The secret of the Earth

One of the questions currently keeping astrobiologists (the people who would like to study life on other planets if only they could find some) awake at night is, what is the crucial difference that allowed the emergence and evolution of life on Earth, while its neighbours remained sterile?

In their violent youth, all the inner planets started out with so much surplus heat energy—from planetary accretion and radioactive decay—that their surfaces melted to form magma oceans hundreds or thousands of kilometres deep. Such oceans lose internal heat to space rapidly—so rapidly that within a few tens of millions of years, the surface of a young planet cools and solidifies to create a hot stagnant lid above a vigorously convecting mantle.

Through this mantle rise plumes of hot buoyant rock, in a “bottom-up” form of tectonics. Close to the surface, these plumes melt to produce a thick, light crust above a thin layer of cooled and rigid mantle. Together, these two layers form a primitive shell known as the lithosphere. As it is less dense overall than the mantle beneath, this lithosphere is buoyant and therefore cannot sink very far before choking any early subduction zones that might form—a condition called “trench lock.”

Planetary evolution is, to a great extent, a story of heat loss. Sooner or later, all rocky, Earth-like planets arrive at a major fork in the road to heat death. Most will continue along the stagnant lid highway, perhaps with occasional overturn of their surfaces. This may have occurred beneath the northern hemisphere of Mars early in its history, creating the so-called “Martian Dichotomy,” or on Venus around 600 million years ago, completely renewing its surface, a style known as “episodic lid” tectonics.

A few planets, however, will take the road less travelled, the road that leads to the creation of moving plates. This was the route chosen by the Earth at the end of the Archean eon, 2.5 billion years ago. The transformation was precipitated by the onset of deep subduction as the cooling lid became denser and eventually “negatively buoyant,” allowing sheets of lithosphere to sink thousands of kilometres into the lower mantle, thereby providing a driving force for the surface plates, to which they were still attached.

Which route a rocky planet (as opposed to a gas giant) chooses depends on factors that are still imperfectly understood. Computer simulations of mantle convection suggest that the choice of stagnant or mobile lid depends ultimately on the flow of heat out of the molten iron core, which thus heats the mantle from below, and the generation of heat within the mantle by radioactive decay of the unstable isotopes ²³⁸U, ²³⁵U, ²³²Th, and ⁴⁰K. The balance between these two determines the overall pattern of mantle convection and the degree of coupling between the overturning mantle and the plates above.

Through time, both sources wane as heat is lost through the surface, affecting buoyancy and viscosity: the chilled lid becomes less buoyant while the mantle beneath becomes more viscous, exerting more drag on the lid. Eventually, a point is reached at which the “hot stagnant lid” becomes unstable. It then fragments and starts to collapse into the interior, marking a transition to the episodic lid mode in which a stagnant lid re-forms after the collapse event, only to be followed, some tens of millions of years later, by another collapse. Further cooling may lead to a second transition from episodic lid to fully mobile lid tectonics, that is, to plate tectonics.

Many now believe that the transformation to plate tectonics was the step that allowed life on Earth to develop beyond the microbial stage. By exchanging elements such as carbon, hydrogen, and phosphorus between the surface and the deep mantle, plate tectonics made the world inhabitable for complex organisms such as wise apes. Plate tectonics also provided the thermostat that maintained conditions at the surface within the narrow limits necessary to retain liquid water and prevent a descent into “frozen planet” mode or, worse, a runaway greenhouse resulting in a Venus-like desert.

Thus, while it appears that microbes can evolve very early in a planet’s evolution—and should therefore be common throughout the galaxy—animals, plants, and fungi are a different matter altogether, requiring billions of years of incubation on worlds that experience a high level of stability. No wonder, then, that some have taken to referring to Earth as the “Goldilocks planet.”

Finding exoplanets with plate tectonics is a difficult, and possibly insurmountable, challenge, even with the largest and most expensive telescopes. Astrobiologists are therefore focussing their efforts on building an inventory of “Earth-like” planets orbiting within the “habitable zones” of their stars, the band in which surface temperatures are estimated to be right for the maintenance of liquid water, thought to be essential for life. However, unless such planets are able to support plate tectonics for billions of years, they are likely to be as sterile as Venus or Mars. Disappointingly, recent calculations suggest that only about a third of the stars in our galaxy even have the correct chemical composition to produce “Earth-like” planets on which a negatively buoyant lid may form.

All in all, the Earth’s mobile lid has turned out to be a very good thing for us and for all multicellular life, but like all good things, it will come to an end. For plate tectonics is its own worst enemy, being such an efficient mode of shedding heat that, in a billion years or so, it will have cooled the Earth sufficiently that the upper mantle will no longer be able to melt as plates are ripped apart at mid-ocean ridges. The consequence will be what Bob Stern calls “ridge lock:” opposing plates will become welded together and the planet will pass into its “cold stagnant lid” phase. At about the same time, the oceans will have evaporated owing to the gradual increase in the Sun’s luminosity, finally exposing the tennis ball seam of mid-ocean ridges just as sea-floor spreading comes to an end.

The Earth therefore occupies the space between two fatal extremes, and has done so for two and a half billion years, long enough for microbes to evolve into Man. The demise of plate-driven convection will, however, spell the end of the world as we know it: without the life-support services provided by plate tectonics, the Earth will once again be inhabited only by microbes, eventually becoming as lifeless as its neighbours. If the inhabitants of a planet orbiting a star in the elliptical galaxy known affectionately as NGC 4709, 200 million light years distant in the constellation Centaurus, were to launch a spacecraft towards Earth today, travelling, like Stephen Hawking’s laser-propelled nanoprobes at a fifth of the speed of light, it would arrive to find a planet rather like Venus: hot and dead.

Featured image credit: Space by Melmak. CC0 via Pixabay.