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 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, ) • 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 • 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)

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, , 1883