While the surface of Venus is one of the most Hellish places in the solar system, scientists have long speculated on the possibility of microbial life in the middle atmosphere of the planet, where there is abundant sunlight and the temperature and pressure are comparable to Earth’s atmosphere at sea level. The report of discovery of “biomarker” gas phosphine in the atmosphere of Venus in 2020 heightened interest in the topic, although the detection has been questioned and alternative abiotic explanations proposed.
A 2008 paper in Astrophysics and Space Science, “On the possibility of microbiota transfer from Venus to Earth”, suggests that if life is present in the atmosphere of Venus, during transits of Venus, which occur in pairs separated by intervals of between 105 to 121 years, interaction of the solar wind with Venus’s atmosphere causes erosion of the atmosphere, with gases, dust, and any possible microbes to be ejected in a comet-like tail pointing away from the Sun. During inferior conjunctions with the Earth, and particularly during transits, where the Sun, Venus, and Earth are almost perfectly aligned, this tail will intersect the Earth’s magnetosphere and may result in some of this material being deposited in Earth’s upper atmosphere. The authors estimate that if life is present in the atmosphere of Venus, each transit event could deposit between 10^{11} and 10^{13} microbes in Earth’s atmosphere. Here is the abstract:
The possibility of the clouds of Venus providing habitats for extremophilic microorganisms has been discussed for several decades. We show here that the action of the solar wind leads to erosion of parts of the atmosphere laden with aerosols and putative microorganisms, forming a comet-like tail in the antisolar direction. During inferior conjunctions that coincide with transits of the planet Venus this comet-like tail intersects the Earth’s magnetopause and injects aerosol particles. Data from ESA’s Venus Express spacecraft and from SOHO are used to discuss the ingress of bacteria from Venus into the Earth’s atmosphere, which we estimate as \sim 10^{11}–10^{13} cells for each transit event.
The authors expand upon this idea in chapter 10, “Microbial transfers from Venus to Earth”, of the 2020 book Cosmic Genetic Evolution.
Interestingly, in 1963, Donald R. Barber of the Norman Lockyer Astronomical
Observatory near Sidmouth, England reported a series of bacterial invasions of rainwater collected and used for washing photographic plates which occurred over a 25 year period, all within 15 to 60 days of an inferior conjunction of Venus with the Earth. He observed the bacteria to have anomalous characteristics and suggested they may have originated on Venus and carried to Earth by the solar wind. A popular treatment of this by Barber appeared in the Analog science fact/fiction magazine in November 1964, “Invasion by Washing Water”, which I recall reading at the time.