Chapter 43 — The Thermostat Beneath Our Feet

Earth has maintained surface temperatures capable of supporting liquid water for approximately 4 billion years—despite the Sun having brightened by approximately 30 percent over that period. This long-term stability is not an accident of orbital mechanics. It is the product of a geochemical feedback system called the carbon-silicate cycle, operating through plate tectonics.

The cycle regulates itself. Atmospheric carbon dioxide dissolves in rainwater to form weak carbonic acid, which weathers silicate rocks on the continents. The weathering products wash to the ocean and are incorporated into carbonate sediments on the seafloor. Plate tectonics subducts these carbonates into the mantle, where high temperatures release the carbon dioxide back to the atmosphere through volcanic activity. When temperatures rise, weathering accelerates, drawing down CO₂ and cooling the climate. When temperatures fall, weathering slows, CO₂ builds up from continued volcanic outgassing, and the climate warms. The thermostat is self-correcting over geological timescales.

The thermostat requires plate tectonics, and plate tectonics requires a precise combination of planetary conditions: sufficient internal heat to drive mantle convection, the right crustal composition and thickness, water in the mantle to reduce viscosity, and a planetary size large enough to retain internal heat over billions of years. Mars, too small, has no active carbon-silicate cycle. Venus, possibly lacking plate tectonics due to a dry interior, experienced runaway greenhouse warming. The same geophysical machinery that keeps Earth's temperature regulated is the geophysical machinery that is extraordinary in our solar system. The thermostat and the planet are the same thing.

Walker, J.C.G., Hays, P.B., Kasting, J.F. "A negative feedback mechanism for the long-term stabilization of Earth's surface temperature." Journal of Geophysical Research: Oceans 86, no. C10 (1981): 9776–9782. Gonzalez, G. and Richards, J.W. The Privileged Planet. Regnery Publishing, 2004. Chapter 5.

Chapter 44 — The Perfect Eclipse

The Sun is approximately 400 times larger than the Moon. It is also approximately 400 times farther from Earth. These two facts together produce the most remarkable geometric coincidence in the solar system: from Earth's surface, the Sun and the Moon have nearly identical apparent sizes in the sky—approximately 0.5 degrees across. During a total solar eclipse the Moon covers the Sun's disk almost exactly, revealing the solar corona and chromosphere while stars become visible in the darkened sky.

This geometry has not been scientifically trivial. In 1919 Arthur Eddington observed a total solar eclipse from the island of Príncipe and measured the deflection of starlight passing near the Sun. His result confirmed Einstein's general theory of relativity and produced one of the most consequential experimental verifications in the history of science. Coronal spectroscopy during total eclipses has driven major advances in solar physics. The element helium was discovered in the solar spectrum during an eclipse in 1868, before it was identified on Earth. The chromosphere and corona are effectively inaccessible to direct observation except during totality.

The coincidence is not static. The Moon is receding from Earth at approximately 3.8 centimeters per year. Hundreds of millions of years ago the Moon's disk was larger; hundreds of millions of years in the future it will be too small to produce total eclipses. The window in which the Moon and Sun appear the same size is the window in which complex life capable of building telescopes and testing physical theories exists on Earth. Gonzalez and Richards observed that the same conditions making Earth habitable—the right-sized Moon for axial stability, the right orbital distance for liquid water—also produce the exceptional observational geometry that made possible the confirmation of general relativity. The parameters converge.

Gonzalez, G. and Richards, J.W. The Privileged Planet. Regnery Publishing, 2004. Chapter 2. Dyson, F.W., Eddington, A.S., Davidson, C. "A Determination of the Deflection of Light by the Sun's Gravitational Field." Philosophical Transactions of the Royal Society A 220 (1920): 291–333.