Why Earth’s Cracked Crust May Be Essential for Life

From a distance, it's not obvious that Earth is full of life. You have to get pretty close to see the biggest forests, and closer still to see the work of humans, let alone microbes. But even from space, the planet itself seems alive. Its landmass is broken apart into seven continents, which are separated by vast waters. Below those oceans, in the unseen depths of our planet, things are even livelier. The Earth is chewing itself up, melting itself down, and making itself anew.

Photo of a diver between two tectonic plates in Silfra. reykjavik. Iceland

A dozen cold, rigid plates slowly slip and slide atop Earth's hot inner mantle, diving beneath one another and occasionally colliding. This process of plate tectonics is one of Earth's defining characteristics. Humans mostly experience it through earthquakes and, more rarely, volcanoes. The lava currently spurting from backyards in Hawaii — a result of a deep-mantle hot spot — is related to tectonic activity.

But there's more to plate tectonics than earthquakes and eruptions. A wave of new research is increasingly hinting that Earth's external motions may be vital to its other defining feature: life. That Earth has a moving, morphing outer crust may be the main reason why Earth is so vibrant, and why no other planet can match its abundance.

"Understanding plate tectonics is a major key to understanding our own planet and its habitability. How do you make a habitable planet, and then sustain life on it for billions of years?" said Katharine Huntington, a geologist at the University of Washington. "Plate tectonics is what modulates our atmosphere at the longest timescales. You need that to be able to keep water here, to keep it warm, to keep life chugging along."

In the past few years, geologists and astrobiologists have increasingly tied plate tectonics to everything else that makes Earth unique. They have shown that Earth's atmosphere owes its longevity, its components, and its incredibly stable Goldilocks-like temperature — not too hot, but not too cold — to the recycling of its crust. Earth's oceans might not exist if water were not periodically subsumed by the planet's mantle and then released. Without plate tectonics driving the creation of coastlines and the motion of the tides, the oceans might be barren, with life-giving nutrients relegated forever to the stygian depths. If plate tectonics did not force slabs of rock to dive underneath one another and back into the Earth, a process called subduction, then the seafloor would be entirely frigid and devoid of interesting chemistry, meaning life might never have taken hold in the first place. Some researchers even believe that without the movement of continents, life might not have evolved into complex forms.

In 2015, James Dohm and Shigenori Maruyama of the Tokyo Institute of Technology coined a new term for this interdependence: the Habitable Trinity. The phrase describes a planet with abundant water, an atmosphere and a landmass — all of which exchange and circulate material — as a prerequisite for life.

Yet understanding how plate tectonics affects evolution — and whether it is a necessary ingredient in that process — hinges on finding answers to some of the hottest questions in geoscience: how and when the plates started moving. Figuring out why this planet has a movable crust could tell geologists more not just about this planet, but about all planets or moons with solid surfaces, and whether they could have life, too.

From Mountains to Trenches

In 2012, the film director James Cameron became the first person to dive solo all the way down the deepest gash on Earth. He touched down 35,756 feet below the ocean surface in the Challenger Deep, a depression within the Mariana Trench, itself a much larger trough at the intersection of two tectonic plates. Cameron collected samples throughout the trench, including evidence of life thriving on the seams of our planet.

As the Pacific plate is dragged down into Earth's mantle, it warms up and releases water trapped within the rock. In a process called serpentinization, the water bubbles out of the plate and transforms the physical properties of the upper mantle. This transformation allows methane and other compounds to percolate out of the mantle through hot springs on the otherwise frigid ocean floor.

Similar processes on early Earth could have supplied the raw ingredients for metabolism, which may have given rise to the first replicating cells. Cameron brought back evidence of such cells' modern descendants: microbial mats, clumps of microbes that thrive beneath nearly seven miles of water, where sunlight can't penetrate and pressure is more than 1,000 times that of sea level.

"It's really exciting, because it links plate tectonics with life," said Keith Klepeis, a geologist at the University of Vermont. "It gives us ideas of what to look for elsewhere in the solar system. It gives us an idea of what early life could have been on Earth."

Cameron's record-setting dive was not the only expedition to demonstrate a connection between plate tectonics and ocean life. Recent research ties plate tectonic activity to the burst of evolution called the Cambrian explosion, 541 million years ago, when a stunning array of new, complex life arose.

In December 2015, researchers in Australia published a study of roughly 300 drill cores from seafloor sites around the globe, some containing samples that were 700 million years old. They measured phosphorus as well as trace elements like copper, zinc, selenium and cobalt — nutrients that are essential for all life. When these nutrients are abundant in the oceans, they can spark rapid plankton growth. The researchers, led by Ross Large of the University of Tasmania, showed that these elements increased in concentration by an order of magnitude around 560 to 550 million years ago.

Large and his team argue that plate tectonics drove this process. Mountains form when continental plates collide and shove rock skyward, where it can more readily be battered by rain. Weathering then slowly leaches nutrients from the mountains into the oceans.

Maybe more surprisingly, Large and his colleagues also found that these elements were low in abundance during more recent periods — and that these periods coincided with mass extinctions. These nutrient-poor periods happened when phosphorus and trace elements were being consumed by the Earth faster than they could be replenished, Large said.

Tectonic activity also plays an essential role in maintaining the long-term stability of Earth's thermostat. Consider the case of carbon dioxide. A planet with too much carbon dioxide could end up like Venus, a planetary blast furnace. Plate activity on Earth has helped to regulate the level of carbon dioxide over the eons.

The same weathering that pulls nutrients from mountaintops down into the oceans also helps to remove carbon dioxide from the atmosphere. The first step of this process happens when atmospheric carbon dioxide combines with water to form carbonic acid — a compound that helps to dissolve rocks and accelerate the weathering process. Rain brings both carbonic acid and calcium from dissolved rocks into the ocean. Carbon dioxide also dissolves directly into the ocean, where it combines with the carbonic acid and dissolved calcium to make limestone, which falls to the ocean floor. Eventually, over unimaginable eons, the sequestered carbon dioxide is swallowed by the mantle.

"That is something that regulates CO2 in the atmosphere on long timescales," Huntington said.

Plate tectonics might even be responsible for another atmospheric ingredient, and arguably the most important: oxygen.

A full 2 billion years before the Cambrian explosion, back in the Archean eon, Earth had hardly any of the air we breathe now. Algae had begun to use photosynthesis to produce oxygen, but much of that oxygen was consumed by iron-rich rocks that used the oxygen to make rust.

According to research published in 2016, plate tectonics then initiated a two-step process that led to higher oxygen levels. In the first step, subduction causes the Earth's mantle to change and produce two types of crust — oceanic and continental. The continental version has fewer iron-rich rocks and more quartz-rich rocks that don't pull oxygen out of the atmosphere.

Then over the next billion years — from 2.5 billion years ago to 1.5 billion years ago — rocks weathered down and pumped carbon dioxide into the air and oceans. The extra carbon dioxide would have aided algae, which then could make even more oxygen — enough to eventually spark the Cambrian explosion.

Plate tectonics may also have given life an evolutionary boost. Robert Stern, a geologist at the University of Texas, Dallas, thinks plate tectonics arose sometime in the Neoproterozoic era, between 1 billion and 540 million years ago. This would have coincided with a period of unusual global cooling around 700 million years ago, which geologists and paleoclimate experts refer to as "snowball Earth." In April, Stern and Nathaniel Miller of the University of Texas, Austin, published research suggesting that plate tectonics would have catastrophically redistributed the continents, disturbing the oceans and the atmosphere. And, Stern argues, this would have had major consequences for life.

"You need isolation and competition for evolution to really get going. If there is no real change in the land-sea area, there is no competitive drive and speciation," Stern said. "That's the plate tectonics pump. Once you get life, you can really make it evolve fast by breaking up continents and continental shelves and moving them to different latitudes and recombining them."

Stern has also argued that plate tectonics might be necessary for the evolution of advanced species. He reasons that dry land on continents is necessary for species to evolve the limbs and hands that allow them to grasp and manipulate objects, and that a planet with oceans, continents and plate tectonics maximizes opportunities for speciation and natural selection.

"I think you can get life without plate tectonics. I think we did. I don't think you can get us without plate tectonics," he said.

Stern imagines a far future in which orbiting telescopes can determine which exoplanets are rocky, and which ones have plate tectonics. Emissaries to distant star systems should aim for the ones without plate tectonics first, he said, the better to avoid spoiling the evolution of complex life on another world.

Cracking Earth's Shell

But everything depends on when the process started, and that's a big open question.

Earth formed about 4.54 billion years ago and started out as an incandescent ball of molten rock. It probably did not have plate tectonics in any recognizable form for at least 1 billion years after its formation, mostly because the newborn planet was too hot, said Craig O'Neill, a planetary scientist at Macquarie University in Australia.

Back then, as now, convection within the planet's inner layers would have moved heat and rock around. Rock in the mantle is squeezed and heated in the crucible of Earth's innards and then rises toward the surface, where it cools and becomes denser, only to sink and start the process again. Picture a lava lamp.

Through convection, vertical motion was happening even on the early Earth. But the mantle at that time was relatively thin and "runny," O'Neill said, and unable to generate the force necessary to break the solid crust.

"Subduction wasn't happening. There was no horizontal motion," Klepeis said. "So there was a time before continents, before the first continent formed" — the time before land, if you will. Earth would have had a so-called "stagnant lid," without disparate plates.

O'Neill published research in 2016 showing that early Earth might have been more like Jupiter's volcanic moon Io, "where you have a volcanically active regime, and not a lot of lateral motion," O'Neill said. As the planet began to cool, plates could more readily couple with the mantle below, causing the planet to transition into an era of plate tectonics.

This raises the question of what cracked the lid and created those plates in the first place.

Some researchers think an intrusion might have gotten things moving. In the past two years, several teams of researchers have proposed that asteroids left over from the birth of the solar system might have cracked Earth's lid. Last fall, O'Neill and colleagues published research suggesting that a bombardment of asteroids, half a billion years after Earth formed, could have started subduction by suddenly shoving the cold outer crust into the hot upper mantle. In 2016, Maruyama and colleagues argued that asteroids would have delivered water along with their impact energy, weakening rocks and enabling plate movement to start.

But it's possible Earth didn't need a helping hand. Its own cooling process may have broken the lid into pieces, like a cake baked in a too-hot oven.

Three billion years ago, Earth may have had short-lived plate tectonic activity in some regions, but it was not widespread yet. Eventually, cooler areas of crust would have been pulled downward, weakening the surrounding crust. As this happened repeatedly, the weak areas would have gradually degraded into plate boundaries. Eventually, they would have formed full tectonic plates driven by subduction, according to a 2014 paper in Nature by David Bercovici of Yale University and Yanick Ricard of the University of Lyon in France.

Or the opposite might have happened: Instead of cold crust pushing down, hot mantle plumes — like the kind that are driving Hawaii's eruptions — could have risen to the surface, percolating through the crust and melting it, breaking the lid apart. Stern and Scott Whattam of Korea University in Seoul showed how this could work in a 2015 study.

According to these theories, plate tectonics may have started and stopped several times before picking up momentum about 3 billion years ago. "If you had to press everyone's buttons and make them take a number, there's a running ballpark in the community that around 3 billion years ago, plate tectonics started emerging," O'Neill said.

Yet it's hard to know for sure because the evidence is so fragmentary.

"Oceanic crust is only 200 million years old. We're just missing the evidence that we need," O'Neill said. "There's a lot of geochemistry that's come a long way since the 1980s, but the same fundamental questions are still there."

The oldest rocks on Earth suggest that some sort of proto-subduction was happening as far back as 4 billion years ago, but these rocks are hard to interpret, O'Neill said. Meanwhile, sometime between 3 billion and 2 billion years ago, Earth's mantle apparently underwent several chemical changes that can be attributed to cooling, changing its convection pattern. Some geologists take this as a recording of the gradual onset and spread of tectonic plates throughout the planet.

"The real answer is we don't know," said Brad Foley, a geophysicist at Pennsylvania State University. "We've got these rocks, but we can't figure out what's the smoking gun that would tell us there is plate tectonics or subduction at this time, or there definitely wasn't."

Plates on Other Planets

So are tectonics essential to life?

Ultimately, the problem is that we have one sample. We have one planet that looks like Earth, one place with water and a slipping and sliding outer crust, one place teeming with life. Other planets or moons may have activity resembling tectonics, but it's not anything close to what we see on Earth.

Take Enceladus, a frozen moon of Saturn that is venting material into space from strange-looking fractures in its global ice crust. Or Venus, a planet that seems to have been resurfaced 500 million years ago but has no plates that we can discern. Or Mars, which has the solar system's largest volcano in Olympus Mons, but whose tectonic history is mysterious. Olympus Mons is found in a great bulging province called Tharsis, which is so gigantic that it might have weighed down Mars' crust enough to cause its poles to wander.

O'Neill has published research showing that a Mars-size planet with abundant water could be pushed into a tectonically active state. And others have argued that some regions in Mars' southern hemisphere resemble seafloor spreading. But researchers agree it hasn't had any action for at least 4 billion years, which is roughly the age of its crust, according to data from orbiters and robots on the surface.

"There is some argument that maybe very, very early on, it could have had plate tectonics, but my view is it probably never did," Foley said.

The InSight Mars lander, which launched in May and is scheduled to arrive on November 26, will help settle the debate. InSight's three instruments aim to measure the thickness and makeup of the Martian crust, mantle and core, providing new clues as to how Mars lost its magnetic field and whether it once had plate tectonics.

"If we can understand other planets, like Venus and Mars, and the moons of Jupiter, it helps us know what to look for here on Earth. It's a reason to keep exploring other planets — it helps us back home," Klepeis said.

While the origins of plate tectonics remain a subject for debate, geologists can agree that at some point, the gears will stop grinding.

O'Neill has come to think of plate tectonics as a middle-age phase for rocky planets. As a planet ages, it may evolve from a hot, stagnant world to a warm, tectonically active one, and finally to a cold, stagnant one again in its later years. We know planets can grow quiet as they cool down; many geologists think this is what happened to Mars, which cooled off faster than Earth because it is so much smaller.

Earth will eventually cool down enough for plate tectonics to wane, and for the planet to settle down into a stagnant-lid state once more. New supercontinents will rise and fall before this happens, but at some point, earthquakes will cease. Volcanoes will shut off for good. Earth will die, just like Mars. Whether the life forms that cover its every crevice will still be here is a question for the future.

fonte: Quanta Magazine
https://www.quantamagazine.org/plate-tectonics-may-be-essential-for-life-20180607/

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