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Claim: A planet does not need plate tectonics to sustain life

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Artist’s concept of Kepler-69c, a super-Earth/NASA

Plus, a friend offers a reply. From ScienceDaily:

There may be more habitable planets in the universe than we previously thought, according to Penn State geoscientists, who suggest that plate tectonics — long assumed to be a requirement for suitable conditions for life — are in fact not necessary.

When searching for habitable planets or life on other planets, scientists look for biosignatures of atmospheric carbon dioxide. On Earth, atmospheric carbon dioxide increases surface heat through the greenhouse effect. Carbon also cycles to the subsurface and back to the atmosphere through natural processes.

“Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment,” said Bradford Foley, assistant professor of geosciences. “Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”

Most of Earth’s volcanoes are found at the border of tectonic plates, which is one reason scientists believed they were necessary for life. Subduction, in which one plate is pushed deeper into the subsurface by a colliding plate, can also aid in carbon cycling by pushing carbon into the mantle.

Planets without tectonic plates are known as stagnant lid planets. On these planets, the crust is one giant, spherical plate floating on mantle, rather than separate pieces. These are thought to be more widespread than planets with plate tectonics. In fact, Earth is the only planet with confirmed tectonic plates.

Foley and Andrew Smye, assistant professor of geosciences, created a computer model of the lifecycle of a planet. They looked at how much heat its climate could retain based on its initial heat budget, or the amount of heat and heat-producing elements present when a planet forms. Some elements produce heat when they decay. On Earth, decaying uranium produces thorium and heat, and decaying thorium produces potassium and heat.

After running hundreds of simulations to vary a planet’s size and chemical composition, the researchers found that stagnant lid planets can sustain conditions for liquid water for billions of years. At the highest extreme, they could sustain life for up to 4 billion years, roughly Earth’s life span to date.

“You still have volcanism on stagnant lid planets, but it’s much shorter lived than on planets with plate tectonics because there isn’t as much cycling,” said Smye. “Volcanoes result in a succession of lava flows, which are buried like layers of a cake over time. Rocks and sediment heat up more the deeper they are buried.”

The researchers found that at high enough heat and pressure, carbon dioxide gas can escape from rocks and make its way to the surface, a process known as degassing. On Earth, Smye said, the same process occurs with water in subduction fault zones.Paper. (open access) – Bradford J. Foley, Andrew J. Smye. Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets. Astrobiology, 2018; 18 (7): 873 DOI: 10.1089/ast.2017.1695 More.

A knowledgeable friend writes to say,

The article focuses on carbon recycling as essential for life. It posits that a “stagnant lid” planet without plate tectonics can still spew carbon into the atmosphere. But it doesn’t really deal with “weathering”: Plate tectonics not only adds carbon to the atmosphere but but removes carbon from it through weathering. This cyclical process provides a “steady state” carbon allowance that helps maintain a balanced climate.

Also, plate tectonics does much more for life than just recycle carbon. Consider this quote from an article last month in Quanta:

“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.”

“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.”

In sum, I don’t think this article has made its case that plate tectonics is not needed for life. See: Why Earth’s Cracked Crust May Be Essential for Life (Quanta)

See also: “Behold, countless Earths sail the galaxies … that is, if you would only believe …

and

Don’t let Mars fool you. Those exoplanets teem with life!

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