Life on Venus

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The atmosphere of Venus as viewed in ultraviolet by the Pioneer Venus Orbiter in 1979. Venuspioneeruv.jpg
The atmosphere of Venus as viewed in ultraviolet by the Pioneer Venus Orbiter in 1979.

The possibility of life on Venus is a subject of interest in astrobiology due to its proximity and similarities to Earth. To date, no definitive proof has been found of past or present life on Venus. Theories have decreased significantly since the early 1960s, when spacecraft began studying the planet and it became clear that its environment is extreme compared to Earth's. However, there is ongoing study as to whether life could have existed on the Venusian surface before a runaway greenhouse effect took hold, and related study as to whether a relict biosphere could persist high in the modern Venusian atmosphere.


Venus's location closer to the Sun than Earth and the extreme greenhouse effect raising temperatures on the surface to nearly 735 K (462 °C; 863 °F), and the atmospheric pressure 90 times that of Earth, make water-based life as we know it unlikely on the surface of the planet. However, a few scientists have speculated that thermoacidophilic extremophile microorganisms might exist in the temperate, acidic upper layers of the Venusian atmosphere. [1] [2] [3] In September 2020 research was published showing the presence of phosphine in the planet's atmosphere, a potential biosignature. [4] [5] [6]

Surface conditions

Main article: Observations and explorations of Venus

Because Venus is completely covered in clouds, human knowledge of surface conditions was largely speculative until the space probe era. Until the mid-20th century, the surface environment of Venus was believed to be similar to Earth, hence it was widely believed that Venus could harbor life. In 1870, the British astronomer Richard A. Proctor said the existence of life on Venus was impossible near its equator, [7] but possible near its poles. Science fiction writers were free to imagine what Venus might be like until the 1960s; see Venus in fiction. Among the speculations on Venus were that it had a jungle-like environment or that it had oceans of either petroleum or carbonated water.

However, microwave observations published by C. Mayer et al. [8] in 1958 indicated a high-temperature source (600 K). Strangely, millimetre-band observations made by A. D. Kuzmin indicated much lower temperatures. [9] Two competing theories explained the unusual radio spectrum, one suggesting the high temperatures originated in the ionosphere, and another suggesting a hot planetary surface.

In 1962, Mariner 2, the first successful mission to Venus, measured the planet's temperature for the first time, and found it to be "about 500 degrees Celsius (900 degrees Fahrenheit)." [10] Since then, increasingly clear evidence from various space probes showed Venus has an extreme climate, with a greenhouse effect generating a constant temperature of about 500 °C (932 °F) on the surface. The atmosphere contains sulfuric acid clouds. In 1968, NASA reported that air pressure on the Venusian surface was 75 to 100 times that of Earth. [11] This was later revised to 92 bars, [12] almost 100 times that of Earth and similar to that of more than 1,000 m (3,300 ft) deep in Earth's oceans. In such an environment, and given the hostile characteristics of the Venusian weather, life as we know it is highly unlikely to occur.

Past habitability potential

Scientists have speculated that if liquid water existed on its surface before the runaway greenhouse effect heated the planet, microbial life may have formed on Venus, but it may no longer exist. [13] Assuming the process that delivered water to Earth was common to all the planets near the habitable zone, it has been estimated that liquid water could have existed on its surface for up to 600 million years during and shortly after the Late Heavy Bombardment, which could be enough time for simple life to form, but this figure can vary from as little as a few million years to as much as a few billion. [14] [15] [16] [17] [18] Recent studies from September 2019 concluded that Venus may have had surface water and a habitable condition for around 3 billion years and may have been in this condition until 700 to 750 million years ago. If correct, this would have been an ample amount of time for the formation of life, [19] and for microbial life to evolve to become aerial. [20]

There has been very little analysis of Venusian surface material, so it is possible that evidence of past life, if it ever existed, could be found with a probe capable of enduring Venus's current extreme surface conditions, [21] [22] although the resurfacing of the planet in the past 500 million years [23] means that it is unlikely that ancient surface rocks remain, especially those containing the mineral tremolite which, theoretically, could have encased some biosignatures. [22]

Suggested panspermia events

It has been speculated that life on Venus may have come to Earth through panspermia. "Current models indicate that Venus may have been habitable. Complex life may have evolved on the highly irradiated Venus, and transferred to Earth on asteroids. This model fits the pattern of pulses of highly developed life appearing, diversifying and going extinct with astonishing rapidity through the Cambrian and Ordovician periods, and also explains the extraordinary genetic variety which appeared over this period." [24]

Present habitability of its atmosphere

Atmospheric conditions

Although there is little possibility of existing life near the surface of Venus, the altitudes about 50 km (31 mi) above the surface have a mild temperature, and hence there are still some opinions in favor of such a possibility in the atmosphere of Venus. [25] [26] The idea was first brought forward by German physicist Heinz Haber in 1950. [27] In September 1967, Carl Sagan and Harold Morowitz published an analysis of the issue of life on Venus in the journal Nature. [21]

In the analysis of mission data from the Venera , Pioneer Venus and Magellan missions, it was discovered that carbonyl sulfide, hydrogen sulfide and sulfur dioxide were present together in the upper atmosphere. Venera also detected large amounts of toxic chlorine just below the Venusian cloud cover. [28] Carbonyl sulfide is difficult to produce inorganically, [26] but it can be produced by volcanism. [29] Sulfuric acid is produced in the upper atmosphere by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapor. [30]

Solar radiation constrains the atmospheric habitable zone to between 51 km (65 °C) and 62 km (−20 °C) altitude, within the acidic clouds. [3] It has been speculated that clouds in the atmosphere of Venus could contain chemicals that can initiate forms of biological activity. [31] [32] It has been speculated that any hypothetical microorganisms inhabiting the atmosphere, if present, could employ ultraviolet light (UV) emitted by the Sun as an energy source, which could be an explanation for the dark lines (called "unknown UV absorber") observed in the UV photographs of Venus. [33] [34] 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. [35]

Potential biomarkers

In August 2019, astronomers reported a newly discovered long-term pattern of UV light absorbance and albedo changes in the atmosphere of Venus and its weather, that is caused by "unknown absorbers" that may include unknown chemicals or even large colonies of microorganisms high up in the atmosphere. [36] [37]

In January 2020, astronomers reported evidence that suggests Venus is currently volcanically active, and the residue from such activity may be a potential source of nutrients for possible microorganisms in the Venusian atmosphere. [38] [39] [40]

Research published in September 2020 indicated the detection of phosphine (PH3) in Venus's atmosphere that was not linked to any known abiotic method of production present or possible under Venusian conditions. [4] [5] [6] It is not expected for a molecule like phosphine to persist in the Venusian atmosphere, since under the ultraviolet radiation, it will eventually react with water and carbon dioxide. PH3 is associated with anaerobic ecosystems on Earth and may indicate life on anoxic exoplanets. Related studies suggested that the detected concentration of phosphine (20 ppb) in the clouds of Venus indicated a "plausible amount of life," and further, that the typical predicted biomass densities were "several orders of magnitude lower than the average biomass density of Earth’s aerial biosphere.” [41] [42] As of 2019, no known abiotic process generates phosphine gas on terrestrial planets (as opposed to gas giants [43] ) in appreciable quantities, so detectable amounts of phosphine could indicate life. [44] [45]

See also

Possible life on other bodies of the Solar System

Related Research Articles

Astrobiology Science concerned with life in the universe

Astrobiology, formerly known as exobiology, is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and if it does, how humans can detect it.

Extraterrestrial life Hypothetical life which may occur outside of Earth and which did not originate on Earth

Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth. Such life might range from simple prokaryotes to beings with civilizations far more advanced than humanity. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as astrobiology.

Venus Second planet from the Sun in the Solar System

Venus is the second planet from the Sun. It is named after the Roman goddess of love and beauty. As the second-brightest natural object in the night sky after the Moon, Venus can cast shadows and can be, on rare occasion, visible to the naked eye in broad daylight. Venus lies within Earth's orbit, and so never appears to venture far from the Sun, either setting in the west just after dusk or rising in the east a bit before dawn. Venus orbits the Sun every 224.7 Earth days. With a rotation period of 243 Earth days, it takes longer to rotate about its axis than any other planet in the Solar System and does so in the opposite direction to all but Uranus. Venus does not have any moons, a distinction it shares only with Mercury among planets in the Solar System.

Life on Mars Scientific assessments on the microbial habitability of Mars

The possibility of life on Mars is a subject of huge interest in astrobiology due to its proximity and similarities to Earth. To date, no proof has been found of past or present life on Mars. Cumulative evidence shows that during the ancient Noachian time period, the surface environment of Mars had liquid water and may have been habitable for microorganisms. The existence of habitable conditions does not necessarily indicate the presence of life.

Circumstellar habitable zone Zone around a star where surface liquid water may exist on a planet

In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

Planetary habitability Extent to which a planet is suitable for life as we know it

Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones the only areas in which life might arise.

Observations and explorations of Venus

Observations of the planet Venus include those in antiquity, telescopic observations, and from visiting spacecraft. Spacecraft have performed various flybys, orbits, and landings on Venus, including balloon probes that floated in the atmosphere of Venus. Study of the planet is aided by its relatively close proximity to the Earth, compared to other planets, but the surface of Venus is obscured by an atmosphere opaque to visible light.

Colonization of Venus Proposed colonization of the planet Venus

The colonization of Venus has been a subject of many works of science fiction since before the dawn of spaceflight, and is still discussed from both a fictional and a scientific standpoint. However, with the discovery of Venus's extremely hostile surface environment, attention has largely shifted towards the colonization of the Moon and Mars instead, with proposals for Venus focused on colonies floating in the upper-middle atmosphere and on terraforming.

Atmosphere of Venus dense and hot, the atmosphere of planet Venus is mostly CO2 with clouds of sulfuric acid

The atmosphere of Venus is the layer of gases surrounding Venus. It is composed primarily of carbon dioxide and is much denser and hotter than that of Earth. The temperature at the surface is 740 K, and the pressure is 93 bar (9.3 MPa), roughly the pressure found 900 m (3,000 ft) underwater on Earth. The Venusian atmosphere supports opaque clouds of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible. Information about the topography has been obtained exclusively by radar imaging. Aside from carbon dioxide, the other main component is nitrogen. Other chemical compounds are present only in trace amounts.

Ocean world Type of planet with a surface completely covered by an ocean of water

An ocean world, ocean planet, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or within a subsurface ocean. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid, such as lava, ammonia or hydrocarbons like on Titan's surface.

Extraterrestrial atmosphere atmosphere of an object other than the Earth

The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus, and Pluto. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.

Habitability of natural satellites Measure of the potential of natural satellites to have environments hospitable to life

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Before the scientific search for and study of extrasolar planets, the possibility was argued through philosophy and science fiction. The mediocrity principle suggests that planets like Earth should be common in the Universe, while the Rare Earth hypothesis suggests that they are extremely rare. The thousands of exoplanetary star systems discovered so far are profoundly different from our solar system, supporting the Rare Earth hypothesis.

Kepler-22b Exoplanet orbiting around Kepler-22

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Habitability of red dwarf systems star

The habitability of red dwarf systems is presumed to be determined by a large number of factors from a variety of sources. Although the low stellar flux, high probability of tidal locking, small circumstellar habitable zones, and high stellar variation experienced by planets of red dwarf stars are impediments to their planetary habitability, the ubiquity and longevity of red dwarfs are factors which provide ample opportunity for any possibility of habitability to be realized. We study how each of the many factors, and the interactions among them, could affect habitability to learn more about the frequency and most likely locations of extraterrestrial life and intelligence.

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Superhabitable planet Hypothetical type of planet that may be better-suited for life than Earth is

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