Plate Tectonics and Planetary Evolution: Earth's Thermoregulation and Moon Formation

per year. In the absence of plate tectonics, moreover, atmospheric carbon dioxide would be irreversibly sequestered in sediments as carbonate, and its greenhouse effect would be reduced to the point that the planet would find itself completely frozen in less than a million years. Besides this, in the absence of plate tectonics, in a few tens of thousands of years the reserves of phosphorus, calcium, magnesium, iron, nickel, cobalt, manganese and copper would be exhausted. A striking example is constituted by the Australian continent. Since volcanic activity has been completely exhausted for at least 60 million years, this continent has extremely poor soils that are cultivable only with the addition of fertilizers. Tectonic activity is finally essential for planetary thermoregulation. On the continental crust, interfacing with the atmosphere, rock silicates react with carbon dioxide, producing silicic acid $\mathrm{H_4SiO_4}$ and calcium/magnesium bicarbonate. Transported to the ocean by rainwater, these form silica ($\mathrm{SiO_2}$) and carbonate, which accumulate on the ocean floor. In subduction areas, silica and carbonates are reconverted into silicates and carbon dioxide, which return to the surface through volcanic emanations and tectonic elevations. On the crust, rain removes silicic acid and bicarbonate, so atmospheric carbon dioxide continues to react with rock silicates; in subduction areas, volcanic activity removes carbon dioxide and silicates in the form of gaseous emanations and magma effusions. Rock weathering is sensitive to temperature, so if surface temperature increases, silicate weathering accelerates and greater quantities of carbon dioxide exit the atmosphere. Conversely, if surface temperature decreases, the reaction slows down and the atmospheric concentration of carbon dioxide increases. Since carbon dioxide is a greenhouse gas, this mechanism functions as a global thermostat that maintains the planet's surface temperature within a range compatible with life. The interruption of any part of the process would stop the thermostat and, within a few million years, Earth would heat up incredibly or freeze completely, depending respectively on the removal of carbon dioxide or its emission. The genesis of the Moon and the birth of life. For centuries, the most varied theories have been expressed about the genesis of the Moon. In the twentieth century, however, from analyses of lunar samples recovered in various space missions, it has been possible to deduce some certain facts. First of all, given its dimensions, the Moon formed relatively late. Tungsten isotopic data require that the Moon formed more than 30 million years after the beginning of the solar system, while it is predicted that most objects of these dimensions formed in the first hundreds of thousands of years. Furthermore, the oldest rocks appear to have formed from an ocean of magma. Finally, the isotopic composition of the Moon's oxygen is identical to that of Earth to within 5 parts per million, while that of almost all asteroidal and planetary objects is different. These now established observations, combined with the orbital characteristics of the Earth-Moon system, unanimously suggest that the Moon broke away from Earth at some point in its evolution and did not form independently from it. We can therefore certainly affirm that the separation of the Moon from Earth represented a major bifurcation in the latter's evolution. In our view, it is also easy to note how such bifurcation is closely associated with another turning point in Earth's evolutionary path, related to the birth of life. Earth, in addition to being the only planet in the solar system endowed with life, is also the only one endowed with a satellite of large relative dimensions and which holds most of the angular momentum of the Earth-Moon system. In fact, there are many clues that suggest this is not a coincidence, but that there exists a causal relationship between the two phenomena. A large satellite the size of the Moon has certainly benefited Earth's climate by stabilizing its ecliptic obliquity. Mars, which does not have a large moon, undergoes chaotic variations in obliquity of even 60°. The stabilization of Earth's obliquity might not have been crucial for the origin of life, but it could have...