Privileged Planet: tectonic plates required (Introduction)

by David Turell , Wednesday, April 03, 2019, 22:15 (2061 days ago) @ David Turell

It is proposed that a planet must have a surface with movable subduction floating plates, or life will not exist. So far early analysis of known planets shows none are like that :

https://www.scientificamerican.com/article/toffee-planets-hint-at-earths-cosmic-rarity/...

"That has not stopped Byrne’steam speculating on what its findings might mean for the myriad super Earths already discovered beyond our solar system. The most striking possibility is that super Earths might not be able to sustain plate tectonics—the drifting of continents and cycling of crustal rock that intimately shapes Earth. Without that process, you can say goodbye to the building of mountains, the creation of oceans and plenty of a planet’s volcanoes, and, just maybe, the evolution of complex life itself.

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"Byrne and his colleagues’ work hinges on defining the point at which rocks deep below a planet’s surface no longer break in a mechanical way and instead begin to move like hot plastic. This point, known as the brittle-ductile transition (BDT), depends on how the pressure and temperature change with depth. For our own world’s crust, the BDT lies about 15.5 miles below the surface, although it varies quite a bit. But what about on super Earths, where greater gravitational forces would correspondingly increase pressures on rock? At what depths would BDTs emerge on such alien planets?

"The calculations revealed the BDT depths for those super Earths to be shockingly shallow, with some scarcely more than a mile beneath the surface. A nearby star, a suffocating atmosphere or an abundance of internal, radioactivity-generated heat could further bake the top of such a world, perhaps raising the BDT all the way to the surface, creating a full-blown toffee world.

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"With that in mind, Robert Stern, a geoscientist at the University of Texas at Dallas, who was not involved in the research, says that this ambitious work deserves credit, because these sorts of numerical models will help paint a picture of what exoplanetary geology may be like. “In my lifetime, I’ve seen the solar system turn from something that belonged to astronomers to something that belonged to geologists,” he says. “We’re not there yet with exoplanets, but you can see this is a step in that direction.”

"Although incremental and provisional, the toffee worlds hypothesis could represent a sizable step indeed, as it directly addresses a question foremost in many an exoplanet-pondering geologist’s mind: Are worlds with plate tectonics common as dirt or vanishingly rare? Either way, the answer has game-changing implications.

"In order for plate tectonics to exist, a planet needs a few ingredients. Water is probably vital, because it weakens the mantle and permits chunks of the planet to slip and slide that otherwise would remain immobile. A world’s plates also must be sufficiently thick and dense to sink into the mantle—a crucial step for initiating and stabilizing the tectonic cycle over eons. Water or no, toffee worlds’ wafer-thin, brittle layers would not be able to dive deep, short-circuiting the “engine” of plate tectonics before it could even start.

"This concept reinforces the notion that plate tectonics is a rare feature in the cosmos, Stern says. After all, as far as we can presently see, Earth is the only planet where it operates.

"Aside from making toffee planets geologically dull, the absence of plate tectonics could also significantly reduce continental erosion and runoff into any oceans. This, Stern explains, would rob toffee worlds of a nutrient pump than may have given life a huge boost on ancient Earth. Plate tectonics also acts as Earth’s thermostat, keeping the planet’s temperature stable on geological timescales by buffering the levels of atmospheric greenhouse gases. Both of these tectonic side effects may be essential for the development of complex life."

Comment: All of these characteristics are well described in the three books that carefully outline how special the Earth is, and HAS TO BE, for life to appear and survive. We can only look at solar systems in this galaxy. We may well be the only Earth here, but we will never know about what might be there in other large galaxies. The question of whether we are alone in the universe can never be answered.