Life on Venus
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The speculation of life currently existing on Venus decreased significantly since the early 1960s, when spacecraft began studying Venus and it became clear that the conditions on Venus are extreme compared to those on Earth.
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), 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 lower-temperature, acidic upper layers of the Venusian atmosphere.
Until the mid 20th century, the surface environment of Venus was believed to be similar to Earth, and hence it was widely believed that Venus could harbor life. In 1870, the British astronomer Richard Proctor said the existence of life on Venus was impossible near its equator, but possible near its poles.
Since the 1960s, 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 on the surface. The atmosphere contains sulfuric acid clouds and the atmospheric pressure at the surface is 90 bar, 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 increasingly hostile characteristics of the Venusian weather, life as we know it is highly unlikely to occur.
In September 1967, Carl Sagan and Harold Morowitz published an analysis of the issue of life on Venus to the journal Nature.
Habitability of its atmosphere
Although there is little possibility of existing life near the surface of Venus, the altitudes about 50 km 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.
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 vapour.
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. 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.
Past habitability potential
It is plausible that microbial life originated on Venus if liquid water existed on its surface prior to the heating of the planet by the runaway greenhouse effect, but no longer exists. 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 few billion. This might also have given enough time for microbial life to evolve to be aerial. 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, although 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.
- All Summer in a Day – short story by Ray Bradbury
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- Life on Enceladus
- Life on Europa
- Life on Mars
- Life on Titan
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