Seite - 38 - in Water, Energy, and Environment - A Primer

anytimescalerelevant tohumanactivities.Theirproductiontakes millions of years, while their depletion through use occurs on a much,much shorter timescale. 5.2 NUCLEARENERGY Nuclear energy, the energy that is released when matter is converted to energy, can be tapped via nuclear fission, the splitting of heavy nuclei into lighter ones, or via nuclear fusion, the fusing of two low atomic weight nuclei to create one of greater atomic weight. In both casesmass is lost in the nuclear process and this lost mass provides large amounts of energy in accord with Einstein’s e=mc2 equation. These nuclear processes are described inChapter 7. 5.3 GEOTHERMALENERGY Another large energy resource is the heat energy stored in the Earth that derives from radioactive decay at the Earth’s core. This geothermal energy is available everywhere onEarth if one drills deep enough and is discussed inChapter 8. 5.4 THESUN Our largest energy resource, the Sun, is a modest-sized star located, on average, 93 million miles (150 million kilometres) from the Earth. Its energy, as does that of other stars, derives from long-lasting fusion reactions in the Sun’s interior. Chapter 7 provides a detailed discussion of the Sun’s primary fusion reaction, the fusing of hydrogen isotopes into helium. The energy created in the Sun is transmitted to the Earth as radiation that enters the Earth’s atmosphere. Through a variety ofmechanisms, this radiative energy becomes available for our use as solar, wind, biomass, hydropower, and ocean energy. These, and another form of energy (tidal energy, driven by the gravitational tug between Earth andMoon) constitute, together Water,Energy, andEnvironment–APrimer38