The writer is a science commentator
In terms of planetary real estate, Venus is no Fifth Avenue. The moonless, ringless planet might mirror Earth in size and rocky composition but it would be murder to live there. The surface, at about 460C, is hot enough to melt lead and the atmospheric pressure is high enough to crush a human.
A suffocating atmosphere of carbon dioxide is scattered with clouds that rain down droplets of sulphuric acid. And yet there is a slender chance that life hovers above this hostile world, which spins in the opposite direction from most planets and has sunrises more than a year apart.
Last week, scientists announced they had detected phosphine in the Venusian clouds. The colourless, toxic gas is produced naturally on Earth in miserly amounts by volcanic activity and lightning, but more generously by some bacteria that do not require oxygen. That makes it a possible “biosignature”, a hallmark of life on other worlds. The finding, sparking excitement and scepticism, reminds us that we have only the shallowest reckoning of the deepest questions: how did life begin and are we alone in the universe?
An international team led by Professor Jane Greaves, at Cardiff university, detected phosphine at an altitude of about 60km above Venus’s surface. The observations, made using the James Clerk Maxwell Telescope in Hawaii, were confirmed using a second telescope in Chile and published in Nature Astronomy. Interestingly, phosphine has been separately detected on Jupiter and Saturn, but ascribed there to extreme pressures and temperatures rather than living processes.
The idea that alien microbes waft around in the clouds of Venus is both intensely appealing and quite mad. “I wouldn’t have bet on current-day Venus being the first place in the solar system to show a potential biological signature for life,” admits Carolin Crawford, public astronomer at Cambridge university. “I still think the cold and arid deserts of Mars, or the underwater oceans of Enceladus or Europa to be the most promising options.” Those moons, of Saturn and Jupiter respectively, bear comparison to habitats on Earth known to harbour extremophiles, the world’s hardiest lifeforms.
In addition, extraordinary claims demand extraordinary proof. While few doubt that phosphine, which breaks down in sunlight, hangs in the Venusian atmosphere, the question is: why? The researchers, having ruled out volcanic and other sources, insist that only alien lifeforms can churn out sufficient phosphine to match observations.
David Rothery, professor of planetary sciences at The Open University, is more guarded. “The scientists can’t think of a way of getting those phosphine levels without it being a byproduct of microbial life. That doesn’t mean there isn’t another way, and it is a long way from proving that there’s life on Venus,” he says. Caution is the watchword: a 1996 announcement that a meteorite contained Martian bacteria turned out to be a false alarm.
Venus might, though, once have been a congenial setting for life, blanketed with oceans that later boiled away in a runaway greenhouse effect. The cloud layer where the phosphine was spotted has a temperature of around 30C and a bearable pressure. So, this Earthlike slice of Venus’s atmosphere offers a tempting scenario: life once existed on Venus; it was mostly wiped out by runaway heat; and against the odds, clung on in the clouds. The BepiColombo mission, due to reach Mercury in 2025, will make two fly-bys of Venus, on October 15 and in August 2021. Mission scientists, including Prof Rothery, are now adding phosphine detection to its list of tasks.
Still, further detection will not, of itself, dispel the doubt. Only physical samples from the clouds could definitively confirm the existence of Venusian life, and whether it resembles life here.
That might vindicate the theory of “panspermia” which holds that life was seeded on Earth by comets, asteroids or space dust carrying lifeforms from elsewhere. Perhaps life started on Venus and spread to Earth, or vice versa, or both rocky planets were coincidentally seeded by the same material.
Or, muses Matthew Bothwell, a Cambridge astronomer who has studied the evolution of galaxies, it might be that life began twice, independently: “Twice in the same solar system suggests life elsewhere in the universe.”
Since the sun is one of at least 100bn in our galaxy, and our galaxy is one of billions across the universe, life should be an unexceptional phenomenon and the cosmos should be teeming. That is why we keep searching, hunting the heavens for unnatural radio signals.
It is our mistake, though, to assume any cosmic companions are intelligent, akin asserts Mr Bothwell, to elephants assuming aliens have trunks. He says, “Intelligence might not be that important or special for life. In fact, it might be an evolutionary dead-end. The cleverer we get, the more dangerous our technologies become. One human being could wipe out life on Earth.”
That seems more thought-provoking than any microbial miasma suspended in the clouds of Venus — that one reason we have never detected any super-smart alien civilisations is because they no longer exist.
Letters in response to this article:
Get alerts on Space exploration when a new story is published