The possibility of life on Venus is a subject of interest in astrobiology due to Venus's proximity and similarities to Earth. To date, no definitive evidence has been found of past or present life there. In the early 1960s, studies conducted via spacecraft demonstrated that the current Venusian environment is extreme compared to Earth's. Studies continue to question whether life could have existed on the planet's surface before a runaway greenhouse effect took hold, and whether a relict biosphere could persist high in the modern Venusian atmosphere.
With extreme surface temperatures reaching nearly and an atmospheric pressure 92 times that of Earth, the conditions on Venus make water-based life as we know it impossible 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.Redfern, Martin (25 May 2004). "Venus clouds 'might harbour life'". BBC News. Retrieved 30 December 2015. In September 2020, research was published that reported the presence of phosphine in the planet's atmosphere, a potential biosignature. However, doubts have been cast on these observations.
As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine) and Mars (via methane) was reported, though whether these gases are present is still unclear. On 2 June 2021, NASA announced two new related missions to Venus: DAVINCI and VERITAS.
Surface conditions
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,Proctor, Richard A., Other Worlds Than Ours: The Plurality of Worlds Studied Under the Light of Recent Scientific Researches. New York : J.A. Hill and Co., 1870. s. 94. but possible near its poles.
Microwave observations published by C. Mayer et al. in 1958 indicated a high-temperature source (600 K). Strangely, millimetre-band observations made by A. D. Kuzmin indicated much lower temperatures. 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)." 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 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. This was later revised to 92 bars, almost 100 times that of Earth and similar to that of more than 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.Bruce Dorminey, "Venus Likely Had Past Life; Next Step Is Finding It", Forbes, 28 March 2016. 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. A study published in 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, and for microbial life to evolve to become aerial.
Since then, there have been more studies and climate models, with different conclusions. For example, a study from 2023 concluded that only hypothetical habitable eras that ended prior to 3 billion years ago are consistent with Ar, O2, H2O, and CO concentrations in Venus’s modern atmosphere. In addition, a study from 2024 found that Venusian volcanic gases have at most a 6% water mole fraction, which is substantially drier than terrestrial magmas degassed at similar conditions, indicating that Venus has never been liquid-water habitable.
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. However, the resurfacing of the planet in the past 500 million years means that it is unlikely that ancient surface rocks remain, especially those containing the mineral tremolite which, theoretically, could have encased some biosignatures.
Studies reported on 26 October 2023 suggest Venus, for the first time, may have had plate tectonics during ancient times, and, as a result, may have had a more habitable environment, and possibly one capable of life forms. However, because dry Venusian eruptions suggest that Venus never had oceans on its surface, Venus has likely been a inhospitable world for its entire history.
Suggested panspermia events It has been speculated that life on Venus may have come to Earth through lithopanspermia, via the ejection of icy bolides that facilitated the preservation of multicellular life on long interplanetary voyages. "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." This theory, however, is a fringe one, and is seen as being unlikely.
Cataclysmic events Between 700 and 750 million years ago, a near-global resurfacing event triggered the release of carbon dioxide from rock on the planet, which transformed its climate. In addition, according to a study from researchers at the University of California, Riverside, Venus would be able to support life if Jupiter had not altered its orbit around the Sun.
Present habitability of its atmosphere Atmospheric conditions Although there is little possibility of existing life near the surface of Venus, the altitudes about 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. Venus as a Natural Laboratory for Search of Life in High Temperature Conditions: Events on the Planet on March 1, 1982 , L. V. Ksanfomality, published in Astronomicheskii Vestnik, Vol. 46, No. 1, 2012 . The idea was first brought forward by German physicist Heinz Haber in 1950. In September 1967, Carl Sagan and Harold Morowitz published an analysis of the issue of life on Venus in the journal Nature.
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. Carbonyl sulfide is difficult to produce inorganically, but it can be produced by volcanism. Sulfuric acid is produced in the upper atmosphere by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapor. The re-analysis of Pioneer Venus data in 2020 has 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.
Solar radiation constrains the atmospheric habitable zone to between 51 km (65 °C) and 62 km (−20 °C) altitude, within the acidic clouds. It has been speculated that clouds in the atmosphere of Venus could contain chemicals that can initiate forms of biological activity and have zones where photophysical and chemical conditions allow for Earth-like phototrophy.
Potential biomarkers
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. 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 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.
In January 2020, astronomers reported evidence that suggests Venus is currently (within 2.5 million years from present) volcanically active, and the residue from such activity may be a potential source of nutrients for possible microorganisms in the Venusian atmosphere.
In 2021, it was suggested the color of "unknown UV absorber" match that of "red oil", a known substance comprising a mix of organic carbon compounds dissolved in concentrated sulfuric acid.
Phosphine Research published in September 2020 indicated the detection of phosphine (PH3) in Venus's atmosphere by Atacama Large Millimeter Array (ALMA) telescope that was not linked to any known abiotic method of production present or possible under Venusian conditions. However, the claimed detection of phosphine was disputed by several subsequent studies. A molecule like phosphine is not expected 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 planets. Related studies suggested that the initially claimed 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." , no known abiotic process generates phosphine gas on terrestrial planets (as opposed to gas giants) in appreciable quantities. The phosphine can be generated by geological process of weathering olivine lavas containing inorganic phosphides, but this process requires an ongoing and massive volcanic activity. Therefore, detectable amounts of phosphine could indicate life. In July 2021, a volcanic origin was proposed for phosphine, by extrusion from the mantle.