Instruct: Given a question, retrieve relevant documents that answer the question.
Query: What is the temperature of Venus' atmosphere?
The atmosphere of Venus is known to have undergone substantial evolution over geologic time. Evidence for this includes the present remarkable contrast between Venus’ atmosphere and the Earth’s: Venus has very little water, a 95% CO atmosphere, a surface temperature of 750 K, and a surface pressure of 90 bar. The early Venusian atmosphere is thought to have undergone either a moist or runaway greenhouse heating episode to produce these conditions, and this would have included hydrodynamic escape of light gases from the upper atmosphere to deplete possibly as much as an ocean of water. Support for this scenario comes from the measured ratio of D/H in Venus' atmosphere of roughly 1.6% from the Pioneer Venus mass spectrometer, orbiting ion mass spectrometer (OIMS) data, and IR spectra of the night-side atmosphere. This large enhancement over cosmic abundances is consistent with the loss of an ocean's worth of HO over geologic time.
The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K. The heat sources necessary to sustain such a high level are not understood, as neither the solar UV nor the auroral activity can provide the necessary energy to maintain these temperatures. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure level may contribute too. In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass and high temperatures explain why the corona extends as far as , or two Uranian radii, from its surface. This extended corona is a unique feature of Uranus. Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings. The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus. Observations show that the ionosphere occupies altitudes from . The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere. The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity. Auroral activity is insignificant as compared to Jupiter and Saturn.
Neptune's more varied weather when compared to Uranus is due in part to its higher internal heating. Although Neptune lies over 50% farther from the Sun than Uranus, and receives only 40% its amount of sunlight, the two planets' surface temperatures are roughly equal. The upper regions of Neptune's troposphere reach a low temperature of . At a depth where the atmospheric pressure equals , the temperature is . Deeper inside the layers of gas, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Depending on the thermal properties of its interior, the heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is more difficult to simultaneously explain Uranus's lack of internal heat while preserving the apparent similarity between the two planets.
Several solutions have been proposed to explain the calm weather on Uranus. One proposed explanation for this dearth of cloud features is that Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low internal thermal flux. Why Uranus's heat flux is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun. Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is times the solar energy absorbed in its atmosphere. In fact, Uranus's heat flux is only  W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m². The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System, colder than Neptune.
Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun, but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is times the solar energy absorbed in its atmosphere. Uranus's heat flux is only , which is lower than the internal heat flux of Earth of about . The lowest temperature recorded in Uranus's tropopause is , making Uranus the coldest planet in the Solar System.
Using the measured stellar luminosity of Gliese 581 is of 0.013 times that of our Sun, it is possible to calculate Gliese 581c's effective temperature a.k.a. black body temperature. (note: this probably differs from its surface temperature). According to Udry's team, the effective temperature for Gliese 581c, assuming an albedo (reflectivity) such as Venus' (0.64), would be , and assuming an Earth-like albedo (0.296), then it would be , a range of temperatures which overlaps with the range that water would be liquid at a pressure of 1 atmosphere. However, the effective temperature and actual surface temperature can be very different due to the greenhouse properties of the planetary atmosphere: for example, Venus has an effective temperature of , but a surface temperature of (mainly due to a 96.5% carbon dioxide atmosphere), a difference of about .
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide and water. Although water is thought to reside deep in the atmosphere, its directly measured concentration is very low. The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three.
Jupiter has the largest planetary atmosphere in the Solar System, spanning over in altitude. Because Jupiter has no surface, the base of its atmosphere is usually considered to be the point at which atmospheric pressure is equal to .
The troposphere, stratosphere, mesosphere, thermosphere, and exosphere are the five layers which make up Earth's atmosphere. 75% of the gases in the atmosphere are located within the troposphere, the lowest layer. In all, the atmosphere is made up of about 78.0% nitrogen, 20.9% oxygen, and 0.92% argon. In addition to the nitrogen, oxygen, and argon there are small amounts of other gases including CO and water vapor. Water vapor and CO allow the earth's atmosphere to catch and hold the Sun's energy through a phenomenon called the greenhouse effect. This allows Earth's surface to be warm enough to have liquid water and support life. In addition to storing heat, the atmosphere also protects living organisms by shielding the earth's surface from cosmic rays—which are often incorrectly thought to be deflected by the magnetic field. The magnetic field—created by the internal motions of the core—produces the magnetosphere which protects Earth's atmosphere from the solar wind. As the earth is 4.5 billion years old, it would have lost its atmosphere by now if there were no protective magnetosphere.
The atmosphere of Jupiter is classified into four layers, by increasing altitude: the troposphere, stratosphere, thermosphere and exosphere. Unlike the Earth's atmosphere, Jupiter's lacks a mesosphere. Jupiter does not have a solid surface, and the lowest atmospheric layer, the troposphere, smoothly transitions into the planet's fluid interior. This is a result of having temperatures and the pressures well above those of the critical points for hydrogen and helium, meaning that there is no sharp boundary between gas and liquid phases. Hydrogen becomes a supercritical fluid at a pressure of around 12 bar.
The atmosphere contains various simple compounds such as water, methane (CH), hydrogen sulfide (HS), ammonia (NH) and phosphine (PH). Their abundances in the deep (below 10 bar) troposphere imply that the atmosphere of Jupiter is enriched in the elements carbon, nitrogen, sulfur and possibly oxygen by factor of 2–4 relative to the Sun. The noble gases argon, krypton and xenon also appear in abundance relative to solar levels (see table), while neon is scarcer. Other chemical compounds such as arsine (AsH) and germane (GeH) are present only in trace amounts. The upper atmosphere of Jupiter contains small amounts of simple hydrocarbons such as ethane, acetylene, and diacetylene, which form from methane under the influence of the solar ultraviolet radiation and charged particles coming from Jupiter's magnetosphere. The carbon dioxide, carbon monoxide and water present in the upper atmosphere are thought to originate from impacting comets, such as Shoemaker-Levy 9. The water cannot come from the troposphere because the cold tropopause acts like a cold trap, effectively preventing water from rising to the stratosphere (see Vertical structure above).
Uranus – seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the gas giants. Uranus's atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of , and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ice and rock.
History of Uranus
The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane, ammonia and water vapour—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. According to later models, suggested by study of ancient minerals, the atmosphere in the late Hadean period consisted largely of water vapour, nitrogen and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, and rapidly lost it during the Hadean period, along with the bulk of the original inert gases. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, giving it a pH of about 5.5. The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry.
The objects of the inner Solar System are composed mostly of rock, the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula. Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide, have melting points up to a few hundred kelvins. They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit. Together, gases and ices are referred to as "volatiles".
The Persian polymath Avicenna claimed to have observed Venus as a spot on the Sun. This is possible, as there was a transit on May 24, 1032, but Avicenna did not give the date of his observation, and modern scholars have questioned whether he could have observed the transit from his location at that time; he may have mistaken a sunspot for Venus. He used his transit observation to help establish that Venus was, at least sometimes, below the Sun in Ptolemaic cosmology, i.e. the sphere of Venus comes before the sphere of the Sun when moving out from the Earth in the prevailing geocentric model.
In a letter to "Nature", Veizer et al.(2000), compared the reconstruction of tropical sea surface temperatures throughout the Phanerozoic eon (the past ~550 Myr) with the variable galactic cosmic rays and concluded that their results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable.
The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of 35% during the Carboniferous period, significantly higher than today's 21%. Two main processes govern changes in the atmosphere: plants use carbon dioxide from the atmosphere, releasing oxygen and the breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which oxidizes and hence reduces the amount of oxygen in the atmosphere. However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen. The exact cause of the variation of the amount of oxygen in the atmosphere is not known. Periods with much oxygen in the atmosphere are associated with rapid development of animals. Today's atmosphere contains 21% oxygen, which is high enough for rapid development of animals.The climate of the late Precambrian showed some major glaciation events spreading over much of the earth. At this time the continents were bunched up in the Rodinia supercontinent. Massive deposits of tillites and anomalous isotopic signatures are found, which gave rise to the Snowball Earth hypothesis. As the Proterozoic Eon drew to a close, the Earth started to warm up. By the dawn of the Cambrian and the Phanerozoic, life forms were abundant in the Cambrian explosion with average global temperatures of about 22 °C.
Kepler-186f orbits its star with about 5% of the Sun's luminosity with an orbital period of 129.9 days and an orbital radius of about 0.40 times that of Earth's (compared to 0.39 AU for Mercury). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from 0.23 to 0.46 AU). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of Mars in our Solar System.
Kepler-186f's location within the habitable zone does not ensure it is habitable; this is also dependent on its atmospheric characteristics, which are unknown. However, Kepler-186f is too distant for its atmosphere to be analyzed by existing telescopes (e.g., NESSI) or next-generation instruments such as the James Webb Space Telescope. A simple climate model — in which the planet's inventory of volatiles is restricted to nitrogen, carbon dioxide and water, and clouds are not accounted for — suggests that the planet's surface temperature would be above if at least 0.5 to 5 bars of CO is present in its atmosphere, for assumed N partial pressures ranging from 10 bar to zero, respectively.
Of official weather stations which are currently in operation, Vostok is the coldest on earth in terms of mean annual temperature. However, it has been disputed that Vostok Station is the coldest known location on earth. The now inactive Plateau Station, located on the central Antarctic plateau, is believed to have recorded an average yearly temperature that was consistently lower than that of Vostok Station during the 37-month period that it was active in the late 1960s, and satellite readings have routinely detected colder temperatures in areas between Dome A and Dome F.
Another such example would be HIP 11915, which has a planetary system containing a Jupiter-like planet orbiting at a similar distance that the planet Jupiter does in the Solar System. To strengthen the similarities, the star is class G5V, has a temperature of 5750 K, has a sun-like mass and radius, and is only 500 million years younger than the Sun. As such, the habitable zone would extend in the same area as the zone in the Solar System, around 1 AU. This would allow an Earth-like planet to exist around 1 AU.
In fact, recent studies of zircons (in the fall of 2008) found in Australian Hadean rock hold minerals that point to the existence of plate tectonics as early as 4 billion years ago. If this holds true, the previous beliefs about the Hadean period are far from correct. That is, rather than a hot, molten surface and atmosphere full of carbon dioxide, Earth's surface would be very much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby reducing greenhouse effects, and leading to a much cooler surface temperature, and the formation of solid rock, and possibly even life.
The fact that Venus is located closer to the Sun than Earth, raising temperatures on the surface to nearly , the atmospheric pressure is ninety times that of Earth, and the extreme impact of the greenhouse effect, make water-based life as currently known unlikely. A few scientists have speculated that thermoacidophilic extremophile microorganisms might exist in the lower-temperature, acidic upper layers of the Venusian atmosphere. The atmospheric pressure and temperature fifty kilometres above the surface are similar to those at Earth's surface. This has led to proposals to use aerostats (lighter-than-air balloons) for initial exploration and ultimately for permanent "floating cities" in the Venusian atmosphere. Among the many engineering challenges are the dangerous amounts of sulfuric acid at these heights.
In 1761, Russian polymath Mikhail Lomonosov observed an arc of light surrounding the part of Venus off the Sun's disc at the beginning of the egress phase of the transit and concluded that Venus has an atmosphere. In 1940, Rupert Wildt calculated that the amount of CO in the Venusian atmosphere would raise surface temperature above the boiling point for water. This was confirmed when Mariner 2 made radiometer measurements of the temperature in 1962. In 1967, Venera 4 confirmed that the atmosphere consisted primarily of carbon dioxide.
Venus has an extremely dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide. The mass of its atmosphere is 93 times that of Earth's, whereas the pressure at its surface is about 92 times that at Earth's—a pressure equivalent to that at a depth of nearly under Earth's oceans. The density at the surface is 65 kg/m, 6.5% that of water or 50 times as dense as Earth's atmosphere at at sea level. The -rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least . This makes Venus's surface hotter than Mercury's, which has a minimum surface temperature of and maximum surface temperature of , even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. This temperature is higher than that used for sterilization.
Despite the harsh conditions on the surface, the atmospheric pressure and temperature at about 50 km to 65 km above the surface of the planet is nearly the same as that of the Earth, making its upper atmosphere the most Earth-like area in the Solar System, even more so than the surface of Mars. Due to the similarity in pressure and temperature and the fact that breathable air (21% oxygen, 78% nitrogen) is a lifting gas on Venus in the same way that helium is a lifting gas on Earth, the upper atmosphere has been proposed as a location for both exploration and colonization.