Life on Venus

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 735 K (462 °C; 863 °F) and an atmospheric pressure 92 times that of Earth, the conditions on Venus 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. 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.

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, 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 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. This was later revised to 92 bars, 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.

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. 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.

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.