ISP205 Lecture #11, Feburary 13, 2001

  1. Review:
    1. Inverse Square Law: 
      brightness decreases with the square of the distance
        
    2. Solar System: most things orbit/spin counterclockwise
      Exceptions: spins of Venus and Uranus
        
    3. Density = Mass / Volume
        
    4. Typical densities:
      Water: 1 g/cm3
      Rock:   2.5-3.5 g/cm3
      Iron:     8 g/cm3
         
    5. There are 2 (3)  types of planets
      1. Terrestrial Planets
        (Mercury, Venus, Earth, Mars) +Moon
        density: 3.9 - 5.5 g/cm3
        made of rock and iron
      2. Giant Planets
        (Jupiter, Saturn, Uranus, Neptun)
        density: 0.7 - 1.6 g/cm3
        made of gas, ice, some rock 
      3. Pluto
        density: 2.0 g
              
  2. Review Radioactivity/ Dating
    1. Radioactivity is the decay of atomic nuclei into another 
      element via emission of a particle (electron or alpha particle)
        
    2. Radioactivity is a random process
         
    3. The half-life is the time it takes for half the nuclei in a sample
      to decay (start counting at any time)
      (plot of data from radioactive dating exercise)
        
    4. Each material has a characteristic probability for the decay 
      of a nucleus and therefore a characteristic half-life that 
      is often well known from laboratory measurements
         
    5. The ratio of undecayed nuclei to decayed nuclei tells you 
      how many half-lives the sample is old.
        
    6. Examples for radioactive nuclei used for dating:
      (the number indicates the total number of protons and neutrons
      in the nucleus)
      235U decays into 207Pb   half-life: 700 Million Years
      40K   decays into   40Ar   half-life: 1.3 billion years
      14C    decays into  14N     half-life: 5730 years
         
    7. Radioactive dating works best for ages that are of the same
      order of magnitude than the half-life.
      (otherwise either the number of decayed nuclei or the number
      of undecayed nuclei is too small to be measured).
         
    8. Radioactive dating of Meteorites yields an age of the solar 
      system of (at least) 4.5 billion years.
              
  3. Other objects in the solar system than the Sun and Planets:
    1. Moons        satellites orbiting a planet
      earths moon sometimes listed among terrestrial planets
      (density 3.3 g/cm3)
         
    2. Rings: dust and rocks orbit in disk around equator of a planet
      all giant planets have rings - saturns rings are most easily visible
      see this picture for other planets rings
        
    3. Asteroids   
      rocky objects that orbit the sun, mainly in asteroid belt
      most of them are in the Asteroid belt 
           (between Mars and Jupiter)
      more than 10000 known (known orbits)
      size up to 1000km (Ceres)
      NEAR mission finished data taking of Eros - 
          controlled crash next week
          Should hit ground Feb 12, 3:04 pm Eastern Time
          See Image of Eros; (size: 33km X 13km X 13km)
      Binary Asteroids !
      Movie of crashing probe
      Last images
          
    4. Comets       
      ice (water, carbon dioxide, carbon monoxide, methyl alcohol)
         
      orbit the sun on highly elliptical orbits
      come from the Oort cloud, ~50000AU away 
                 from sun (pluto: 40 AU !)
                 like Hale Bopp - will be back in 2380 years !
      and from Kuiper Belt        just beyond Plutos orbit
      (see picture)
          
      more than 1000 known  (5-10/yr discovered)
                         
    5. Dust            tiny grains, for example rock

      we have special names for things that hit a planet:
    6. Meteors      dust grains, burning up in earths atmosphere
    7. Meteorites   any bigger piece hitting a planet or moon
        
  4. Geological Activity
  5. Planet Earth
    1. Radius: 6378 km (largest terrestrial planet)
    2. How can we learn about earths interior ?
      1. Density measurement
      2. Seismology
        1. Detect waves triggered by earthquakes at different
          locations on earth
        2. S-waves: shear waves do not penetrate liquid
        3. P-waves: presure waves do penetrate liquid
          demo s,p waves
          (picture seismology)
      3. Volcanoes
      4. Magnetic field
        DEMO: fieldlines
        picture earth magnetic field
        • Earth has a magnetic field with the north and south 
          poles roughly aligned with the rotational axis
        • Magnetic field traps charged particles from the sun
              because they spiral round the field lines
              this causes Northern Lights (aurorae) 
              (picture, polar lights !!! experiment !!)
        • Origin: Liquid iron and earths rotation lead to currents that 
           generate a magnetic field (Dynamo effect).
          DEMO: current generates magnetic field
       
    3. Basic properties and composition:
      1. Crust, continental:  20-70 km, solid rock
                   ocean floor: 6 km, solid rock
      2. Mantle: 2900 km, somewhat liquid rock (flows slowly)
        Temperature up to 3000 oK
      3. Core:    remaining 3500 km, iron with some nickel and sulfur
        Temperature: 5000-7000 oK
      4. Outer core: Liquid
      5. Inner Core: (inner 1200 km radius) probably solid 
        because of high pressure
      6. Origin of heat in earths interior: 
        Earth is cooling but slowly because of size
        Initial heating through:
        1. Gravitational energy release during formation
          and fractionation
        2. Large impact rate of "planetesimals"
        3. Radioactivity
                 
    4. Atmosphere:
      1. Thin layer of gas around earths surface
      2. Properties:
        • Pressure at bottom: 1 bar (1 kg/cm2)
        • Composition today: 78% Nitrogen, 21% Oxygen,
          1% Argon plus traces of other gases (CO2, Water)
      3. Earths gravity just strong enough to keep heavier
        molecules on earth without too much leakage at
        current temperatures.
        Hydrogen and Helium escaped !
      4. Layers:
        • 0 - 10 km: Troposphere (wheather, clouds, ...)
          Temperature at 10km: 50 oC below freezing
        • 10 km - 50 km: Stratosphere
        • Ozone Layer at top of Stratosphere: shields against
          UV radiation
        • above 50 km : Mesosphere
        • above 80 km : Ionosphere (gas ionized from UV 
              radiation)
      5. Origin of Atmosphere
        1. Origin of first atmosphere (most likely) 
          1. , water etc. incorporated in earth by
            impact of comets or planetesimals
            (small pieces of matter that stuck together to
            form the planets in the early solar system) 
          2. Gases are then released by volcanism and
            heating from high impact rate of planetesimals
          3. The first atmosphere was very hot (steamy)
          4. The first atmosphere was most likely very rich in 
            CO2 with no oxygen
        2. Since about 2 billion years oxygen has been added
          as waste product of plant life
    5. The Greenhouse effect
      1. Visible light can penetrate the atmosphere. On dark ground
        it heats and is converted to infrared radiation that is blocked
        by the atmosphere. This is called the greenhouse effect.
        picture
      2. Water vapor and CO2 gas block infrared, but not visible light
        and are the main greenhouse gases on earth.
      3. Without the greenhouse effect the earths temperature would be
        below freezing
      4. Runaway greenhouse effect:
        High temperatures release water vapor and CO2 from rocks.
        That leads to a stronger greenhouse effect and higher temperatures.
        An additional factor can be the melting of polar ice that reflects
             sunlight as visible light.
      5. Earths distance from the sun is just right for a moderate greenhouse
        effect.
      6. For the first time human activities contribute to the greenhouse
        effect (picture)