1.
Extraterrestrial atmosphere
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The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earths atmosphere. In addition to Earth, many of the other objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus, several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere, due to its small size, Mercury has no substantial atmosphere. Its extremely thin atmosphere mostly consists of an amount of helium and traces of sodium, potassium. These gases derive from the wind, radioactive decay, meteor impacts. Mercurys atmosphere is not stable and is constantly being refreshed because of its atoms escaping into space as a result of the planets heat, Venus atmosphere is mostly composed of carbon dioxide. It contains minor amounts of nitrogen and other elements, including compounds based on hydrogen, nitrogen, sulfur, carbon. The atmosphere of Venus is much hotter and denser than that of Earth, as greenhouse gases warm a lower atmosphere, they cool the upper atmosphere, leading to compact thermospheres. By some definitions, Venus has no stratosphere, the troposphere begins at the surface and extends up to an altitude of 65 kilometres. At the top of the troposphere, temperature and pressure reach Earth-like levels, winds at the surface are a few metres per second, reaching 70 m/s or more in the upper troposphere. The stratosphere and mesosphere extend from 65 km to 95 km in height, the thermosphere and exosphere begin at around 95 kilometres, eventually reaching the limit of the atmosphere at about 220 to 250 km. The air pressure at Venus surface is about 92 times that of the Earth, the enormous amount of CO2 in the atmosphere creates a strong greenhouse effect, raising the surface temperature to around 470 °C, hotter than that of any other planet in the Solar System. The Martian atmosphere is thin and composed mainly of carbon dioxide, with some nitrogen. The average surface pressure on Mars is 0. 6-0.9 kPa and this results in a much lower atmospheric thermal inertia, and as a consequence Mars is subject to strong thermal tides that can change total atmospheric pressure by up to 10%. The thin atmosphere also increases the variability of the planets temperature, Martian surface temperatures vary from lows of approximately −140 °C during the polar winters to highs of up to 20 °C in summers. The Mars Reconnaissance Orbiter, though spanning a much shorter dataset, shows no warming of planetary average temperature, MCS MY28 temperatures are an average of 0.9 and 1.7 K cooler than TES MY24 measurements. Locally and regionally, however, changes in pits in the layer of carbon dioxide at the Martian south pole observed between 1999 and 2001 suggest the south polar ice cap is shrinking
2.
Extraterrestrial skies
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In astronomy, an extraterrestrial sky is a view of outer space from the surface of a world other than Earth. The only extraterrestrial sky that has been observed and photographed by astronauts is that of the Moon. The skies of Titan, Mars, and Venus have been observed indirectly by space probes designed to land on the surface, characteristics of extraterrestrial skies appear to vary substantially due to a number of factors. An extraterrestrial atmosphere, if present, has a bearing on visible characteristics. The atmospheres density and chemical composition can contribute to differences in colour, opacity, astronomical objects may also be visible and can include natural satellites, rings, star systems and nebulas and other planetary system bodies. Because Mercury has little or no atmosphere, a view of the planets skies would be no different from viewing space from orbit, Mercury has a southern pole star, α Pictoris, a magnitude 3.2 star. It is fainter than Earths Polaris, on average, the visible diameter of the Sun on Mercury is 2.5 times as large as it appears from Earth, and its total brightness is more than 6 times as great. Because of the eccentric orbit, the Suns apparent size in the sky would vary from 2.2 times that from Earth at aphelion. Mercury has a 3,2 spin–orbit resonance and this means that although a sidereal day lasts around 58.7 Earth days, a solar day lasts around 176 Earth days. The on-sky distance over which the Sun appears to be travelling in this direction is slightly larger than the apparent size of the Sun itself. The apparent motion reverses because approximately four days prior to perihelion, at perihelion, Mercurys orbital angular velocity slightly exceeds the rotational velocity, so the Sun appears to be retrograde. Four days after perihelion, the Suns normal apparent motion resumes, because of its spin–orbit resonance, Mercury presents one of two spots of its surface to the Sun on alternate perihelia. After the Sun, the second brightest object in the Mercurian sky is Venus, the reason for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so we see only its night side. Indeed, even when Venus is brightest in the Earths sky, for a Mercurian observer, on the other hand, Venus is closest when it is in opposition to the Sun and is showing its full disk. The apparent magnitude of Venus is as bright as −7.7, the Earth and the Moon are also very prominent, their apparent magnitudes being about −5 and −1.2 respectively. The maximum apparent distance between the Earth and the Moon is about 15′, all other planets are visible just as they are on Earth, but somewhat less bright at opposition. The zodiacal light is more prominent than it is from Earth. The atmosphere of Venus is so thick that the Sun is not distinguishable in the sky
3.
Thermosphere
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The thermosphere is the layer of the Earths atmosphere directly above the mesosphere. The exosphere is above that but is a layer of the atmosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, taking its name from the Greek θερμός meaning heat, the thermosphere begins about 85 kilometres above the Earth. At these high altitudes, the atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation, temperatures are highly dependent on solar activity, and can rise to 2,000 °C. Radiation causes the particles in this layer to become electrically charged, enabling radio waves to be refracted. The highly diluted gas in this layer can reach 2,500 °C during the day. Even though the temperature is so high, one would not feel warm in the thermosphere, a normal thermometer might be significantly below 0 °C, at least at night, because the energy lost by thermal radiation would exceed the energy acquired from the atmospheric gas by direct contact. In the anacoustic zone above 160 kilometres, the density is so low that molecular interactions are too infrequent to permit the transmission of sound, the dynamics of the thermosphere are dominated by atmospheric tides, which are driven by the very significant diurnal heating. Atmospheric waves dissipate above this level because of collisions between the gas and the ionospheric plasma. The International Space Station orbits the Earth within the middle of the thermosphere and it is convenient to separate the atmospheric regions according to the two temperature minima at about 12 km altitude and at about 85 km. The density of the Earths atmosphere decreases nearly exponentially with altitude, the total mass of the atmosphere is M = ρA H ≃1 kg/cm2 within a column of one square centimeter above the ground. 80% of that mass is concentrated within the troposphere, the mass of the thermosphere above about 85 km is only 0. 002% of the total mass. Therefore, no significant energetic feedback from the thermosphere to the atmospheric regions can be expected. Turbulence causes the air within the atmospheric regions below the turbopause at about 110 km to be a mixture of gases that does not change its composition. Its mean molecular weight is 29 g/mol with molecular oxygen and nitrogen as the two dominant constituents, the lighter constituents atomic oxygen, helium, and hydrogen successively dominate above about 200 km altitude and vary with geographic location, time, and solar activity. The ratio N2/O which is a measure of the density at the ionospheric F region is highly affected by these variations. These changes follow from the diffusion of the constituents through the major gas component during dynamic processes
4.
Mesosphere
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The mesosphere is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the mesopause. In the mesosphere, temperature decreases as the altitude increases, the upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures below −143 °C. The stratosphere, mesosphere and lowest part of the thermosphere are collectively referred to as the middle atmosphere, the mesopause, at an altitude of 80–90 km, separates the mesosphere from the thermosphere—the second-outermost layer of the Earths atmosphere. This is also around the altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies. Above this level the atmosphere becomes non-uniform, the heights of different chemical species differ by their molecular masses. Within the mesosphere, temperature decreases with increasing height, due to decreasing solar heating and increasing cooling by CO2 radiative emission, the top of the mesosphere, called the mesopause, is the coldest part of Earths atmosphere. Temperatures in the mesosphere fall as low as −101 °C, varying according to latitude. The main dynamic features in this region are strong winds, atmospheric tides, internal atmospheric gravity waves. Most of these tides and waves start in the troposphere and lower stratosphere, in the mesosphere, gravity-wave amplitudes can become so large that the waves become unstable and dissipate. This dissipation deposits momentum into the mesosphere and largely drives global circulation, noctilucent clouds are located in the mesosphere. The upper mesosphere is also the region of the known as the D layer. The D layer is present during the day, when some ionization occurs with nitric oxide being ionized by Lyman series-alpha hydrogen radiation. The ionization is so weak that when night falls, and the source of ionization is removed, the mesosphere has been called the ignorosphere because it is poorly studied relative to the stratosphere and the thermosphere. A5 km deep sodium layer is located between 80–105 km, made of unbound, non-ionized atoms of sodium, the sodium layer radiates weakly to contribute to the airglow. The sodium has a concentration of 400,000 atoms per cubic centimeter. This band is regularly replenished by sodium sublimating from incoming meteors, astronomers have begun utilizing this sodium band to create guide stars as part of the adaptive optical correction process used to produce ultra-sharp ground-based observations. Millions of meteors enter the Earths atmosphere, averaging 40 tons per year, the mesosphere lies above altitude records for aircraft and balloons, and below the minimum altitude for orbital spacecraft. It has only been accessed through the use of sounding rockets, as a result, it is the least-understood part of the atmosphere
5.
Stratosphere
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The stratosphere is the second major layer of Earths atmosphere, just above the troposphere, and below the mesosphere. About 20% of the mass is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and cooler layers closer to the Earth, the increase of temperature with altitude is a result of the absorption of the Suns ultraviolet radiation by the ozone. This is in contrast to the troposphere, near the Earths surface, the border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the starts at 18 km, at mid latitudes, it starts at 10–13 km and ends at 50 km, at the poles. Temperatures vary within the stratosphere with the seasons, in particular with the polar night, the greatest variation of temperature takes place over the poles in the lower stratosphere, those variations are largely steady at lower latitudes and higher altitudes. The mechanism describing the formation of the layer was described by British mathematician Sydney Chapman in 1930. Molecular oxygen absorbs high energy sunlight in the UV-C region, at shorter than about 240 nm. The oxygen atoms produced combine with oxygen to form ozone. Ozone in turn is photolysed much more rapidly than molecular oxygen as it has an absorption that occurs at longer wavelengths. Ozone photolysis produces O and O2, the oxygen atom product combines with atmospheric molecular oxygen to reform O3, releasing heat. The rapid photolysis and reformation of ozone heats the stratosphere resulting in a temperature inversion and this increase of temperature with altitude is characteristic of the stratosphere, its resistance to vertical mixing means that it is stratified. Within the stratosphere temperatures increase with altitude, the top of the stratosphere has a temperature of about 270 K, overall the attenuation of solar UV at wavelengths that damage DNA by the ozone layer allows life to exist on the surface of the planet outside of the ocean. All air entering the stratosphere must pass through the tropopause, the minimum that divides the troposphere and stratosphere. The rising air is literally freeze dried, the stratosphere is a dry place. The top of the stratosphere is called the stratopause, above which the temperature decreases with height and we now know that there are additional ozone loss mechanisms, and that these mechanisms are catalytic meaning that a small amount of the catalyst can destroy a great number of ozone molecules. The first is due to the reaction of hydroxyl radicals OH· with ozone, OH is formed by the reaction of electronically excited oxygen atoms produced by ozone photolysis, with water vapor. While the stratosphere is dry, additional water vapor is produced in situ by the oxidation of methane
6.
Troposphere
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The troposphere is the lowest portion of Earths atmosphere, and is also where nearly all weather takes place. It contains approximately 75% of the mass and 99% of the total mass of water vapor. The average depths of the troposphere are 20 km in the tropics,17 km in the mid latitudes, the lowest part of the troposphere, where friction with the Earths surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km deep depending on the landform, atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. The tropopause is a layer, where the air temperature ceases to decrease with height. Most of the phenomena we associate with day-to-day weather occur in the troposphere, by volume, dry air contains 78. 09% nitrogen,20. 95% oxygen,0. 93% argon,0. 04% carbon dioxide, and small amounts of other gases. Air also contains an amount of water vapor. The chemical composition of the troposphere is essentially uniform, with the exception of water vapor. The source of vapor is at the surface through the processes of evaporation. Thus the proportion of water vapor is normally greatest near the surface, the pressure of the atmosphere is maximum at sea level and decreases with altitude. This is because the atmosphere is nearly in hydrostatic equilibrium. The temperature of the troposphere generally decreases as altitude increases, the rate at which the temperature decreases, − d T / d z, is called the environmental lapse rate. The ELR is nothing more than the difference in temperature between the surface and the tropopause divided by the height. The reason for this difference is that the ground absorbs most of the suns energy. Meanwhile, the radiation of heat at the top of the results in the cooling of that part of the atmosphere. The ELR assumes the atmosphere is still, but as air is heated it becomes buoyant, when a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air expands, it pushes the surrounding air outward. As energy transfer to a parcel of air by way of heat is very slow, such a process is called an adiabatic process
7.
Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
8.
Planet
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The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
9.
Natural satellite
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A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan
