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The atmosphere of Pluto is the tenuous layer of gases surrounding
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
. It consists mainly of
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(N2), with minor amounts of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
(CH4) and
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
(CO), all of which are vaporized from their ices on Pluto's surface. It contains layered haze, probably consisting of heavier compounds which form from these gases due to high-energy radiation. The atmosphere of Pluto is notable for its strong and not completely clear seasonal changes caused by peculiarities of the orbital and axial rotation of Pluto. The surface pressure of the atmosphere of Pluto, measured by ''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
'' in 2015, is about (), roughly 1/100,000 of Earth's atmospheric pressure. The temperature on the surface is , but it quickly grows with altitude due to methane-generated
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
. Near the altitude it reaches , and then slowly decreases. Pluto is the only
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
with a known atmosphere. Its closest analog is the
atmosphere of Triton The atmosphere of Triton is the layer of gases surrounding Triton. The surface pressure is only 14 microbars (1.4 Pa or 0.0105mmHg), of the surface pressure on Earth, and it is composed of nitrogen, similar to those of Titan and Earth. It extend ...
, although in some aspects it resembles even the
atmosphere of Mars 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 no ...
. The atmosphere of Pluto has been studied since the 1980s by way of earth-based observation of
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
s of stars by Pluto and
spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
. In 2015, it was studied from a close distance by the spacecraft ''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
''.


Composition

The main component of the atmosphere of Pluto is
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
. The
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
content, according to measurements by ''New Horizons'', is 0.25%. For
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
, the Earth-based estimates are (2010) and (2015). Under the influence of high-energy cosmic radiation, these gases react to form more complex compounds (not volatile at Pluto's surface temperatures), including
ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petroc ...
(C2H6),
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene ...
(C2H4),
acetylene Acetylene ( systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
(C2H2), heavier
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s and
nitrile In organic chemistry, a nitrile is any organic compound that has a functional group. The prefix '' cyano-'' is used interchangeably with the term ''nitrile'' in industrial literature. Nitriles are found in many useful compounds, including me ...
s and
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on a ...
(HCN) (the amount of ethylene is about 0.0001%, and the amount of acetylene is about 0.0003%). These compounds slowly precipitate on the surface. They probably also include
tholins Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon ...
, which are responsible for the brown color of Pluto (like some other bodies in the outer solar system). The most volatile compound of the atmosphere of Pluto is nitrogen, the second is carbon monoxide and the third is methane. The indicator of volatility is
saturated vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
( sublimation pressure). At temperature (close to minimum value for surface of Pluto) it is about for nitrogen, for carbon monoxide and for methane. It quickly increases with temperature, and at (close to the maximum value) approaches to , and respectively. For heavier-than-methane hydrocarbons,
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
,
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
,
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
and hydrogen cyanide, this pressure remains negligibly low (about or still lower), which indicates absence of volatility at Pluto's conditions (at least in cold lower atmosphere). Methane and carbon monoxide, due to their lower abundance and volatility, could be expected to demonstrate stronger deviations from pressure equilibrium with surface ices and bigger temporal and spatial variations of concentration. But actually the concentration of, at least, methane, does not depend noticeably on height (at least, in the lower 20–30 km), longitude or time. But temperature dependence of the volatilities of methane and nitrogen suggest that the concentration of methane will decrease as Pluto moves further from the Sun. It is notable that the observed concentration of methane is 2 orders of magnitude higher than expected from Raoult's law on the basis of its concentration in surface ice and the ratio of the sublimation pressures of methane and nitrogen. Reasons of this discrepancy are unknown. It could be due to the existence of separate patches of relatively clean methane ice, or due to an increased methane content in the uppermost layer of usual mixed ice. Seasonal and orbital changes of insolation result in migration of surface ices: they sublimate in some places and condensate in others. According to some estimates, this causes meter-sized changes of their thickness. This (but also changes in viewing geometry) results in appreciable changes of the brightness and color of Pluto. Methane and carbon monoxide, despite their low abundance, are significant for the thermal structure of the atmosphere: methane is a strong heating agent and carbon monoxide is a cooling one (although amount of this cooling is not completely clear).


Haze

''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
'' discovered in the atmosphere of Pluto a multi-layered
haze Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classificati ...
, which covers the entirety of the dwarf planet and reaches altitude over 200 km. The best images show about 20 layers of the haze. Horizontal extent of the layers is no less than 1000 km. The thickness of the layers varies from 1 to >10 km, and vertical distance between them is about 10 km. In northern regions the haze is 2-3 times denser than near the equator. Despite the very low density of the atmosphere, the haze is rather appreciable: it even scatters enough light to allow photographing some details of Pluto's night side. Long shadows from mountains are seen on the haze. Its normal
optical depth 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 throug ...
is estimated as 0.004 or 0.013 (thus, it diminishes the intensity of a vertical beam of light by or ; for grazing light the effect is much stronger). Scale height of the haze is ; it approximately coincides with scale height of pressure in the middle atmosphere. On the heights of it diminishes to 30 km. Size of the haze particles is unclear. Its blue color points to a particle radius near 10 nm, but the ratio of brightnesses at different phase angles indicates a radius exceeding 100 nm. This can be explained by aggregation of small (tens of nm) particles into larger (hundreds of nm) clusters. The haze probably consists of particles of non-volatile compounds, which are synthesized from atmospheric gases under influence of cosmic high-energy radiation. The layers show the presence of
atmospheric wave An atmospheric wave is a periodic disturbance in the fields of atmospheric variables (like surface pressure or geopotential height, temperature, or wind velocity) which may either propagate ('' traveling wave'') or not (''standing wave''). Atmos ...
s (presence of which is also suggested by observations of occultations), and such waves can be created by wind blowing over Pluto's rough surface. The haze is the most probable reason for a kink in the curve of light intensity vs. time obtained by ''New Horizons'' during the flight through Pluto's shadow (see image on right) – below altitude the atmosphere attenuates light much stronger than above. A similar kink was observed during stellar occultation in 1988. Firstly it was also interpreted as weakening of light by haze, but now it is thought to be mainly a result of strong
inverse Inverse or invert may refer to: Science and mathematics * Inverse (logic), a type of conditional sentence which is an immediate inference made from another conditional sentence * Additive inverse (negation), the inverse of a number that, when a ...
temperature gradient in lower atmosphere. During later occultations (when the atmosphere of Pluto was already denser) this kink was absent. Another evidence of the haze was obtained in 2002 due to a new occultation. The stellar light which managed to reach Earth during the occultation (due to refraction in Pluto's atmosphere), demonstrated an increase of intensity with wavelength.At least, in infrared range — from 0.75 to 2 µm. This was interpreted as a reliable evidence of light scattering by
aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthropogen ...
s (similar to the reddening of rising Sun). However, this feature was absent during later eclipses (including 29 June 2015), and on 14 July 2015, ''New Horizons'' found the haze to be blue. In the final batch of images received from ''New Horizons'', a number of potential clouds were observed.


Temperature and thermal structure

Pluto has no or almost no
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. Fro ...
; observations by ''New Horizons'' suggest only a thin tropospheric
boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary cond ...
. Its thickness in the place of measurement was 4 km, and the temperature was 37±3 K. The layer is not continuous. Above it lays a layer with fast increase of temperature with height, the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
. The temperature gradient is estimated to be 2.2, or 5.5 degrees per km. It is a result of
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
, caused by
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
. The mean temperature of the surface is (measured in 2005), and the mean value for the whole atmosphere is (2008). At height the temperature reaches its maximum (;
stratopause The stratopause (formerly mesopeak) is the level of the atmosphere which is the boundary between two layers: the stratosphere and the mesosphere. In the stratosphere, the temperature increases with altitude, and the stratopause is the region where ...
) and then slowly decreases (about ;
mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
). Causes of this decrease are unclear; it could be related to the cooling effect of
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
, or
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on a ...
, or other reasons. Above 200 km the temperature reaches approximately and then remains constant. The temperature of the upper layers of the atmosphere does not show noticeable temporal changes. In 1988, 2002 and 2006 it was approximately constant and equal to (with uncertainty about ), despite a twofold increase in pressure. Dependence on latitude or morning/evening conditions is also absent: temperature is the same above every part of the surface. It is consistent with theoretical data, which predict fast mixing of the atmosphere. But there is evidence for small vertical heterogeneities in temperature. They reveal themselves in sharp and brief spikes of brightness during stellar occultations. Amplitude of these heterogeneities is estimated to be on the scale of a few km. They can be caused by atmospheric
gravity waves In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere ...
or turbulence, which can be related to
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...
or wind. Interaction with the atmosphere significantly influences the temperature of the surface. Calculations show that the atmosphere, despite its very low pressure, can significantly diminish diurnal variations in temperature. But there still remain temperature variations of about  – partly because of cooling of the surface due to sublimation of ices.


Pressure

Pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
of the atmosphere of Pluto is very low and strongly time-dependent. Observations of stellar
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
s by Pluto show that it increased about 3 times between 1988 and 2015, even though Pluto has been moving away from the Sun since 1989. This is probably caused by Pluto's north pole coming into sunlight in 1987, which intensified evaporation of nitrogen from the northern hemisphere,In these sources this pole is called "south" according to nomenclature of that time. whereas its southern pole is still too warm for condensation of nitrogen. Absolute values of surface pressure are difficult to obtain from occultation data, because this data does not usually reach the lowest layers of the atmosphere. So, surface pressure has to be extrapolated, and this is somewhat ambiguous due to the height dependence of temperature and, consequently, pressure not being completely clear . The radius of Pluto must also be known, but it was poorly constrained before 2015. So, precise values of Pluto's surface pressure were impossible to calculate in previous times. For some occultations since 1988, pressure was calculated for a reference level of from the center of Pluto (which later turned out to be 88±4 km from the surface). Curves of pressure vs. distance from the center, obtained from occultations in 1988 and 2002, in combination with the now known radius of Pluto () give values of about for 1988 and for 2002. Spectral data provided values in 2008 and in 2012 for the distance from the center (1±4 km from the surface). An occultation on 4 May 2013 gave data almost precisely for the surface level (1190 km from the center, or 3±4 km from the surface): . An occultation on 29/30 June 2015, just 2 weeks before ''New Horizons'' encounter, provided a surface pressure of . The first direct and reliable data about the lowermost layers of the atmosphere of Pluto were obtained by ''New Horizons'' on 14 July 2015 due to radio-occultation measurements. The surface pressure was estimated to be ( at entry of the spacecraft behind Pluto and at the exit). This is consistent with occultation data from previous years, although some of the previous calculations based on this data gave about 2 times higher results. The stellar occultation 17 July 2019 have shown the Pluto`s atmospheric pressure have dropped about 30% from maximal values in 2015, reaching 0.967 Pa. 6 June 2020 further pressure decline to 0.91 Pa was measured. The scale height of pressure in Pluto's atmosphere varies significantly with height (in other words, the height dependence of the pressure deviates from
exponential Exponential may refer to any of several mathematical topics related to exponentiation, including: *Exponential function, also: **Matrix exponential, the matrix analogue to the above *Exponential decay, decrease at a rate proportional to value *Expo ...
). This is caused by strong height variations of temperature. For the lowermost layer of the atmosphere the scale height is about , and for heights  — .


Seasonal changes

Due to
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values bet ...
, in the
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...
Pluto receives 2.8 times less heat than in perihelion.Square of ratio of distances in aphelion and perihelion: (49.30 au / 29.66 au)2 = 2.76. It should cause strong changes in its atmosphere, although details of these processes are not clear. Firstly it was thought that in aphelion the atmosphere must largely freeze out and fall on the surface (this is suggested by strong temperature dependence of sublimation pressure of its compounds), but more elaborated models predict that Pluto has a significant atmosphere year-round. Pluto's last passage through its perihelion was on 5 September 1989. As of 2015, it is moving away from the Sun and its overall surface illumination is decreasing. However, the situation is complicated by its big
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orb ...
(122.5°), which results in long polar
days A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two ...
and nights on large parts of its surface. Shortly before the perihelion, on 16 December 1987, Pluto underwent
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
, and its north poleDue to reverse direction of axial rotation of Pluto, naming of its poles is somewhat ambiguous. Since 2009, International Astronomical Union defines north (more precisely, ''positive'') pole of Pluto on basis of direction of the rotation: it is that pole, from whose side Pluto appears to rotate counterclockwise
Archinal et al., 2011
. It is oriented to southern side of the solar system.
came out of the polar night, which had lasted 124 Earth years. Data, existing as of 2014, allowed the scientists to build a model of seasonal changes in Pluto's atmosphere. During the previous aphelion (1865) significant quantity of volatile ices were present in both the northern and southern hemispheres. Approximately at the same time, the equinox occurred and the southern hemisphere became tilted towards the Sun. Local ices began to migrate to the northern hemisphere, and around 1900 the southern Hemisphere became largely devoid of ices. After the following equinox (1987), the southern hemisphere turned away from the Sun. Nonetheless, its surface was already substantially heated, and its big
thermal inertia In thermodynamics, a material's thermal effusivity, thermal inertia or thermal responsivity is a measure of its ability to exchange thermal energy with its surroundings. It is defined as the square root of the product of the material's thermal ...
(provided by non-volatile water ice) greatly slowed down its cooling. That is why gases, which now intensively evaporate from northern hemisphere, cannot quickly condense in the southern, and keep accumulating in the atmosphere, increasing its pressure. Around , the southern hemisphere will cool enough to permit intensive condensation of the gases, and they will migrate there from the northern hemisphere, where is polar day. It will last till equinox near aphelion (about 2113). The northern hemisphere will not lose its volatile ices completely, and their evaporation will supply the atmosphere even at the aphelion. The overall change of atmospheric pressure in this model is about 4 times; the minimum was reached near , and the maximum will be near 2030. The full temperature range is only several degrees. In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016. In 2021, astronomers at the
Southwest Research Institute Southwest Research Institute (SwRI), headquartered in San Antonio, Texas, is an independent and nonprofit applied research and development (R&D) organization. Founded in 1947 by oil businessman Tom Slick, it provides contract research and devel ...
confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.


Escape

Early data suggested that Pluto's atmosphere loses molecules () of nitrogen per second, an amount corresponding to the loss of a surface layer of volatile ices several hundred meters or several kilometers thick during the lifetime of the Solar System. However, subsequent data from ''New Horizons'' revealed that this figure was overestimated by at least four orders of magnitude; Pluto's atmosphere is currently losing only 1×1023 molecules of nitrogen and 5×1025 molecules of methane every second. This presumes a loss of several centimeters of nitrogen ice and several dozen meters of methane ice during the lifetime of the Solar System. Molecules with high enough velocity, which escape into outer space, are ionized by solar
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted, possibly forming a shock wave upstream of Pluto. The ions are "picked up" by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail. The Solar Wind around Pluto (SWAP) instrument on the ''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
'' spacecraft made the first measurements of this region of low-energy atmospheric ions shortly after its closest approach on 14 July 2015. Such measurements will enable the SWAP team to determine the rate at which Pluto loses its atmosphere and, in turn, will yield insight into the evolution of the Pluto's atmosphere and surface. The reddish-brown cap of the north pole of
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the ...
, the largest of Pluto's moons (
Mordor Macula Mordor Macula is the informal name for a large red area about in diameter near the north pole of Charon, Pluto's largest moon. It is named after the black land called Mordor in J.R.R. Tolkien's '' The Lord of the Rings''. Origin The origin of ...
), may be composed of tholins,
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
s produced from
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
,
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and other gases released from the atmosphere of Pluto and transferred over about distance to the orbiting moon. Models show that Charon can receive about 2.5% of gases lost by Pluto.


History of study

As early as the 1940s,
Gerard Kuiper Gerard Peter Kuiper (; ; born Gerrit Pieter Kuiper; 7 December 1905 – 23 December 1973) was a Dutch astronomer, planetary scientist, selenographer, author and professor. He is the eponymous namesake of the Kuiper belt. Kuiper is ...
searched for evidence of the atmosphere in the spectrum of Pluto, without success. In the 1970s, some astronomers forwarded the hypothesis of a thick atmosphere and even oceans of
neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypt ...
: according to some views of those times, all other gases that are abundant in the Solar System would either freeze or
escape Escape or Escaping may refer to: Computing * Escape character, in computing and telecommunication, a character which signifies that what follows takes an alternative interpretation ** Escape sequence, a series of characters used to trigger some s ...
. However, this hypothesis was based on a heavily overestimated mass of Pluto. No observational data about its atmosphere and chemical composition existed at the time. The first strong, though indirect evidence of the atmosphere appeared in 1976. Infrared photometry by the 4-meter Nicholas U. Mayall Telescope revealed methane ice on Pluto's surface, which must sublimate significantly at Plutonian temperatures. Existence of the atmosphere of Pluto was proven via stellar
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
. If a star is occulted by a body without an atmosphere, its light disappears sharply, but occultations by Pluto show a gradual decrease. This is mainly due to
atmospheric refraction Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of ligh ...
(not
absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which s ...
or scattering). The first such observations were made on 19 August 1985 by
Noah Brosch Noah Brosch (born 1948) is an Israeli astronomer, astrophysicist and space researcher. Biography Noah Brosch was born in Bucharest (Romania) in 1948 and immigrated with his family to Israel in 1963. Brosch studied at Tel Aviv University (BSc ...
and Haim Mendelson of the
Wise Observatory The Florence and George Wise Observatory (IAU code 097) is an astronomical observatory owned and operated by Tel Aviv University. It is located west of the town of Mitzpe Ramon in the Negev desert near the edge of the Ramon Crater, and it is ...
in Israel. But quality of the data was rather low due to unfavorable observational conditions (in addition, the detailed description was published only 10 years later). On 9 June 1988 the existence of the atmosphere was convincingly proven by occultation observations from eight sites (the best data were obtained by the
Kuiper Airborne Observatory The Gerard P. Kuiper Airborne Observatory (KAO) was a national facility operated by NASA to support research in infrared astronomy. The observation platform was a highly modified Lockheed C-141A Starlifter jet transport aircraft (s/n: 6110, re ...
). Scale height of the atmosphere was measured, making it possible to calculate the ratio of the temperature to the mean molecular mass. The temperature and pressure themselves were impossible to calculate at the time due to an absence of data on the chemical composition of the atmosphere and a large uncertainty in the radius and mass of Pluto. The question of composition was answered in 1992 via infrared spectra of Pluto by the 3.8-meter
United Kingdom Infrared Telescope The United Kingdom Infra-Red Telescope (UKIRT) is a 3.8 metre (150 inch) infrared reflecting telescope, the second largest dedicated infrared (1 to 30 micrometres) telescope in the world. It is located on Mauna Kea, Hawai'i as part of Mau ...
. The surface of Pluto turned out to be covered mainly by
nitrogen ice Solid nitrogen is a number of solid forms of the element nitrogen, first observed in 1884. Solid nitrogen is mainly the subject of academic research, but low-temperature, low-pressure solid nitrogen is a substantial component of bodies in the ou ...
. Since
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
is, in addition, more volatile than
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
, this observation implied a prevalence of nitrogen also in the atmosphere (although gaseous nitrogen was not seen in the spectrum). Furthermore, a small admixture of frozen
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
was discovered. The same year observations by the 3.0-meter NASA Infrared Telescope Facility revealed the first conclusive evidence of gaseous methane. Understanding the state of the atmosphere requires knowing the surface temperature. Best estimates are derived from measurements of
thermal emission Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...
of Pluto. The first values, calculated in 1987 from observations by
IRAS The Infrared Astronomical Satellite ( Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten ...
, were about , with subsequent studies suggesting . In 2005, observations by the
Submillimeter Array The Submillimeter Array (SMA) consists of eight diameter radio telescopes arranged as an interferometer for submillimeter wavelength observations. It is the first purpose-built submillimeter interferometer, constructed after successful interfe ...
succeeded in distinguishing the emissions of Pluto and Charon, and the average temperature of Pluto's surface was measured to be (). It was approximately colder than expected; the difference may be due to cooling from the sublimation of nitrogen ice,. Further research revealed that the temperature is strongly different in different regions: from 40 to . Around the year 2000, Pluto entered star-rich fields of the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
, where it will reside until the 2020s. The first stellar occultations after 1988 were on 20 July and 21 August 2002 by teams led by Bruno Sicardy of the
Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histo ...
and
James L. Elliot James Ludlow Elliot (June 17, 1943 – March 3, 2011) was an American astronomer and scientist who, as part of a team, discovered the rings around the planet Uranus. Elliot was also part of a team that observed global warming on Triton, the la ...
of
MIT The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
. Atmospheric pressure turned out to be about 2 times higher than in 1988. The next occultation observed was on 12 June 2006, with later ones occurring more frequently. Processing of these data shows that the pressure continues to increase. An occultation of an exceptionally bright star, about 10 times brighter than the Sun itself, was observed on 29/30 June 2015 – only 2 weeks before the ''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
'' encounter. On 14 July 2015 the ''New Horizons'' spacecraft made the first explorations of the atmosphere of Pluto from close distance, including radio occultation measurements and observations of weakening of solar radiation during flight through Pluto's shadow. It provided the first direct measurements of parameters of the lower atmosphere. The surface pressure turned out to be .


Notes


See also

* Climate zones on Pluto


References


External links


Some literature in the Astrophysics Data System

Video (00:17) of Alice occultation (Pluto's atmosphere passes in front of sun)
(NASA; ''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
'', 14 July 2015). * * * Some of raw images of ''New Horizons'' showing sun-lighted atmosphere
1234
{{Atmospheres Discoveries by James L. Elliot
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
Pluto