Future of Earth

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Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7 billion years from now

The biological and geological future of Earth can be extrapolated based on the estimated effects of several long-term influences. These include the chemistry at Earth's surface, the cooling rate of the planet's interior, gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. An uncertain factor is the influence of human technology such as climate engineering, which could cause significant changes to the planet. For example, the current Holocene extinction is being caused by technology, and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.

Over time intervals of hundreds of millions of years, random celestial events pose a global risk to the biosphere, which can result in mass extinctions. These include impacts by comets or asteroids and the possibility of a near-Earth supernova—a massive stellar explosion within a 100-light-year (31-parsec) radius of the Sun. Other large-scale geological events are more predictable. Milankovitch's theory predicts that the planet will continue to undergo glacial periods at least until the Quaternary glaciation comes to an end. These periods are caused by the variations in eccentricity, axial tilt, and precession of Earth's rotation and orbit. As part of the ongoing supercontinent cycle, plate tectonics will probably create a supercontinent in 250–350 million years. Sometime in the next 1.5–4.5 billion years, Earth's axial tilt may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.

The luminosity of the Sun will steadily increase, causing a rise in the solar radiation reaching Earth and resulting in a higher rate of weathering of silicate minerals. This will affect the carbonate–silicate cycle, which will reduce the level of carbon dioxide in the atmosphere. About 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C₃ carbon fixation photosynthesis used by trees. Some plants use the C₄ carbon fixation method to persist at carbon dioxide concentrations as low as ten parts per million. However, in the long term, plants will likely die off altogether. The extinction of plants would cause the demise of almost all animal life since plants are the base of much of the animal food chain.

In about one billion years, solar luminosity will be 10% higher, causing the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics and the entire carbon cycle will end. Then, in about 2–3 billion years, the planet's magnetic dynamo may cease, causing the magnetosphere to decay, leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in Earth's surface temperature will cause a runaway greenhouse effect, creating conditions more extreme than present-day Venus and heating Earth's surface enough to melt it. By that point, all life on Earth will be extinct. Finally, the planet will likely be absorbed by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded beyond the planet's current orbit.

Human influence

Main articles: Anthropocene, Conservation biology, Climate change, Nuclear warfare, and Human impact on the environment

Horne foundry copper smelter in Rouyn-Noranda, Canada, graphically demonstrating human-generated gaseous emissions

Humans play a key role in the biosphere, with the large human population dominating many of Earth's ecosystems. This has resulted in a widespread, ongoing mass extinction of other species during the present geological epoch, now known as the Holocene extinction. The large-scale loss of species caused by human influence since the 1950s has been called a biotic crisis, with an estimated 10% of the total species lost as of 2007. At current rates, about 30% of species are at risk of extinction in the next hundred years. The Holocene extinction event is the result of habitat destruction, the widespread distribution of invasive species, poaching, and climate change. In the present day, human activity has had a significant impact on the surface of the planet. More than a third of the land surface has been modified by human actions, and humans use about 20% of global primary production. The concentration of carbon dioxide in the atmosphere has increased by close to 50% since the start of the Industrial Revolution.

The consequences of a persistent biotic crisis have been predicted to last for at least five million years. It could result in a decline in biodiversity and homogenization of biotas, accompanied by a proliferation of species that are opportunistic, such as pests and weeds. Novel species may emerge; in particular taxa that prosper in human-dominated ecosystems may rapidly diversify into many new species. Microbes are likely to benefit from the increase in nutrient-enriched environmental niches. No new species of existing large vertebrates are likely to arise and food chains will probably be shortened.

Anti-nuclear weapons protest march in Oxford, 1980

There are multiple scenarios for known risks that can have a global impact on the planet. From the perspective of humanity, these can be subdivided into survivable risks and terminal risks. Risks that humans pose to themselves include climate change, the misuse of nanotechnology, a nuclear holocaust, warfare with a programmed superintelligence, a genetically engineered disease, or a disaster caused by a physics experiment. Similarly, several natural events may pose a doomsday threat, including a highly virulent disease, the impact of an asteroid or comet, runaway greenhouse effect, and resource depletion. There may be the possibility of an infestation by an extraterrestrial lifeform. The actual odds of these scenarios occurring are difficult if not impossible to deduce.

Should the human species become extinct, then the various features assembled by humanity will begin to decay. The largest structures have an estimated decay half-life of about 1,000 years. The last surviving structures would most likely be open-pit mines, large landfills, major highways, wide canal cuts, and earth-fill flank dams. A few massive stone monuments like the pyramids at the Giza Necropolis or the sculptures at Mount Rushmore may still survive in some form after a million years.^[a]

Cataclysmic astronomical events

The Barringer Meteorite Crater in Flagstaff, Arizona, showing evidence of the impact of celestial objects upon Earth

As the Sun orbits the Milky Way, wandering stars such as Gliese 710 may approach close enough to have a disruptive influence on the Solar System. A close stellar encounter could cause a significant reduction in the perihelion distances of comets in the Oort cloud—a hypothetical spherical region of icy bodies orbiting within half a light-year of the Sun. Such an encounter could trigger a 40-fold increase in the number of comets reaching the inner Solar System. Impacts from these comets could trigger a mass extinction of life on Earth. These disruptive encounters are estimated to occur an average of once every 45 million years. There is a 1% chance every billion years that a star will pass within 100 AU of the Sun, potentially disrupting the Solar System. The mean time for the Sun to collide with another star in the solar neighborhood is approximately 30 trillion (3×10¹³) years, which is much longer than the estimated age of the Universe, at approximately 13.8 billion years. This can be taken as an indication of the low likelihood of such an event occurring during the lifetime of the Earth. Based on results from the Gaia telescope's second data release from April 2018, an estimated 694 stars will approach the Solar System to less than 5 parsecs in the next 15 million years. Of these, 26 have a good probability to come within 1.0 parsec (3.3 light-years) and 7 within 0.5 parsecs (1.6 light-years).

The energy released from the impact of an asteroid or comet with a diameter of 5–10 km (3–6 mi) or larger is sufficient to create a global environmental disaster and cause a statistically significant increase in the number of species extinctions. Among the deleterious effects resulting from a major impact event is a cloud of fine dust ejecta blanketing the planet, blocking some direct sunlight from reaching the Earth's surface thus lowering land temperatures by about 15 °C (27 °F) within a week and halting photosynthesis for several months (similar to a nuclear winter). The mean time between major impacts is estimated to be at least 100 million years. During the last 540 million years, simulations demonstrated that such an impact rate is sufficient to cause five or six mass extinctions and 20 to 30 lower severity events. This matches the geologic record of significant extinctions during the Phanerozoic Eon. Such events can be expected to continue.

A supernova is a cataclysmic explosion of a star. Within the Milky Way galaxy, supernova explosions occur on average once every 40 years. During the history of Earth, multiple such events have likely occurred within a distance of 100 light-years; known as a near-Earth supernova. Explosions inside this distance can contaminate the planet with radioisotopes and possibly impact the biosphere. Gamma rays emitted by a supernova react with nitrogen in the atmosphere, producing nitrous oxides. These molecules cause a depletion of the ozone layer that protects the surface from ultraviolet (UV) radiation from the Sun. An increase in UV-B radiation of only 10–30% is sufficient to cause a significant impact on life; particularly to the phytoplankton that form the base of the oceanic food chain. A supernova explosion at a distance of 26 light-years will reduce the ozone column density by half. On average, a supernova explosion occurs within 32 light-years once every few hundred million years, resulting in a depletion of the ozone layer lasting several centuries. Over the next two billion years, it is predicted that there will be about 20 supernova explosions and one gamma ray burst that will have a significant impact on the planet's biosphere.

The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods. This does not significantly affect the stability of the Solar System over intervals of a few million years or less, but over billions of years, the orbits of the planets become unpredictable. Computer simulations of the Solar System's evolution over the next five billion years suggest that there is a small (less than 1%) chance that a collision could occur between Earth and either Mercury, Venus, or Mars. During the same interval, the odds that Earth will be scattered out of the Solar System by a passing star are on the order of 1 in 100,000 (0.001%). In such a scenario, the oceans would freeze solid within several million years, leaving only a few pockets of liquid water about 14 km (9 mi) underground. There is a remote chance that Earth will instead be captured by a passing binary star system, allowing the planet's biosphere to remain intact. The odds of this happening are about 1 in 3 million.

Orbit and rotation

The gravitational perturbations of the other planets in the Solar System combine to modify the orbit of Earth and the orientation of its rotation axis. These changes can influence the planetary climate. Despite such interactions, highly accurate simulations show that overall, Earth's orbit is likely to remain dynamically stable for billions of years into the future. In all 1,600 simulations, the planet's semimajor axis, eccentricity, and inclination remained nearly constant.

Glaciation

An artist's impression of ice age Earth at glacial maximum.

Historically, there have been cyclical ice ages in which glacial sheets periodically covered the higher latitudes of the continents. Ice ages may occur because of changes in ocean circulation and continentality induced by plate tectonics. The Milankovitch theory predicts that glacial periods occur during ice ages because of astronomical factors in combination with climate feedback mechanisms. The primary astronomical drivers are a higher than normal orbital eccentricity, a low axial tilt (or obliquity), and the alignment of the northern hemisphere's summer solstice with the aphelion. Each of these effects occur cyclically. For example, the eccentricity changes over time cycles of about 100,000 and 400,000 years, with the value ranging from less than 0.01 up to 0.05. This is equivalent to a change of the semiminor axis of the planet's orbit from 99.95% of the semimajor axis to 99.88%, respectively.

Earth is passing through an ice age known as the Quaternary glaciation, and is presently in the Holocene interglacial period. This period would normally be expected to end in about 25,000 years. However, the increased rate at which humans release carbon dioxide into the atmosphere may delay the onset of the next glacial period until at least 50,000–130,000 years from now. On the other hand, a global warming period of finite duration (based on the assumption that fossil fuel use will cease by the year 2200) will probably only impact the glacial period for about 5,000 years. Thus, a brief period of global warming induced by a few centuries' worth of greenhouse gas emission would only have a limited impact in the long term.

Obliquity

The rotational offset of the tidal bulge exerts a net torque on the Moon, boosting it while slowing the Earth's rotation (not to scale).

The tidal acceleration of the Moon slows the rotation rate of the Earth and increases the Earth-Moon distance. Friction effects—between the core and mantle and between the atmosphere and surface—can dissipate the Earth's rotational energy. These combined effects are expected to increase the length of the day by more than 1.5 hours over the next 250 million years, and to increase the obliquity by about a half degree. The distance to the Moon will increase by about 1.5 Earth radii during the same period.

Based on computer models, the presence of the Moon appears to stabilize the obliquity of the Earth, which may help the planet to avoid dramatic climate changes. This stability is achieved because the Moon increases the precession rate of the Earth's rotation axis, thereby avoiding resonances between the precession of the rotation and precession of the planet's orbital plane (that is, the precession motion of the ecliptic). However, as the semimajor axis of the Moon's orbit continues to increase, this stabilizing effect will diminish. At some point, perturbation effects will probably cause chaotic variations in the obliquity of the Earth, and the axial tilt may change by angles as high as 90° from the plane of the orbit. This is expected to occur between 1.5 and 4.5 billion years from now.

A high obliquity would probably result in dramatic changes in the climate and may destroy the planet's habitability. When the axial tilt of the Earth exceeds 54°, the yearly insolation at the equator is less than that at the poles. The planet could remain at an obliquity of 60° to 90° for periods as long as 10 million years.

Geodynamics

Pangaea was the last supercontinent to form before the present.

Tectonics-based events will continue to occur well into the future and the surface will be steadily reshaped by tectonic uplift, extrusions, and erosion. Mount Vesuvius can be expected to erupt about 40 times over the next 1,000 years. During the same period, about five to seven earthquakes of magnitude 8 or greater should occur along the San Andreas Fault, while about 50 events of magnitude 9 may be expected worldwide. Mauna Loa should experience about 200 eruptions over the next 1,000 years, and the Old Faithful Geyser will likely cease to operate. The Niagara Falls will continue to retreat upstream, reaching Buffalo in about 30,000–50,000 years. Supervolcano events are the most impactful geological hazards, generating over 1,000 km³ of fragmented material and covering thousands of square kilometers with ash deposits. However, they are comparatively rare, occurring on average every 100,000 years.

In 10,000 years, the post-glacial rebound of the Baltic Sea will have reduced the depth by about 90 m (300 ft). The Hudson Bay will decrease in depth by 100 m over the same period. After 100,000 years, the island of Hawaii will have shifted about 9 km (5.6 mi) to the northwest. The planet may be entering another glacial period by this time.

Continental drift

Further information: Continental drift

The theory of plate tectonics demonstrates that the continents of the Earth are moving across the surface at the rate of a few centimeters per year. This is expected to continue, causing the plates to relocate and collide. Continental drift is facilitated by two factors: the energy generated within the planet and the presence of a hydrosphere. With the loss of either of these, continental drift will come to a halt. The production of heat through radiogenic processes is sufficient to maintain mantle convection and plate subduction for at least the next 1.1 billion years.

At present, the continents of North and South America are moving westward from Africa and Europe. Researchers have produced several scenarios about how this will continue in the future. These geodynamic models can be distinguished by the subduction flux, whereby the oceanic crust moves under a continent. In the introversion model, the younger, interior, Atlantic Ocean becomes preferentially subducted and the current migration of North and South America is reversed. In the extroversion model, the older, exterior, Pacific Ocean remains preferentially subducted and North and South America migrate toward eastern Asia.

As the understanding of geodynamics improves, these models will be subject to revision. In 2008, for example, a computer simulation was used to predict that a reorganization of the mantle convection will occur over the next 100 million years, creating a new supercontinent composed of Africa, Eurasia, Australia, Antarctica and South America to form around Antarctica.

Regardless of the outcome of the continental migration, the continued subduction process causes water to be transported to the mantle. After a billion years from the present, a geophysical model gives an estimate that 27% of the current ocean mass will have been subducted. If this process were to continue unmodified into the future, the subduction and release would reach an equilibrium after 65% of the current ocean mass has been subducted.

Introversion

A rough approximation of Pangaea Ultima, one of the four models for a future supercontinent

Christopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of the Paleomap Project. In their scenario, 50 million years from now the Mediterranean Sea may vanish, and the collision between Europe and Africa will create a long mountain range extending to the current location of the Persian Gulf. Australia will merge with Indonesia, and Baja California will slide northward along the coast. New subduction zones may appear off the eastern coast of North and South America, and mountain chains will form along those coastlines. The migration of Antarctica to the north will cause all of its ice sheets to melt. This, along with the melting of the Greenland ice sheets, will raise the average ocean level by 90 m (300 ft). The inland flooding of the continents will result in climate changes.

As this scenario continues, by 100 million years from the present, the continental spreading will have reached its maximum extent and the continents will then begin to coalesce. In 250 million years, North America will collide with Africa. South America will wrap around the southern tip of Africa. The result will be the formation of a new supercontinent (sometimes called Pangaea Ultima), with the Pacific Ocean stretching across half the planet. Antarctica will reverse direction and return to the South Pole, building up a new ice cap.

Extroversion

The first scientist to extrapolate the current motions of the continents was Canadian geologist Paul F. Hoffman of Harvard University. In 1992, Hoffman predicted that the continents of North and South America would continue to advance across the Pacific Ocean, pivoting about Siberia until they begin to merge with Asia. He dubbed the resulting supercontinent, Amasia. Later, in the 1990s, Roy Livermore calculated a similar scenario. He predicted that Antarctica would start to migrate northward, and East Africa and Madagascar would move across the Indian Ocean to collide with Asia.

In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years. This marks the completion of the current supercontinent cycle, wherein the continents split apart and then rejoin each other about every 400–500 million years. Once the supercontinent is built, plate tectonics may enter a period of inactivity as the rate of subduction drops by an order of magnitude. This period of stability could cause an increase in the mantle temperature at the rate of 30–100 °C (54–180 °F) every 100 million years, which is the minimum lifetime of past supercontinents. As a consequence, volcanic activity may increase.

Supercontinent

The formation of a supercontinent can dramatically affect the environment. The collision of plates will result in mountain building, thereby shifting weather patterns. Sea levels may fall because of increased glaciation. The rate of surface weathering can rise, increasing the rate at which organic material is buried. Supercontinents can cause a drop in global temperatures and an increase in atmospheric oxygen. This, in turn, can affect the climate, further lowering temperatures. All of these changes can result in more rapid biological evolution as new niches emerge.

The formation of a supercontinent insulates the mantle. The flow of heat will be concentrated, resulting in volcanism and the flooding of large areas with basalt. Rifts will form and the supercontinent will split up once more. The planet may then experience a warming period as occurred during the Cretaceous period, which marked the split-up of the previous Pangaea supercontinent.

Solidification of the outer core

Fluid convection in the outer core produces a dynamo effect

The iron-rich core region of the Earth is divided into a 2,440 km (1,520 mi) diameter solid inner core and a 6,960 km (4,320 mi) diameter liquid outer core. The rotation of the Earth creates convective eddies in the outer core region that cause it to function as a dynamo. This generates a magnetosphere about the Earth that deflects particles from the solar wind, which prevents significant erosion of the atmosphere from sputtering. As heat from the core is transferred outward toward the mantle, the net trend is for the inner boundary of the liquid outer core region to freeze, thereby releasing thermal energy and causing the solid inner core to grow. This iron crystallization process has been ongoing for about a billion years. In the modern era, the radius of the inner core is expanding at an average rate of roughly 0.5 mm (0.02 in) per year, at the expense of the outer core. Nearly all of the energy needed to power the dynamo is being supplied by this process of inner core formation.

The inner core is expected to consume most or all of the outer core 3–4 billion years from now, resulting in an almost completely solidified core composed of iron and other heavy elements. The surviving liquid envelope will mainly consist of lighter elements that will undergo less mixing. Alternatively, if at some point plate tectonics cease, the interior will cool less efficiently, which would slow down or even stop the inner core's growth. In either case, this can result in the loss of the magnetic dynamo. Without a functioning dynamo, the magnetic field of the Earth will decay in a geologically short time period of roughly 10,000 years. The loss of the magnetosphere will cause an increase in erosion of light elements, particularly hydrogen, from the Earth's outer atmosphere into space, resulting in less favorable conditions for life.

Solar evolution

See also: Stellar evolution and Formation and evolution of the Solar System

The energy generation of the Sun is based upon thermonuclear fusion of hydrogen into helium. This occurs in the core region of the star using the proton–proton chain reaction process. Because there is no convection in the solar core, the helium concentration builds up in that region without being distributed throughout the star. The temperature at the core of the Sun is too low for nuclear fusion of helium atoms through the triple-alpha process, so these atoms do not contribute to the net energy generation that is needed to maintain hydrostatic equilibrium of the Sun.

At present, nearly half the hydrogen at the core has been consumed, with the remainder of the atoms consisting primarily of helium. As the number of hydrogen atoms per unit mass decreases, so too does their energy output provided through nuclear fusion. This results in a decrease in pressure support, which causes the core to contract until the increased density and temperature bring the core pressure into equilibrium with the layers above. The higher temperature causes the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium.

Evolution of the Sun's luminosity, radius and effective temperature compared to the present Sun. After Ribas (2010).

The result of this process has been a steady increase in the energy output of the Sun. When the Sun first became a main sequence star, it radiated only 70% of the current luminosity. The luminosity has increased in a nearly linear fashion to the present, rising by 1% every 110 million years. Likewise, in three billion years the Sun is expected to be 33% more luminous. The hydrogen fuel at the core will finally be exhausted in five billion years, when the Sun will be 67% more luminous than at present. Thereafter, the Sun will continue to burn hydrogen in a shell surrounding its core until the luminosity reaches 121% above the present value. This marks the end of the Sun's main-sequence lifetime, and thereafter it will pass through the subgiant stage and evolve into a red giant.

By this time, the collision of the Milky Way and Andromeda galaxies should be underway. Although this could result in the Solar System being ejected from the newly combined galaxy, it is considered unlikely to have any adverse effect on the Sun or its planets.

Climate impact

See also: Faint young Sun paradox and Medea hypothesis

In the far future, most of Earth's land will likely be a barren desert, like the Atacama desert in Chile.

The rate of weathering of silicate minerals will increase as rising temperatures speed chemical processes up. This, in turn, will decrease the level of carbon dioxide in the atmosphere, as reactions with silicate minerals convert carbon dioxide gas into solid carbonates. Within the next 600 million years from the present, the concentration of carbon dioxide will fall below the critical threshold needed to sustain C₃ photosynthesis: about 50 parts per million. At this point, trees and forests in their current forms will no longer be able to survive. This decline in plant life is likely to be a long-term decline rather than a sharp drop. Plant groups will likely die one by one well before the 50 parts per million level is reached. The first plants to disappear will be C₃ herbaceous plants, followed by deciduous forests, evergreen broad-leaf forests and finally evergreen conifers. However, C₄ carbon fixation can continue at much lower concentrations, down to above 10 parts per million; thus, plants using C₄ photosynthesis may be able to survive for at least 0.8 billion years and possibly as long as 1.2 billion years from now, after which rising temperatures will make the biosphere unsustainable. Researchers at Caltech have suggested that once C₃ plants die off, the lack of biological production of oxygen and nitrogen will cause a reduction in Earth's atmospheric pressure, which will counteract the temperature rise, and allow enough carbon dioxide to persist for photosynthesis to continue. This would allow life to survive up to 2 billion years from now, at which point water would be the limiting factor. Currently, C₄ plants represent about 5% of Earth's plant biomass and 1% of its known plant species. For example, about 50% of all grass species (Poaceae) use the C₄ photosynthetic pathway, as do many species in the herbaceous family Amaranthaceae.

When the carbon dioxide levels fall to the limit where photosynthesis is barely sustainable, the proportion of carbon dioxide in the atmosphere is expected to oscillate up and down. This will allow land vegetation to flourish each time the level of carbon dioxide rises due to tectonic activity and respiration from animal life; however, the long-term trend is for the plant life on land to die off altogether as most of the remaining carbon in the atmosphere becomes sequestered in the Earth. Plants—and, by extension, animals—could survive longer by evolving other strategies such as requiring less carbon dioxide for photosynthetic processes, becoming carnivorous, adapting to desiccation, or associating with fungi. These adaptations are likely to appear near the beginning of the moist greenhouse (see further).

The loss of higher plant life will result in the eventual loss of oxygen as well as ozone due to the respiration of animals, chemical reactions in the atmosphere, and volcanic eruptions. Modeling of the decline in oxygenation predicts that it may drop to 1% of the current atmospheric levels by one billion years from now. This decline will result in less attenuation of DNA-damaging UV, as well as the death of animals; the first animals to disappear would be large mammals, followed by small mammals, birds, amphibians and large fish, reptiles and small fish, and finally invertebrates.

Before this happens, it is expected that life would concentrate at refugia of lower temperatures such as high elevations where less land surface area is available, thus restricting population sizes. Smaller animals would survive better than larger ones because of lesser oxygen requirements, while birds would fare better than mammals thanks to their ability to travel large distances looking for cooler temperatures. Based on oxygen's half-life in the atmosphere, animal life would last at most 100 million years after the loss of higher plants. Some cyanobacteria and phytoplankton could outlive plants due to their tolerance for carbon dioxide levels as low as 1 ppm, and may survive for around the same time as animals before carbon dioxide becomes too depleted to support any form of photosynthesis.

In their work The Life and Death of Planet Earth, authors Peter D. Ward and Donald Brownlee have argued that some form of animal life may continue even after most of the Earth's plant life has disappeared. Ward and Brownlee use fossil evidence from the Burgess Shale in British Columbia, Canada, to determine the climate of the Cambrian Explosion, and use it to predict the climate of the future when rising global temperatures caused by a warming Sun and declining oxygen levels result in the final extinction of animal life. Initially, they expect that some insects, lizards, birds, and small mammals may persist, along with sea life; however, without oxygen replenishment by plant life, they believe that animals would probably die off from asphyxiation within a few million years. Even if sufficient oxygen were to remain in the atmosphere through the persistence of some form of photosynthesis, the steady rise in global temperature would result in a gradual loss of biodiversity.

As temperatures rise, the last of animal life will be driven toward the poles, possibly underground. They would become primarily active during the polar night, aestivating during the polar day due to the intense heat. Much of the surface would become a barren desert and life would primarily be found in the oceans. However, due to a decrease in the amount of organic matter entering the oceans from land as well as a decrease in dissolved oxygen, sea life would disappear too, following a similar path to that on Earth's surface. This process would start with the loss of freshwater species and conclude with invertebrates, particularly those that do not depend on living plants such as termites or those near hydrothermal vents such as worms of the genus Riftia. As a result of these processes, multicellular life forms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years, leaving only the prokaryotes.

An alternate scenario that may occur is that, according to a 2024 study in The Planetary Science Journal, assuming that weathering is only weakly correlated with rising temperatures, multicellular life may survive much longer, beyond 1 billion years for a while. In this scenario, silicate weathering does not increase fast enough to deplete atmospheric carbon dioxide. Such levels would only fall below the threshold for plants utilizing C₃ photosynthesis leading to their extinction in 800 million years. However, carbon dioxide levels would remain sustainable for C₄ photosynthesis, only declining below present levels in 1.1 billion years. As such, the remaining plant life and subsequently animal life will survive for substantially longer. By 1.6 billion years from now, however, as the Earth's surface temperature rises to about 338 K (65 °C; 149 °F), the maximum tolerance temperature as documented for a symbiont of Dichanthelium lanuginosum, these processes will eventually force weathering to accelerate and will lead to carbon dioxide levels falling below the threshold for C₄ plants. The end result would be carbon dioxide starvation, oxygen depletion, and then extinction of the remaining plant and animal life in 1.86 billion years.

Loss of oceans

The atmosphere of Venus is in a "super-greenhouse" state. Earth in a few billion years could likely resemble present Venus.

One billion years from now, about 27% of the modern ocean will have been subducted into the mantle. If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the present day surface reservoir would remain at the surface. Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C; 116 °F). The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans. At this point, models of the Earth's future environment demonstrate that the stratosphere would contain increasing levels of water. These water molecules will be broken down through photodissociation by solar UV, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's seawater in about 1 to 1.5 billion years from the present, depending on the model.

There will be one of two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere while water vapor starts to accumulate in the stratosphere (if the oceans evaporate very quickly), and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere (if the oceans evaporate too slowly). In this ocean-free era, there would continue to be surface reservoirs as water is steadily released from the deep crust and mantle, which could contain an amount of water equivalent to several times that present in the Earth's oceans. Some water may be retained at the poles and there may be occasional rainstorms, but for the most part, the planet would be a desert with large dunefields covering its equator, and a few salt flats on what was once the ocean floor, similar to the ones in the Atacama Desert in Chile.

With no water to serve as a lubricant, plate tectonics would likely stop and the most visible signs of geological activity would be shield volcanoes located above mantle hotspots. In these arid conditions the planet may retain some microbial and possibly even multicellular life. Most of these microbes will be halophiles and life could find refuge in the atmosphere as has been proposed to have happened on Venus. However, the increasingly extreme conditions will likely lead to the extinction of the prokaryotes between 1.6 billion years and 2.8 billion years from now, with the last of them living in residual ponds of water at high latitudes and heights or in caverns with trapped ice. However, underground life could last longer.

What proceeds after this depends on the level of tectonic activity. A steady release of carbon dioxide by volcanic eruption could cause the atmosphere to enter a "super-greenhouse" state like that of the planet Venus. But, as stated above, without surface water, plate tectonics would probably come to a halt and most of the carbonates would remain securely buried until the Sun becomes a red giant and its increased luminosity heats the rock to the point of releasing the carbon dioxide. However, as pointed out by Peter Ward and Donald Brownlee in their book The Life and Death of Planet Earth, according to NASA Ames scientist Kevin Zahnle, it is highly possible that plate tectonics may stop long before the loss of the oceans, due to the gradual cooling of the Earth's core, which could happen in just 500 million years. This could potentially turn the Earth back into a water world, and even perhaps drowning all remaining land life.

The loss of the oceans could be delayed until 2 billion years in the future if the atmospheric pressure were to decline. A lower atmospheric pressure would reduce the greenhouse effect, thereby lowering the surface temperature. This could occur if natural processes were to remove the nitrogen from the atmosphere. Studies of organic sediments have shown that at least 100 kilopascals (0.99 atm) of nitrogen has been removed from the atmosphere over the past four billion years, which is enough to effectively double the current atmospheric pressure if it were to be released. This rate of removal would be sufficient to counter the effects of increasing solar luminosity for the next two billion years.

By 2.8 billion years from now, the surface temperature of the Earth will have reached 422 K (149 °C; 300 °F), even at the poles. At this point, any remaining life will be extinguished due to the extreme conditions. What happens beyond this depends on how much water is left on the surface. If all of the water on Earth has evaporated by this point (via the "moist greenhouse" at ~1 Gyr from now), the planet will stay in the same conditions with a steady increase in the surface temperature until the Sun becomes a red giant. If not and there are still pockets of water left, and they evaporate too slowly, then in about 3–4 billion years, once the amount of water vapor in the lower atmosphere rises to 40%, and the luminosity from the Sun reaches 35–40% more than its present-day value, a "runaway greenhouse" effect will ensue, causing the atmosphere to warm and raising the surface temperature to around 1,600 K (1,330 °C; 2,420 °F). This is sufficient to melt the surface of the planet. However, most of the atmosphere is expected to be retained until the Sun has entered the red giant stage.

With the extinction of life, 2.8 billion years from now, it is expected that Earth's biosignatures will disappear, to be replaced by signatures caused by non-biological processes.

Red giant stage

The size of the current Sun (now in the main sequence) compared to its estimated size during its red giant phase

Once the Sun changes from burning hydrogen within its core to burning hydrogen in a shell around its core, the core will start to contract, and the outer envelope will expand. The total luminosity will steadily increase over the following billion years until it reaches 2,730 times its current luminosity at the age of 12.167 billion years. Most of Earth's atmosphere will be lost to space. Its surface will consist of a lava ocean with floating continents of metals and metal oxides and icebergs of refractory materials, with its surface temperature reaching more than 2,400 K (2,130 °C; 3,860 °F). The Sun will experience more rapid mass loss, with about 33% of its total mass shed with the solar wind. The loss of mass will mean that the orbits of the planets will expand. The orbital distance of Earth will increase to at most 150% of its current value (that is, 1.5 AU (220 million km; 140 million mi)).

The most rapid part of the Sun's expansion into a red giant will occur during the final stages, when the Sun will be about 12 billion years old. It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180 million km; 110 million mi). Earth will interact tidally with the Sun's outer atmosphere, which would decrease Earth's orbital radius. Drag from the chromosphere of the Sun would reduce Earth's orbit. These effects will counterbalance the impact of mass loss by the Sun, and the Sun will likely engulf Earth in about 7.59 billion years from now.

The drag from the solar atmosphere may cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross Earth's Roche limit, meaning that tidal interaction with Earth would break apart the Moon, turning it into a ring system. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow the Earth, the planet may be left moonless.

The ablation and vaporization caused by Earth's fall on a decaying trajectory towards the Sun may remove Earth's mantle, leaving just the core, which will finally be destroyed after at most 200 years. Earth's sole legacy will be a very slight increase (0.01%) of the solar metallicity following this event.

Beyond and ultimate fate

The Helix Nebula, a planetary nebula similar to what the Sun will produce in 8 billion years

After fusing helium in its core to carbon, the Sun will begin to collapse again, evolving into a compact white dwarf star after ejecting its outer atmosphere as a planetary nebula. The predicted final mass is 54% of the present value, most likely consisting primarily of carbon and oxygen.

Currently, the Moon is moving away from Earth at a rate of 4 cm (1.6 inches) per year. In 50 billion years, if the Earth and Moon are not engulfed by the Sun, they will become tidally locked into a larger, stable orbit, with each showing only one face to the other. Thereafter, the tidal action of the Sun will extract angular momentum from the system, causing the orbit of the Moon to decay and the Earth's rotation to accelerate. In about 65 billion years, it is estimated that the Moon may collide with the Earth, due to the remaining energy of the Earth–Moon system being sapped by the remnant Sun, causing the Moon to slowly move inwards toward the Earth.

Beyond this point, the ultimate fate of the Earth (if it survives) depends on what happens. On a time scale of 10¹⁵ (1 quadrillion) years the remaining planets in the Solar System will be ejected from the system by close encounters with other stellar remnants, and Earth will continue to orbit through the galaxy for around 10¹⁹ (10 quintillion) years before it is ejected or falls into a supermassive black hole. If Earth is not ejected during a stellar encounter, then its orbit will decay via gravitational radiation until it collides with the Sun in 10²⁰ (100 quintillion) years. If proton decay can occur and Earth is ejected to intergalactic space, then it will last around 10³⁸ (100 undecillion) years before evaporating into radiation.

Atmosphere of Venus

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Atmosphere_of_Venus 

 

Atmosphere of Venus

Cloud structure in Venus's atmosphere revealed by ultraviolet observations

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 (467 °C, 872 °F), and the pressure is 93 bar (9.3 MPa; 1,350 psi), roughly the pressure found 900 m (3,000 ft) 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 (≈360 km/h or 220 mph) 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 (≈10 km/h or 6.2 mph) 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 (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.

History

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:

I have often wonder’d that when I have viewed Venus at her nearest to the Earth, when she resembled an Half-moon, just beginning to have something like Horns, through a Telescope of 45 or 60 Foot long, she always appeared to me all over equally lucid, that I can’t say I observ’d so much as one spot in her, tho in Jupiter and Mars, which seem much less to us, they are very plainly perceived. For if Venus had any such thing as Sea and Land, the former must necessarily show much more obscure than the other, as anyone may satisfy himself, that from a very high Mountain will but look down upon our Earth. I thought that perhaps the too brisk Light of Venus might be the occasion of this equal appearance; but when I used an Eye-glass that was smok’d for the purpose, it was still the same thing. What then, must Venus have no Sea, or do the Waters there reflect the Light more than ours do, or their Land less? or rather (which is most probable in my opinion) is not all that Light we see reflected from an Atmosphere surrounding Venus, which being thicker and more solid than that in Mars or Jupiter, hinders our seeing any thing of the Globe it self, and is at the same time capable of sending back the Rays that it receives from the Sun?

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.

Colourized image (Venera 9, 1975), 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

Composition of the atmosphere of Venus. The chart on the right is an expanded view of the trace elements that all together do not even make up a tenth of a percent.

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. 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 (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. The ratio is about 0.015–0.025, which is 100–150 times higher than the terrestrial value of 1.6×10⁻⁴. According to some measurements, in the upper atmosphere of Venus D/H ratio is 1.5 higher than in the bulk atmosphere.

Atmospheric composition

Absorption spectrum of a simple gas mixture corresponding to Earth's atmosphere

The composition of the atmosphere of Venus based on HITRAN data created using HITRAN on the Web system.

Green colour—water vapour, red—carbon dioxide, WN—wavenumber (other colours have different meanings, shorter wavelengths on the right, longer on the left).

Phosphine

In 2020, there was considerable discussion regarding whether phosphine (PH₃) 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

Comparison of Atmosphere Compositions – Venus, 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).

1761 drawing by Mikhail Lomonosov in his work on the discovery of atmosphere of Venus

The atmospheric pressure at the surface of Venus is about 92 times that of the Earth, similar to the pressure found 900 m (3,000 ft) below the surface of the ocean. The atmosphere has a mass of 4.8×10²⁰ 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 (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), 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 (per unit area) Mercury does. The average temperature on the surface is above the melting points of lead (600 K, 327 °C), tin (505 K, 232 °C), and zinc (693 K, 420 °C). 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.

Atmosphere

Height (km)	Temp. (°C)	Atmospheric pressure (atm)
0	462	92.10
5	424	66.65
10	385	47.39
15	348	33.04
20	306	22.52
25	264	14.93
30	222	9.851
35	180	5.917
40	143	3.501
45	110	1.979
50	75	1.066
55	27	0.531 4
60	−10	0.235 7
65	−30	0.097 65
70	−43	0.036 90
80	−76	0.004 760
90	−104	0.000 373 6
100	−112	0.000 026 60

The troposphere on Venus contains 99% of the atmosphere by mass. 90% of the atmosphere of Venus is within 28 km (17.5 mi) of the surface; by comparison, 90% of the atmosphere of Earth is within 16 km (10 mi) of the surface. At a height of 50 km (31 mi) 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 (50 mi) 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 (20 °C) and 310 K (37 °C), 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 (0 °C) to 323 K (50 °C) and the air pressure the same as habitable regions of Earth. As CO₂ is heavier than air, the colony's air (nitrogen and oxygen) could keep the structure floating at that altitude like a dirigible.

Circulation

Composite image of the polar vortex of Venus in strong red (red image is an infrared view of hot high clouds and the grey image is an ultra-violet view of lower clouds)

The circulation in Venus's troposphere follows the so-called cyclostrophic flow. 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. Venus's windspeeds can be directly measured only in the upper troposphere (tropopause), between 60 and 70 km, altitude, which corresponds to the upper cloud deck. The cloud motion is usually observed in the ultraviolet part of the spectrum, where the contrast between clouds is the highest. The linear wind speeds at this level are about 100 ± 10 m/s at lower than 50° latitude. They are retrograde in the sense that they blow in the direction of the retrograde rotation of the planet. The winds quickly decrease towards the higher latitudes, eventually reaching zero at the poles. Such strong cloud-top winds cause a phenomenon known as the super-rotation of the atmosphere. In other words, these high-speed winds circle the whole planet faster than the planet itself rotates. The super-rotation on Venus is differential, which means that the equatorial troposphere super-rotates more slowly than the troposphere at the midlatitudes. The winds also have a strong vertical gradient. They decline deep in the troposphere with the rate of 3 m/s per km. The winds near the surface of Venus are much slower than that on Earth. They actually move at only a few kilometres per hour (generally less than 2 m/s and with an average of 0.3 to 1.0 m/s), but due to the high density of the atmosphere at the surface, this is still enough to transport dust and small stones across the surface, much like a slow-moving current of water.

Meridional (north-south) component of the atmospheric circulation in the atmosphere of Venus. Note that the meridional circulation is much lower than the zonal circulation, which transports heat between the day and night sides of the planet

All winds on Venus are ultimately driven by convection. Hot air rises in the equatorial zone, where solar heating is concentrated and flows to the poles. Such an almost-planetwide overturning of the troposphere is called Hadley circulation. However, the meridional air motions are much slower than zonal winds. The poleward limit of the planet-wide Hadley cell on Venus is near ±60° latitudes. Here air starts to descend and returns to the equator below the clouds. This interpretation is supported by the distribution of the carbon monoxide, which is also concentrated in the vicinity of ±60° latitudes. Poleward of the Hadley cell a different pattern of circulation is observed. In the latitude range 60°–70° cold polar collars exist. They are characterized by temperatures about 30–40 K lower than in the upper troposphere at nearby latitudes. The lower temperature is probably caused by the upwelling of the air in them and by the resulting adiabatic cooling. Such an interpretation is supported by the denser and higher clouds in the collars. The clouds lie at 70–72 km altitude in the collars—about 5 km higher than at the poles and low latitudes. A connection may exist between the cold collars and high-speed midlatitude jets in which winds blow as fast as 140 m/s. Such jets are a natural consequence of the Hadley-type circulation and should exist on Venus between 55 and 60° latitude.

Odd structures known as polar vortices lie within the cold polar collars. They are giant hurricane-like storms four times larger than their terrestrial analogs. Each vortex has two "eyes"—the centres of rotation, which are connected by distinct S-shaped cloud structures. Such double eyed structures are also called polar dipoles. Vortices rotate with the period of about 3 days in the direction of general super-rotation of the atmosphere. The linear wind speeds are 35–50 m/s near their outer edges and zero at the poles. The temperature at the cloud-tops in each polar vortex is much higher than in the nearby polar collars, reaching 250 K (−23 °C). The conventional interpretation of the polar vortices is that they are anticyclones with downwelling in the centre and upwelling in the cold polar collars. This type of circulation resembles a winter polar anticyclonic vortex on Earth, especially the one found over Antarctica. The observations in the various infrared atmospheric windows indicate that the anticyclonic circulation observed near the poles penetrates as deep as to 50 km altitude, i.e. to the base of the clouds. The polar upper troposphere and mesosphere are extremely dynamic; large bright clouds may appear and disappear over the space of a few hours. One such event was observed by Venus Express between 9 and 13 January 2007, when the south polar region became brighter by 30%. This event was probably caused by an injection of sulfur dioxide into the mesosphere, which then condensed, forming a bright haze. The two eyes in the vortices have yet to be explained.

False colour near-infrared (2.3 μm) image of the deep atmosphere of Venus obtained by Galileo, red areas are signatures of the hot surface radiating through the atmosphere relatively unhindered. The dark spots are clouds silhouetted against the very hot lower atmosphere emitting thermal infrared radiation.

The first vortex on Venus was discovered at the north pole by the Pioneer Venus mission in 1978. 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. In the first layer the temperature is nearly constant at 230 K (−43 °C). This layer coincides with the upper cloud deck. In the second layer, the temperature starts to decrease again, reaching about 165 K (−108 °C) at the altitude of 95 km, where mesopause begins. It is the coldest part of the Venusian dayside atmosphere. In the dayside mesopause, which serves as a boundary between the mesosphere and thermosphere and is located between 95 and 120 km, temperature increases to a constant—about 300–400 K (27–127 °C)—value prevalent in the thermosphere. In contrast, the nightside Venusian thermosphere is the coldest place on Venus with temperature as low as 100 K (−173 °C). It is even called a cryosphere.

The circulation patterns in the upper mesosphere and thermosphere of Venus are completely different from those in the lower atmosphere. 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 temperature of this layer—230 K (−43 °C)—is far higher than the typical temperature found in the nightside thermosphere—100 K (−173 °C). The air circulated from the dayside also carries oxygen atoms, which after recombination form excited molecules of oxygen in the long-lived singlet state (¹Δ_g), which then relax and emit infrared radiation at the wavelength 1.27 μm. This radiation from the altitude range 90–100 km is often observed from the ground and spacecraft. 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.

The Venus Express probe has shown through stellar occultation that the atmospheric haze extends much further up on the night side than the day side. On the day side the cloud deck has a thickness of 20 km and extends up to about 65 km, whereas on the night side the cloud deck in the form of a thick haze reaches up to 90 km in altitude—well into mesosphere, continuing even further to 105 km as a more transparent haze. 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. The ionosphere almost coincides with the thermosphere. The high levels of the ionization are maintained only over the dayside of the planet. Over the nightside the concentration of the electrons is almost zero. The ionosphere of Venus consists of three layers: v1 between 120 and 130 km, v2 between 140 and 160 km and v3 between 200 and 250 km. There may be an additional layer near 180 km. The maximum electron volume density (number of electrons in a unit of volume) of 3×10¹¹ m⁻³ is reached in the v2 layer near the subsolar point. The upper boundary of the ionosphere (the ionopause) is located at altitudes 220–375 km and separates the plasma of the planetary origin from that of the induced magnetosphere. The main ionic species in the v1 and v2 layers is O₂⁺ ion, whereas the v3 layer consists of O⁺ ions. The ionospheric plasma is observed to be in motion; solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.

The ionosphere of Venus and its interaction with the solar wind.

Induced magnetosphere

Venus interacts with the solar wind. Components of the induced magnetosphere are shown.

Venus is known not to have a magnetic field. The reason for its absence is not at all clear, but it may be related to a reduced intensity of convection in the Venusian mantle. Venus only has an induced magnetosphere formed by the Sun's magnetic field carried by the solar wind. This process can be understood as the field lines wrapping around an obstacle—Venus in this case. The induced magnetosphere of Venus has a bow shock, magnetosheath, magnetopause and magnetotail with the current sheet.

At the subsolar point the bow shock stands 1900 km (0.3 R_v, where R_v is the radius of Venus) above the surface of Venus. This distance was measured in 2007 near the solar activity minimum. Near the solar activity maximum it can be several times further from the planet. The magnetopause is located at the altitude of 300 km. The upper boundary of the ionosphere (ionopause) is near 250 km. Between the magnetopause and ionopause there exists a magnetic barrier—a local enhancement of the magnetic field, which prevents the solar plasma from penetrating deeper into the Venusian atmosphere, at least near solar activity minimum. The magnetic field in the barrier reaches up to 40 nT. The magnetotail continues up to ten radii from the planet. It is the most active part of the Venusian magnetosphere. There are reconnection events and particle acceleration in the tail. The energies of electrons and ions in the magnetotail are around 100 and 1000 eV respectively.

Due to the lack of the intrinsic magnetic field on Venus, the solar wind penetrates relatively deep into the planetary exosphere and causes substantial atmosphere loss. 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

Infrared image of the night-side of Venus, showing different layers of clouds at altitudes of 35 to 50 km in different colours. Red coloured clouds are the highest, green and blue are below. Carbon dioxide and monoxide areas are absorbing infrared signatures.

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 geometric albedo, a common measure of reflectivity, is the highest of any planet in the Solar System. This high reflectivity potentially enables any probe exploring the cloud tops sufficient solar energy such that solar cells can be fitted anywhere on the craft. The density of the clouds is highly variable with the densest layer at about 48.5 km, reaching 0.1 g/m³ similar to the lower range of cumulonimbus storm clouds on Earth.

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.

Photograph taken by the uncrewed Galileo 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.

CO₂ → CO + O

SO₂ + O → SO₃

2 SO₃ + 4 H₂O → 2 H₂SO₄ · H₂O

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.

Close-up view of a cloud front encircling Venus's south polar vortex in morning infrared and ultra-violet light

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, but the debate is ongoing, with volcanic lightning and sprites also under discussion. 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. However, the Venus Express findings are incompatible with data from the JAXA Akatsuki spacecraft which indicate a very low flash rate. 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.

The mechanism generating lightning on Venus, if present, remains unknown. Whilst the sulfuric acid cloud droplets can become charged, the atmosphere may be too electrically conductive for the charge to be sustained, preventing lightning.

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.). Lightning could contribute to the production of carbon monoxide and oxygen gas by converting sulfur and sulfur dioxide into sulfuric acid, and water and sulfuric dioxide to sulfur to sustain clouds. Regardless of how frequent lightning on Venus is, it is important to study as it can be a potential hazard for spacecraft.

Throughout the 1980s, it was thought that the cause of the night-side glow ("ashen light") on Venus was lightning however, there may be the possibility that Venus lightning would be too weak to cause it.

Possibility of life

Main article: Life on Venus

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.

The Venusian atmosphere has been found to be sufficiently out of equilibrium as to require further investigation. Analysis of data from the Venera, Pioneer, and Magellan missions has found hydrogen sulfide (later disputed) and sulfur dioxide (SO₂) together in the upper atmosphere, as well as carbonyl sulfide (OCS). The first two gases react with each other, implying that something must produce them. Carbonyl sulfide is difficult to produce inorganically, but it is present in the Venusian atmosphere. However, the planet's volcanism could explain the presence of carbonyl sulfide. In addition, one of the early Venera probes detected large amounts of toxic chlorine just below the Venusian cloud deck.

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. In 2016, disulfur dioxide was identified as a possible candidate for causing the so far unknown UV absorption of the Venusian atmosphere. 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 or true signal with much over-estimated amplitude, compatible with 1 ppb concentration of phosphine. The re-analysis of ALMA dataset in April 2021 have recovered the 20 ppb phosphine signal, with signal-to-noise ratio of 5.4, and by August 2021 it was confirmed the suspected contamination by sulfur dioxide was contributing only 10% to the tentative signal in phosphine spectral line band.

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

Main article: Transit of Venus

Venus transits the face of the Sun on June 8, 2004, providing valuable information on the upper atmosphere through spectroscopic 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.

The upper atmosphere of Venus can be measured from Earth when the planet crosses the sun in a rare event known as a solar transit. The last solar transit of Venus occurred in 2012. Using quantitative astronomical spectroscopy, scientists were able to analyze sunlight that passed through the planet's atmosphere to reveal chemicals within it. As the technique to analyse light to discover information about a planet's atmosphere only first showed results in 2001, 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

Main article: List of missions to Venus

Recent and current spaceprobes

This image shows Venus in ultraviolet, 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.

The JAXA probe Akatsuki (Venus Climate Orbiter), launched in May 2010, studied the planet for a period of two years, including the structure and activity of the atmosphere, but failed to enter Venus orbit in December 2010. A second attempt to achieve orbit succeeded 7 December 2015. 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.

Venus In-Situ Explorer 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, and the idea has been featured from time to time since the early 2000s. Venus has a high albedo, and reflects most of the sunlight that shines on it making the surface quite dark, the upper atmosphere at 60 km has an upward solar intensity of 90%, meaning that solar panels on both the top and the bottom of a craft could be used with nearly equal efficiency. 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.

Artist's concept of the planned DAVINCI+ 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.