Earth
Our Home in Space

March 11, 2011, jeb

Outline Six Regions of the Earth Hydrosphere The Tides Atmosphere Magnetosphere Internal Structure of the Earth Core Mantle Crust Earth's Evolution Continental Drift Earth's Basic Properties Other sources on the Earth

orbiting 150 million kilometers from the Sun.

oldest rocks found on surface about 3.9 billion years old
• based on radioactive dating

Six Regions of the Earth

• Core
  • metallic (mostly nickel and iron)
• Mantle
  • rocky structure (80% of Earth's volume)
• Crust
  • thin surface covering mantle
• Hydrosphere
  • liquid oceans
• Atmosphere
  • nitrogen and oxygen
• Magnetosphere
  • charged particles trapped in Earth's magnetic field

Hydrosphere

• Earth is only planet with large quantities of surface water
  • 70 % of surface covered by oceans
• Surface water is deformed by tidal force of Moon (and Sun)
  • enormous forces of tides continuously eroding surface
• Earth rotation rate is slowing as a result of tidal force
  • slowed from 400 days/year to 365 in 500 million years

The Tides

• Both the Moon and the Sun exert tidal forces on the Earth
• The tidal influence of the Moon is twice as large as the Sun
  due to its closer distance
• The combined effect of the Sun and the Moon produce variations in the high and low tides
• The tidal bulge does not point directly toward the Moon
  due to effects of friction
• The rotation rate of the Earth is slowing down
  • fossil record
    • length of day increases 0.0015 seconds per century
    • day was just 22 hours long 500,000,000 years ago
  • slowing of rotation caused by drag on rotation due to offset in tidal bulge

Atmosphere

• 78% Nitrogen, 21% Oxygen, few % Water
• Radiation shield (ozone)
• Burns up most asteroids
• Insulates surface, keeps warm (greenhouse effect (see below) => CO₂)
• Oxygen came from life which appeared 3.5 billion yrs ago
• Blue sky results from preferential scattering of blue light (see below)

• Early Earth atmosphere - common gases of early Solar System
  • hydrogen
  • helium
  • methane
  • ammonia
  • water vaper
• These early gases, being light, escaped from Earth (over 1/2 billion years)
  • especially hydrogen and helium
• Secondary atmosphere created from volcanic activity - lots of Nitrogen
• Life appeared over 3.5 billion years ago -> oxygen appears
• Now - -
  • 78 % N₂
  • 21 % O₂

• Troposphere - below 12 kilometers
• Stratosphere - 12 kilometers to 40-50 kilometers
• Ozone Layer - around 50 kilometers
• Mesosphere - 50 kilometers to 80 kilometers
• Ionosphere - above 80 kilometers

• Ozone layer straddles boundary between the stratosphere and the mesosphere
  • At about 50 km, incoming solar radiation is absorbed by ozone (O₃) and nitrogen
• Chlorofluorocarbons (CFCs) can destroy atmospheric ozone (ozone -> oxygen)
  • Antarctic Ozone Hole

• Convection
  • occurs when cool fluid overlies warm fluid
• Greenhouse Effect
  • less energy leaves the Earth than enters due to change of the nature of the radiation
    recall the nature of electromagnetic radiation
    • visible sunlight enters and is absorbed
    • reradiated infrared radiation is trapped
• Global Warming
  • Atmospheric CO₂ levels over past thousand years
  • Average global temperature

• When light passes through the atmosphere, the blue wavelengths of light are preferentially scattered by molecules in the atmosphere.
  • this is known as Rayleigh scattering, after the British physicist who first investigated the phenomenon.
• This is also the reason sunsets and sunrises are reddish

Magnetosphere

• Earth's magnetic field
  • => circulating internal iron (magnetic) fluid
• Van Allen Belts
  • Trapped charged particles
  • Aurora
  • Aurora from the Space Shuttle
• Earth's magnetosphere is greatly distorted by the solar wind
• Magnetosphere protects the Earth's surface from potentially destructive incoming charged particles
  • strong magnetic field deflects these charged particles
  • could life have arisen on Earth without a magnetosphere?

Internal Structure of the Earth

• P- waves (pressure or primary waves)
  • refracted at the surface of the core
• S- waves (shear or secondary waves)
  • cannot pass through the liquid core

Core

• Core is high density (12,000 kg/m³ = 12 g/cm³) => 5000 K
• Rich in Nickel and Iron
• Much of the core is in liquid state
  but near center of the Earth, the pressure is so high, the core is solid
• rapidly rotating, conducting liquid in core, explains the Earth's magnetic field
  • dynamo theory
  • core rotates slightly faster than bulk of Earth

Mantle

• Mantle is medium density (5,000 kg/m³) => T rises to 4000K

Crust

• Crust is low density (3,000 kg/m³) (water is 1,000 kg/m³ = 1 g/cm³)
• Plate tectonics (continental drift)

Earth's Evolution

• Models of formation based on scientific evidence says Earth formed 4.6 billion years ago by gravitationally capturing material from its surroundings
• When Earth formed it was hot and largely molten (liquid form) due to debris bombardment and continued gravitational infall
• Heavy material gradually sank toward center and interior heated more
  • 4 billion years ago Earth's differentiation was underway
    • Intense bombardment continued heating the surface, and radioactivity throughout the Earth provided additional energy to heat and liquify
    • Heavy material sank over time toward the center of the Earth while lighter material rose toward the surface.
• Earth cooled from outside (surface crust) in
• Radioactivity continues today to heat interior
• Earth has a metallic liquid core

Continental Drift

• Earth's surface => a dozen plates
• Plates slowly move => few centimeters/year
• Interactions at plate boundaries
  
  plates make up Earth's lithosphere
  lithosphere slides over the asthenosphere
  constant recycling and transformation of crust material by the plates is called the rock cycle
  
  
  • => large deformations
  • => earthquakes (Kobe, 1995)
  • => sudden volcanic eruptions (Mt. St. Helens, 1980)
  • => ongoing eruptions (Kilauea, Hawaii)
  • => mountain building (Himalayas and Mt. Everest)
  • => San Andreas and associated faults in California
    caused by North American and Pacific plates
  • Earthquakes, and effects such as Tsunamis, contribute over long time scales to requirements of life
    (see New York Times Science Section, January 11, 2005)
• Plate motion driven by convection in mantle
  
  
• Locations of the continents over time (based on current drift rates and directions)
  • Present
  • 65 million years ago
  • 130 million years ago
  • 200 millions years ago
    • geologists think single gargantuan landmass (Pangaea) dominated the planet
  
  
  
• Seafloor is spreading
  • Evidence of magnetic reversals

Planet order from Sun Orbital period Semi-major axis Mass Radius Density Surface Gravity Escape speed Rotational Period Inclination Surface Magnetic Field Surface Temperature

3 1 year (365.25 days) 150 x 106 km (1 AU) 5.97 x 1024 kg 6378 km 5520 kg/m3 = 5.52 g/cm3 9.8 m/sec2 11.2 km/s (25,000 mph) 23.9 hours 23.45° 0.5 gauss 290 K

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Other sources on the Earth

Welcome to the Planets - Earth
Views of the Solar System - Earth
StarDate Guide to Earth
The Nine Planets - Earth