The phrase “a hard nut to crack” aptly describes coconuts, Brazil nuts, and spies trained to resist interrogation. A new research study by Yale University geophysicist Jun Korenaga demonstrates that planets are hard to crack, too.1 In fact, planets are much harder to crack than planetary astronomers presumed previously.
For life, advanced or primitive, to exist for more than a few million years, a planet’s crust must easily crack into moveable plates that can slide both past and underneath one another. Geophysicists and planetary astronomers call these phenomena plate tectonics.
Without plate tectonics nutrients critical for long-term life support cannot be recycled. Without plate tectonics a planet’s atmosphere cannot sustain all the ingredients that life demands. Without plate tectonics there exists no possibility for compensating for the host star’s changing luminosity so that the planet’s surface temperature remains at levels life can tolerate.
In his paper Korenaga challenges the long-held belief that nearly all super-Earths (rocky planets between 1–10 times Earth’s mass) will manifest enduring plate tectonic activity. This belief was founded on the presumption that a high enough temperature difference between the uppermost and bottommost layers of the planet’s mantle would generate a sufficient contrast in viscosity as to guarantee ongoing plate tectonic activity. Korenaga proves that even if the temperature difference were as unrealistically high as 1,700° Centigrade (3,100° Fahrenheit) no plate tectonics would occur unless the effective friction coefficient at and near the crust-mantle boundary is very low. In fact, Korenaga shows that for a fixed value of the effective friction coefficient the mass of the planet has virtually no bearing at all on whether or not a planet will exhibit plate tectonics. The only factor that matters is the value of the effective friction coefficient at and near the crust-mantle boundary.
Korenaga demonstrates that unless the effective friction coefficient is less than about 0.1 no hope exists for a planet to generate plate tectonics. For Earth, the effective friction coefficient = 0.03. The only way to drive a planet’s effective friction coefficient below 0.1 is for that planet to possess a large amount of long-lasting surface liquid water. The presence of such water will generate hydration chemical reactions that will produce chemical lubricants. On Earth, for example, hydration reactions generate talc at the critical plate and core-mantle boundaries.
Korenaga concludes that a rocky planet must possess a large amount of long-lasting surface liquid water in order to avoid a permanent “stagnant lid,” a planetary crust that never cracks into plates that can slide by or underneath one another. He does not address what form that surface liquid water must take for advanced life to be possible.
Maintaining an adequate amount of surface liquid water is no easy matter. As I explained in my book The Creator and the Cosmos,2 unless a planet’s features are very carefully fine-tuned, the surface liquid water will turn to permanent ice or vapor. In either case plate tectonics grinds to a halt.
Another not-so-easy matter is maintaining the exact amount of surface liquid water advanced life demands. Too little water will produce insufficient chemical lubricants to generate adequate plate tectonic activity. Too much water means either that islands and continents will never appear on the planet’s surface or that they will appear at size levels too small to adequately circulate nutrients, compensate for the host star’s changing luminosity, and maintain the necessary atmospheric ingredients that advanced life requires.
As I describe in previous Today’s New Reasons to Believe articles, too much water is by far the more likely scenario for planets with mass and surface temperature similar to Earth’s. As mentioned in my book More Than a Theory, the quantity and elevation levels of islands and continents appearing on a planet’s surface must vary at highly specified rates throughout the planet’s history for advanced life to exist.3 (See figure 1.)
Korenaga’ research piles up yet more evidence for the rare Earth doctrine. This doctrine states that while planets the size and mass of Earth (some of which may have plentiful surface water) may prove abundant, planets with the just-right characteristics and composition to support advanced life will prove either rare or nonexistent. Such a doctrine is consistent with the Bible’s message that God supernaturally designed Earth and its life for the specific benefit of human beings.
- Jun Korenaga, “On the Likelihood of Plate Tectonics on Super-Earths: Does Size Matter?” Astrophysical Journal Letters 725 (December 10, 2010): L43–L46.
- Though no longer available as a print edition, The Creator and the Cosmos is available as an ebook from Logos Bible Software.
- Hugh Ross, More Than a Theory (Grand Rapids: Baker, 2009): 150–64.