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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 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 • • • • 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
