- 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
- 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
- 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
Crust and Surface
- Crust: The outer layer, comprising
the continents and seafloor, made of lighter rocks containing
- 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
- 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
- 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.
* The only other bodies known to have auroras are Jupiter,
Ganymede, and Saturn.
- 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.
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!].
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012