Instruct: Given a question, retrieve passages that can help answer the question.
Query: What is Venus' atmosphere made of?
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.
The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope.
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).
At ultraviolet and visible wavelengths, Uranus's atmosphere is bland in comparison to the other giant planets, even to Neptune, which it otherwise closely resembles. When "Voyager 2" flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet. One proposed explanation for this dearth of features is that Uranus's internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus's tropopause is , making Uranus the coldest planet in the Solar System.
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 .
Since the atmospheres, albedo and greenhouse effects of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black-body temperature of the Earth is 255.3 K (−18 °C or 0 °F ). It is the greenhouse gases that keep the Earth warmer. Venus has a black-body temperature of only 184.2 K (−89 °C or −128 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ). Though the atmosphere of Venus traps more heat than Earth's, NASA lists the black-body temperature of Venus based on the fact that Venus has an extremely high albedo (Bond albedo 0.90, Visual geometric albedo 0.67), giving it a lower black body temperature than the more absorbent (lower albedo) Earth.
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 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.
Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).
Prior to the early 1960s, the atmosphere of Venus was believed by astronomers to have an Earth-like temperature. When Venus was understood to have a thick carbon dioxide atmosphere with a consequence of a very large greenhouse effect, some scientists began to contemplate the idea of altering the atmosphere to make the surface more Earth-like. This hypothetical prospect, known as terraforming, was first proposed by Carl Sagan in 1961, as a final section of his classic article in the journal "Science" discussing the atmosphere and greenhouse effect of Venus. Sagan proposed injecting photosynthetic bacteria into the Venus atmosphere, which would convert the carbon dioxide into reduced carbon in organic form, thus reducing the carbon dioxide from the atmosphere.
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.
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.
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.
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
Ganymede (Jupiter III) is the largest and most massive moon of Jupiter and in the Solar System. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of and is 8% larger than the planet Mercury, although only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
Today, astronomers have access to a continuous record of Jupiter's atmospheric activity thanks to telescopes such as Hubble Space Telescope. These show that the atmosphere is occasionally wracked by massive disturbances, but that, overall, it is remarkably stable. The vertical motion of Jupiter's atmosphere was largely determined by the identification of trace gases by ground-based telescopes. Spectroscopic studies after the collision of Comet Shoemaker–Levy 9 gave a glimpse of the Jupiter's composition beneath the cloud tops. The presence of diatomic sulfur (S) and carbon disulfide (CS) was recorded—the first detection of either in Jupiter, and only the second detection of S in any astronomical object— together with other molecules such as ammonia (NH) and hydrogen sulfide (HS), while oxygen-bearing molecules such as sulfur dioxide were not detected, to the surprise of astronomers.
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.
Among the notable discoveries was COROT-7b, discovered in 2009 which became the first exoplanet shown to have a rock or metal-dominated composition.
The stability of the atmosphere in Earth is not a consequence of chemical equilibrium. Oxygen is a reactive compound, and should eventually combine with gases and minerals of the Earth's atmosphere and crust. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event. Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume. Traces of methane (at an amount of 100,000 tonnes produced per year) should not exist, as methane is combustible in an oxygen atmosphere.
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of . Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm, only slightly less than Earth's density of 5.515 g/cm. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm versus Earth's 4.4 g/cm.
The existence of an atmosphere on Venus was concluded by Mikhail Lomonosov on the basis of his observation of the transit of Venus of 1761 from the Imperial Academy of Sciences of St. Petersburg. He used a two-lens achromat refractor and a weak solar filter (smoked glass) and reported seeing a bump or bulge of light ("Lomonosov's arc") off the solar disc as Venus began to exit the Sun. Lomonosov attributed that effect to refraction of solar rays through an atmosphere; he also reported the appearance of a sliver around the part of Venus that had just entered the Sun's disk during the initial phase of transit. In 2012, Pasachoff and Sheehan reported, based on knowing what Venus's atmosphere would look like because of Pasachoff and Schneider's observations of the 2004 transit of Venus, that what Lomonosov reported was not Venus's atmosphere. To make a decisive test, a group of researchers carried out experimental reconstruction of Lomonosov's discovery of Venusian atmosphere with antique refractors during the transit of Venus on 5–6 June 2012. They observed the "Lomonosov's arc" and other aureole effects due to Venus's atmosphere and concluded that Lomonosov's telescope was fully adequate to the task of detecting the arc of light around Venus off the Sun's disc during ingress or egress if proper experimental techniques as described by Lomonosov in his 1761 paper are employed.
The refined density of TRAPPIST-1b, along with the planet's transmission spectrum, indicate that its atmosphere is made mainly of vaporized water. The atmosphere has likely risen the planet's surface temperature to about 750-1500 K, which would require an atmospheric pressure between 10 (10) to 10 (10,000) bar, potentially thicker than Venus's atmosphere. TRAPPIST-1b's atmosphere is likely very similar to that of Venus.
The renowned astronomer Carl Sagan proposed the planetary engineering of Venus in an article published in the journal "Science" in 1961. Sagan imagined seeding the atmosphere of Venus with algae, which would convert water, nitrogen and carbon dioxide into organic compounds. As this process removed carbon dioxide from the atmosphere, the greenhouse effect would be reduced until surface temperatures dropped to "comfortable" levels. The resulting carbon, Sagan supposed, would be incinerated by the high surface temperatures of Venus, and thus be sequestered in the form of "graphite or some involatile form of carbon" on the planet's surface. However, later discoveries about the conditions on Venus made this particular approach impossible. One problem is that the clouds of Venus are composed of a highly concentrated sulfuric acid solution. Even if atmospheric algae could thrive in the hostile environment of Venus's upper atmosphere, an even more insurmountable problem is that its atmosphere is simply far too thick—the high atmospheric pressure would result in an "atmosphere of nearly pure molecular oxygen" and cause the planet's surface to be thickly covered in fine graphite powder. This volatile combination could not be sustained through time. Any carbon that was fixed in organic form would be liberated as carbon dioxide again through combustion, "short-circuiting" the terraforming process.
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.