Cosmos - Carl Sagan [123]
Some of the rarer elements are generated in the supernova explosion itself. We have relatively abundant gold and uranium on Earth only because many supernova explosions had occurred just before the solar system formed. Other planetary systems may have somewhat different amounts of our rare elements. Are there planets where the inhabitants proudly display pendants of niobium and bracelets of protactinium, while gold is a laboratory curiosity? Would our lives be improved if gold and uranium were as obscure and unimportant on Earth as praseodymium?
The origin and evolution of life are connected in the most intimate way with the origin and evolution of the stars. First: The very matter of which we are composed, the atoms that make life possible, were generated long ago and far away in giant red stars. The relative abundance of the chemical elements found in the Cosmos matches the relative abundance of atoms generated in stars so well as to leave little doubt that red giants and supernovae are the ovens and crucibles in which matter has been forged. The Sun is a second- or third-generation star. All the matter in it, all the matter you see around you, has been through one or two previous cycles of stellar alchemy. Second: The existence of certain varieties of heavy atoms on the Earth suggests that there was a nearby supernova explosion shortly before the solar system was formed. But this is unlikely to be a mere coincidence; more likely, the shock wave produced by the supernova compressed interstellar gas and dust and triggered the condensation of the solar system. Third: When the Sun turned on, its ultraviolet radiation poured into the atmosphere of the Earth; its warmth generated lightning; and these energy sources sparked the complex organic molecules that led to the origin of life. Fourth: Life on Earth runs almost exclusively on sunlight. Plants gather the photons and convert solar to chemical energy. Animals parasitize the plants. Farming is simply the methodical harvesting of sunlight, using plants as grudging intermediaries. We are, almost all of us, solar-powered. Finally, the hereditary changes called mutations provide the raw material for evolution. Mutations, from which nature selects its new inventory of life forms, are produced in part by cosmic rays—high-energy particles ejected almost at the speed of light in supernova explosions. The evolution of life on Earth is driven in part by the spectacular deaths of distant, massive suns.
Imagine carrying a Geiger counter and a piece of uranium ore to some place deep beneath the Earth—a gold mine, say, or a lava tube, a cave carved through the Earth by a river of molten rock. The sensitive counter clicks when exposed to gamma rays or to such high-energy charged particles as protons and helium nuclei. If we bring it close to the uranium ore, which is emitting helium nuclei in a spontaneous nuclear decay, the count rate, the number of clicks per minute, increases dramatically. If we drop the uranium ore into a heavy lead canister, the count rate declines substantially; the lead has absorbed the uranium radiation. But some clicks can still be heard. Of the remaining counts, a fraction come from natural radioactivity in the walls of the cave. But there are more clicks than can be accounted for by radioactivity. Some of them are caused by high-energy charged particles penetrating the roof. We are listening to cosmic rays, produced in another age in the depths of space. Cosmic rays, mainly electrons and protons, have bombarded the Earth for the entire history of life on our planet. A star destroys itself thousands of light-years away and produces cosmic rays that spiral through the Milky