ISP205 Lecture #13, Feburary 20, 2001

Terrestrial Planets

  1. Review: Moon
    1. Mostly rock with small solid iron core
    2. Highlands very old (4.1-4.2 billion years old)
    3. Lowlands somewhat younger (3.3-3.8 billion years)
    4. Craters circular because of high speed of meteorites
      and explosion like impact
    5. Highlands much more heavily cratered - impact 
      rate in early solar system was much higher and 
      leveled out around 4 billion years ago
    6. No atmosphere because escape velocity is too low
    7. Origin of the moon: most likely "Giant Impact"
      Why ? Explains compositional differences and similarities
      because mostly mantle material is ejected into space
    8. Other theories less likely:
      1. Capture (requires 3rd body)
      2. Sister (cannot explain composition differences - like 
        absence of metals) 
      3. Fission (doesn't work - can also not explain composition
        differences)
  2. Overview terrestrial planets
    (see pictures)  
    1. Earth has 1 Moon
    2. Mars has 2 Moons - Phobos and Deimos (pics)
      maybe captured asteroids deflected by Jupiter from outer
      asteroid belt into Mars orbit.
  3. Surface features and geological activity
    1. Mercury
      (only visited by one space craft: Mariner 10, 1974)
      1. Heavily cratered surface, similar to Moon
      2. Large basins with indication of lava floods
        (some similarity to the moon, and similar ages)
        but no activity since ~4 billion years
      3. Long scarps across surface (probably 4 billion years old)
        cracks from cooling and shrinking. (pic)
        (unique feature of Mercury)
      4. Radar images imply water ice just beneath polar
        region surface.
      5. Very weak magnetic field
    2. Venus
      1. Because of clouds surface only visible with Radar
        (clouds are made of sulfuric acid)
      2. Extensive radar mapping (98%) has been done by Magellan
        1990-1994 from orbit (see complete map)
      3. Some "high lands" that resemble continents: Aphrodite
        and Ishtar
      4. Venus is geological active with the surface being 
        on average only 500 mio years old
      5. Main activity are volcanoes (pictures)
      6. It seems that most of the surface appeared in a 
        short time, and that since then the geological 
        activity was much lower.
        (for example very few craters are filled with lava)
      7. Cracks from tectonic forces
      8. No magnetic field
    3. Earth ( we know that one ...)
    4. Mars
      1. Young, lightly cratered  lowlands mostly in southern hemisphere
        (~3 billion years old) 
        and older highlands (heavily cratered, ~ 4 billion years old)
        (still somewhat younger than moon highlands) (picture)
      2. Geological activity mostly ceased 3-4 billion years ago
        but: some Volcanoes might still be active occasionally 
        (suggested by crater counts on volcanoe slopes)
      3. Largest Volcano in the solar system: Olympus Mons
      4. Spectacular Canyon: Valle Marineris (7km deep and
        100 km long).
      5. Icy polar caps - permanent caps made of water ice,
        seasonal extensions of CO2 ice (dry ice)
      6. There are indications that Mars had liquid water at some 
        point in its history:
        1. Curved channels that resemble water carved canyons
          on earth: Nanedi Valle
          this indicates contineous flow of fluid - water ?
        2. These features are mainly seen in heavily cratered
          regions. What does that mean ?
        3. Also hints on very recent outflows:
          (see recent pictures from Global surveyor 1997)
          Apparently outflows show sharp rocks not covered
          at all with sediments or sand - some scientists
          concluded these features are very young 
          (as young as a few years)
      7. Canals ?
        1. Mars Canals first claimed by 1877 by Schiaparelli
        2. "Confirmed" by Percival Lowell who observed 183
          canals, many of them observed in more than 20
          observing nights. 67 of Schiarpellis 79 canals "found"
          (see picture)
        3. Today we know that canals were an optical illusion.
      8. Faces ?
        Also an optical illusion
      9. Bacteria ?
        Maybe ...

         

      10. No magnetic field
  4. Planet interiors  (Redshift demo)
    1. The smaller a planet the faster it cools
    2. Mercury smallest terrestrial planet: 
      huge solid (partially molten?) iron core and thick crust
    3. According to one hypothesis mercury lost most of its 
      rocks in a giant impact
      (explains also more elliptical orbit and tilt or orbital plane of 70)
    4. Mercury has a weak magnetic field - probably residual from iron
    5. Mars next smallest: cooled slower and had some volcanic 
      activity in the distant past before crust became too thick.
    6. Only small solid iron core and no magnetic field
    7. Venus almost as big as earth - has a liquid iron core but 
      still thicker crust. 
      Venus has active volcanoes, but less than on earth.
    8. Venus has no magnetic field because of the extremely slow rotation
    9. Earth has the thinnest crust and the highest interior temperatures.  
  5. Surface conditions
    (how would be life on mercury, venus, or mars ?)
    1. Mercury
      1. Siderial day: 59 days
      2. Year: 88 days
      3. Solar day: 176 days (longest day in solar system)
        (see you at noon tomorrow can mean a long time)
      4. Until 1965 astronomers thought Mercury rotates
        once a (Mercury)year. New measurements with
        Doppler Radar (DEMO)
      5. Seasons
        1. Seasons entirely from variation of the distance 
          to the sun (varies from 46 Mio km to 70 Mio. km)
          therefore big difference in heat intensity
        2. No axis tilt
      6. Temperatures: largest temperature differences in solar system
        Night: -173oC (-280F), Day peak: 430oC (800F)
        Reason: no atmosphere, close to sun and very long days and nights
      7. No atmosphere
      8. Water ? Ice has been found beneath poles (with radar)
    2. Venus
      1. Siderial day: 243 days "clockwise"
      2. Year: 225 days
      3. Solar day: 117 days 
        (because Venus spins opposite to orbit, solar day gets
        shortened by spin)
      4. No seasons:
        • Axis of rotation only tilted by 3o (earth 23o)
        • Almost circular orbit
      5. Temperatures
        Constant everywhere at ~460oC (860 F)
        (note that lead and zinc melt already)
        because of thick atmosphere
      6. Atmospheric pressure: 90 bar
        (= 90 kg/cm2 - or a truck on your head - 300 cm2)
      7. Atmosphere: 96% CO2, 3.5% N2
      8. Always cloudy (Sulfuric acid clouds)
        therefore no direct sunlight
        during day: dim, reddish illumination
        (see pictures from Venerea probes)
      9. No wind and water and therefore very little erosion
    3. Mars
      1. Siderial day: 1 day (roughly same as earth)
      2. Year: 1.9 (tropical) years
      3. Solar day only 2 minutes shorter than siderial day
      4. Seasons: 
        • axis tilt 25o (similar to earths 23o)
        • distance to the sun varies because of eccentric
          orbit and influences temperatures as well
      5. Temperatures:
        1. -100oC to possibly about freezing on a hot summer 
          day in a good place
      6. Very thin atmosphere (0.007 bar)
      7. Atmosphere similar to Venus: 95% CO2, 2.7% N2)
      8. Some water ice and CO2 clouds and dust clouds
      9. Dust clouds create red sky and lead to "inverted sunsets"
        (see pictures of Mars surface)
  6. Atmospheres
    1. Why are climates so different ? 
      Distance to sun according to inverse square law leads to 
      7 times  more heating on Mercury
      2 times  more heating on Venus
      2 times  less heating on Mars

      this cannot explain the large differences between
      Venus, Earth, and Mars
    2. Most likely Venus, Earth, and Mars had similar atmospheres
      in the beginning, and therefore similar greenhouse effects,
      and similar surface conditions
         
    3. Venus experienced a runaway greenhouse effect:
      1. Slightly more heat from sun
        (calculate difference from inverse square law)
      2. Evaporation of water and release of CO2 from rocks
      3. Increase of greenhouse gases and thicker atmosphere
      4. higher temperature - go back to 2.
        (DEMO: runaway equilibrium)
      5. Water vapor decomposed in H and O by UV light
      6. O bound in rocks, H escaped - irreversible loss of water
    4. Mars experienced a runaway refrigerator effect
      1. Contineous loss of atmosphere because of lower
        gravity
      2. Temperatures get lower
      3. Water vapor freezes out, also some CO2
      4. Less greenhouse gases
      5. Goto 2
    5. The earths atmosphere was drastically changed by life