The Terrestrial Planets

• In order of distance from the Sun:
  • Mercury
  • Venus
  • Earth
  • Mars
• In order of decreasing mass:
  • Earth
  • Venus
  • Mars
  • Mercury
• Many Very Early Men Ate Juicy Steaks Using No Plates

What They Have in Common

• Differentiation - interiors are differentiated - denser closer to the center.
• Three basic processes responsible for surface features:
  • Impact craters - mostly in early history of solar system
  • Volcanism - internal heat from radioactivity
  • Tectonics - vertical or horizontal?
• Erosion only on the Earth??

Mercury

• Smallest of the terrestrial planets (0.055 M_Å , only 4.4 times as massive as the Moon, but density close to Earth’s).
• Difficult to observe, since never more than 30º from the Sun
  • led to early mistakes about its rotation

Rotation of Mercury

• Period of revolution: P_rev = 88 days
• Period of rotation: P_rot = 59 days
  • P_rot = 2/3 P_rev  
• An example of commensurate periods.
  • One period is a simple integer multiple (or fraction of two integers) of the other.
  • Commensurate periods arise from tidal friction.
  • Moon is another example (P_rot = P_rev).
• Fairly eccentric orbit, e = 0.21.
  • One bulge always points towards Sun at perihelion
  • One solar day = 2P_rev = 176 days

Mercury

• High mean density
  • 5.4 g cm^-3
  • Mostly core, very little mantle
  • massive Fe core (mantle lost in collision or via evaporation?)
• Surface is similar to lunar far side, except:
  • scarps, some 100’s of kilometers long
    • due to shrinkage of planet by 1-3 km
    • Note direction of light
  • not saturated with craters
    • implies surface formed later than the Moon’s
    • Remember: age and number of craters related - the fewer the craters, the younger the age of the surface
• Has very thin atmosphere
  • High temperature (T ~ 700 K) precludes a permanent atmosphere
  • Transient atmosphere of solar-wind helium
• Weak magnetic field
  • Strength 10 times weaker than Earth’s, but detectable (the Fe core must be important)
  • Magnetic field can deflect the solar wind

Venus

• Most brilliant object in the sky (except for Sun and Moon).
• Easy to observe (greatest elongation ~47º).
• Very slow rotator
  • P_rot = 243 days (retrograde)
  • no detectable magnetic field
• Shrouded in heavy high-altitude clouds
• In terms of mass, radius, and density, Venus is a near-twin of the Earth

Venus’ Atmosphere

• Pressure is ~100 times ´ of Earth
• Very high surface temperature (~750 K)
  • Same temperature as Mercury - why?
  • Hot enough to melt lead
  • High Temp due to Greenhouse effect.
• CO₂ is 96%, N₂ is 3%
• Clouds are mostly sulfuric acid

The Greenhouse Effect

• Glass is opaque in the infrared, so heat is trapped.
• Greenhouse heats up until rate of heating equals rate of cooling.
• Remember the example of the Earth:
  • The Greenhouse Effect works on Venus because of the CO₂ atmosphere
  • CO₂ is transparent to optical light, but strongly absorbs infrared light.

Why Are Venus and Earth So Different?

• Earth has liquid water surface (~75%).
  • Water dissolves CO₂ out of atmosphere
  • Formation of solid carbonates (e.g., limestone), i.e. compounds that use up the CO_{2 .}  
• Venus is too hot for liquid water
  • CO₂ in its atmosphere is not adsorbed.
  • Atmosphere is 96% CO₂
• Earth’s atmosphere would be like the atmosphere of Venus if the solid carbonates were put back into the atmosphere.
• The atmosphere of Venus would be like that of Earth if you could take the CO₂ out of the atmosphere.
• Probably the two planets started out with similar atmospheres

Surface of Venus

• Mapped by radar
• Less surface variation than on Earth
• No water, not much erosion
• High temperature
• No deep oceanic basins
• There are high, large plateaus - "continents?"
• Belts of mountains, volcanoes
• Few areas with extensive cratering - surface relatively young
• Very large craters (small objects burn up in dense atmosphere)
• Large lava domes form where magma leaks through surface cracks.
• Very large volcanoes form because of little plate motion.
• Recall that Hawaiian volcanoes are limited in size because of plate drifting over hot spot.

Age of Venus’ Surface

• No rock samples, so use crater density
• Few craters are less than 30 km diameter because of thick atmosphere
• Large craters are fewer even than on the earth implies age < 500 Myr
• Craters show little or no modification by tectonic motion or volcanoes
• Resurfacing history is unique in solar system - vertical tectonics

The "How" of Venusian Resurfacing

• Shield volcanoes
• Some compression zones - mountains
• Effects of high surface temp. on lithosphere?
• Absence of water -> more resistance to movement
• Theories:
  • Catastrophic resurfacing every few 100 Myr
  • Episodic volcanism or other tectonic activity on short time scales?
  • Combination of two ideas

Mars

• Mars has superficial similarities to Earth
  • Equator inclined ~24º to orbital plane.
    • Mars has seasons, like the Earth
  • Length of solar day ~24^h
  • Transparent atmosphere with clouds and dust storms
    • But the surface pressure of the Martian atmosphere is 2% that of the Earth, and is 95% CO₂
    • Less dense than Earth
    • distance to sun is 1.5 AU

Martian Surface

• Surface more complex than moon or Mercury
  • Ancient, heavily cratered terraine
  • Hugh channels due to water flow
  • Hugh shield volcanoes with much lava
  • Deep Canyon
• Cratered surface, modified by wind erosion.
• Surface composition:
  • About 44% silicon dioxide (sand)
  • About 19% iron oxide (rust)
• Large volcanoes, such as Olympus Mons
• • enormous shield volcano Þ little plate motion
  • little cratering Þ fairly young surface
  • sharp cliffs at edge Þ wind erosion
• Earth's shield volcanoes (Hawaii) don't get so big – WHY?

Big Volcanoes on Mars - Olympus Mons

• Olympus Mons is 600 km across and 25 km high
• Olympus Mons is much larger than similar features on Earth.
• Similar features on Earth would sink into crust.

• Olympus Mons is biggest volcano is solar system
  • Crust more rigid than Earth’s, gravity weaker
  • On Earth or Venus it would sink
  • no plate motions -> lava accumulates in one spot
• Volcanism played more important role over longer time period than Moon or Mercury.

Surface of Mars

• Evidence of water flows (floods)
  • water probably now in a permafrost below surface
• Some water flows must have been fairly massive (e.g., tear-drop shaped islands in plains).
• Valles Marineris ("Valley of the Mariners", or "The Grand Canyon of Mars")

Valles Marineris

• 5000 km long by 100 km wide, 6 km deep
• due to faulting and wind erosion (atmosphere is denser in canyon)

Martian Seasons

• Seasons on Mars are affected by the planet’s large orbital eccentricity (e = 0.093).  As a result, they are:
  • moderated in the North
  • enhanced in the South
  • Temperature extremes generate strong surface winds

Dust Storms

• Strong surface winds produce large-scale dust storms that on occasion cover the entire planet.

Martian Seasons and Variations in Southern Polar Cap

• Polar caps change with seasons
  • change quickly, so must be thin
  • in winter, cold enough to freeze CO₂
    • cap large, mostly CO₂ frost
  • in summer, CO₂ melts
    • cap small, probably mostly residual water
• At Martian atmospheric pressure, CO₂ freezes at 150 K; water freezes at 190 K.

Martian Clouds

• Clouds are also sometimes seen in the atmosphere of Mars

Satellites of Mars

• The Earth and Mars are the only terrestrial planets with satellites.
• Mars has two very small satellites, which are probably captured asteroids:
  • Phobos ("Fear")
    • 20 × 23 ×28 km
  • Deimos ("Terror")
    • 12 km (roughly spherical)

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