Comparative Planetology of
the Terrestrial Planets
Basic Characteristics
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Inside the Frost Line
Smaller than the Gas Giants
Rocky surfaces
High densities (3-5.5 gm/cm3)
Small atmospheric mass
Few or no moons
Densities
•  Metals 5+ gm/cm3
•  Rock 3-5 gm/cm3
•  Ice 1-2 gm/cm3
Density of water is 1 gm/cm3
Surface Characteristics
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Rock (or liquid water)
Ice caps (Earth, Mars)
Cratered surfaces
Evidence for tectonic activity
Mercury
Distance from Sun: 0.39 AU
Orbital Period: 88 days
Eccentricity:
0.21
Rotation Period: 58.6 days
(3 days = 2 years)
Radius: 0.38 Earth radii
Mass: 0.06 Earth masses
Density: 5.4 gm/cm3
Moons: 0
Old, cratered
surface
No maria
Lower crater
density than
Luna
Scarps/Cliffs
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Up to 3 km relief
Unique to Mercury
Not seen on Moon
Suggestive of
crustal shrinking
Inferring
Planetary
History
from its
surface
Mercury Flyover
Messenger Spacecraft
Mercury Vertical Relief
Shakespeare Basin. Crater Janacek (48 km diameter)
3km vertical relief
Gravity Anomalies
Red: high gravity
Centered on Caloris
Basin
Liquids
Caloris
Basin
Impact
basin
1500 km
diameter
colorenhanced
Water Ice on Mercury
Ice on Mercury?
Recall: Tp~ (L*/d2)1/4
•  TE = 247K è TMercury ~
395K
•  Actual temperature:
100 – 700 K
•  No atmosphere; no
greenhouse
•  Poles permanently
shadowed (i=7o)
Venus
Distance from Sun: 0.72 AU
Orbital Period: 224 days
Eccentricity:
0.007
Rotation Period: 243 days
Radius: 0.95 Earth radii
Mass: 0.82 Earth masses
Density: 5.3 gm/cm3
Moons: 0
Venus
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Cloud-covered
Not a steamy tropical paradise!
Equilibrium temperature 290K
Actual surface temperature: 737K
–  Strong runaway greenhouse
–  Hottest planet in the solar system
Radar Image
80% plains
2 “continents”
Crater Counts
~1000 craters
3-280 km
Little erosion
Resurfaced ~
300-600 Mya
Maat Mons
11 km high
Lava Flows
Tectonic Activity
•  More volcanos than Earth
–  167 volcanos over 100 km wide
•  Less subduction
•  Ongoing vulcanism:
–  Variable SO2 concentrations
–  Lightning. Associated with volcanic ash?
The Surface
Venera 13
The Surface
Venera 14
The
Tick
Earth
Distance from Sun: 1.0 AU
Orbital Period: 365 days
Eccentricity: 0.017
Rotation Period: 1 day
Radius: 1.0 Earth radii
Mass: 1.0 Earth masses
Density: 5.5 gm/cm3
Moons: 1
Mars
Distance from Sun: 1.52 AU
Orbital Period: 1.88 years
Eccentricity: 0.09
Rotation Period: 1.03 days
Radius: 0.53 Earth radii
Mass: 0.11 Earth masses
Density: 3.9 gm/cm3
Moons: 2
Superlatives of Mars
•  Largest Volcano – Olympus Mons
Superlatives of Mars
•  Largest Rift Valley – Valles Marineris
Dendritic Channels – Flowing Water
Earth
Mars
Dendritic Channels
Earth
Canyons
Attractions of Mars
•  Running water
•  Ice
•  Weather
–  Dust devils
–  Frost
•  Craters
•  Erosions
•  But no canals
Moon
(Luna)
Distance from Sun: 1.0 AU
Distance from Earth: 0.03 AU
Orbital Period: 27.3 days
Rotation Period: 27.3 days
Radius: 0.27 Earth radii
Mass: 0.012 Earth masses
Density: 3.3 gm/cm3
Mare Orientale
Hemispheric Differences
•  Maria on front
•  Back entirely
highlands
•  Back higher than
front
•  Compositional
differences
There is no Dark Side of the
Moon
Lunar Rilles
Volcanic, not hyrodrologic
Lunar History
•  Pre-Nectarian (Hadean on Earth)
•  Nectarian 3.8-4 Gya
–  Maria form
•  Imbrian 3.2- 3.8 Gya
–  Imbrium basin; Mare Orientale formed
•  Eratosthenian 3.2 – 1.1 Gya
–  Crater rays obliterated
•  Copernican <1.1 Gya
The Terrestrial Planets
Crater Shapes
Craters
•  Typical impact speeds ~ orbital velocity
(30 km/s for Earth)
•  E=1/2 mv2
•  Size typically 10 times size of impactor
•  Depth 10-20% of radius
•  Sometimes a central peak
Linné
2.4 km
<100 Mya
Tycho
85 km
109 Mya
Copernicus
93 km; 800 Mya
Comparison of impact
craters on 2 planets
and 2 moons
Inferences from Volcanos
Plate Tectonics
Planetary Tectonics
Mercury, Mars, Moon:
•  Small volume to surface ratio (~R)
•  Cooled rapidly
•  Not very active
Venus:
•  Similar size to Earth
•  Should be active
•  Lack of water affects crustal motion
Erosion
•  Mercury, Moon: micrometeoroid
impacts, solar wind
•  Mars: wind, (water)
•  Venus:
•  Earth: wind, water, vegetation
Last Resurfacing
Requires subduction
•  Moon: 4 Gya
•  Mercury: 3.5 Gya
•  Mars: 3 Gya
•  Venus: 0.3-0.6 Gya
•  Earth: in process
Planetary Magnetic Fields
Mercury:
•  Yes, despite small size
•  Due to large core?
Venus:
•  No, due to slow rotation
Mars, Moon:
•  No, because core froze
Planetary Atmospheres
•  Next week
Take-Aways
•  All terrestrial planets started out the
same
•  Subsequent evolution driven by
–  Planet mass
–  Rotation rate
–  Distance from Sun
Other Uses for Terrestrial
Planets
•  Mercury: eccentric orbit near Sun. Orbital precession
is a test of General Relativity.
•  Venus:
–  Phases convinced Galileo of heliocentric model
–  parallax during transit gave first measurement of the AU.
•  First successful measurement in 1769 by Captain Cook
from Tahiti plus observers in Europe.
•  Transits 2004, 2012; next pair 2117/2125
•  Mars: used by Kepler to determine Kepler’s laws
Venus Parallax
Transit of Venus March, John Philip Sousa, 1883