Study: Geological Habitability Parameters Imply Earth is Special and Advanced Life Extremely Rare
Looking forward to the August 27 release of the new edition of The Privileged Planet, which you can pre-order now, we’ve been considering issues of habitability — on Mars (here and here), and on the moons Enceladus and Titan. Now comes a new paper in Scientific Reports, “The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations.” It argues that extraterrestrial “advanced communicative civilizations” (or ACCs) must be extremely rare in the universe. Their logic is simple: ACCs would require planets with continents, oceans, and plate tectonics, but these major geological features of Earth are likely to be very rare elsewhere. If these features are required for ACCs, and if they are rare, then such advanced civilizations must also be very rare.
An excellent article at Nautilus, “The Odds That Aliens Exist Just Got Worse,” explains the paper’s basic findings: “The likelihood that other technologically sophisticated societies exist is smaller than previously thought, because basic amenities we take for granted on Earth — continents, oceans, and plate tectonics — are cosmically rare.” But if we take these findings out of the context of current obsessions over aliens, then the implication, of course, is simply that Earth is very privileged because the presence of continents, oceans, and plate tectonics on our planet is extremely rare.
Unknowns of the Drake Equation
As a geologist, what I find most fascinating about this Scientific Reports article is that it adds new geological factors to the Drake equation. That is the famous equation that is used to roughly calculate the number of technologically advanced extraterrestrial civilizations in our galaxy. The Nautilus article notes that the values of some factors in the classical formulation of the Drake equation are very difficult to estimate, particularly:
the fraction of potentially habitable planets on which life likely has emerged (a variable that’s completely unconstrained, since only one case — ours — is known); the fraction of those planets on which intelligent life has developed (a criterion that often elicits dark humor about whether human life qualifies); the fraction of that fraction that have sent signals into deep space (again, just one known example, out-going calls only); and the length of time those civilizations have been sending such signals (to be determined).
Some argue that even the low ends of reasonable estimated ranges for these factors imply that ET life should be sending telecommunications to us Earthlings. So why aren’t we getting any signals? This absence of ET life communicating with us is known as the “Fermi Paradox.”
How to Resolve the Fermi Paradox?
There are many possible ways to understand why we aren’t hearing from ET life — not the least of which is that, as the best evidence suggests, life is extremely unlikely to arise via natural chemical means. But the Nautilus article explains that the study tried to resolve the paradox by arguing that very few planets have the requisite geological factors needed for advanced life to arise:
Bringing a geologic perspective to the problem, Stern and Gerya propose to resolve the paradox by adding two more factors to the already unwieldy Drake equation: the fraction of habitable planets with distinct continents and oceans; and the fraction of those planets with a plate tectonic system that has operated for at least 500 million years. The values of these terms are very small, they argue, because the development of distinct landmasses and water bodies, and the tectonic habit of crustal recycling — characteristics of Earth that we take for granted — are unlikely outcomes in the evolution of rocky planets.
With these new factors, the number of advanced civilizations in our galaxy that might communicate with us falls to … almost zero.
The technical paper explains that a planet where ACCs evolve requires ocean basins, continental dry land, and plate tectonics: “Although primitive life must evolve in the sea, advanced communicative civilizations must evolve on dry land.” In light of these requirements, it proposes geological terms for resolving the Fermi Paradox:
We resolve the Fermi Paradox (1) by adding two additional terms to the Drake Equation: foc (the fraction of habitable exoplanets with significant continents and oceans) and fpt (the fraction of habitable exoplanets with significant continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by demonstrating that the product of foc and fpt is very small (< 0.00003–0.002).
Note the value at the end: “< 0.00003–0.002.” If you add a factor with that value to the Drake equation, it suggests that ACC life elsewhere in the galaxy is extremely rare — if it exists at all.
Plate Tectonics Necessary for Life
There are multiple reasons why plate tectonics is necessary for life — something I spoke about in my talk at Discovery Institute’s 2022 Dallas Conference on Science and Faith.
Thus, I wholeheartedly agree with the Nautilus article when it states:
Most geologists will agree with Stern’s and Gerya’s argument that plate tectonics should be included as a criterion for long-term planetary habitability. Earth’s tectonic system allows the planet’s atmosphere and hydrosphere to remain in communication with its interior, in a remarkable, self-perpetuating cycle. Subducted ocean crust — seafloor that slips down into Earth’s interior — carries water back into the mantle, and at shallow depths, this water lowers the melting temperature of mantle rock, giving rise to unusual magmas that create the continental crust — what we surface dwellers live on — which is rich in rare elements, like phosphorus, that are critical to life.
At greater depths, subducted water acts to decrease the viscosity of the mantle, allowing it to churn, or convect, more vigorously — which in turn drives plate motion. When the Earth’s mantle exports heat via convection, it encourages the liquid iron outer core to convect as well, and this generates Earth’s protective magnetic field, which shields the surface environment from harmful cosmic radiation. Without plate tectonics, continents would quickly be eroded to sea level. But tectonic collisions continuously rejuvenate Earth’s topography, providing rivers with more energy to transport nutrient-rich sediments to shallow marine environments. In other words, plate tectonics is entangled with all the phenomena that support life on Earth.
So plate tectonics is vital for generating our magnetic field (necessary for life), maintaining the presence of life-necessary elements in the oceans, and creating both continents and oceans (which, if you haven’t noticed, are also vital for advanced life). This is all very reasonable, as is the study’s conclusion that plate tectonics is probably very rare on planets in the universe.
One reason for this is that all of the other rocky planets in our solar system have something called “stagnant lid” tectonics. That’s where the lithosphere of a planet is composed of a single plate and only vertical tectonic movement occurs due to upwelling from the mantle, but not horizontal plate movement. Earth is the only known planet to have horizontal plate tectonics movement, suggesting it is rare:
How Old Is Plate Tectonics on Earth?
One thing that is likely to get some pushback is the study’s claim that modern-style plate tectonics on Earth did not commence until the Neoproterozoic, which lasted from about 1 billion years ago until the beginning of the Cambrian period (about 540 million years ago). Many geologists, including many who work in paleomagnetism (the field of my PhD research), would argue that plate tectonics began very early in Earth’s history. That’s because we see evidence of continents on the move going all the way back into the Archean. But this controversy should not affect the paper’s basic claim. Here’s why:
The value of their fpt factor, added to the Drake equation, assumes that plate tectonics has only been operating on Earth for about 0.5 Ga (Ga = gigaannum, or a billion years). If plate tectonics has been operating for longer than that, it would presumably make Earth even more special, because plate tectonics would be longer lasting. In other words, in estimating the specialness of Earth, the paper’s calculations are conservative. In any case, however long plate tectonics has been operating on our planet, it seems that any other planets with plate tectonics, continents, and oceans are rare, and very special indeed.
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