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?

BASIC NUMBERS

THE TERRESTRIAL PLANETS
Property	Earth	Mercury	Venus	Mars
Orbital Radius (AU)	1.00	0.39	0.72	1.52
Orbital Period (years)	1.00	0.24	0.62	1.88
Orbital Eccentricity	0.0017	0.206	0.007	0.093
Rotation Period (days)	1.00	59	243R	1.02
Mass / Mearth	1.00	0.055	0.82	0.11
Surface Gravity / gearth	1.00	0.38	0.91	0.38
Max surface temperature	300 K	700 K	750 K	244 K
Atmospheric Pressure / Pearth	1.00	0	90	0.007
Moons	1	0	0	2

INVESTIGATING MERCURY

• 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?

INVESTIGATING VENUS

• 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.

RUNAWAY GREENHOUSE

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 warming, etc.
• 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.

INVESTIGATING MARS

• Can see interesting features from Earth, but Earth and Martian atmospheres limit detail.
• 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.
  • Captured asteroids?
• Cratered terrain in southern hemisphere:
  • Ancient.
• Northern hemisphere lower, smoother:
  • Younger.
  • 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?
• Atmosphere:
  • 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) nighttime minimum.
• 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 of water.
• Mars probably started with thicker CO₂ atmosphere, more greenhouse warming, warm enough surface for liquid water.
• Eventually CO₂ dissolved in water, surface cooled, water froze.
• Water now in ice caps. Maybe subsurface layer of ice elsewhere on planet?
• 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.