Comparative Planetology of the Terrestrial Planets Basic Characteristics
• 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
• 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
• 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
color- enhanced Water Ice on Mercury Ice on Mercury?
2 1/4 Recall: Tp~ (L*/d )
• 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
• 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 Earth Dendritic Channels 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