WHY GERMANS GROW TOMATOES IN URINE?

A fish tank brimming with urine is the first thing you see when you enter Jens Hauslage’s cramped office at the German space agency, DLR, near Cologne. It sits on a shelf by his desk, surrounded by the usual academic clutter of books, charts and scientific papers.

Rising from the centre of the tank are two transparent plastic cylindrical columns – around a metre in height. Spreading from the top of each tube is a bushy, healthy-looking tomato plant with green leaves, flowers and even a few bright red tomatoes.

Though the laboratory is full of tanks containing urine, there is little smell thanks to it being broken down by bacteria (Credit: Richard Hollingham)

“Are they edible?” I ask, half expecting to be told to stay well clear.

“Sure,” says Hauslage, a plant physiologist, casually twisting one of the tomatoes from its stem and handing over the euro-sized fruit.

I put it in my mouth. To be brutally honest, it’s not the nicest tomato I have ever tried: the skin is a little tough and the taste is slightly bitter. But it is, nevertheless, a healthy, edible tomato.

Recycled sweat

It’s hardly surprising the tomatoes in Hauslage’s office aren’t the tastiest. The plants have been specially bred to be grown in space. This experimental tank of urine, pipes and plants is the original prototype for a satellite designed to prove that tomatoes could be cultivated successfully on the Moon or Mars.

Right now, almost all the food on the International Space Station (ISS) is ferried up in cargo ships from Earth. The only exception is a few lettuce and cabbage leaves astronauts have managed to grow in a hydroponics solution. Most of the water on board the ISS, however, comes from astronaut urine. Liquid waste from washing, sweat and the toilet is almost totally recycled using a complex processing system. Today’s urine is tomorrow’s coffee.

But what if you could use the useful salts in astronaut urine to grow food? If humans are ever to live for long periods on the Moon or Mars, they will need a self-sustaining food supply. “You will need more than protein bars,” says Hauslage.

In Hauslage’s lab, there is plenty more urine swilling around – in vats, funnels and hoses  

“The Earth is a closed biological system with plants producing oxygen and food, then you have the animals and the microbes to produce all the degradation processes in the soil,” he says. “Without these systems, no sustainable long-term life-support system will be viable.”

In his lab, there is plenty more urine swilling around – in vats, funnels and hoses. Most of the urine is artificial – mixed so the scientists know the exact chemical composition for their experiments – but some comes from human volunteers. The walls are lined with grey plastic drain pipes, attached to plastic boxes full of bubbling artificial urine. At the centre of the room, larger terracotta-coloured pipes rise from plastic barrels of urine.

Jens Hauslage's lab is full of tanks growing tomatoes in urine - a concept that could one day feed astronauts (Credit: Richard Hollingham)

“The smaller columns can handle the urine of one person for a day,” explains Hauslage. “The big columns can handle the urine of four to six people.”

Each of these columns is packed with pumice stone – solidified lava that is riddled with holes – home to rich colonies of bacteria. These microbes feed on the urine pumped through the pipes, some bacteria convert ammonia into nitrites and others convert this into nitrate salts – fertiliser.

This is a controlled laboratory version of the nitrogen cycle that takes place naturally in soils and watercourses on Earth. As well as urine, this closed-loop biological system could be used to process leftover food or leaves that drop from a plant.

Considering we are in a room filled with gallons of urine, there is no smell  

Curiously, considering we are in a room filled with gallons of urine, there is no smell. “The degradation of urine into carbon dioxide and ammonia is really fast,” says Hauslage, “and the bacteria inside our filters are fast too.”

Having developed the technology in the lab, the DLR science team is now taking it into orbit. Later this year, the space agency will launch its Eu:cropis mission (the acronym stands, somewhat tortuously, for Euglena and Combined Regenerative Organic Food Production in Space), a metre-wide cylindrical satellite containing two miniature greenhouses.

Launched on a SpaceX Falcon 9 rocket, the satellite will orbit the Earth carrying tomato seeds, a tank of synthetic urine and bacterial colonies. The spacecraft will spin to simulate the gravity of the Moon for the first greenhouse. After six months the spin rate will be increased to simulate Martian gravity when the second greenhouse is brought online.

Astronauts exploring Mars will need more than protein bars to keep them going (Credit: iStock)

Initially oxygen will be produced by a colony of green algae – the Euglena of the acronym – although eventually the tomato plants will produce more oxygen than carbon dioxide.

“After launch we’ll spin the satellite and water the system,” says Hauslage. “The tomatoes will germinate and we’ll feed the system with urine to produce tomatoes.”

The plants will be carefully monitored with an array of 16 cameras, with data sent back to Earth four times a day. Although this isn’t the first time tomatoes have flown in space, it is the first satellite dedicated to growing plants in a closed system.

If the mission proves successful, then this could provide a way forward for growing plants on other worlds. But how about also using the other human waste product: faeces?

In The Martian, we see Matt Damon living off potatoes on Mars, cultivated in a soil of human excrement. Currently solid waste on the ISS is bagged up and packed into supply ships to be burnt-up in the Earth’s atmosphere.

The tomatoes have a bitter taste but are perfectly edible (Credit: Richard Hollingham)

“For me as a scientist, faeces is not quite gold like urine,” says Hauslage, “but it contains a lot of potassium which we need for a good fertiliser.” There are, however, dangerous pathogens in this solid waste that would need careful handling and processing.

Nevertheless, it is likely that nothing in a future colony on the Moon or Mars will be thrown away – everything will need to be recycled. And feeding dozens of colonists on a remote world is going to prove a challenge.

“You need a lot of calories, so you have to produce a lot of potatoes, cucumbers, tomatoes and so on and you need protein and fat,” says Hauslage. The latter food group is a particular challenge, but his team is already on the case: “We’ve also got an experiment producing fat with algae solution using our urine water.”

Yum. Just don’t expect food grown in space to win any Michelin Stars.

WHY VITAMIN PILLS DONT WORK AND EVEN WORSE TO U

For Linus Pauling, it all started to go wrong when he changed his breakfast routine. In 1964, at the age of 65, he started adding vitamin C to his orange juice in the morning. It was like adding sugar to Coca Cola, and he believed – wholeheartedly, sometimes vehemently  – that it was a good thing.

Before this, his breakfasts were nothing to write about. Just that they happened early every morning before going to work at California Institute of Technology, even on weekends. He was indefatigable, and his work was fruitful.

At the age of 30, for instance, he proposed a third fundamental way that atoms are held together in molecules, melding ideas from both chemistry and quantum mechanics. Twenty years later, his work into how proteins (the building blocks of all life) are structured helped Francis Crick and James Watson decode the structure of DNA (the code of said building blocks) in 1953. 

The next year, Pauling was awarded a Nobel Prize in Chemistry for his insights into how molecules are held together. As Nick Lane, a biochemist from University College London, writes in his 2001 book Oxygen, “Pauling… was a colossus of 20th Century science, whose work laid the foundations of modern chemistry.”

Linus Pauling was one of our most influential scientists, yet his belief in the power of antioxidants may have set us down a dangerous path (Credit: Getty Images)

But then came the vitamin C days. In his 1970 bestselling book, How To Live Longer and Feel Better, Pauling argued that such supplementation could cure the common cold. He consumed 18,000 milligrams (18 grams) of the stuff per day, 50 times the recommended daily allowance.

In the book’s second edition, he added flu to the list of easy fixes. When HIV spread in the US during the 1980s, he claimed that vitamin C could cure that, too.

In 1992, his ideas were featured on the cover of Time Magazine under the headline: “The Real Power of Vitamins”. They were touted as treatments for cardiovascular diseases, cataracts, and even cancer. “Even more provocative are glimmerings that vitamins can stave off the normal ravages of ageing,” the article claimed.

Sales in multivitamins and other dietary supplements boomed, as did Pauling’s fame.

But his academic reputation went the other way. Over the years, vitamin C, and many other dietary supplements, have found little backing from scientific study. In fact, with every spoonful of supplement he added to his orange juice, Pauling was more likely harming rather than helping his body. His ideas have not just proven to be wrong, but ultimately dangerous. 

Antioxidants were meant to delay the ravages of ageing, but there's little evidence that supplements bring any noticeable benefits (Credit: Getty Images)

Pauling was basing his theories on the fact that vitamin C is an antioxidant, a breed of molecules that includes vitamin E, beta-carotene, and folic acid. Their benefits are thought to arise from the fact that they neutralise highly reactive molecules called free-radicals.

In 1954, Rebeca Gerschman then at the University of Rochester, New York, first identified these molecules as a possible danger – ideas expanded upon by Denham Harman, from the Donner Laboratory of Medical Physics at UC Berkeley in 1956, who argued that free radicals can lead to cellular deterioration, disease and, ultimately, ageing.

Throughout the 20th Century, scientists steadily built on his ideas and they soon became widely accepted.

Here’s how it works. The process starts with mitochondria, those tiny combustion engines that sit within our cells. Inside their internal membranes food and oxygen are converted into water, carbon dioxide, and energy. This is respiration, a mechanism that fuels all complex life.

‘Leaky watermills’

But it isn’t so simple. In addition to food and oxygen, a continuous flow of negatively charged particles called electrons is also required. Like a subcellular stream downhill powering a series of watermills, this flow is maintained across  four proteins, each embedded in the internal membrane of the mitochondria, powering the production of the end product: energy.

This reaction fuels everything we do, but it is an imperfect process. There is some leakage of electrons from three of the cellular watermills, each able to react with oxygen molecules nearby. The result is a free radical, a radically reactive molecule with a free electron. 

Oxygen is the breath of life, but it also holds the potential to make us old, decrepit, and then dead

In order to regain stability, free radicals wreak havoc on the structures around them, ripping electrons from vital molecules such as DNA and proteins in order to balance its own charge. Although inconceivably small in scale, the production of free radicals, Harman and many others posited, would gradually take its toll on our entire bodies, causing mutations that can lead to ageing and age-related diseases such as cancer.

In short, oxygen is the breath of life, but it also holds the potential to make us old, decrepit, and then dead.

Clinical trials are the only ways to reveal the effects of a drug - and investigations into antioxidants have produced some shocking results (Credit: Alamy)

Shortly after free radicals were linked to ageing and disease, they were seen as enemies that should be purged from our bodies. In 1972, for example, Harman wrote, “Decreasing [free radicals] in an organism might be expected to result in a decreased rate of biological degradation with an accompanying increase in the years of useful, healthy life. It is hoped that [this theory] will lead to fruitful experiments directed toward increasing the healthy human lifespan.” 

He was talking about antioxidants, molecules that accept electrons from free radicals thereby diffusing the threat. And the experiments he hoped for were sown, nurtured, and replicated over the next few decades. But they bore little fruit.

The results were the largely the same: an excess of antioxidants didn’t quell the ravages of ageing, nor stop the onset of disease

In the 1970s and into the 80s, for example, many mice – our go-to laboratory animal – were prescribed a variety of supplementary antioxidants in their diet or via an injection straight into the bloodstream. Some were even genetically modified so that the genes coding for certain antioxidants were more active than non-modified lab mice. 

Although different in method, the results were the largely the same: an excess of antioxidants didn’t quell the ravages of ageing, nor stop the onset of disease.

“They never really proved that they were extending lifespan, or improving it,” says Antonio Enriquez from the Spanish National Centre for Cardiovascular Research in Madrid. “Mice don’t care for [supplements] very much.”

Far from protecting us from disease, one study found that vitamin supplements increased the incidence of lung cancer among smokers (Credit: Alamy)

What about humans? Unlike our smaller mammalian kin, scientists can’t take members of society into labs and monitor their health over their lifetime, while controlling for any extraneous factors that could bias the results at the end. But what they can do is set up long-term clinical trials.

The premise is pretty simple. First, find a group of people similar in age, location, and lifestyle. Second, split them into two subgroups. One half receives the supplement you’re interested in testing, while the other receives a blank – a sugar pill, a placebo. Third, and crucially to avoid unintentional bias, no one knows who was given which until after the trial; not even those administering the treatment. 

The incidence of lung cancer increased by 16% in the group given vitamin supplements

Known as a double-blind control trial, this is the gold standard of pharmaceutical research. Since the 1970s, there have been many trials like this trying to figure out what antioxidant supplementation does for our health and survival. The results are far from heartening.

In 1994, for example, one trial followed the lives of 29,133 Finish people in their 50s. All smoked, but only some were given beta-carotene supplements. Within this group, the incidence of lung cancer increased by 16%.

A similar result was found in postmenopausal women in the U.S. After 10 years of taking folic acid (a variety of B vitamin) every day their risk of breast cancer increased by 20% relative to those women who didn’t take the supplement. 

It gets worse. One study of more than 1,000 heavy smokers published in 1996 had to be terminated nearly two years early. After just four years of beta-carotene and vitamin A supplementation, there was a 28% increase in lung cancer rates and a 17% increase in those who died.

These aren’t trivial numbers. Compared to placebo, 20 more people were dying every year when taking these two supplements. Over the four years of the trial, that equates to 80 more deaths. As the authors wrote at the time, “The present findings provide ample grounds to discourage use of supplemental beta-carotene and the combination of beta-carotene and vitamin A.”

Fatal ideas

Of course, these notable studies don’t tell the full story. There are some studies that do show benefits of taking antioxidants, especially when the population sampled doesn’t have access to a healthy diet. 

But, according a review from 2012 that noted the conclusions of 27 clinical trials assessing the efficacy of a variety of antioxidants, the weight of evidence does not fall in its favour.

Just seven studies reported that supplementation led to some sort of health benefit from antioxidant supplements, including reduced risk of coronary heart disease and pancreatic cancer. Ten studies didn’t see any benefit at all – it was as if all patients were given the sugar pill also (but, of course, they weren’t). That left another 10 studies that found many patients to be in a measurably worse state after being administered antioxidants than before, including an increased incidence of diseases such as lung and breast cancer.

The idea that antioxidant supplements are a miracle cure is completely redundant – Antonio Enriquez

“The idea that antioxidant [supplementation] is a miracle cure is completely redundant,” says Enriquez. Linus Pauling was largely unaware of the fact that his own ideas could be fatal. In 1994, before the publication of many of the large-scale clinical trials, he died of prostate cancer. Vitamin C certainly wasn’t the cure-all that he cantankerously claimed it was up until his last breath. But did it contribute to a heightened risk? 

Dosing up on vitamin C does not even help us fight the common cold (Credit: Alamy)

We’ll never know for sure. But given that multiple studies have linked excess antioxidants to cancer, it certainly isn’t out of the question. A study published in 2007 from the US National Cancer Institute, for instance, found that men that took multivitamins were twice as likely to die from prostate cancer compared to those who didn’t. And in 2011, a similar study on 35,533 healthy men found that vitamin E and selenium supplementation increased prostate cancer by 17%.

Ever since Harman proposed his great theory of free radicals and ageing, the neat separation of antioxidants and free radicals (oxidants) has been deteriorating. It has aged.

Antioxidant is only a name, not a fixed definition of nature. Take vitamin C, Pauling’s preferred supplement. At the correct dose, vitamin C neutralises highly charged free radicals by accepting their free electron. It’s a molecular martyr, taking the hit upon itself to protect the cellular neighbourhood. 

But by accepting an electron, the vitamin C becomes a free radical itself, able to damage cell membranes, proteins and DNA. As the food chemist William Porter wrote in 1993, “[vitamin C] is truly a two-headed Janus, a Dr Jekyll-Mr Hyde, an oxymoron of antioxidants.”

Thankfully, in normal circumstances, the enzyme vitamin C reductase can return vitamin C’s antioxidant persona. But what if there’s so much vitamin C that it simply can’t keep up with supply? Although such simplifying of complex biochemistry is in itself problematic, the clinical trials above provide some possible outcomes.  

Divide and conquer

Antioxidants have a dark side. And, with increasing evidence that free radicals themselves are essential for our health, even their good side isn’t always helpful.

Without free radicals, cells would continue to grow and divide uncontrollably

We now know that free radicals are often used as molecular messengers that send signals from one region of the cell to another. In this role, they have been shown to modulate when a cell grows, when it divides in two, and when it dies. At every stage of a cell’s life, free radicals are vital.

Without them, cells would continue to grow and divide uncontrollably. There’s a word for this: cancer.

We would also be more prone to infections from outside. When under stress from an unwanted bacterium or virus, free radicals are naturally produced in higher numbers, acting as silent klaxons to our immune system. In response, those cells at the vanguard of our immune defense – macrophages and lymphocytes – start to divide and scout out the problem. If it is a bacterium, they will engulf it like Pac-Man eating a blue ghost.

It is trapped, but it is not yet dead. To change that, free radicals are once again called into action. Inside the immune cell, they are used for what they are infamous for: to damage and to kill. The intruder is torn apart.

From start to finish, a healthy immune response depends on free radicals being there for us, within us. As geneticists Joao Pedro Magalhaes and George Church wrote in 2006: “In the same way that fire is dangerous and nonetheless humans learned how to use it, it now appears that cells evolved mechanisms to control and use [free radicals].”

Put another way, freeing ourselves of free radicals with antioxidants is not a good idea. “You would leave the body helpless against some infections,” says Enriquez.

Few would dispute that a balanced diet is essential for good health, but most of us don't need supplements to meet our nutritional needs (Credit: Getty Images)

Thankfully, your body has systems in place to keep a your inner biochemistry as stable as possible. For antioxidants, this generally involves filtering any excess out of the bloodstream into urine for disposal. “They go in the toilet,” says Cleva Villanueva from Instituto Politécnico Nacional, Mexico City, in an email.

“We’re very good at balancing things out so that the affect [of supplementation] is moderate whatever you do, which we should be grateful for,” says Lane. Our bodies have been selected to balance the risk of oxygen ever since the first microbes started to breathe this toxic gas. We can’t change billions of years of evolution with a simple pill.

No one would deny that vitamin C is vital to a healthy lifestyle, as are all antioxidants, but unless you are following doctor's orders, these supplements are rarely going to be the answer for a longer life when a healthy diet is also an option. “Administration of antioxidants is justified only when it is evident that there is a real deficiency of a specific antioxidant,” says Villanueva. “The best option is to get antioxidants from food because it contains a mixture of antioxidants that work together.”

“Diets rich in fruits and vegetables have been shown generally to be good for you,” says Lane. “Not invariably, but generally that’s agreed to be the case.” Although often attributed to antioxidants, the benefits of such a diet, he says, might also hail from a healthy balance of pro-oxidants and other compounds whose roles aren’t yet fully understood.

After decades of unlocking the baroque biochemistry of free radicals and antioxidants, hundreds of thousands of volunteers, and millions of pounds spent on clinical trials, the best conclusion that 21st Century science has to offer is also found within a child’s classroom – eat your five-a-day.

THE UNXPECTED INGREDIENT FOR LIFE

From microbes to marsupials, life abounds on Earth. But even if you stripped the planet of all its inhabitants, Earth would still "live".

Its molten core churns, generating a magnetic field that envelops the planet. Erupting volcanoes spew gases and pave new lands with fresh lava. Earth's surface is a jigsaw puzzle of continent-sized rocky plates that push, rub, and clash against one another – powerful processes that build mountains and reshape landscapes. 

Earth is not just a vessel for life; the planet itself is alive. But its geological metabolism – and especially the dynamism of its tectonic plates – is also responsible for making it a habitable world. If the planet were a cold, dead, and inert space rock, life as we know it probably could not exist. At least on today's Earth, geology and biology go hand in hand.

Of all planets, Earth is the only one known to have plate tectonics. It is also the only one known to harbour life. But whether that means plate tectonics is required for life, no one knows for sure.

Earth is a mosaic of tectonic plates (Credit: Karsten Schneider/SPL)

Astronomers have discovered thousands of planets beyond the Solar System, some of which could be habitable. And plate tectonics could boost the likelihood for life – especially for more complex organisms. If aliens do in fact exist, then they might also live on an active planet, rife with ground-shifting geological activity just like on Earth.

But as spacecraft exploring the Solar System have discovered, Earth is not unique when it comes to geological activity. Even though neither the moon nor Mars has plate tectonics, both worlds experience "moonquakes" and "marsquakes." 

Jupiter's moon Europa is a contender for life (Credit: Nasa/JPL/Caltech/Seti)

Some of Jupiter's moons have active volcanoes and geysers. Mercury has a magnetic field, suggesting at least part of its core is molten. Even Pluto – once thought to be a relatively dormant ice world – turns out to be full of towering ice mountains and glaciers, a landscape more dynamic than scientists expected. 

Subduction can dig deep ocean trenches or induce volcanic eruptions

Still, geological activity alone is not the same as plate tectonics. Earth is the only planet in the Solar System with an outer crust broken into several plates like a cracked eggshell. These rigid tectonic plates, extending a couple of hundred kilometres deep at most, float on the more malleable mantle below. 

Other worlds in the Solar System have ancient surfaces that are pockmarked with craters millions or even billions of years old. But on Earth, tectonic plates shift and slide, constantly renewing the surface. At mid-ocean ridges, rising magma forms new crust as it pushes two plates apart.

When two plates press into each other, one can get subsumed underneath. This process of subduction can dig deep ocean trenches or induce volcanic eruptions. Sometimes, like in the Himalayas, continental plates thrust themselves into each other, and with nowhere to go but up, they build mountains.

This is all essential for life on Earth.

Without plate tectonics, Earth could become a hot and inhospitable world like Venus (Credit: SSV/MIPL/Magellan Team/NASA)

These processes carry carbon in and out of Earth's interior, and by doing so, regulate the amount of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas: too much of it, and the atmosphere traps too much heat.

"The surface temperature increases and Earth eventually becomes a planet like Venus," says Jun Korenaga, a geophysicist at Yale University, US. Too little, and all the heat would escape, leaving Earth inhospitably cold.

Plate tectonics helps keep volcanism active for a long time

The carbon cycle therefore acts as a global thermostat, regulating itself when needed (although it does not take into account the excess carbon dioxide that is driving human-caused climate change). A warmer climate also results in more rain, which helps extract more carbon dioxide out of the atmosphere.

The gas is dissolved in raindrops, which fall on exposed rock. Chemical reactions between the rainwater and rock release the carbon and minerals like calcium from the rock. The water then flows through rivers and streams, eventually reaching the ocean, where the carbon forms carbonate rocks and organic objects like seashells.

The carbonate settles on the bottom of the ocean, on a tectonic plate that gets subducted, carrying the carbon into Earth's interior. Volcanoes then belch the carbon back into the atmosphere as carbon dioxide.

After hundreds of millions of years, the cycle is finally complete. 

Plate tectonics help carry carbon in and out of Earth's interior (Credit: Karsten Schneider/SPL)

Plate tectonics plays a part in every aspect of this cycle. Not only does subduction deliver carbon back into Earth's mantle, but tectonic activity brings fresh rock to the surface. That exposed rock is crucial for the chemical reactions that release minerals. Mountains, formed from plate tectonics, channel air upward, where it cools, condenses, and forms raindrops – which help extract carbon from the atmosphere.

Plate tectonics could have created diverse environments that sparked evolution

Then there are the volcanoes. "Plate tectonics helps keep volcanism active for a long time," says Brad Foley, a geophysicist at Penn State University, US. "If we didn't have volcanism sending back carbon dioxide into the atmosphere, then the planet could get very cold. It would freeze over." 

Maintaining a warm climate is key for a habitable planet. But plate tectonics contributes other things as well. For example, research has suggested that erosion and weathering processes remove elements like copper, zinc, and phosphorous from rock and carry them to the sea.

These elements are important nutrients for organisms like plankton. In the past, they could have been responsible for bursts in biodiversity, such as the Cambrian explosion 540 million years ago. Evidence also suggests that periods with little erosion – and therefore fewer available nutrients in the ocean – coincided with mass extinction events. 

Hydrothermal vents are home to diverse ecosystems (Credit: Ken Macdonald/SPL)

By moving continents around, plate tectonics could also have created diverse environments that sparked evolution. Over millions of years, continents drift across Earth's surface, going from one climate zone to another. Without plate tectonics, Earth would not have its diverse geography, which provides a wide range of habitats.  

The engine that generates the magnetic field is a churning, molten core of iron

Plate tectonics is also responsible for hydrothermal vents on the ocean floor. Near a plate boundary, seawater can seep into cracks, where magma heats it to hundreds of degrees, ejecting the hot water back into the ocean. Hydrothermal vents, discovered in the late 1970s, are home to diverse ecosystems, and some scientists have suggested that similar vents gave rise to the first life on Earth.

The constant plate motions may even play a role in Earth's magnetic field. The field might have acted as a shield that prevented the solar wind from stripping away the atmosphere – another possible requirement for life. The engine that generates the magnetic field is a churning, molten core of iron. Those turbulent motions are due to a process called convection, in which the hotter liquid rises while the cooler stuff sinks. Whether or not convection takes place in Earth's core – and so whether it creates a magnetic field – depends on the planet's cooling rate.

"If you have plate tectonics, then that tends to cool the interior faster than if you didn't have it," says Peter Driscoll, a geophysicist at the Carnegie Institution of Washington. A faster cooling rate allows for convection and, in turn, a magnetic field. Mars and Venus, for example, do not have plate tectonics. Nor do they have liquid cores, magnetic fields, or life – that we know of, anyway.

But while plate tectonics is important for life on Earth today, what about extraterrestrial life?

Several thousand known exoplanets have now been discovered (Credit: Nasa/Jpl-Caltech)

Astronomers estimate that as many as a hundred billion planets populate the galaxy. This includes plenty of Earth-sized ones within the so-called habitable zone of their star, the region where it is not too hot nor too cold for liquid water to potentially exist on the surface. They have even found such a planet around Proxima Centauri, the closest star to the Solar System.

Being in the habitable zone and having liquid water are the most important factors for whether life could exist on a planet. But after that, plate tectonics, among other features, come into play.

It can slowly drip downward like molasses, dropping carbon deep into the interior

"Plate tectonics is extremely helpful for life," says Norm Sleep, a geophysicist at Stanford University, US. If a planet had plate tectonics, he says, "habitability would be greatly enhanced." 

Of course, any discussion of habitability on other planets is inherently speculative. There is only one known example of a habitable world, and that is Earth.

"Plate tectonics is critical for the life we know and love as humans," says Lindy Elkins-Tanton, a planetary scientist at Arizona State University, US. But "it's not necessarily required for life in a broader sense."

On Earth, for instance, plate tectonics' most important role is regulating the carbon cycle. But on another planet, plate tectonics might not be necessary to maintain such a cycle.

Volcanoes in Hawaii can erupt without tectonic activity (Credit: Mint Images Limited /Alamy)

Some volcanoes, such as the ones that make up the Hawaiian Islands, do not require tectonic activity.

"With that volcanism, there's still a way to have carbon dioxide added to the atmosphere," Foley says. "That volcanism is also creating fresh rock that can weather, so you have the ability to do both parts of the carbon cycle."

Still, returning that carbon back into the planet's interior without subduction gets tricky. A planet without plate tectonics, called a stagnant-lid planet, is encased in a rigid crust that locks in carbon. However, the deeper layers of the crust are warmer and softer. It is also denser than the mantle, so if it is soft enough, it can slowly drip downward like molasses, dropping carbon deep into the interior, where it can be expelled again via volcanoes.

You have to take any prediction of the early Earth with a grain of salt

But even if some sort of carbon cycle is possible, it might not last as long, and the planet will have a shorter window for habitability. Without plate tectonics, Foley says, volcanism might die out sooner. 

Some researchers say that, even on Earth, life might not have needed plate tectonics. In 2016, Craig O'Neill, a planetary scientist at Macquarie University in Sydney, developed computer models that suggest Earth did not have plate tectonics in the distant past – even when life first appeared 4.1 billion years ago. If life emerged on Earth without plate tectonics, then that would imply tectonic activity is not required for life.

But that conclusion is premature, other researchers say. "You have to take any prediction of the early Earth with a grain of salt," says Foley. Different assumptions with the model can give completely different answers.

How Earth's surface may once have appeared (illustration) (Credit: Richard Bizley/SPL)

In the end, researchers agree that plate tectonics could help coax life into existence. But no one can say for sure whether it is necessary. "We don't understand enough about plate tectonics to understand whether it's critical for habitability," Elkins-Tanton says. Scientists did not develop the theory until the latter half of the 20th Century, and they do not fully understand it on Earth, let alone on other planets. 

We could barely detect it on our planet, and we're standing right on it

One complicating factor on Earth is the intimate relationship between plate tectonics and life. "These geological cycles are making the Earth more habitable," Sleep says, but biology is also important. "Life has had four billion years to evolve traits that adapt itself to being able to live on a planet with plate tectonics." Maybe life on Earth came to rely on plate tectonics simply because evolution steered it that way.

Even if plate tectonics were required for life, astronomers probably would not be able to tell whether a planet has plate tectonics in the first place. Planets outside the Solar System are far away, and even the best telescopes can only tease out the chemical composition of planetary atmospheres, which is already a remarkable feat. But short of interstellar travel, it is virtually impossible to measure plate tectonics on another planet.

"We could barely detect it on our planet, and we're standing right on it," Elkins-Tanton says.

Plate tectonics is just one of many factors that may influence habitability. Scientists may not determine the formula for life until they actually discover ET. For now, Earth will remain the only world that is truly alive