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.

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.”

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.

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 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.

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?

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.

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.

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.

___
http://www.bbc.com/earth/story/20170111-the-unexpected-ingredient-necessary-for-life