10 Earth radii.[3] This region of deflection is called a bow shock.

Depending on planet size and atmospheric composition, however, a lack of magnetic field does not fully determine the fate of a planet's atmosphere. Venus, for instance, has no powerful magnetic field. Its close proximity to the Sun also increases the speed and number of particles, and would presumably cause the atmosphere to be stripped almost entirely, much like that of Mars. Despite this, the atmosphere of Venus is two orders of magnitude denser than Earth's.[4] Recent models indicate that stripping by solar wind accounts for less than 1/3 of total non-thermal loss processes.[4]

While Venus and Mars have no magnetosphere to protect the atmosphere from solar winds, photoionizing radiation (sunlight) and the interaction of the solar wind with the atmosphere of the planets causes ionization of the uppermost part of the atmosphere. This ionized region in turn induces magnetic moments that deflect solar winds much like a magnetic field. This limits solar-wind effects to the uppermost part of the atmosphere, roughly 1.2–1.5 planetary radii away from the planet, or an order of magnitude closer to the surface than Earth's magnetic field creates. Beyond this region, called a bow shock, the solar wind is slowed to subsonic velocities.[3] Nearer to the surface, solar-wind dynamic pressure reaches a balance with the pressure from the ionosphere, in a region called the ionopause. This interaction typically prevents solar-wind stripping from being the dominant loss process of the atmosphere.

At planets with a magnetic field, like Earth, ions can escape along the vertical magnetic field lines in the polar regions. Thus, while a magnetic field protects a planet from some atmospheric escape processes, it also enables others that do not occur on unmagnetized planets. In total, it has been found that a magnetic field is not necessary to protect a planet from losing its atmosphere, and that the absence of an intrinsic magnetic field can lead to somewhat lower escape rates.[5]

### Comparison of non-thermal loss processes based on planet and particle mass

Argon Isotope Ratios are a signature of atmospheric escape on Mars (Curiosity rover) (April, 2013)[6][7]

The dominant non-thermal loss processes depends on the planetary body. The relative significance of each process depends on planetary mass, atmospheric composition, and distance from the sun. The dominant non-thermal loss processes for Venus and Mars, two terrestrial planets neither with magnetic fields, are dissimilar. The dominant non-thermal loss process on Mars is from solar winds, as the atmosphere is not dense enough to shield itself from the winds during peak solar activity.[4] Venus is somewhat shielded from solar winds because of its denser atmosphere and as a result, solar pick-up is not its dominant non-thermal loss process. Smaller bodies without magnetic fields are more likely to suffer from solar winds, as the planet is too small to have sufficient gravity to produce a dense enough atmosphere and stop solar wind pick-up.

The dominant loss process for Venus' atmosphere is through electric force field acceleration. Stellar EUV and XUV photoionize molecular and atomic species in the upper atmosphere, producing free electrons. The electrons are less massive than their parent ions, and as a result are more easily accelerated up to and beyond the escape velocity of the planet, leaving the ionosphere of Venus and flowing out into the stellar wind.[4] This charge separation between the escaping, low-mass electrons and significantly heavier, positively-charged ions sets up a polarization electric field. That electric field, in turn, acts to pull the positively charged ions along behind the escaping electrons, out of the atmosphere. As a result, H+ ions are accelerated beyond escape velocity. Other important loss processes on Venus are photochemical reactions driven by Venus's proximity to the Sun. Photochemical reactions rely on the splitting of molecules into constituent atoms, often with a significant portion of the kinetic energy carried off in the less massive particle with sufficiently high kinetic energy to escape. Oxygen, relative to hydrogen, was assumed to not be of sufficiently low mass to escape through this mechanism. However, a Japanese research team in 2017 found ions from earth have been discovered on the moon which matched earth's oxygen profile perfectly.[8]

### Phenomena of non-thermal loss processes on moons with atmospheres

Several natural satellites in the Solar System have atmospheres and are subject to atmospheric loss processes. They typically have no magnetic fields of their own, but orbit planets with powerful magnetic fields. Many of these moons lie within the magnetic fields generated by the planets and are less likely to undergo sputtering and pick-up. The shape of the bow shock, however, allows for some moons, such as Titan, to pass through the bow shock when their orbits take them between the Sun and their primary. Titan spends roughly half of its transit time outside of the bow-shock and being subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the transit of Titan, causing neutral hydrogen to escape.[9] The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud.[10] 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.

### Impact erosion

The impact of a large meteoroid can lead to the loss of atmosphere. If a collision is energetic enough, it is possible for ejecta, including atmospheric molecules, to reach escape velocity. Just one impact such as the Chicxulub event does not lead to a significant loss, but the terrestrial planets went through enough impacts when they were forming for this to matter.[11]

## Sequestration

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. It also occurs on Earth when carbon dioxide is sequestered in sediments, or cycled through the oceans. The dry ice caps on Mars are also an example of sequestration.

One mechanism for sequestration is chemical; for example, most of the carbon dioxide of the Earth's original atmosphere has been chemically sequestered into carbonate rock. Very likely a similar process has occurred on Mars. Oxygen can be sequestered by oxidation of rocks; 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.

## Dominant atmospheric escape and loss processes on Earth

Earth is too large to lose a significant proportion of its atmosphere through Jeans escape. The current rate of loss is about 3 kg of hydrogen and 50 g of helium per second.[2] The exosphere is the high-altitude region where atmospheric density is sparse and Jeans escape occurs. Jeans escape calculations assuming an exosphere temperature of 1,800 K [13] show that to deplete O+ ions by a factor of e (2.718...) would take nearly a billion years. 1,800 K is higher than the actual observed exosphere temperature; at the actual average exosphere temperature, depletion of O+ ions would not occur even over a trillion years. Furthermore, most oxygen on Earth is bound as O2, which is too massive to escape Earth by Jeans escape.[2]

Earth's magnetic field protects it from solar winds and prevents escape of ions, except near the magnetic poles where charged particles stream towards the earth along magnetic field lines. The gravitational attraction of Earth's mass prevents other non-thermal loss processes from appreciably depleting the atmosphere. Yet Earth's atmosphere is two orders of magnitude less dense than that of Venus at the surface. Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere and lithosphere. H2O vapor is sequestered as liquid H2O in oceans, greatly decreasing the atmospheric density. With liquid water running over the surface of Earth, CO2 can be drawn down from the atmosphere and sequestered in sedimentary rocks. Some estimates indicate that nearly all carbon on Earth is contained in sedimentary rocks, with the atmospheric portion being approximately 1/250,000 of Earth's CO2 reservoir. If both of the reservoirs were released to the atmosphere, Earth's atmosphere would be even denser than Venus's atmosphere. Therefore, the dominant “loss” mechanism of Earth's atmosphere is not escape to space, but sequestration. However, in 1 billion years' time, 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).

Space activities do release measurable mass, the International Space Station alone consumed 7,000 kg per year of propellant over the last 20 years; roughly 7,000 kg of this being hydrogen that was vented and at that altitude, escape is almost certain. Of course, thats merely a rounding error when considering the vast mass of earth itself.

## References

1. ^ Muriel Gargaud, Encyclopedia of Astrobiology, Volume 3, Springer Science & Business Media, May 26, 2011, p. 879.
2. ^ a b c David C. Catling and Kevin J. Zahnle, The Planetary Air Leak, Scientific American, May 2009, p. 26 (accessed 25 July 2012)
3. ^ a b Shizgal, B. D.; Arkos, G. G. (1996). "Nonthermal escape of the atmospheres of Venus, Earth, and Mars". Reviews of Geophysics. 34 (4): 483–505. Bibcode:1996RvGeo..34..483S. doi:10.1029/96RG02213.
4. ^ a b c d Lammer, H.; Lichtenegger, H. I. M.; Biernat, H. K.; Erkaev, N. V.; Arshukova, I. L.; Kolb, C.; Gunell, H.; Lukyanov, A.; Holmstrom, M.; Barabash, S.; Zhang, T. L.; Baumjohann, W. (2006). "Loss of hydrogen and oxygen from the upper atmosphere of Venus". Planetary and Space Science. 54 (13–14): 1445–1456. Bibcode:2006P&SS...54.1445L. doi:10.1016/j.pss.2006.04.022.
5. ^ Gunell, H.; Maggiolo, R.; Nilsson, H.; Stenberg Wieser, G.; Slapak, R.; Lindkvist, J.; Hamrin, M.; De Keyser, J. (2018). "Why an intrinsic magnetic field does not protect a planet against atmospheric escape". Astronomy and Astrophysics. 614: L3. Bibcode:2018A&A...614L...3G. doi:10.1051/0004-6361/201832934.
6. ^ Webster, Guy (April 8, 2013). "Remaining Martian Atmosphere Still Dynamic". NASA. Retrieved April 9, 2013.
7. ^ Wall, Mike (April 8, 2013). "Most of Mars' Atmosphere Is Lost in Space". Space.com. Retrieved April 9, 2013.
8. ^ http://www.space.com/35502-moon-has-oxygen-from-earth-plants.html
9. ^ Lammer, H.; Stumptner, W.; Bauer, S. J. (1998). "Dynamic escape of H from Titan as consequence of sputtering induced heating". Planetary and Space Science. 46 (9–10): 1207–1213. Bibcode:1998P&SS...46.1207L. doi:10.1016/S0032-0633(98)00050-6.
10. ^ Wilson, J. K.; Mendillo, M.; Baumgardner, J.; Schneider, N. M.; Trauger, J. T.; Flynn, B. (2002). "The dual sources of Io's sodium clouds". Icarus. 157 (2): 476–489. Bibcode:2002Icar..157..476W. doi:10.1006/icar.2002.6821.
11. ^ Melosh, H.J.; Vickery, A.M. (April 1989). "Impact erosion of the primordial atmosphere of Mars". Nature. 338: 487–489. Bibcode:1989Natur.338..487M. doi:10.1038/338487a0. PMID 11536608.
12. ^ https://www.sciencedaily.com/releases/2016/12/161207093031.htm
13. ^ Space Studies Board, Division on Engineering and Physical Sciences (Jan 15, 1961). "The Atmospheres of Mars and Venus". National Academies Press.

Atmosphere

An atmosphere (from Modern Greek ἀτμός (atmos), meaning 'vapour', and σφαῖρα (sphaira), meaning 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.

The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%) , carbon dioxide (0.04%) and other gases in trace amounts. Oxygen is used by most organisms for respiration; nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction of nucleotides and amino acids; and carbon dioxide is used by plants, algae and cyanobacteria for photosynthesis. The atmosphere helps to protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. The current composition of the Earth's atmosphere is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.

The term stellar atmosphere describes the outer region of a star and typically includes the portion above the opaque photosphere. Stars with sufficiently low temperatures may have outer atmospheres with compound molecules.

Atmosphere of Mars

The atmosphere of the planet Mars is composed mostly of carbon dioxide. The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar). It ranges from a low of 30 pascals (0.0044 psi; 0.30 mbar) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi; 11.55 mbar) in the depths of Hellas Planitia. This pressure is well below the Armstrong limit for the unprotected human body. Mars's atmospheric mass of 25 teratonnes compares to Earth's 5148 teratonnes; Mars has a scale height of 11.1 kilometres (6.9 mi) versus Earth's 8.5 kilometres (5.3 mi).The Martian atmosphere consists of approximately 96% carbon dioxide, 1.9% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane, among other gases, for a mean molar mass of 43.34 g/mol. There has been renewed interest in its composition since the detection of traces of methane in 2003 that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.The atmosphere is quite dusty, giving the Martian sky a light brown or orange-red color when seen from the surface; data from the Mars Exploration Rovers indicate suspended particles of roughly 1.5 micrometres in diameter.On 16 December 2014, NASA reported detecting an unusual increase, then decrease, in the amounts of methane in the atmosphere of the planet Mars. Organic chemicals have been detected in powder drilled from a rock by the Curiosity rover. Based on deuterium to hydrogen ratio studies, much of the water at Gale Crater on Mars was found to have been lost during ancient times, before the lakebed in the crater was formed; afterwards, large amounts of water continued to be lost.On 18 March 2015, NASA reported the detection of an aurora that is not fully understood and an unexplained dust cloud in the atmosphere of Mars.On 4 April 2015, NASA reported studies, based on measurements by the Sample Analysis at Mars (SAM) instrument on the Curiosity rover, of the Martian atmosphere using xenon and argon isotopes. Results provided support for a "vigorous" loss of atmosphere early in the history of Mars and were consistent with an atmospheric signature found in bits of atmosphere captured in some Martian meteorites found on Earth. This was further supported by results from the MAVEN orbiter circling Mars, that the solar wind is responsible for stripping away the atmosphere of Mars over the years.In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25 times brighter than any observed earlier due to a massive, and unexpected, solar storm in the middle of the month.On 1 June 2018, NASA scientists detected signs of a dust storm (see image) on the planet Mars which may affect the survivability of the solar-powered Opportunity rover since the dust may block the sunlight (see image) needed to operate; as of 12 June, the storm is the worst ever recorded at the surface of the planet, and spanned an area about the size of North America and Russia combined (about a quarter of the planet); as of 13 June, Opportunity was reported to be experiencing serious communication problem(s) due to the dust storm; a NASA teleconference about the dust storm was presented on 13 June 2018 at 01:30 pm/et/usa and is available for replay. In July 2018, researchers reported that the largest single source of dust on the planet Mars comes from the Medusae Fossae Formation.On 7 June 2018, NASA announced a cyclical seasonal variation in atmospheric methane.

Callisto (moon)

Callisto (Jupiter IV) is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System not to be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand nearly still in Callisto's sky. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.Callisto is composed of approximately equal amounts of rock and ices, with a density of about 1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.The surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.

Earth mass

Earth mass (ME or M⊕, where ⊕ is the standard astronomical symbol for planet Earth) is the unit of mass equal to that of Earth.

The current best estimate for Earth mass is M⊕ = 5.9722×1024 kg, with a standard uncertainty of

6×1020 kg (relative uncertainty 10−4).

It is equivalent to an average density of 5515 kg⋅m−3.

The Earth mass is a standard unit of mass in astronomy that is used to indicate the masses of other planets, including rocky terrestrial planets and exoplanets. One Solar mass is close to 333,000 Earth masses.

The Earth mass excludes the mass of the Moon. The mass of the Moon is about 1.2% of that of the Earth, so that the mass of the Earth+Moon system is close to 6.0456×1024 kg.

Most of the mass is accounted for by iron and oxygen (c. 32% each), magnesium and silicon (c. 15% each), calcium, aluminium and nickel (c. 1.5% each).

Precise measurement of the Earth mass is difficult, as it is equivalent to measuring the gravitational constant, which is the fundamental physical constant known with least accuracy, due to the relative weakness of the gravitational force.

The mass of the Earth was first measured with any accuracy (within about 20% of the correct value) in the Schiehallion experiment in the 1770s, and within 1% of the modern value in the Cavendish experiment of 1798.

Exoplanet

An exoplanet (UK: , US: ) or extrasolar planet is a planet outside the Sun's Solar System. The first evidence of an exoplanet was noted in 1917, but was not recognized as such. The first scientific detection of an exoplanet was in 1988; it was confirmed to be an exoplanet in 2012. The first confirmed detection occurred in 1992. As of 1 January 2019, there are 3,946 confirmed planets in 2,945 systems, with 650 systems having more than one planet.There are many methods of detecting exoplanets. The High Accuracy Radial Velocity Planet Searcher (HARPS) has discovered about a hundred exoplanets since 2004, while the Kepler space telescope, since 2009, has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which up to 40% may be false positives.

In several cases, multiple planets have been observed around a star.

About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets further away in galaxies beyond the local Milky Way galaxy, may exist. The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.Besides exoplanets, there are also rogue planets, which do not orbit any star. These tend to be considered separately, especially if they are gas giants, in which case they are often counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way possibly number in the billions (or more).

Geology of Pluto

The geology of Pluto consists of the characteristics of the surface, crust, and interior of Pluto. Because of Pluto's distance from Earth, in-depth study from Earth is difficult. Many details about Pluto remained unknown until 14 July 2015, when New Horizons flew through the Pluto system and began transmitting data back to Earth. When it did, Pluto was found to have remarkable geologic diversity, with New Horizons team member Jeff Moore saying that it "is every bit as complex as that of Mars". The final New Horizons Pluto data transmission was received on 25 October 2016.

HD 219134 b

HD 219134 b (or HR 8832 b) is one of at least five exoplanets orbiting HR 8832, a main-sequence star in the constellation of Cassiopeia. As of July 2015, super-Earth HD 219134 b, with a size of about 1.6 R⊕, and a density of 6.4 g/cm3, was reported as the closest rocky exoplanet to the Earth, at 21.25 light-years away. The exoplanet was initially detected by the instrument HARPS-N of the Italian Telescopio Nazionale Galileo via the radial velocity method and subsequently observed by the Spitzer telescope as transiting in front of its star. The exoplanet has a mass of about 4.5 times that of Earth and orbits its host star every three days. In 2017, it was found that the planet likely hosts an atmosphere.

HD 219134 c

HD 219134 c, also known as HR 8832 c, is a hot, dense, rocky exoplanet orbiting around the K-type star HR 8832 in the constellation of Cassiopeia. Originally thought to be a little less than three times the mass of Earth, it is now known to be over 4 times the mass and 51% larger in radius, suggesting a rocky composition with a higher quantity of iron than Earth. The exoplanet was initially detected by the instrument HARPS-N of the Italian Telescopio Nazionale Galileo via the radial velocity method. Transits of the planet were observed by the Spitzer Space Telescope in 2017. Later that year, it was predicted that HD 219134 c has an atmosphere.

The Hadean ( ) is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. As of 2016, the ICS describes its status as "informal". Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the "Priscoan period", from priscus, the Latin word for "ancient". Other, older texts simply refer to the eon as the Pre-Archean.

Hydrodynamic escape

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.

The classical thermal escape mechanism is when individual molecules from the high velocity tail of the Maxwell–Boltzmann distribution

reach the escape velocity and overcome the gravity field. This is known as Jeans escape and depends on the temperature of the planet's exosphere and the strength of its gravity field. It can be shown that for cold giant gas planets such as Jupiter and Saturn there is no thermal driven atmospheric escape of significance while for smaller and warmer planets such as Earth only light atoms may escape in this manner (heavier atoms stay).Hydrodynamic escape occurs if there is a strong thermal driven atmospheric escape of light atoms which through drag effects (collisions) also drive off heavier atoms—a bulk flow type of escape of the upper atmosphere, a so-called "blowoff". The heaviest species of atom that can be removed in this manner is called the cross-over mass.

It requires a large source of energy at a certain altitude to maintain a significant hydrodynamic escape. Solar radiation is seldom enough for known present day atmospheres in the Solar System. It is speculated that early atmospheres of Earth, Venus and Mars have experienced periods of hydrodynamic escape due to the heat input from planetary accretion processes.Exoplanets that are extremely close to their parent star, such as hot Jupiters can experience significant hydrodynamic escape 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.

James Jeans

Sir James Hopwood Jeans (11 September 1877 – 16 September 1946) was an English physicist, astronomer and mathematician.

Kevin J. Zahnle

Kevin J. Zahnle is a planetary scientist at the NASA Ames Research Center and a Fellow of the American Geophysical Union. He studies impact processes, atmospheric escape processes, geochemical modelling of atmophiles, and photochemical modelling.

He earned his PhD in 1985 at the University of Michigan under Prof. James C. G. Walker. He then completed postdoctoral fellowships at both Stanford and NRC at NASA Ames before coming to NASA Ames Research Center in 1988.

List of plasma physics articles

This is a list of plasma physics topics.

Makemake

Makemake (minor-planet designation 136472 Makemake) is a dwarf planet and perhaps the largest Kuiper belt object in the classical population, with a diameter approximately two-thirds that of Pluto. Makemake has one known satellite, S/2015 (136472) 1. Makemake's extremely low average temperature, about 30 K (−243.2 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.Makemake was discovered on March 31, 2005, by a team led by Michael E. Brown, and announced on July 29, 2005. Initially, it was known as 2005 FY9 and later given the minor-planet number 136472. Makemake was recognized as a dwarf planet by the International Astronomical Union (IAU) in July 2008. Its name derives from Makemake in the mythology of the Rapa Nui people of Easter Island.

Ocean planet

An ocean planet, ocean world, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or subsurface. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid, such as lava (the case of Io) or ammonia (the case of Titan's inner ocean).

Earth is the only known astronomical object to have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water. For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor

may be used as a proxy. The characteristics of ocean worlds —or ocean planets— provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to originate and host life.

Pluto

Pluto (minor planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered.

Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet.

Pluto is the largest and second-most-massive (after Eris) known dwarf planet in the Solar System, and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.5 AU).

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.

On July 14, 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km (12,000 mi) to the orbiting moon.

Ruth Murray-Clay

Ruth Murray-Clay is a professor at the University of California Santa Cruz who studies the formation of planetary systems.

Terraforming of Mars

Terraforming of Mars is a hypothetical process of planetary engineering by which the surface and climate of Mars would be deliberately changed to make large areas of the environment hospitable to humans, thus making the colonization of Mars safer and sustainable.

There are a few proposed terraforming concepts, some of which present prohibitive economic and natural resource costs. As of 2019 it is not feasible, using existing technology, to terraform Mars. Any climate change induced in the near term is proposed to be driven by greenhouse warming produced by an increase in atmospheric CO2 and a consequent increase in atmospheric water vapor, but Mars does not retain enough carbon dioxide that could practically be put back into the atmosphere to warm it.

Wind

Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects, an important one being its velocity (wind speed); another the density of the gas involved; another its energy content or wind energy. Wind is also a great source of transportation for seeds and small birds; with time things can travel thousands of miles in the wind.

In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high-speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termed squalls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, and hurricane. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Within the tropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds.

In human civilization, the concept of wind has been explored in mythology, influenced the events of history, expanded the range of transport and warfare, and provided a power source for mechanical work, electricity and recreation. Wind powers the voyages of sailing ships across Earth's oceans. Hot air balloons use the wind to take short trips, and powered flight uses it to increase lift and reduce fuel consumption. Areas of wind shear caused by various weather phenomena can lead to dangerous situations for aircraft. When winds become strong, trees and human-made structures are damaged or destroyed.

Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind also affects the spread of wildfires. Winds can disperse seeds from various plants, enabling the survival and dispersal of those plant species, as well as flying insect populations. When combined with cold temperatures, wind has a negative impact on livestock. Wind affects animals' food stores, as well as their hunting and defensive strategies.

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