Summary of Our Improbable Universe
Chapter 5: Mother Earth
The heavy elements, from which out bodies are made, did not exist in the very early universe. First generation stars generated these elements deep in their cores. In supernova explosions these elements were blasted into space. At the same time the heavy metals like gold, lead, and uranium were built up from iron. The vapors of these elements condensed in space to form simple molecules and microscopic particles of dust. Together with unburned hydrogen and helium, they formed giant molecular clouds. The stepping stones to life that were required to set this stage were described in earlier chapters.
After billions of years, the gas and dust in a giant molecular cloud in our vicinity of the Milky Way Galaxy, was disturbed by the shock wave from another supernova explosion. The resulting increase in density triggered the gradual gravitational collapse of a region of that cloud into the solar nebula that became our sun and solar system. As the process proceeded the nebula became hotter and hotter from the gravitational compression. This heating is the same type of phenomena that occurs during the compression stroke of a diesel engine. The hot gasses resisted the compression until the dust could radiate away the excess heat.
The mild turbulence of the parent cloud led to rapid rotation as the solar nebula contracted. This is similar to the way that a figure skater spins rapidly by drawing in her arms. The centrifugal acceleration, of the rapid rotation, inhibited contraction at the equator but not at the poles. Therefore the cloud contracted into a spinning disk. Disks like this have been observed by the Hubble space telescope in the Orion nebula. At the center of every disk is a sphere of gas known as a proto star. These spheres are destined to become stars. The material in these disks will condense into planetary systems like our own. Almost all of the proto stars that have been observed by the Hubble telescope have these disks. Therefore planetary systems in which life can evolve are very common.
As the proto star that formed our sun contracted it heated. When the temperature at its center reached about five million degrees it caught fire. At first it burned its scant supply of high octane deuterium into helium. The infant star became an infant terrible. Its outer layers expanded out to the orbit of the Earth and singed the inner planets. This drove almost all of the light elements like hydrogen and helium out of the inner solar system and left behind rocky planets like our own. Without this scalding, our Earth would be a gaseous planet like Jupiter and life would have been impossible. The parts per million supply of deuterium that was created in the first three minutes of the Big Bang was yet another stepping stone to our existence (but it isn't on the list of 14 because it may not be needed elsewhere).
The initial supply of deuterium was soon used up and the Sun settled down into the slow burning of hydrogen into deuterium and helium. The molten Earth cooled and continued to accumulate material. The hotter interior drove internal convection. The hot rock churned like a pot of boiling water. At the surface it released trapped gases like water vapor, carbon dioxide, and nitrogen. Over millions of years the surface cooled enough for the molten rock to solidify into a crust. Further cooling allowed the water vapor to rain out of the atmosphere and form the ocean.
As the cooling process proceeded the Earth became less volcanic. The volcanism would have become rare billions of years ago if the interior did not possess a source of heat beyond its initial store. The decay of natural radio active materials like potassium 40, uranium and thorium have provided enough internally generated heat to keep the interior in a molten state and volcanoes going. Without volcanism, the Earth would have become a dead planet like Mars.
Because Mars is very much smaller than Earth, it did not have a strong enough heat engine to maintain volcanism. When its volcanoes extinguished it lost its source of carbon dioxide. The gradual removal of carbon dioxide from its atmosphere by the precipitation of carbonate rocks, like lime stone, eventually depleted its atmosphere of carbon dioxide. The resulting loss of a green house effect cooled its upper atmosphere. This allowed water vapor to circulate into the upper atmosphere. Most of it then leaked into space as hydrogen gas when it was broken apart by ultra-violet light from the sun. In this way the oceans of Mars were gradually lost to space.
Earth would have lost its carbon dioxide and then its oceans billions of years ago if it had possessed an inadequate quantity of heat from natural radioactivity. Furthermore, the production of sugar from carbon dioxide by plants would be halted. The primary foundation of the food chain of the planet would be destroyed. Therefore the existence of long lived, naturally radioactive elements is yet another stepping stone to intelligent life. Bacterial life could get started without it but the life of the biosphere would have been too short for us to have evolved without elements like potassium 40, uranium, and thorium.
The universe that resulted from the Big Bang could easily have been incapable of producing adequate quantities of deuterium and long lived naturally radioactivity for Earth to have been a planet on which intelligent life could evolve. Without the rapid burning of deuterium at the birth of the sun the Earth would have far to much gas and vapor from the start. Without the slow release of gas and vapor that long lived radioactivity stimulates, the atmosphere and the ocean would be lost. But these stepping stones, and at least twelve other physical properties, were in a perfect configuration to allow the evolution of biological life.