By Ewan Jones
One of the biggest science stories in the last couple of weeks was thankfully unrelated to Covid-19. Instead, it involved the detection of phosphine gas in the clouds of Venus. Unlike other gasses that are often used as ‘biosignatures’ – signs of life on other planets – such as methane (which can also be produced geologically), phosphine gas has no known non-biological source, implying that some form of life may have been involved in its production.
On Earth, microbes release phosphine gas in oxygen-starved conditions. In order to produce the amount of phosphine observed in Venus’ atmosphere, these terranic microbes would only require 10% of their maximum productivity, however other routes of production, such as through volcanic eruptions or meteorites, would provide levels of phosphine 1,000 times lower than those observed.
Interestingly, Venus is sometimes referred to as “Earth’s twin”, since both are almost the same size, have the same mass, and have almost identical compositions. In fact, computer modelling of the planet’s ancient climate by NASA suggests that Venus may have once been habitable for up to two billion years of its early history, with water oceans that produced a thick cloud layer to protect the nascent planet.
However, modern conditions on Venus are incredibly hellish, with temperatures above 450C, little water, and immense clouds of sulphuric acid; most likely due to a runaway greenhouse gas effect as a result of a carbon dioxide buildup in its atmosphere. Although conditions on Venus may now seem incredibly anti-life, it is possible that organisms having evolved during its Edenic period may have clung on as the planet warmed, struggling to this day.
However, if the detected phosphine gas was created by these forms of life in such grim conditions, we may need to reconsider just what we designate as ‘life’ in the first place. Unfortunately, the conditions on Venus may be so extreme that the presence of life must be ruled out, and the use of phosphine as a biosignature should be reconsidered, with these findings implying an as-of-yet undiscovered pathway for phosphine synthesis in the absence of life.
However, all may not be lost – a paper published by Seager et al. in the Journal of Astrobiology proposes a life cycle for a Venusian aerial biosphere whereby microbial life survives in a ‘dessicated’, or dried-out, form in the lower, uninhabitable atmosphere, and are then propelled upwards into the clouds via diffusion to be rehydrated and continue their life cycle. This ability to desiccate is not restricted to science-fiction either: the tardigrade, an Earthly microbe, is able to completely dry itself out in order to survive incredibly extreme environments – even the vacuum of space!
We are constantly discovering new niches filled with life where we never would have expected it, such as undersea vents and boiling lakes, so an organism that has evolved to survive in droplets of sulphur isn’t too big of a stretch!
Luckily for us, we won’t even need to wait for a Venus mission to be launched before we can physically collect samples of the phosphine gas for ourselves – in a complete coincidence, the European Space Agency and the Japan Aerospace Exploration Agency had already planned a Venus fly-by next month as part of their BepiColombo mission to Mercury, which launched in 2018. Could this be the moment we discover extraterrestrial life?
Image: NASA Goddard Photo and Video