Theme 2

• Earth


  • Called the 'Goldilocks Planet' as it is neither too hot nor too cold and just the correct distance from the Sun for water to be able to exist in a liquid state
  • Being a rocky dense planet, it has strong gravity and it is this that has helped it retain the dense atmosphere
  • The atmosphere protects from dangerous UV rays and also helps to moderate the temperature at the Earth's surface
  • The Earth's magnetic field deflects away charged particles from the solar wind
• Uniformitarianism


  Principle used to interpret geological processes operating on planetary bodies within the solar system
• Earth structure


  • Crust
  • Mantle
  • Core
• Continental crust


  • Average thickness 35 km
  • Composition: granitic
  • Density: 2.7
  • Maximum age: 4,000 Ma
• Oceanic crust


  • Average thickness 7 km
  • Composition: basaltic
  • Density: 2.9
  • Maximum age: 200 Ma
• Mantle


  • Average thickness 2,900 km
  • Composition: peridotite
  • Density: 3.3-5.5
• Core


  • Average thickness 3,450 km
  • Composition: iron and nickel
  • Density: 9.9-12.6
• Meteorites


  Provide evidence for the composition of the Earth
• Meteorite types


  • Stony meteorites (density 3.6, rich in olivine, similar to upper mantle)
  • Stony-iron meteorites (density 4.8, contain olivine, nickel and iron, similar to lower mantle/outer core)
  • Iron meteorites (density 7.7, made up of iron and nickel, similar to inner core)
• Meteorites are rare and poorly preserved on Earth but common and well preserved on the Moon and Mars
• On Earth the atmosphere causes most meteorites to burn up before hitting the surface, weathering and erosion quickly fills in craters with sediment and vegetation soon re-establishes itself. 71% of the Earth's surface is ocean so many may land in the sea, never to be discovered. These would be destroyed at subduction zones.
• On the Moon and Mars there is little or no atmosphere so meteorites do not burn up and impact the surface. Craters formed are preserved as there is an absence of water, so there is no weathering and erosion (except for wind erosion on Mars). As there are no oceans, all craters are visible and there is no vegetation to recolonise and mask the features.
• Impacts from meteorites/comets may have had a significant effect on the evolution of the Earth and its biosphere.
• The Moon


  • Formed 4,500 Ma as another small planet (Theia) collided with Earth and was engulfed by it, increasing its mass
  • The Moon helps regulate Earth's orbit around the Sun and also causes tidal cycles
  • The Moon is tectonically inactive, there are no active volcanoes, no plate tectonic activity and no rock cycle processes except for meteorite impacts
• Major landforms on the Moon

  • Terrae (lighter coloured areas representing the lunar highlands/original crust)
  • Mare (darker regions representing large basaltic lava flows, probably caused by meteorite bombardment)
  • Meteorite craters with debris lines
• The largest crater on the Moon is the South Pole-Aitken Basin which is 2,500 km wide but just 13 km deep and is thought to have been formed by a very large meteorite impacting the Moon at a shallow angle with a glancing blow.
• Generally, the larger the meteorite craters on the Moon, the older they are, more recent ones are smaller. Cross cutting relationships of overlapping craters can be used to establish their relative ages.
• Mars


  • A rocky planet approximately half the size of the Earth and gravity about one third of the Earth
  • Has an atmosphere of 1% mass of Earth's which is made up of mainly carbon dioxide
  • Currently volcanically inactive and there is no active plate tectonic activity, suggesting the interior of Mars has cooled more than Earth and there is not enough heat within to drive volcanic or plate tectonic activity
  • The only erosive force occurring today is wind
• Mars was very different in the geological past. It has a number of extinct volcanoes with Olympus Mons (active 115-25 Ma) being the largest volcano in the solar system which is 600 km wide and 22 km high.
• Olympus Mons developed because the heat/magma source feeding the volcano remained fixed in place for millions of years as there were no plate movements, and the weak gravity allowed the structure to continue to grow. As there was no water on Mars by this time, the processes of weathering and erosion (except wind) did not wear it down.
• Although tectonically inactive today, there is some evidence that the Valles Marineris, a canyon 4000 km long, 200 km wide and 8 km deep may represent the initial stage in the formation of a divergent plate boundary with a rift valley structure running along its centre. Further tectonic activity is indicated by the 150km left-lateral displacement of a meteorite crater on the northern rim of the canyon.
• Mars clearly had water in its early history and probably a much thicker atmosphere than today. Evidence for this lies in a variety of rock types and geological structures photographed by the various Mars rovers and satellites.
• Evidence of water on Mars

  • Branching river channels with a dendritic/tree like pattern
  • Channels that eventually split into many distributaries forming a structure resembling a delta
  • Moderately sorted sediments with sub rounded to rounded clasts suggesting water transport and deposition
  • Desiccation cracks in fine-grained sediment (mud) on the bottom of lake/crater
• Sand dunes on Mars

  • Much larger than those on Earth, some reaching up to 700 metres in height
  • Internal structure shows large scale cross bedding (dune bedding) exactly the same as their equivalents on Earth, interpreted as being formed by sand blown in a series of mega-ripples by a unidirectional current
  • Sand dunes cover large areas on Mars as there is an absence of moisture and vegetation to anchor the sand