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• Accretion Accretion happens when gravity attracts tiny bits of matter towards an object. This results in a gradual increase in the object’s size. In relation to the solar system’s formation, the objects increased in size until they turned into planets. As the objects
• There are two hypotheses on how the structure of Earth was formed, which both involve accretion: homogeneous and heterogeneous accretion hypotheses.
• Homogeneous Accretion Hypothesis


  The formation of Earth began after the condensation of fine particles of the primitive nebula about 4.6 billion years ago. When these particles accreted, they formed a homogeneous primordial Earth. Thus, early Earth had a uniform solid composition.
• Primary components of early Earth

  • Iron
  • Magnesium
  • Nickel
  • Silicates
  • Radioactive elements (uranium, thorium)
• Formation of Earth's core

  1. Gravitational contraction and decay of radioactive elements increased temperature

  2. Iron and nickel melted and sank towards the center due to high density

  3. Less dense silicates were displaced and moved upwards

  4. Iron and nickel accumulated and formed Earth's core
• Formation of Earth's crust

  1. After cooling down, a thin layer of solid rock formed the crust, including the continental and ocean basins

  2. In between the core and the crust is the mantle, which is made up of semi-molten silicate rocks and other minerals
• It took many years for iron and nickel to accumulate and reach the center of about 4000 miles deep
• During this time, Earth's surface experienced turmoil, violent earthquakes, continual volcanic eruptions, and flowing lava covering the surface
• Heterogeneous Accretion Hypothesis


  The core has formed at the same time as Earth, so early Earth had its basic layered structure with a core, mantle, and crust
• Heterogeneous accretion


  1. As the nebula cooled down, its particles condensed depending on their condensation points

  2. Oxides of aluminum and calcium condensed first

  3. Iron and nickel condensed next

  4. Silicates condensed further

  5. Condensed particles collided and accreted

  6. Particles from initial condensation accreted first

  7. Aluminum and calcium oxides accreted first

  8. Iron and nickel accreted to form Earth's core

  9. Outermost layer composed of silicates and volatile particles, including water
• Freshwater Freshwater, despite the name, may still contain small amounts of salt. Freshwater can be present in the form of rain and snow, and it can even be found in permanently frozen soil known as permafrost. It is commonly stored in rivers, streams, ponds, lakes, marshes, glaciers, and polar caps.
• Density Density is measured as mass per unit of volume (in g/cm³). The density of water changes with several factors such as temperature and salinity. Water is densest at 3.98°C and is least dense at freezing temperature
• Cell membrane


  • Separates cells from the outside environment
  • Selectively takes in important substances
  • Prevents toxic products from entering the cell
• Consistency of water in maintaining liquid form at room temperatures

  Allows the flow and transfer of substances from the cell to its environment and vice versa
• Heat capacity


  The amount of energy (heat) needed to raise the temperature of a substance by one degree
• Sand heats up faster than the sea

  Water needs more energy (heat) before its temperature is raised by one degree
• Water's high heat capacity


  • It is an excellent coolant since a high amount of heat energy can be absorbed without getting too hot
  • Allows the fireproof balloon trick to be possible, as the water inside needs a higher amount of heat before its temperature increases and causes the balloon to burst
• Without water's high heat capacity, the atmosphere will be extremely cold during winter and extremely hot during summer
• Hypothesis on the origin of water on Earth

  Prevailing hypothesis suggests water came from comets that collided with Earth
• In 2000, scientists investigating the LINEAR S-4 comet had discovered that water from the comet had the same isotopic composition as the water in the seas
• Isotopes


  Atoms with the same number of protons but different numbers of neutrons
• Other studies suggest that water was already present within Earth since its formation, as volatiles trapped in magma and manifested as liquid water during degassing after the crust had formed
• The truth may also be a mixture of both hypotheses
• Albedo


  The ability of a material to reflect light
• High albedo


  Material can reflect light more than a material with low albedo
• Black surfaces


  Have low albedo since they absorb more sunlight and its associated heat than light-colored surfaces
• Albedo scale


  0 to 1
• Values closer to 0


  Material absorbs all the light
• Value of 1


  Material reflects all the light
• Low albedo materials


  • Asphalt
• High albedo materials


  • White paint
  • Ice
• Albedo


  The percentage of solar energy that is reflected back to space by the clouds, atmosphere, and light-colored areas (deserts and areas covered with ice and snow)
• Earth's albedo is 30% of the solar energy that reaches the Earth
• The remaining 70% of the solar energy is absorbed by the atmosphere, plants, and oceans
• Absorbed energy


  • Drives wind and ocean currents
  • Distributes heat throughout the planet
• More sunlight shines on the equatorial regions than the polar regions
• All the solar energy absorbed by the atmosphere, land, and oceans must be radiated back to space
• Energy in should always equal energy out
• Changes in Earth's orbit and tilt

  Led to a series of ice ages over the last million years
• Earth's action


  Affects the amount of radiation coming from the sun throughout the year