The Earth

  General Properties

  • Size: Radius about 6500 km or 4000 miles (for one way to measure it think about Eratosthenes' idea); It bulges at the equator because of its rotation.
  • Motion: Its distance from the Sun is about 1 AU or 150,000 Mkm, the period of revolution 365 days; Rotation takes 23 h 56 min.
  • Unique features: The Earth is like an oasis in the solar system, with its variety of landscapes and environments, in particular the hydrosphere, and the moderate temperature.


  • Structure: Made of layers of different density, on average 5.5 times as dense as water (we can measure the mass from orbiting bodies), and denser on average than the surface rocks; Our main source of information on the interior geological structure is seismic waves.
  • Heat and differentiation: The Earth was initially heated by its gravitational collapse and the high rate of impacts, but the interior is heated mostly by radioactivity; The Earth was even hotter and more molten in the past, and the heavier materials sank deeper, giving rise to the different layers.
  • Core: [3500 km thick;] Very hot, of dense metallic material (Iron, Nickel, ...); The inner 10% is solid (hot but very high pressure), the rest is molten and generates the Earth's magnetic field with the currents produced by the Earth's rotation.
  • Mantle: [3000 km thick;] Dense, rocky material; Inner part solid rock, outer part hot, soft magma - Convection.

  • Consequences: The boundaries between plates are geologically active, with a high rate of earthquakes (going on all the time!) and volcanoes (lava from the mantle's magma).

  Crust and Surface Activity

  • Crust: The outer layer, comprising the continents and seafloor, made of lighter rocks containing silicates.
  • Plate tectonics: The crust is divided into about a dozen, roughly 50-km thick plates, that move due to the convection in the mantle by a few cm/year.
  • Consequences: Continental drift, that we can track byusing fixed points in the sky as reference; The pattern of continents changes in time; More than 200 Myr ago all current continents were part of Pangaea, and even earlier the changes may have occurred faster than now, because of uneven clumps in the mantle.


  • Composition: Nitrogen (78%), Oxygen (21%), Argon (1%), CO2 (the right amount) ... Temperature, pressure, and density tend to decrease with height.
  • Origin: Initially produced by outgassing from rocks, then modified by life, with various factors contributing to equilibrium and feedback.
  • Parts: Troposphere (15 km, where weather is produced by convection, heated by the ground with IR); stratosphere (up to 50 km, ozone, UV protection); ionosphere (radio broadcasting); 99% of the mass in contained in a 100-km layer, of roughly uniform composition due to constant mixing [except for water and ozone].
  • Effects: Breathing; Weather; Protection from radiation; Warmth from greenhouse effect (trapped IR radiation and energy balance); Erosion by air and rainwater and transport of dust.


  • Origin: The Earth's magnetic field; The planet is like a huge magnet, which changes over time – the "North Pole" on the surface moves northward by about 40 m/day.
  • Composition: Charged particles trapped inside the Van Allen Belts, extending thousands of km out.
  • Effects: Protects us from the charged particles of the solar wind and some cosmic rays; Produces the auroras,* visible as both Northern and Southern Lights.

  • Protection: The atmosphere protects us from excessive harmful radiation from space, and the Earth's magnetic field is largely responsible for deflecting energetic particles (mainly the solar wind) toward the poles, where they produce auroras.

  Influence of Space on Earth

  • Constant effects: Radiation and solar wind from the Sun; Tides produced by the Moon; A constant rain of small solar system debris and meteorites and cosmic rays; [Can they lead to mutations? Do their cycles relate to ice ages?].
  • Impacts: There is evidence of many large asteroids and comet impacts, some with global consequences.
  • Supernova explosions: According to some scientists, this is what will one day destroy life on Earth (one that is waiting to happen is eta Carinae, but it is too distant for that).
  • Plus: The Earth probably "wasn't meant" to have a natural satellite, but it has one, the Moon; and also tons of smaller debris [the thousands of pieces of 17,000-mph space junk we placed in orbit ourselves!].

* The only other bodies known to have auroras are Jupiter, Ganymede, and Saturn.

page by luca bombelli <bombelli at>, modified 29 sep 2012