W1-2 Energy

• All processes in the Earth system are driven by energy.

External Energy Sources

• External energy sources include the sun and gravity/tides
• The sun radiates heat due to thermonuclear reactions (fusion) in its core.
• Fusion converts matter to energy.
• Energy released by fusion in the sun is in the form of gamma rays (98%) and neutrinos.
• Gamma rays are responsible for the fraction of the Sun's energy that reaches Earth.
• The sun consists of six concentric layers.
  • The Core: the site of all nuclear fusion reactions comprised of 62% He and 38% H
  • The Radiative Layer: energy released from the core moves across via radiation.
  • The Convective Layer: energy moves across via convection.
  • The Photosphere: the visible portion of the sun that emits light.
  • The Chromosphere: a low-density layer of very hot gas.
  • The Corona: the outermost layer of even lower density gas.
• Radiation energy released in the Sun's core has a very short wavelength and is extremely energetic.
• As gamma rays move outward from the core they are repeatedly absorbed and reemitted as longer-wavelength, lower-energy radiation.
• The energy flux from the Sun varies with wavelength, and the shape of the Sun's spectral curve matches that of a blackbody radiator.
  • A blackbody is a perfect absorber of light.
• The spectrum of electromagnetic radiation emitted by the Sun is not the same as the spectrum of solar radiation that reaches the Earth's surface.
• Gases in the Earth's atmosphere selectively absorb some wavelengths of solar radiation.
• The ozone layer absorbs very short wavelength (UV) radiation, this energy is transformed into heat, warming the stratosphere.
• The Sun is responsible for 99.985% of all energy in the Earth system
• External energy also comes in as a result of gravity which is the mutual physical attraction between the Earth and the Moon.
• The gravitational pull that the Moon exerts on Earth is balanced by an equal and opposite inertial force created by Earth's movement.
• The side of Earth nearest the Moon is pulled toward the Moon by gravity, while the side of Earth farthest from the Moon is pulled away by inertial force.
  • This produces a periodic distortion called a tide, which takes the form of a flattening distortion (ellipsoid).
• The Earth is elastic, meaning it has the capacity to deform reversibly.
• The internal resistance (friction) caused by the elastic deformation within the planet is translated into heat and one of Earth's internal heat energy sources.
• Earth tides (body tides) are distinct from ocean tides, although both are caused by gravity.

Internal Energy Sources

• In addition to energy from the sun and earth tides, energy sources come from within the planet itself.
• Terrestrial (geothermal) energy sources have a much smaller input than the sun, but greater than the tidal contribution.
• The increase in temperature as depth increases in the Earth is the geothermal gradient.
• The geothermal gradient varies from place to place and becomes less pronounced with depth.
• By extrapolation, the Earth's core temperature is about 5300 K, almost as hot as the Sun's surface.
• Heat energy flows out through solid rocks at the Earth's surface by conduction.
• Volcanoes involve the movement of hot material from inside the planet to outside, so some heat energy reaches the Earth's surface by convection.
• Convection is a very efficient way for the Earth to transfer heat from its interior to the surface.
• Convective heat transfer provides the driving force behind plate tectonics.
• Several sources for Earth's internal terrestrial energy exist:
  • Radiogenic heat: the main source, accounting for ~66% of interior heat flow, is produced by the decay of radioactive elements
  • Accretionary heat: internal heat left over from the formation of the Earth by countless particles colliding and sticking together
  • Tidal heating: heat generated by internal friction from the constant distortion of the planet
  • Core formation: heat from the gravitational potential energy of the dense core material sinking into the center of the planet plus heat released as the innermost material solidified.

Earth's Energy Cycle

• The energy cycle encompasses the inputs, outputs, pathways, and reservoirs for the energy that drives all other Earth system cycles.
• Energy may be added or subtracted and transferred from one reservoir to another, functioning like a budget, overall the transactions must balance.
• Energy In:
  • Incoming solar radiation powers the winds, rainfall, ocean currents, waves, and the rest of the hydrologic cycle, and photosynthesis.
  • Earth's internal heat energy drives the tectonic cycle, causing lithospheric plates to shift, uplift mountains, cause earthquakes, and volcanic eruptions.
• Energy Out:
  • Earth loses energy by reflection (albedo) and by degradation and reradiation.
  • 40% of solar radiation is reflected by the top of the atmosphere, clouds, ocean surfaces, continents, and ice and snow.
  • Absorbed solar radiation undergoes irreversible degradation through transfer from reservoir to another and conversion from one form to another, eventually ending up as heat, reradiated into space.
  • Earth's outgoing radiation is also selectively absorbed by gases in the atmosphere, causing the greenhouse effect.

Energy and Society

• Humans tap into energy from Earth's reservoirs to extract power for:
  • Transportation
  • Home and office use
  • Industrial use
• The global population consumes 3.0 x 10^20 Joules annually, equivalent to burning 10 barrels of oil per person per year.
• Four extensively developed energy sources include:
  • Fossil fuels
  • Biomass energy
  • Hydroelectric energy
  • Nuclear energy
• Five other sources being developed more extensively include:
  • Solar energy
  • Wind energy
  • Waves
  • Tides
  • Geothermal energy