# Atmospheres/Astronomy

Atmospheric astronomy has three basic aspects: astronomy conducted through an atmosphere, astronomy of an atmosphere, and astronomy conducted using an atmosphere.

Uranus is seen by Voyager 2. Credit: NASA/JPL/Voyager mission.

Gaseous objects have at least one chemical element or compound present in the gaseous state. These gaseous components make up at least 50 % of the detectable portion of the gaseous object. Atmospheric astronomy determines whether gaseous objects have layers or spherical portions predominantly composed of gas.

Within these spherical portions may occur various gaseous meteors such as clouds, winds, or streams.

## Astronomy

Def. the number of molecules of a compound per a molecule of hydrogen is called the mixing ratio.

## Planetary sciences

An orange sunset in the Mahim Bay is shown around the Haji Ali Dargah in India. Credit: Humayunn Peerzaada.

The Earth's shadow at sunrise is seen over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. Note: the lowest blue-grey area is not the sky but the surface of the Pacific Ocean. Credit: .

Earth's shadow and Belt of Venus at sunset, looking east from the Marin Headlands just north of San Francisco. (Note: there is a thin greyish cloud layer partially obscuring the horizon in this image.) Credit: .

Seeing an orange Sun due to atmospheric effects and feeling the warmth of its rays is probably a student's first encounter with an apparent astronomical orange radiation source.

The Earth's shadow or Earth shadow (also sometimes known as the dark segment) are names for the shadow that the Earth itself casts on its atmosphere. This shadow is often visible from the surface of the Earth, as a dark band in the sky near the horizon. This atmospheric phenomenon can sometimes be seen twice a day, around the times of sunset and sunrise.

The effect of the Earth's shadow on the atmosphere is quite often visible in the sky, and yet often goes unrecognized. This shadow is visible to observers as it falls on the atmosphere of the Earth during the twilight hours. When the weather conditions and the observer's viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band.

Assuming the sky is clear, the Earth's shadow is visible in the opposite half of the sky to the sunset or sunrise, and is seen right above the horizon as a dark blue band.

In the image at right, the Earth's shadow at sunrise is seen over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. The lowest blue-grey area is not the sky but the surface of the Pacific Ocean.

At left is the Earth's shadow at sunset, looking east from the Marin Headlands just north of San Francisco. There is a thin greyish cloud layer partially obscuring the horizon in this image.

The Earth's shadow (as it is cast onto the atmosphere) can be observed during the twilight hours, assuming the sky is clear and the horizon is relatively unobstructed. At sunset the Earth's shadow is visible opposite the sunset in the eastern sky, just above the horizon. The shadow shows as a dark blue band that stretches over 180° of the horizon.[1][2] It is most noticeable at the antisolar point, exactly opposite the sunset.

At sunrise, the Earth's shadow is seen in a similar way, but in the western sky. The Earth's shadow is best observed when there is a low horizon (such as over the sea), and when the sky conditions are very clear. In addition, the higher up an observer is standing to view the horizon, the sharper the shadow appears.[1][2]

At sunrise, the Earth's shadow can be seen to set as the sun itself rises, and at sunset, the Earth's shadow rises as the sun sets.[1]

## Colors

The Earth has a blue halo when seen from space. Credit: NASA Earth Observatory.

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space. This is shown in the image at right.

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

The bluish color of water is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.[3]

## Theoretical atmospheric astronomy

Here's a theoretical definition:

Def. performing astronomical observations to study the gases surrounding the Earth or any astronomical object is called atmospheric astronomy.

## Backgrounds

"The light blue background is the dayglow emission (less than 1 kR) caused by the interaction between the photoelectrons generated by solar UV radiation and atmospheric molecules and atoms."[4] This background occurs when imaging an Earth aurora from space using ultraviolet astronomy at the VUV wavelengths (135.6 ± 1.5 nm and 149.3 ± 1.5 nm).

"Isophotes of [blue (395.0-485.0 nm)] background starlight brightness (integrated starlight, diffuse galactic light, cosmic light) have been constructed near the celestial [NCP and SCP], ecliptic [NEP and SEP], and galactic [NGP and SGP] poles from observations accumulated by the Pioneer 10 [and 11] imaging photopolarimeter from beyond the asteroid belt."[5]

"Diffuse galactic [blue] light refers to starlight scattered by interstellar grains. Integrated light from extragalactic sources is referred to as cosmic light."[5]

"Earth-based observers are limited in their perception of background starlight, due to the presence of airglow line and continuum emission, zodiacal light, and the effects of atmospheric extinction and scattering."[5]

Some of the blue background data were collected on September 30, 1973 (day/year 279/73).[5]

"To determine the brightnesses, isophotes were generated from six days of merged data, chosen to optimize sky coverage. The days used and their heliocentric distances in AU are: 354/72 (3.27), 149/73 (4.23), 237/73 (4.64), 279/73 (4.81), 21/74 (5.08), 68/74 (5.15)."[5]

Conversion of units over the Pioneer blue (B) bandpass is[5]

${\displaystyle 1S_{10}(V)_{B}=1.16x10^{-8}ergcm^{-2}s^{-1}sr^{-1}nm^{-1}.}$

For example,

NCP ${\displaystyle S_{10}(V)_{B}=56,}$  NEP ${\displaystyle S_{10}(V)_{B}=66,}$  and NGP ${\displaystyle S_{10}(V)_{B}=31.}$ [5]

Whereas,

SCP ${\displaystyle S_{10}(V)_{B}=74,}$  SEP ${\displaystyle S_{10}(V)_{B}=128,}$  and SGP ${\displaystyle S_{10}(V)_{B}=26.}$ [5]

## Meteors

Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km.[6] Millions of meteors occur in the Earth's atmosphere every day.

### Aerometeors

Def. a discrete unit of air, wind, or mist traveling or falling through or partially through an atmosphere is called an aerometeor.

Def. a "wind whose direction and speed are determined by a balance of the horizontal pressure gradient force and the force due to the earth's rotation to the left in the northern hemisphere and to the right in the southern hemisphere"[7] is called a geostrophic wind.

Def. a "warm dry wind blowing down the side of a mountain"[8] is called a foehn, or foehn wind, or chinook.

"The chinook generally blows from the southwest, but its direction may be modified by topography. When it sets in after a spell of intense cold, the temperature may rise by 20–40°F in 15 minutes due to replacement of a cold air mass with a much warmer air mass in minutes."[9]

"Wind shear is a change in wind direction, wind speed, or both, along a given direction in space (e.g., along a horizontal or vertical distance)."[10]

Def. a "strong, abrupt rush of wind"[11] is called a gust.

### Hydrometeors

Def. any "or all of the forms of water particles, whether liquid or solid, that fall from the atmosphere""[12] are called precipitation.

Def. "precipitation products of the condensation of atmospheric water vapour"[13] are called hydrometeors.

"Condensation or sublimation of atmospheric water vapor produces a hydrometeor. It forms in the free atmosphere, or at the earth's surface, and includes frozen water lifted by the wind. Hydrometeors which can cause a surface visibility reduction, generally fall into one of the following two categories:

1. Precipitation. Precipitation includes all forms of water particles, both liquid and solid, which fall from the atmosphere and reach the ground; these include: liquid precipitation (drizzle and rain), freezing precipitation (freezing drizzle and freezing rain), and solid (frozen) precipitation (ice pellets, hail, snow, snow pellets, snow grains, and ice crystals).
2. Suspended (Liquid or Solid) Water Particles. Liquid or solid water particles that form and remain suspended in the air (damp haze, cloud, fog, ice fog, and mist), as well as liquid or solid water particles that are lifted by the wind from the earth’s surface (drifting snow, blowing snow, blowing spray) cause restrictions to visibility. One of the more unusual causes of reduced visibility due to suspended water/ice particles is whiteout, while the most common cause is fog."[10]

## Astrochemistry

This composition diagram shows the evolution/cycles of various elements in Earth's atmosphere. Credit: Phillipe Rekacewicz.`{{free media}}`

"The biogenic volatile organic compounds (BVOC) include the quantitatively dominant group of the isoprenoids [isoprene, monoterpenes (e.g. α-pinene), sesquiterpenes] and oxygen-containing species (OVOC), such as alcohols, carbonyls and organic acids."[14]

## Atmospheres

This is a graph of the global mean atmospheric water vapor superimposed on an outline of the Earth. Credit: NASA.

Def. the "gases surrounding the Earth or any astronomical body"[15] are called an atmosphere.

Being outside in the day light to look upward when the Sun is off to the East or West, you may see that the sky is blue depending on the weather. If the wind is blowing and the temperature is above freezing, you may be buffeted by an invisible force. The atmosphere of Earth, assuming that's the planet's solid surface you're bipedally standing on, is composed of small particles called molecules. While the wind may be physically assaulting you, it's actually these molecules, lots of them, that are contacting your skin. The main reason you're holding your shape is that your internal fluids and solids are pushing back.

These molecules in many instances are in turn made up of atoms of chemical elements. At your geographical location, specified in [a geographic coordinate system] latitude and longitude, this gaseous envelope extends upward. The atmosphere of Earth changes with altitude. At high enough altitude the composition changes significantly, as does the temperature and pressure. If you've packed the appropriate instruments on board for the trip upward, you may have noticed something else has changed.

## Planetary astronomy

"The spectrum of gaseous methane at 77 K in the 1.1-2.6 µm region [is] a benchmark for planetary astronomy".[16]

## Venus

This is an image of Venus in true color. The surface is obscured by a thick blanket of clouds. Credit: NASA/Ricardo Nunes, http://www.astrosurf.com/nunes.`{{free media}}`

The image shows Venus Express data with an artist's impression of the tear-drop shaped ionosphere. Credit: ESA/Wei et al. (2012).

"During a rare period of very low density solar outflow, the ionosphere of Venus was observed to become elongated downstream, rather like a long-tailed comet. ... Under normal conditions, the solar wind has a density of 5 - 10 particles per cubic cm at Earth's orbit, but occasionally the solar wind almost disappears, as happened in May 1999. ... A rare opportunity to examine what happens when a tenuous solar wind arrives at Venus came 3 - 4 August 2010, following a series of large coronal mass ejections on the Sun. NASA's STEREO-B spacecraft, orbiting downstream from Venus, observed that the solar wind density at Earth's orbit dropped to the remarkably low figure of 0.1 particles per cubic cm and persisted at this value for an entire day."[17]

"The observations show that the night side ionosphere moved outward to at least 15 000 km from Venus' centre over a period of only a few hours," said Markus Fraenz, also from the Max Planck Institute for Solar System Research, who was the team leader and a co-author of the paper.[17] "It may possibly have extended for millions of kilometres, like an enormous tail."[17]

"Although we cannot determine the full length of the night-side ionosphere, since the orbit of Venus Express provides limited coverage, our results suggest that Venus' ionosphere resembled the teardrop-shaped ionosphere found around comets, rather than the more symmetrical, spherical shape which usually exists."[17]

"The side of Venus' ionosphere that faces away from the sun can billow outward like the tail of a comet, while the side facing the star remains tightly compacted, researchers said. ... "As this significantly reduced solar wind hit Venus, Venus Express saw the planet’s ionosphere balloon outwards on the planet’s ‘downwind’ nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions," ESA officials said in a statement today (Jan. 29). It only takes 30 to 60 minutes for the planet's comet-like tail to form after the solar wind dies down. Researchers observed the ionosphere stretch to at least 7,521 miles (12,104 kilometers) from the planet, said Yong Wei, a scientist at the Max Planck Institute in Katlenburg, Germany who worked on this research."[18]

Because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind.[19]

The clouds of Venus are capable of producing lightning much like the clouds on Earth.[20] The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. In 2006–07 Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.[20] In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of the planet.[21][22]

Another discovery made by the Venus Express probe in 2011 is that an ozone layer exists high in the atmosphere of Venus.[23]

Venus has an extremely dense atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The atmospheric mass is 93 times that of Earth's atmosphere, while the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure equivalent to that at a depth of nearly 1 kilometer under Earth's oceans. The density at the surface is 65 kg/m³ (6.5% that of water).

## Earth

This is a map of global long term monthly average surface air temperatures in Mollweide projection. Credit: PZmaps.

Clouds are shown along a jet stream over Canada. Credit: NASA.

This image shows a cumulus cloud above Lechtaler Alps, Austria. Credit: Glg.

A spectrum is taken of blue sky clearly showing solar Fraunhofer lines and atmospheric water absorption band. Credit: .

The Earth has a blue halo when seen from space. Credit: .

The map at right shows the global long term monthly average surface air temperature for Earth.

The zones of Earth's climates, going from the equator upward (and downward) are Tropical, Dry, Moderate, Continental and Polar. There are subzones within these zones.

Def. "any of the high-speed, high-altitude air currents that circle the Earth in a westerly direction"[24] is called a jet stream.

Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude).[25] The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around 7–12 km (23,000–39,000 ft) above sea level, and the higher and somewhat weaker subtropical jets at around 10–16 km (33,000–52,000 ft). The Northern Hemisphere and the Southern Hemisphere each have both a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.

Def. a "large white puffy cloud"[26] is called a cumulus cloud.

Def. "[a] visible mass of

1. water droplets suspended in the air ...
2. dust,
3. steam ...
4. smoke ...
5. a group or swarm"[27] is called a cloud.

Def. device "that moves so as to aim a collector of solar energy at the sun as it crosses the sky"[28] is called a solar tracker.

Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky. Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky radiation.

"[P]referential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".[29]

## Mars

This is a color image made from the first post-sunset sequence of calibrated color images, with the color balance set to approximate what the sunset color would have looked like to the human eye. Credit: NASA/JPL/Texas A&M/Cornell.

"A color image was made [at lower right] from the first post-sunset sequence of calibrated color images, with the color balance set to approximate what the sunset color would have looked like to the human eye. The color seen in this first post-sunset image was then used to colorize each image in the sequence. Approximately one-minute gaps between consecutive color images meant the Sun's position changed within each color set, so the images had to be manually shifted to compensate for this motion. In this fashion, the position and brightness of the Sun are taken from each individual image, but the color is taken from a single set of images. The images were then combined into a movie where one color set fades gracefully into the next. Analysis of the five color sets shows that there were only small color variations during the sunset, so most of the real variations are captured in the movie."[30]

"The rapid dimming of the Sun near the horizon is due to the dust in the sky. There is nearly twice as much dust as there was when the Mars Pathfinder spacecraft, which landed on Mars in 1997, imaged the sunset. This causes the Sun to be many times fainter. The sky above the Sun has the same blue tint observed by Pathfinder and also by Viking, which landed on Mars in 1976. This is because dust in the martian atmosphere scatters blue light forward toward the observer much more efficiently than it scatters red light forward. Therefore, a "halo" of blueish sky color is always observed close to the Sun. We're only seeing half of this halo in the movie, because the other half is below the horizon."[30]

## Callisto

The diagram illustrates an induced magnetic field around Callisto. Credit: .

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide[31] and probably molecular oxygen,[32] as well as by a rather intense ionosphere.[33]

Callisto has a very tenuous atmosphere composed of carbon dioxide.[31] It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 x 10-12 bar (0.75 µPa) and particle density 4 x 108 cm−3. Because such a thin atmosphere would be lost in only about 4 days (see atmospheric escape), it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust,[31] which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs.

Oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements.[34] At the same time HST was able to detect condensed oxygen trapped on the surface of Callisto.[35]

Callisto's ionosphere was first detected during Galileo flybys;[33] its high electron density of 7–17 x 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than CO2).[32]

## Europa

Europa has a tenuous atmosphere composed primarily of oxygen.

Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope, first described in 1995, revealed that Europa has a tenuous atmosphere composed mostly of molecular oxygen (O2).[36][37] The surface pressure of Europa's atmosphere is 0.1 μPa, or 10−12 times that of the Earth.[38] In 1997, the Galileo spacecraft confirmed the presence of a tenuous ionosphere (an upper-atmospheric layer of charged particles) around Europa created by solar radiation and energetic particles from Jupiter's magnetosphere,[39][40] providing evidence of an atmosphere.

The molecular hydrogen that escapes Europa's gravity, along with atomic and molecular oxygen, forms a torus (ring) of gas in the vicinity of Europa's orbit around Jupiter. This "neutral cloud" has been detected by both the Cassini and Galileo spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon Io. Models predict that almost every atom or molecule in Europa's torus is eventually ionized, thus providing a source to Jupiter's magnetospheric plasma.[41]

## Ganymede

NASA's Hubble Space Telescope found ozone's spectral "fingerprint" during observations of Ganymede made by Keith Noll and colleagues at the Space Telescope Science Institute in Baltimore, Maryland. Credit: NASA.

"Though ozone may be diminishing on Earth, it is being manufactured one-half billion miles away, on Jupiter's largest satellite, Ganymede."[42]

"NASA's Hubble Space Telescope found ozone's spectral "fingerprint" during observations of Ganymede made by Keith Noll and colleagues at the Space Telescope Science Institute in Baltimore, Maryland. These Hubble Faint Object Spectrograph results were presented at the American Astronomical Society's 27th Annual Meeting of the Division of Planetary Sciences in Kona, Hawaii."[42]

## Io

Gases above Io's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). Credit: NASA/JPL/University of Arizona.

During an eclipse of Jupiter's moon Io on January 1, 2001, NASA's Cassini spacecraft recorded glows from auroras and volcanoes on Io. Credit: NASA/JPL/University of Arizona.

At right is an "eerie view of Jupiter's moon Io in eclipse ... acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter."[43]

"North is to the top of the picture, and Jupiter is towards the right. The resolution is 13.5 kilometers (8 miles) per picture element. The images were taken on May 31, 1998 at a range of 1.3 million kilometers (800,000 miles) by Galileo's onboard solid state imaging camera system during the spacecraft's 15th orbit of Jupiter."[43]

"The camera on Cassini captured images of eclipsed Io in several colors ranging from the near-ultraviolet to the near-infrared. A black-and-white movie clip of 48 clear-filter frames spanning two hours during the eclipse was released on February 5 (PIA02882). Here, two colors have been added to show the type of evidence used by imaging scientists in determining the source of Io's auroral glows. The color pictures were taken at lower resolution -- 120 kilometers (75 miles) per pixel rather than 60 kilometers(37 miles) per pixel -- and less frequently than the clear-filter images. White dots near the equator are volcanoes, some of which are much brighter than the faint atmospheric glows. The brightest of them is the volcano Pele."[44]

"Emissions of light (at wavelengths of 595 to 645 nanometers) likely arise from a tenuous atmosphere of oxygen. These glows would appear red to the eye and are consequently colored red in the movie. Emissions in near-ultraviolet wavelengths (between 300 and 380 nanometers), corresponding wavelength to the bright blue visible glows one would expect from sulfur dioxide. They have been colored blue in the movie. The blue glows are restricted to areas deep down in the atmosphere near the surface of Io, while the red glows are much more extensive, reaching heights of up to 900 kilometers (560 miles). This would be expected if the blue glows are indeed produced by sulfur dioxide, since sulfur dioxide molecules are heavier than oxygen atoms, so are more closely bound to the surface by gravity. The prominent blue and red regions near the equator of Io dance across the moon with the changing orientation of Jupiter's magnetic field, illustrating the relationship between Io's auroras and the electric currents that excite them."[44]

## Saturn

This is a Cassini image in natural color of the gaseous object Saturn. Credit: NASA/JPL/Space Science Institute.

The image shows Saturn's northern hemisphere from the Cassini spacecraft with Mimas in front. Credit: NASA/JPL/Space Science Institute.

The planet exhibits a pale yellow hue due to ammonia crystals in its upper atmosphere. Its exterior is predominantly composed of gas and it lacks a definite surface. The planet primarily consists of hydrogen. The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium.[45] The proportion of helium is significantly deficient compared to the abundance of this element in the Sun.[46] Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been detected in Saturn's atmosphere.[47][48][49]

"Saturn's northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths."[50]

## Titan

This natural color image shows Titan's upper atmosphere. Credit: NASA/JPL/Space Science Institute.

Titan has a vortex at its south pole. Credit: Cassini Imaging Team, ISS, JPL, ESA, NASA.

"This natural color image [at right] shows Titan's upper atmosphere -- an active place where methane molecules are being broken apart by solar ultraviolet light and the byproducts combine to form compounds like ethane and acetylene. The haze preferentially scatters blue and ultraviolet wavelengths of light, making its complex layered structure more easily visible at the shorter wavelengths used in this image."[51]

"Lower down in the atmosphere, the haze turns into a globe-enshrouding smog of complex organic molecules. This thick, orange-colored haze absorbs visible sunlight, allowing only perhaps 10 percent of the light to reach the surface. The thick haze is also inefficient at holding in and then re-radiating infrared (thermal) energy back down to the surface. Thus, despite the fact that Titan has a thicker atmosphere than Earth, the thick global haze causes the greenhouse effect there to be somewhat weaker than it is on Earth."[51]

"Images taken with the Cassini spacecraft wide-angle camera using red, green and blue spectral filters were combined to create this natural color view. The images were obtained at a distance of approximately 9,500 kilometers (5,900 miles) from Titan on March 31, 2005. The image scale is approximately 400 meters (1,300 feet) per pixel."[51]

The second image at right is of the south pole.

"What's happening over the south pole of Titan? A vortex of haze appears to be forming, although no one is sure why. The [second at right] natural-color image shows the light-colored feature. The vortex was found on images taken last month when the robotic Cassini spacecraft flew by the unusual atmosphere-shrouded moon of Saturn. Cassini was only able to see the southern vortex because its orbit around Saturn was recently boosted out of the plane where the rings and moons move. Clues as to what created the enigmatic feature are accumulating, including that Titan's air appears to be sinking in the center and rising around the edges. Winter, however, is slowly descending on the south of Titan, so that the vortex, if it survives, will be plunged into darkness over the next few years."[52]

## Uranus

Uranus's southern hemisphere in approximate natural colour (left) and in shorter wavelengths (right), shows its faint cloud bands and atmospheric "hood" as seen by Voyager 2. Credit: NASA.

Uranus's atmosphere [has a] primary composition of hydrogen and helium [and] contains more "ices" such as water, ammonia, and methane, along with traces of hydrocarbons.[53] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[54] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[53][55] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[53] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.[53][56][57] The mixing ratio is much lower in the upper atmosphere owing to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[58] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. ... Along with methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[59] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).[58][60][61] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.[60][61][62]

In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[63] Their boundary is located at about -45 degrees of latitude. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.[63][64] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[65] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[63] In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[64] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[64] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of latitude.[66]

There are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[67] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.[68] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.[67] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[67] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[65] while the northern hemisphere demonstrated increasing activity,[69] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[70] This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.[66]

### Uranian astrognosy

The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[53] The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.[71][57] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude.[53][72] The tropopause region is responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.[72][55]

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere.[73] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,[74] which form in this part of the atmosphere as a result of methane photolysis.[59] Heat is also conducted from the hot thermosphere.[74] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300 km corresponding to a pressure range of 10 to 0.1 mbar (1000 to 10 kPa) and temperatures of between 75 and 170 K.[58][60] The most abundant hydrocarbons are methane, acetylene and ethane with mixing ratios of around 10−7 relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.[58][60][62] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.[60] The abundance ratio of water is around 7×109.[61] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10 mBar level) forming haze layers,[59] which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.[58][75]

The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K.[53][75] The heat sources necessary to sustain such a high value are not understood, since neither solar far UV and extreme UV radiation nor auroral activity can provide the necessary energy. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure level may contribute too.[73][75] In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass together with the high temperatures explain why the corona extends as far as 50 000 km or two Uranian radii from the planet.[73][75] This extended corona is a unique feature of Uranus.[75] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.[73] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.[57] Observations show that the ionosphere occupies altitudes from 2 000 to 10 000 km.[57] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.[75][76] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[77] Auroral activity is insignificant as compared to Jupiter and Saturn.[75][78]

It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.[53][55]

## Neptune

Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia, and methane. Traces of methane in the outermost regions in part account for the planet's blue appearance.[79]

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.[80] A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,[81] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.[79]

Neptune's atmosphere is sub-divided into two main regions; the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars (10 kPa).[53] The stratosphere then gives way to the thermosphere at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa).[53] The thermosphere gradually transitions to the exosphere.

Neptune's thermosphere is at an anomalously high temperature of about 750 K.[82][75]

In 2007 it was discovered that the upper troposphere of Neptune's south pole was about 10 °C warmer than the rest of Neptune, which averages approximately -200 °C (70 K).[83] The warmth differential is enough to let methane, which elsewhere lies frozen in Neptune's upper atmosphere, leak out as gas through the south pole and into space. The relative "hot spot" is due to Neptune's axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.[84]

## Stellar atmospheres

"In all the stars analysed ... it is very unlikely that the lithium observed in the atmosphere was produced, even in part, inside the stars themselves, and then transported up into the atmospheres."[85]

## Physics

"It is the speed of the dominant group that remains because of the self-canceling rapid oscillations away from the point of stationary phase."[86]

## Temperature

Def. a "measure of cold or heat ... [a] property of macroscopic amounts of matter that serves to gauge the average intensity of the random actual motions of the individually mobile particulate constituents"[87] is called temperature.

## Aerology

Def. "the branch of meteorology involving the observation of the atmosphere by means of balloons, airplanes, etc"[88] is called aerology.

## Aeronomy

The composite shows upper atmospheric lightning and electrical discharge phenomena. Credit: Abestrobi.

Def. "the atmosphere of the Earth and the other planets with reference to their chemical composition, physical properties, relative motion, and responses to radiation from space"[89] is called aeronomy.

Aeronomy is the study of the upper layers of the atmosphere, where dissociation and ionization are important.

Aeronomy is the science of the upper region of the atmosphere, where dissociation and ionization are important.[90]

## Climatology

Climatology is represented by this global warming map. Credit: Robert A. Rohde.

Climatology "is the study of climate, scientifically defined as weather conditions averaged over a period of time,[91] and is a branch of the atmospheric sciences.

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and numerous other meteorological elements in a given region over long periods of time.

## Astrognosy

This graph shows the relationship of the atmosphere and ionosphere to electron density. Credit: Bhamer, updated to SVG by tiZom.

Def. the "lower levels of the atmosphere extending from [a solid or liquid] surface up to the tropopause ... characterized by convective air movements and a large vertical temperature change"[92] is called a troposphere.

Def. the "zone of transition between the troposphere and the stratosphere"[93] is called a tropopause.

Def. the "region ... where temperature increases along with the altitude ... [extending] from the tropopause ... to ... the mesosphere"[94] is called a stratosphere.

Def. a "layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere"[95] is called a mesosphere.

Def. in "the atmosphere, the boundary between the mesosphere and the ionosphere,[96] [...] where the temperature reaches its minimum value"[97] is called a mesopause.

Def. a "layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere"[98] is called a thermosphere.

### Ionospheres

In this diagram, the prominent features in the ionosphere-thermosphere system and their coupling to the different energy inputs show the complex temporal and spatial phenomena that are generated. Credit: NASA.

Upon reaching the top of the mesosphere, the temperature starts to rise, but air pressure continues to fall. This is the beginning of the ionosphere, a region dominated by chemical ions. Many of them are the same chemicals such as nitrogen and oxygen in the atmosphere below, but an ever increasing number are hydrogen ions (protons) and helium ions. These can be detected by an ion spectrometer. The process of ionization removes one or more electrons from a neutral atom to yield a variety of ions depending on the chemical element species and incidence of sufficient energy to remove the electrons.

Def. the "part of the Earth's atmosphere beginning at an altitude of about 50 kilometers [31 miles] and extending outward 500 kilometers [310 miles] or more"[99] or the "similar region of the atmosphere of another planet"[100] is called an ionosphere.

"As a spacecraft travels through the solar system, a targeted radio signal sent back to Earth can be aimed through the ionosphere of a nearby planet. Plasma in the ionosphere causes small but detectable changes in the signal that allow scientists to learn about the upper atmosphere."[101]

### Exospheres

Def. "the uppermost layer of a planet's atmosphere"[102] or "an extremely thin atmosphere""[103] is called an exosphere.

Into the exosphere or outer space, temperature rises from around 1,500°C (centigrade) to upwards of 100,000 K (kelvin).

### Meteorology

Meteorology is represented by cyclone Catarina. Credit: NASA.

Def. "the study of the atmosphere and its phenomena, especially with weather and weather forecasting"[104] is called meteorology.

Def. the "study of the theoretical effects of astronomical bodies and forces on the Earth’s atmosphere"[105] and on the atmosphere of other astronomical objects is called astrometeorology.

Def. a "system of winds rotating around a center of low atmospheric pressure ... [t]he more or less violent small-scale circulations such as tornadoes, waterspouts, and dust devils"[106] is called a cyclone.

## Technology

The image provides a view of Atlantis's payload bay for the Atmospheric Laboratory for Applications and Science (ATLAS-1). Credit: NASA.

The Atmospheric Laboratory for Applications and Science (ATLAS-1) [(April 2, 1992) is] on Spacelab pallets mounted in orbiter's cargo bay. The non-deployable payload, equipped with 12 instruments from the United States, France, Germany, Belgium, Switzerland, The Netherlands and Japan, conducted studies in atmospheric chemistry, solar radiation, space plasma physics and ultraviolet astronomy. ATLAS-1 instruments were: Atmospheric Trace Molecule Spectroscopy (ATMOS); Grille Spectrometer; Millimeter Wave Atmospheric Sounder (MAS); Imaging Spectrometric Observatory (ISO); Atmospheric Lyman-Alpha Emissions (ALAE); Atmospheric Emissions Photometric Imager (AEPI); Space Experiments with Particle Accelerators (SEPAC); Active Cavity Radiometer (ACR); Measurement of Solar Constant (SOLCON); Solar Spectrum (SOLSPEC); Solar Ultraviolet Spectral Irradiance Monitor (SUSIM); and Far Ultraviolet Space Telescope (FAUST). Other payloads [aboard] included [the] Shuttle Solar Backscatter Ultraviolet (SSBUV) experiment.

## Hypotheses

1. The atmosphere of Venus is that of a volcanic planet.

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