Atmosphere of Triton
Encyclopedia
The atmosphere
of Triton
extends 800 kilometers above Triton's surface. The atmosphere mainly is composed of nitrogen
, similar to Titan's atmosphere
and Earth's atmosphere
. The surface pressure
is only 14 microbars, which is 1/70,000th of the surface pressure on Earth
. Originally, it was thought Triton had a thick atmosphere. Voyager 2
observed the atmosphere up close in 1989. Recent observations of the atmosphere have shown an increase in temperature.
is the main gas in Triton's atmosphere. The two other known components are methane
and carbon monoxide
, which abundances are a few hundredths of a percent of that of the nitrogen. Carbon monoxide, which was discovered only in 2010 by the ground based observations, is slightly more abundant than methane. The abundance of methane relative to nitrogen increased by four to five times since 1986 due to the global warming observed on Triton, which passed its solstice
in 2001.
Other possible components of the Triton's atmosphere include argon
and neon
. Since they were not detected in the ultraviolet
spectra of Triton obtained by Voyager 2 in 1989, their abundances are unlikely to exceed a few percent. In addition to the gases mentioned above, the upper atmosphere contains significant amounts of both molecular and atomic hydrogen
, which is produced by the photolysis of methane. This hydrogen quickly escapes into the space serving as a source of plasma in the magnetosphere of Neptune.
Other Solar System planets and moons with similarly composed atmospheres include the Earth
, Titan
, Pluto
and, possibly, .
. The surface temperature was at least because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. The most likely temperature was as of 1989. Later in 1990s it probably increased by about 1 K owning to the general global warming on Triton (see below).
Convection
near Triton's surface heated by the Sun creates a troposphere
(a "weather region") rising to an altitude of about 8 km. In it temperature decreases with height reaching a minimum of about 36 K at the tropopause
. There is no stratosphere
, defined as a layer where heating from the warmer troposphere and thermosphere is balanced by radiative cooling. Higher regions include the thermosphere
(8–850 km) and exosphere
(above 850 km). In the thermosphere the temperature rises reaching a constant value of about 95 kelvin
s above 300 km. The upper atmosphere continuously leaks into the space due to the weak gravity of Triton. The loss rate is about 1 nitrogen molecules per second, which equals about 0.3 kg/s.
Nitrogen
ice particles form clouds in the troposphere a few kilometers above the surface of Triton. Above them a haze is present extending up to 30 km from the surface. It is believed to be composed largely of hydrocarbon
s and nitrile
s created by the action of the Sun's and stellar ultraviolet light on methane.
In 1989 Voyager 2 discovered that near the surface there are winds blowing to the east or north-east with a speed of about 5–15 m/c. Their direction was determined by observations of dark streaks located over the southern polar cap, which generally extend from the south-west to north-east. These winds are thought to be related to the sublimation of nitrogen ice from the southern cap as there was summer in the southern hemisphere in 1989. The gaseous nitrogen moves northward and is deflected by the Coriolis force to the east forming an anticyclone near the surface. The tropospheric winds are capable of moving material of over a micrometre in size thus forming the steaks.
Eight kilometers high in the atmosphere near the tropopause, the winds change direction. They now flow to the west and are driven by differences in temperature between the poles and equator. These high winds may distort Triton's atmosphere making it asymmetric. An asymmetry was actually observed during star occultations by Triton in 1990s.
, but like on Mars, a denser early atmosphere is postulated.
five hours after closest approach to Neptune
in 1989. During the flyby, Voyager 2 took measurements of the atmosphere, finding methane and nitrogen in the atmosphere.
were made of the occultation
of stars by Triton's limb
. These observations indicated the presence of a denser atmosphere than was inferred from Voyager 2 data. The surface pressure in the late 1990s is thought to have increased to at least 19 μbar or, possibly, even to 40 μbar.
Other observations have shown an increase in temperature by 5% from 1989 to 1998. One of the scientists involved in investigation of Triton, James L. Elliot
, said:
These observations indicate Triton is having an unusually warm summer season that only happens once every few hundred years, near solstices. Theories for this warming include the sublimation of frost on Triton's surface and a decrease in ice albedo, which would allow more heat to be absorbed. Another theory argues the changes in temperature are a result of deposition of dark, red material from geological processes on the moon. Because Triton's Bond albedo
is among the highest within the Solar System
, it is sensitive to small variations in spectral albedo
.
.
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
of Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
extends 800 kilometers above Triton's surface. The atmosphere mainly is composed of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, similar to Titan's atmosphere
Atmosphere of Titan
The atmosphere of Titan is known as the only fully developed atmosphere that exists on a natural satellite in our solar system.-History:The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas Solà, who observed distinct limb darkening on Titan in 1903, and...
and Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
. The surface pressure
Surface pressure
Surface pressure is the atmospheric pressure at a location on Earth's surface. It is directly proportional to the mass of air over that location....
is only 14 microbars, which is 1/70,000th of the surface pressure on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. Originally, it was thought Triton had a thick atmosphere. Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
observed the atmosphere up close in 1989. Recent observations of the atmosphere have shown an increase in temperature.
| Gas | Partial pressure in 1986, μbar |
Partial pressure in 2010, μbar |
|---|---|---|
| N2 | ||
| CH4 | ||
| CO | ? |
Composition
NitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
is the main gas in Triton's atmosphere. The two other known components are methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, which abundances are a few hundredths of a percent of that of the nitrogen. Carbon monoxide, which was discovered only in 2010 by the ground based observations, is slightly more abundant than methane. The abundance of methane relative to nitrogen increased by four to five times since 1986 due to the global warming observed on Triton, which passed its solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
in 2001.
Other possible components of the Triton's atmosphere include argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
. Since they were not detected in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
spectra of Triton obtained by Voyager 2 in 1989, their abundances are unlikely to exceed a few percent. In addition to the gases mentioned above, the upper atmosphere contains significant amounts of both molecular and atomic hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, which is produced by the photolysis of methane. This hydrogen quickly escapes into the space serving as a source of plasma in the magnetosphere of Neptune.
Other Solar System planets and moons with similarly composed atmospheres include the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
and, possibly, .
Structure
Triton's atmosphere is well structured and global. The atmosphere extends up to 800 kilometers above the surface, where the exobase is located, and had a surface pressure of about 14 microbars as of 1989. This is only 1/70,000th of the surface pressure on EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. The surface temperature was at least because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. The most likely temperature was as of 1989. Later in 1990s it probably increased by about 1 K owning to the general global warming on Triton (see below).
Convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
near Triton's surface heated by the Sun creates a troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
(a "weather region") rising to an altitude of about 8 km. In it temperature decreases with height reaching a minimum of about 36 K at the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
. There is no stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
, defined as a layer where heating from the warmer troposphere and thermosphere is balanced by radiative cooling. Higher regions include the thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...
(8–850 km) and exosphere
Exosphere
The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...
(above 850 km). In the thermosphere the temperature rises reaching a constant value of about 95 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s above 300 km. The upper atmosphere continuously leaks into the space due to the weak gravity of Triton. The loss rate is about 1 nitrogen molecules per second, which equals about 0.3 kg/s.
Weather
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
ice particles form clouds in the troposphere a few kilometers above the surface of Triton. Above them a haze is present extending up to 30 km from the surface. It is believed to be composed largely of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s and nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
s created by the action of the Sun's and stellar ultraviolet light on methane.
In 1989 Voyager 2 discovered that near the surface there are winds blowing to the east or north-east with a speed of about 5–15 m/c. Their direction was determined by observations of dark streaks located over the southern polar cap, which generally extend from the south-west to north-east. These winds are thought to be related to the sublimation of nitrogen ice from the southern cap as there was summer in the southern hemisphere in 1989. The gaseous nitrogen moves northward and is deflected by the Coriolis force to the east forming an anticyclone near the surface. The tropospheric winds are capable of moving material of over a micrometre in size thus forming the steaks.
Eight kilometers high in the atmosphere near the tropopause, the winds change direction. They now flow to the west and are driven by differences in temperature between the poles and equator. These high winds may distort Triton's atmosphere making it asymmetric. An asymmetry was actually observed during star occultations by Triton in 1990s.
Before Voyager 2
Before Voyager 2 arrived, a nitrogen and methane atmosphere with a density as much as 30% that of the Earth had been suggested. This proved to be a great overestimate, similar to the predictions of the atmospheric density of MarsMars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
, but like on Mars, a denser early atmosphere is postulated.
Voyager 2
Voyager 2 flew past TritonTriton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
five hours after closest approach to Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
in 1989. During the flyby, Voyager 2 took measurements of the atmosphere, finding methane and nitrogen in the atmosphere.
Later observations
In the 1990s, observations from EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
were made of the occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
of stars by Triton's limb
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...
. These observations indicated the presence of a denser atmosphere than was inferred from Voyager 2 data. The surface pressure in the late 1990s is thought to have increased to at least 19 μbar or, possibly, even to 40 μbar.
Other observations have shown an increase in temperature by 5% from 1989 to 1998. One of the scientists involved in investigation of Triton, James L. Elliot
James L. Elliot
James Ludlow Elliot 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 largest moon of Neptune....
, said:
"At least since 1989, Triton has been undergoing a period of global warming. Percentage-wise, it's a very large increase."
These observations indicate Triton is having an unusually warm summer season that only happens once every few hundred years, near solstices. Theories for this warming include the sublimation of frost on Triton's surface and a decrease in ice albedo, which would allow more heat to be absorbed. Another theory argues the changes in temperature are a result of deposition of dark, red material from geological processes on the moon. Because Triton's Bond albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...
is among the highest within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, it is sensitive to small variations in spectral albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
.
Triton Watch
The Triton Watch program uses astronomers to monitor changes in the atmosphere. It was created from funds from NASANASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
.