Atmosphere of Venus

Christiaan Huygens was the first to hypothesize the existence of an atmosphere on Venus. In the Book II of Cosmotheoros, published in 1698, he writes: Decisive evidence for the atmosphere of Venus was provided by Mikhail Lomonosov, based on his observation of the transit of Venus in 1761 in a small observatory near his house in Saint Petersburg, Russia. ==Structure and composition==

Composition The atmosphere of Venus is composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide. The amount of nitrogen in the atmosphere is relatively small compared to the amount of carbon dioxide, but because the atmosphere is so much thicker than that on Earth, its total nitrogen content is roughly four times higher than Earth's, even though on Earth nitrogen makes up about 78% of the atmosphere. The atmosphere contains a range of compounds in small quantities, including some based on hydrogen, such as hydrogen chloride (HCl) and hydrogen fluoride (HF). There is carbon monoxide, water vapour and atomic oxygen as well. with the remainder being mostly bound up in water vapour and sulfuric acid (H2SO4). Strong evidence of significant hydrogen loss over the historical evolution of the planet is the very high D–H ratio measured in the Venusian atmosphere. According to some measurements, in the upper atmosphere of Venus D/H ratio is 1.5 higher than in the bulk atmosphere. On review, an interpolation error was discovered that resulted in multiple spurious spectroscopic lines, including the spectral feature of phosphine. Re-analysis of data with the fixed algorithm either do not result in the detection of the phosphine or detected it with much lower concentration of 1 ppb. Another re-analysis of archived infrared spectral measurements by the NASA Infrared Telescope Facility in 2015 did not reveal any phosphine in the Venusian atmosphere, placing an upper limit for phosphine concentration at 5 ppb—a quarter of the spectroscopic value reported in September. In 2022, no phosphine detection with an upper limit concentration of 0.8 ppb was announced for Venusian altitudes of 75–110 km. In September 2024, the preliminary analysis of the JCMT-Venus data has confirmed the existence of phosphine in the atmosphere of Venus, with the concentration 300 ppb at altitude 55 km. Further data processing is still needed to measure phosphine concentration deeper in the Venusian cloud deck. Ammonia The ammonia in the atmosphere of Venus was tentatively detected by two atmospheric probes - Venera 8 and Pioneer Venus Multiprobe, although the detection was rejected that time due to poorly characterized sensors behavior in Venusian environment and ammonia believed to be chemically unstable in the strongly oxidizing atmosphere of Venus. Troposphere , Mars, Earth (past and present). The atmosphere is divided into a number of sections depending on altitude. The densest part of the atmosphere, the troposphere, begins at the surface and extends upwards to 65 km. The winds are slow near the surface, but at the top of the troposphere the temperature and pressure reaches Earth-like levels and clouds pick up speed to 100 m/s (360 km/h). The density of the air at the surface is 65 kg/m3, that covers the entire surface of Venus. This sea of supercritical carbon dioxide transfers heat very efficiently, buffering the temperature changes between night and day (which last 56 terrestrial days). Particularly higher atmospheric pressures in Venus's past might have created an even more fluid-like layer of supercritical carbon dioxide shaping Venus's landscape; altogether, it is unclear how the supercritical environment behaves and is shaped. The large amount of CO2 in the atmosphere together with water vapour and sulfur dioxide create a strong greenhouse effect, trapping solar energy and raising the surface temperature to around 740 K (467 °C), On the night side of Venus clouds can still be found at 80 km (50 mi) above the surface. As crewed ships sent to Venus would be able to compensate for differences in temperature to a certain extent, anywhere from about 50 to 54 km or so above the surface would be the easiest altitude in which to base an exploration or colony, where the temperature would be in the crucial "liquid water" range of 273 K (0 °C) to 323 K (50 °C) and the air pressure the same as habitable regions of Earth. Its windspeeds are roughly determined by the balance of the pressure gradient and centrifugal forces in almost purely zonal flow. In contrast, the circulation in the Earth's atmosphere is governed by the geostrophic balance. The cloud motion is usually observed in the ultraviolet part of the spectrum, where the contrast between clouds is the highest. The super-rotation on Venus is differential, which means that the equatorial troposphere super-rotates more slowly than the troposphere at the midlatitudes. All winds on Venus are ultimately driven by convection. Vortices rotate with the period of about 3 days in the direction of general super-rotation of the atmosphere. A discovery of the second large "double-eyed" vortex at the south pole of Venus was made in the summer of 2006 by Venus Express, which came with no surprise. Images from the Akatsuki orbiter revealed something similar to jet stream winds in the low and middle cloud region, which extends from 45 to 60 km in altitude. The wind speed maximized near the equator. In September 2017, JAXA scientists named this phenomenon "Venusian equatorial jet". Upper atmosphere and ionosphere The mesosphere of Venus extends from 65 km to 120 km in height, and the thermosphere begins at approximately 120 km, eventually reaching the upper limit of the atmosphere (exosphere) at about 220 to 350 km. The exosphere begins when the atmosphere becomes so thin that the average number of collisions per air molecule is less than one. The mesosphere of Venus can be divided into two layers: the lower one between 62 and 73 km and the upper one between 73 and 95 km. At altitudes 90–150 km the Venusian air moves from the dayside to nightside of the planet, with upwelling over sunlit hemisphere and downwelling over dark hemisphere. The downwelling over the nightside causes adiabatic heating of the air, which forms a warm layer in the nightside mesosphere at the altitudes 90–120 km. The nightside upper mesosphere and thermosphere of Venus is also the source of non-local thermodynamic equilibrium emissions of CO2 and nitric oxide molecules, which are responsible for the low temperature of the nightside thermosphere. In 2011, the spacecraft discovered that Venus has a thin ozone layer at an altitude of 100 km. Venus has an extended ionosphere located at altitudes 120–300 km. . Induced magnetosphere Venus is known not to have a magnetic field. At the subsolar point the bow shock stands 1900 km (0.3 Rv, where Rv is the radius of Venus) above the surface of Venus. This distance was measured in 2007 near the solar activity minimum. The loss happens mainly via the magnetotail. Currently the main ion types being lost are O+, H+ and He+. The ratio of hydrogen to oxygen losses is around 2 (i.e. almost stoichiometric for water) indicating the ongoing loss of water. ==Clouds==

Venusian clouds are thick and are composed mainly (75–96%) of sulfuric acid droplets. These clouds obscure the surface of Venus from optical imaging, and reflect about 75% of the sunlight that falls on them. The cloud cover is such that it reflects more than 60% of the solar light Venus receives, leaving the surface with typical light levels of 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds". The equivalent visibility is about three kilometers, but this will likely vary with the wind conditions. Little to no solar energy could conceivably be collected by solar panels on a surface probe. In fact, due to the thick, highly reflective cloud cover, the total solar energy received by the surface of the planet is less than that of the Earth, despite its proximity to the Sun. space probe en route to Jupiter in 1990 during a Venus flyby. Smaller-scale cloud features have been emphasized and a bluish hue has been applied to show that it was taken through a violet filter. Sulfuric acid is produced in the upper atmosphere by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapour. Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and monatomic oxygen. Monatomic oxygen is highly reactive; when it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapour, another trace component of Venus's atmosphere, to yield sulfuric acid. : : : Surface level humidity is less than 0.1%. Venus's sulfuric acid rain never reaches the ground, but is evaporated by the heat before reaching the surface in a phenomenon known as virga. It is theorized that early volcanic activity released sulfur into the atmosphere and the high temperatures prevented it from being trapped into solid compounds on the surface as it was on the Earth. Besides sulfuric acid, cloud droplets can contain a wide array of sulfate salts, raising pH of droplet to 1.0 in one of scenarios explaining the sulfur dioxide measurements. In 2009, a prominent bright spot in the atmosphere was noted by an amateur astronomer and photographed by Venus Express. Its cause is currently unknown, with surface volcanism advanced as a possible explanation. Lightning The clouds of Venus may be capable of producing lightning, The Soviet Venera 9 and 10 orbiters obtained ambiguous optical and electromagnetic evidence of lightning. There have been attempts to observe lightning from the Venera 11, 12, 13, and 14 landers, however no lightning activity was recorded, but very low frequency (VLF) waves were detected during descent. The European Space Agency's Venus Express in 2007 detected whistler waves which could be attributed to lightning. Their intermittent appearance suggests a pattern associated with weather activity. According to the whistler observations, the lightning rate is at least half of that on Earth and may possibly be similar. Recent work from a Parker Solar Probe flyby indicates that the direction of the whistler waves is towards Venus rather than away, indicating a non-planetary origin. The Pioneer Venus Orbiter (PVO) was equipped with an electric field detector specifically to detect lightning and the Venera 13 and 14 missions included a radio receiver and point discharge sensor to search for thunderstorms. Other missions equipped with instruments that could search for lightning included Venera 9 which had a visible spectrometer; Pioneer which had a star sensor; and VEGA which had a photometer. Lightning could potentially contribute to atmospheric chemistry, through making the molecules present in the atmosphere (carbon dioxide, nitrogen gas, sulfur dioxide, sulfuric acid and water) dissociate into atoms and ions, which then recombine to form new molecules (carbon oxides and suboxides, sulfur oxides, oxygen, elemental sulfur, nitrogen oxides, sulfuric acid clusters, carbon soot, etc.). however, there may be the possibility that Venus lightning would be too weak to cause it. ==Possibility of life==

Due to the harsh conditions on the surface, little of the planet has been explored; in addition to the fact that life as currently understood may not necessarily be the same in other parts of the universe, the extent of the tenacity of life on Earth itself has not yet been shown. Creatures known as extremophiles exist on Earth, preferring extreme habitats. Thermophiles and hyperthermophiles thrive at temperatures reaching above the boiling point of water, acidophiles thrive at a pH level of 3 or below, polyextremophiles can survive a varied number of extreme conditions, and many other types of extremophiles exist on Earth. The surface temperature of Venus (over 450 °C) is far beyond the extremophile range, which extends only tens of degrees beyond 100 °C. However, the lower temperature of the cloud tops means that life could plausibly exist there, the same way that bacteria have been found living and reproducing in clouds on Earth. Any such bacteria living in the cloud tops, however, would have to be hyper-acidophilic, due to the concentrated sulfuric acid environment. Microbes in the thick, cloudy atmosphere could be protected from solar radiation by the sulfur compounds in the air. It has been proposed that microbes at this level could be soaking up ultraviolet light from the Sun as a source of energy, which could be a possible explanation for the "unknown UV absorber" seen as dark patches on UV images of the planet. The existence of this "unknown UV absorber" prompted Carl Sagan to publish an article in 1963 proposing the hypothesis of microorganisms in the upper atmosphere as the agent absorbing the UV light. In 2012, the abundance and vertical distribution of these unknown ultraviolet absorbers in the Venusian atmosphere have been investigated from analysis of Venus Monitoring Camera images, but their composition is still unknown. The dark patches of "unknown UV absorbers" are prominent enough to influence the weather on Venus. In 2021, it was suggested the color of "unknown UV absorber" match that of "red oil" a known substance comprising a mixed organic carbon compounds dissolved in concentrated sulfuric acid. In September 2020, research studies led by Cardiff University using the James Clerk Maxwell and ALMA radio telescopes noted the detection of phosphine in Venus's atmosphere that was not linked to any known abiotic method of production present, or possible under Venusian conditions. It is extremely hard to make, and the chemistry in the Venusian clouds should destroy the molecules before they could accumulate to the observed amounts. The phosphine was detected at heights of at least 48 km above the surface of Venus, and was detected primarily at mid-latitudes with none detected at the poles of Venus. Scientists note that the detection itself could be further verified beyond the use of multiple telescopes detecting the same signal, as the phosphine fingerprint described in the study could theoretically be a false signal introduced by the telescopes or by data processing. The detection was later suggested to be a false positive The re-analysis of ALMA dataset in April 2021 have recovered the 20 ppb phosphine signal, with signal-to-noise ratio of 5.4, ==Evolution==

Through studies of the present cloud structure and geology of the surface, combined with the fact that the luminosity of the Sun has increased by 25% since around 3.8 billion years ago, it is thought that the early environment of Venus was more like that of Earth with liquid water on the surface. At some point in the evolution of Venus, a runaway greenhouse effect occurred, leading to the current greenhouse-dominated atmosphere. The timing of this transition away from Earthlike is not known, but is estimated to have occurred around 4 billion years ago. The runaway greenhouse effect may have been caused by the evaporation of the surface water and the rise of the levels of greenhouse gases that followed. Venus's atmosphere has therefore received a great deal of attention from those studying climate change on Earth. There are no geologic forms on the planet to suggest the presence of water over the past billion years. However, there is no reason to suppose that Venus was an exception to the processes that formed Earth and gave it its water during its early history, possibly from the original rocks that formed the planet or later on from comets. The common view among research scientists is that water would have existed for about 600 million years on the surface before evaporating, though some such as David Grinspoon believe that up to 2 billion years could also be plausible. This longer timescale for the persistence of oceans is also supported by General Circulation Model simulations incorporating the thermal effects of clouds on an evolving Venusian hydrosphere. The early Earth during the Hadean eon is believed by most scientists to have had a Venus-like atmosphere, with roughly 100 bar of CO2 and a surface temperature of 230 °C, and possibly even sulfuric acid clouds, until about 4.0 billion years ago, by which time plate tectonics were in full force and together with the early water oceans, removed the CO2 and sulfur from the atmosphere. Early Venus would thus most likely have had water oceans like the Earth, but any plate tectonics would have ended when Venus lost its oceans. Its surface is estimated to be about 500 million years old, so it would not be expected to show evidence of plate tectonics. ==Observations and measurement from Earth==

measurements from Earth 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 CO2 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. this was the first opportunity to gain conclusive results in this way on the atmosphere of Venus since observation of solar transits began. This solar transit was a rare opportunity considering the lack of information on the atmosphere between 65 and 85 km. The solar transit in 2004 enabled astronomers to gather a large amount of data useful not only in determining the composition of the upper atmosphere of Venus, but also in refining techniques used in searching for extrasolar planets. The atmosphere of mostly CO2, absorbs near-infrared radiation, making it easy to observe. During the 2004 transit, the absorption in the atmosphere as a function of wavelength revealed the properties of the gases at that altitude. The Doppler shift of the gases also enabled wind patterns to be measured. A solar transit of Venus is an extremely rare event, and the last solar transit of the planet before 2004 was in 1882. The most recent solar transit was in 2012; the next one will not occur until 2117. ==Space missions==

Recent and current spaceprobes , seen by the Akatsuki mission. The Venus Express spacecraft formerly in orbit around the planet probed deeper into the atmosphere using infrared imaging spectroscopy in the 1–5 μm spectral range. Designed specifically to study the planet's climate, Akatsuki is the first meteorology satellite to orbit Venus (the first for a planet other than Earth). One of its five cameras known as the "IR2" will be able to probe the atmosphere of the planet underneath its thick clouds, in addition to its movement and distribution of trace components. With a highly eccentric orbit (periapsis altitude of 400 km and apoapsis of 310,000 km), it was able to take close-up photographs of the planet, and also confirm the presence of both active volcanoes as well as lightning. proposed by NASA's New Frontiers program Proposed missions The Venus In-Situ Explorer, proposed by NASA's New Frontiers program is a proposed probe which would aid in understanding the processes on the planet that led to climate change, as well as paving the way towards a later sample return mission. A craft called the Venus Mobile Explorer has been proposed by the Venus Exploration Analysis Group (VEXAG) to study the composition and isotopic measurements of the surface and the atmosphere, for about 90 days. The mission has not been selected for launch. After missions discovered the reality of the harsh nature of the planet's surface, attention shifted towards other targets such as Mars. There have been a number of proposed missions afterward, however, and many of these involve the little-known upper atmosphere. The Soviet Vega program in 1985 dropped two balloons into the atmosphere, but these were battery-powered and lasted for only about two Earth days each before running out of power. Since then, there has been no exploration of the upper atmosphere. In 2002, the NASA contractor Global Aerospace proposed a balloon that would be capable of staying in the upper atmosphere for hundreds of Earth days as opposed to two. A solar flyer has also been proposed by Geoffrey A. Landis in place of a balloon, In addition to this, the slightly lower gravity, high air pressure and slow rotation allowing for perpetual solar power make this part of the planet ideal for exploration. The proposed flyer would operate best at an altitude where sunlight, air pressure, and wind speed would enable it to remain in the air perpetually, with slight dips down to lower altitudes for a few hours at a time before returning to higher altitudes. As sulfuric acid in the clouds at this height is not a threat for a properly shielded craft, this so-called "solar flyer" would be able to measure the area in between 45 km and 60 km indefinitely, for however long it takes for mechanical error or unforeseen problems to cause it to fail. Landis also proposed that rovers similar to Spirit and Opportunity could possibly explore the surface, with the difference being that Venus surface rovers would be "dumb" rovers controlled by radio signals from computers located in the flyer above, only requiring parts such as motors and transistors to withstand the surface conditions, but not weaker parts involved in microelectronics that could not be made resistant to the heat, pressure and acidic conditions. Russian space science plans include the launch of the Venera-D (Venus-D) probe in 2029. The main scientific goals of the Venera-D mission are investigation of the structure and chemical composition of the atmosphere and investigation of the upper atmosphere, ionosphere, electrical activity, magnetosphere, and escape rate. It has been proposed to fly together with Venera-D an inflatable aircraft designed by Northrop Grumman, called Venus Atmospheric Maneuverable Platform (VAMP). The High Altitude Venus Operational Concept (HAVOC) is a NASA concept for a crewed exploration of Venus. Rather than traditional landings, it would send crews into the upper atmosphere, using dirigibles. Other proposals from the late 2010s include VERITAS, Venus Origins Explorer, VISAGE, and VICI. In June 2018, NASA also awarded a contract to Black Swift Technologies for a concept study of a Venus glider that would exploit wind shear for lift and speed. probe's descent stages through Venus's atmosphere In June 2021, NASA selected the DAVINCI+ mission to send an atmospheric probe to Venus in the late 2020s. DAVINCI+ will measure the composition of Venus's atmosphere to understand how it formed and evolved, as well as determine whether the planet ever had an ocean. The mission consists of a descent sphere that will plunge through the planet's thick atmosphere, measuring noble gases and other elements to understand Venus's climate change. This will be the first U.S.-led mission to Venus's atmosphere since 1978. == See also ==