LECTURE 17: THE (OTHER) TERRESTRIAL PLANETS
- What methods do we have for learning about planets?
- What do we learn from Mercury's heavily cratered surface?
- What is unusual about Mercury's iron content?
- How do we observe the surface of Venus?
- What do we know about geological activity on Venus?
- Why does Venus have such a thick atmosphere?
- What are the main features of the Martian surface and atmosphere?
- What is the evidence that Mars used to have liquid water, and
why is that an especially interesting fact?
|THE TERRESTRIAL PLANETS
Orbital Radius (AU)
Orbital Period (years)
Rotation Period (days)
Mass / Mearth
Surface Gravity / gearth
Max surface temperature
Atmospheric Pressure / Pearth
- Hard to see from Earth: small, close to Sun.
- Most info from Mariner 10 mission, flew by three times.
- Provided relatively high resolution images, mass measurement.
DISTINCTIVE PROPERTIES OF MERCURY
- Heavily cratered:
- Old, inactive surface.
- Highlands formed during era of heavy bombardment.
- Lava basins formed near end of heavy bombardment.
- No plate tectonics:
- Not enough internal energy to drive plate motions?
- Wrong structure?
- Essentially no atmosphere:
- Hot surface temperature (at noon, anyway), and weak gravity.
- Mercury day = 2/3 of Mercury year:
- Gravity of Sun on tidally stretched planet in elliptical orbit.
- Surface temperature at noon: 700 K (800 F).
- Surface temperature at night: 100 K (-280 F).
- Magnetic field weak but not zero:
- Some liquid iron in core?
- Highest iron content in solar system:
- Mantle knocked off in giant impact?
- Easier to see from Earth, BUT
- Covered in thick, opaque clouds, can't see surface.
- Solution: radar
- Transmit radio waves to planet.
- Timing of reflection gives distance to surface.
- Intensity gives some information about composition.
- Doppler shift gives rotation of planet.
- Radar mapping from flyby missions and orbiters, American and Soviet.
Most comprehensively, the Magellan mission (U.S.).
- Several atmosphere probes and landers (mainly Soviet). Hard
because of high temperature and atmospheric pressure at surface.
DISTINCTIVE PROPERTIES OF VENUS
- Slow rotation, opposite direction from orbit.
(1 Venus day = 243 Earth days.)
- Giant impact?
- Complicated tidal interaction?
- Relatively low cratering:
- Surface "repaved" within last 0.5 Gyr (500 million years).
- Volcanos, but no sign of plate tectonics:
- Lack of water makes sub-crust layer insufficiently plastic?
- Crust not stiff enough for plates?
- Surface temperature 750 K (891 F)! Massive, CO2 atmosphere,
with surface pressure 90 x Earth (equivalent to 1 km down in ocean).
- Consequence of runaway greenhouse effect.
Why is Venus atmosphere and surface temperature so different from Earth?
- Earth: CO2 dissolved in ocean, solidified in rock.
- Sun 30% less luminous at birth, steady increase over 4.5 Gyr.
- Venus may have started with H2O atmosphere and oceans.
- Sun warming leads to water evaporation leads to increased greenhouse
warming leads to more water evaporation leads to more greenhouse
- Eventually CO2 boils out of oceans, bakes out of rocks.
- Still more greenhouse warming, oceans evaporate.
- H2O eventually broken apart by sunlight, lost from atmosphere.
- End result: planet with heavy CO2 atmosphere, strong
greenhouse warming, no water.
- Can see interesting features from Earth, but Earth and Martian atmospheres
- Visited by flybys, orbiters, landers.
- Viking landers, Pathfinder rover study chemistry of surface.
- Past year: Spirit and Opportunity rovers studying surface in more detail,
looking for evidence of past or recent water.
DISTINCTIVE PROPERTIES OF MARS
- Polar ice caps grow and shrink with season. Water ice
and CO2 ice ("dry ice").
- 24-hour day (nearly).
- Two small (10-20 km) moons.
- Cratered terrain in southern hemisphere:
- Northern hemisphere lower, smoother:
- Ancient ocean floor?
- Volcanic activity, but no plate tectonics:
- Smaller than Earth, cooled more easily, thicker solid crust.
- Used to have magnetic field, but doesn't any more:
- Former liquid iron core has cooled and solidified?
- Thin (0.007 Earth pressure, like 30 km altitude on Earth).
- Mostly CO2.
- Pressure changes with season, as CO2 freezes on polar ice caps.
- Clouds and dust devils.
- Surface temperature: 244 K (-20 F) daytime maximum, 187 K (-123 F)
- Once had flowing water!
WATER ON MARS
- Lots of geological evidence (channels, flood plains)
for past flows of water, visible from orbit.
- Spirit and Opportunity rovers have found numerous examples of
minerals and rock formations that likely formed in the presence
- Mars probably started with thicker CO2 atmosphere,
more greenhouse warming, warm enough surface for liquid water.
- Eventually CO2 dissolved in water, surface cooled,
- Water now in ice caps. Maybe subsurface layer of ice elsewhere
- Liquid water past means early Mars might have had life.
- Conceivable that life originated on Mars, jumped to Earth via
meteors knocked from Martian surface.
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Updated: 2005 May 14 [dhw]