It remains one of the most unexpected scientific discoveries of the year. To their astonishment, British scientists last week revealed they had uncovered strong evidence that phosphine – a toxic, rancid gas produced by microbes – exists in the burning, acid-drenched atmosphere of Venus.
By rights, it should simply not be there. “All the geological and photochemical routes we can think of are far too underproductive to make the phosphine we have seen,” said Cardiff University astronomer Professor Jane Greaves, leader of the team who made the discovery. And that conclusion leaves scientists with the bizarre prospect that microbial activity – the key source of phosphine on Earth – may be occurring in the searing, acidic clouds that swathe Venus.
Not surprisingly, the news that there may be bugs on Venus made front-page headlines. It also adds a bizarre new planetary focus for scientists hunting alien life on nearby planets – a search that is now leading them to increasingly strange and unexpected parts of the solar system, from the frozen moons of Jupiter to the methane-filled lakes of Titan, the largest moon of Saturn.
While astronomers have yet to detect alien lifeforms, most are confident of success one day, though their journey will have taken them through some intriguing highs and some despairing lows. For example, in the late 19th century, and for much of the 20th, most astronomers were confident they would one day find life in the solar system.
And two worlds looked especially promising: Mars and Venus, our nearest planetary neighbours. Telescopes showed Venus was permanently shrouded in clouds – so it was assumed this was a world covered in steamy jungles filled with exotic animal life. At the same time, observations of Mars suggested seasonal changes in vegetation were occurring there with some astronomers arguing they could actually see signs of canals that had been constructed on the planet.
“Then, in the early 1960s, we sent our first space probes to the planets and discovered that Mars was a frozen, dead desert and that Venus was a hellhole,” said astrobiologist Lewis Dartnell, of the University of Westminster. “The prospects of finding alien life in the solar system took a nosedive.”
Astrobiologists like myself have renewed hope we have a chance of finding life on other worlds in our solar systemLewis Dartnell
In the case of Mars, the planet was found to have a painfully thin atmosphere and was being battered by ultraviolet radiation, while Venus was revealed to have a dense, super-heated atmosphere of carbon dioxide lurking below thick clouds of sulphuric acid. The place was so hot, lead would melt on its surface. Astronomers’ hopes of finding alien life on worlds near Earth were crushed.
But in recent years, these hopes have started to rally. On Earth, carbon-based organisms called extremophiles have been found living in some extraordinarily hostile places: in nuclear waste; in highly acidic waters; in undersea vents where temperatures and pressures reach colossal levels; and on panels bolted to the outside of the international space station and exposed to the vacuum of space for years.
Life is not quite so fragile as was once supposed, it has been discovered. And if microbes could survive grim conditions on Earth, perhaps some could endure the harsh environments of Mars or other hostile parts of the solar system.
In addition, US interplanetary probes that began to explore the solar system’s remoter edges returned data that also raised hopes. Jupiter was found to possess moons that have deep underground oceans while Titan was discovered to have an atmosphere containing organic chemicals, the building blocks of life, and had lakes of hydrocarbons such as ethane and methane on its surface.
For good measure, when US space engineers did get around to sending new missions to Mars, these revealed ancient creeks and river beds through which water had once coursed freely. Perhaps, life had evolved on the red planet and may still be clinging on in underground pockets.
“It was a double whammy,” said Dartnell. “Planetary science expanded and found new possible homes of life while the biological sciences showed organisms could survive in much harsher environments than we had previously supposed. That has given astrobiologists like myself renewed hope we have a chance of finding life on other worlds in our solar system.”
These forms of life – if they exist at all – are definitely not going to be made up of intelligent beings capable of building canals or of animals that populate alien jungles, scientists stress. Most expect they will come in the form of fairly simple organisms. Nevertheless their discovery would have enormous repercussions.
At present, humans know of only one world that supports life: our own. It could be that this is a rare fluke and that life has emerged only once in our galaxy’s history – here on Earth. On the other hand, the opposite could be true. Life may be common and widespread in the cosmos. And if astronomers find that life has appeared independently on a second location in our own solar system, that would suggest it is not rare at all and may have emerged on billions of planets in our galaxy.
But what are the most promising locations for finding life in our own solar system? Where should we focus our search for aliens? Venus has just offered itself up as a highly unexpected new candidate for investigation. But where else should we be looking?
Astronomers point to several other promising locations, each very different from the others – though the first one is straightforward: Mars. It was similar to Earth several billion years ago and life may have appeared there around this time, just as it did on our planet. It may linger on in underground deposits.
It is also a fairly easy planet to reach and will soon be visited by a fleet of spacecraft that include the US robot rover Perseverance, which is scheduled to land on Mars next year and will begin collecting rocks to return to Earth at some time over the next decade. These samples would then be studied by scientists for fossil signals that would show whether life had once evolved on the red planet.
“With Perseverance on its way to Mars, the prospect of collecting samples for a return to Earth is becoming really exciting, not only because we will have access to all the analytical capabilities we ever want, but also because of the long term benefits,” said Open University astrobiologist Susanne Schwenzer. “The Apollo samples from the Moon continue to be a rich resource for research and allow us to discover new things decades after they first arrived in our laboratories. And that will be equally true of the Martian rocks we bring back to Earth,” she said.
By contrast, the frozen world of Europa, one of the main moons of Jupiter, offers very different prospects of providing signs of life. Entirely coated in ice, Europa is the smoothest body in the solar system. It has no mountains nor hills, only ridges a few hundred metres high provide deviations along its burnished surface. But beneath Europa’s glassy landscape, there is an ocean of salty water, space probes have discovered. “When you’re looking for alien life, you look for water. And Europa has got oceans of it, which makes it a very promising place to visit,” says Dartnell.
In 2024, the US is scheduled to launch its Europa Clipper, which will sweep over the icy moon in 2030. This could then be followed by a separate lander mission that will search for signs of biological activity from material that might have bubbled up from Europa’s underground ocean. It will not be an easy task, however – with nearby Jupiter’s intense radiation field posing particular problems, a point stressed by astronomer Leigh Fletcher, of Leicester University.
“Organic chemicals and other compounds washed up on Europa’s surface are constantly being bombarded by radiation and that is going to damage any primitive living material that is present. It won’t survive long and that is going to be tough for any spacecraft which is trying to find out what’s occurring on Europa.”
By contrast, Enceladus, a tiny moon of Saturn, provides an easier and more promising target, according to some astronomers. The US Cassini space probe revealed it is actually venting geysers of water – rich in salt and organic matter – from an underground ocean into space. If there are lifeforms there, they should be easy to pick up.
Liquid water and organic material are the two prime ingredients of life, and they exist in abundance on EnceladusCharles Cockell
“It’s simply the most promising place in the solar system to find life,” said astrobiologist Professor Charles Cockell, of Edinburgh University. “There may be water and organics on Mars but these are probably deep underground. But on Enceladus they are gushing out into space. All we have to do is sweep them up.”
No missions to Enceladus have yet been scheduled but scientists at the Jet Propulsion Laboratory in California are developing proposals – though it is acknowledged that a trip there would be a lengthy business. “It will take decades from the mission’s inception to actually get samples back to Earth,” added Cockell.
“However, it will be worth the wait even if we find no evidence for biological activity in Enceladus. Liquid water and organic material are the two prime ingredients of life and they exist in abundance there. However, if we find that life has not evolved there, despite the presence of water and organics, that suggests it must be a very difficult thing to set in motion.”
Saturn possesses another moon that also offers hopes of sustaining life – Titan. It is soaking wet, not with water but with liquid hydrocarbons such as methane and ethane. “There are lakes of the stuff on Titan and although it is extremely cold out there, there is a chance that hydrocarbon chemistry could result in the evolution of lifeforms,” says Dartnell. “It is a fascinating prospect.”
And that leaves Venus. In fact, most astronomers discount the prospect of life thriving there – despite the discovery of phosphine in its atmosphere. Those rancid puffs of gas are most likely being produced by some geochemical process that is currently unknown to science. “I doubt if there are any lifeforms on Venus. It is far too hostile a place for that,” added Cockell. “In any case, there are plenty of other, far more promising places to look in the solar system.”