Atmosphere of Venus
The atmosphere of Venus is the very dense layer of gases surrounding the planet Venus. Venus's atmosphere is composed of 96.5% carbon dioxide and 3.5% nitrogen, with other chemical compounds present only in trace amounts. It is much denser and hotter than that of Earth; the temperature at the surface is 740 K, and the pressure is, roughly the pressure found under water on Earth. The atmosphere of Venus supports decks of opaque clouds of sulfuric acid that cover the entire planet, preventing, until recently, optical Earth-based and orbital observation of the surface. Information about surface topography was originally obtained exclusively by radar imaging. However, the Parker Solar Probe was able to capture images of the surface using IR and nearby visible light frequencies, confirming the topography.
Aside from the very surface layers, the atmosphere is in a state of vigorous circulation. The upper layer of troposphere exhibits a phenomenon of super-rotation, in which the atmosphere circles the planet in just four Earth days, much faster than the planet's sidereal day of 243 days. The winds supporting super-rotation blow at a speed of 100 m/s or more. Winds move at up to 60 times the speed of the planet's rotation, while Earth's fastest winds are only 10% to 20% rotation speed. However, wind speed decreases with decreasing elevation to less than 2.8 m/s on the surface. Near the poles are anticyclonic structures called polar vortices. Each vortex is double-eyed and shows a characteristic S-shaped pattern of clouds. Above there is an intermediate layer of mesosphere which separates the troposphere from the thermosphere. The thermosphere is also characterized by strong circulation, but very different in its nature—the gases heated and partially ionized by sunlight in the sunlit hemisphere migrate to the dark hemisphere where they recombine and downwell.
Unlike Earth, Venus lacks a magnetic field. Its ionosphere separates the atmosphere from outer space and the solar wind. This ionized layer excludes the solar magnetic field, giving Venus a distinct magnetic environment. This is considered Venus's induced magnetosphere. Lighter gases, including water vapour, are continuously blown away by the solar wind through the induced magnetotail. It is speculated that the atmosphere of Venus up to around 4 billion years ago was more like that of the Earth with liquid water on the surface. A runaway greenhouse effect may have been caused by the evaporation of the surface water and subsequent rise of the levels of other greenhouse gases.
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 are 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 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.
History
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.
File:USSR Venera 9 1975 Venus ground colorized by Don P. Mitchell.png|thumb|Colourized image, the colour of the Venusian sky is at the surface orange-yellow due to rayleigh scattering or a blue absorber in the lower atmosphere, being white at higher altitudes, while the surface itself is rather black.
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 and hydrogen fluoride. There is carbon monoxide, water vapour and atomic oxygen as well. Hydrogen is in relatively short supply in the Venusian atmosphere. A large amount of the planet's hydrogen is theorised to have been lost to space, with the remainder being mostly bound up in water vapour and sulfuric acid. 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. The ratio is about 0.015–0.025, which is 100–150 times higher than the terrestrial value of 1.6. According to some measurements, in the upper atmosphere of Venus D/H ratio is 1.5 higher than in the bulk atmosphere.
Phosphine
In 2020, there was considerable discussion regarding whether phosphine might be present in trace amounts in Venus's atmosphere. This would be noteworthy as phosphine is a potential biomarker indicating the presence of life. This was prompted by an announcement in September 2020 that this compound had been detected in trace amounts. No known abiotic source present on Venus could produce phosphine in the quantities detected. 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.The announcement promoted re-analysis of Pioneer Venus data which found part of chlorine and all of hydrogen sulfide spectral features are instead phosphine-related, meaning lower than thought concentration of chlorine and non-detection of hydrogen sulfide. 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
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.The atmospheric pressure at the surface of Venus is about 92 times that of the Earth, similar to the pressure found below the surface of the ocean. The atmosphere has a mass of 4.8 kg, about 93 times the mass of the Earth's total atmosphere. The density of the air at the surface is 65 kg/m3, which is 6.5% that of liquid water on Earth. The pressure found on Venus's surface is high enough that the carbon dioxide is technically no longer a gas, but a supercritical fluid. This supercritical carbon dioxide forms a kind of sea, with 6.5% the density of water, 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. 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, hotter than any other terrestrial planet in the Solar System, even that of Mercury despite being located farther out from the Sun and receiving only 25% of the solar energy Mercury does. The average temperature on the surface is above the melting points of lead, tin, and zinc. The thick troposphere also makes the difference in temperature between the day and night side small, even though the slow retrograde rotation of the planet causes a single solar day to last 116.5 Earth days. The surface of Venus spends 58.3 days in darkness before the sun rises again behind the clouds.
The troposphere on Venus contains 99% of the atmosphere by mass. 90% of the atmosphere of Venus is within 28 km of the surface; by comparison, 90% of the atmosphere of Earth is within 16 km of the surface. At a height of 50 km the atmospheric pressure is approximately equal to that at the surface of Earth. On the night side of Venus clouds can still be found at 80 km above the surface.
The altitude of the troposphere most similar to Earth is near the tropopause—the boundary between troposphere and mesosphere. It is located slightly above 50 km. According to measurements by the Magellan and Venus Express probes, the altitude from 52.5 to 54 km has a temperature between 293 K and 310 K, and the altitude at 49.5 km above the surface is where the pressure becomes the same as Earth at sea level. 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 to 323 K and the air pressure the same as habitable regions of Earth. As CO2 is heavier than air, the colony's air could keep the structure floating at that altitude like a dirigible.