Recap

• N = N* fs fp nh fl fi fc L/T

– N* = Recap

• N = N* fs fp nh fl fi fc L/T 11 – N* = 4 x 10

– fs Recap

• N = N* fs fp nh fl fi fc L/T 11 – N* = 4 x 10

– fs ~ 0.2

– fp = Recap

• N = N* fs fp nh fl fi fc L/T 11 – N* = 4 x 10

– fs ~ 0.2

– fp = (1)

– Nh = The Habitable Zone in the Solar System Location depends on assumptions about atmospheric composition and albedo • Inner edge: • 0.84 – 0.95 AU

• Outer edge: • 1.37 – 1.67 AU ______0.9 – 1.5 AU

Kasting, J.F., Whitmire, D.P. & Reynolds, R.T. Science 101, 108 (1993) Comparative Planetology of the Terrestrial Planets The Characteristics of Earth

Today

Surface: • 71% water • 29% land

Atmosphere:

•77% N2 •21% O2 •0.1% H2O •+Ar, CO2, Ne

•Albedo: 0.39 Characteristics of Earth

A terrestrial, rocky planet • Solid Fe-Ni inner core • Molten outer core • Mantle: silicates • Crust (lithosphere): igneous rocks Tectonically - active • Radioactive decay and settling heats core • Mantle convects heat to surface • Crust floats on mantle • Crust is created and destroyed The Terrestrial Atmosphere

Equilibrium temperature: 247K

Mean temperature: 287K

Atmospheric O2 is chemically unstable

Life requires disequlibria The Dynamic Earth

Molten Core à planetary magnetic field - this protects us from solar and galactic cosmic rays

Plate tectonics à refreshes atmosphere

Oceans à sequester CO2

Subduction à removes CO2 to mantle The A Double Planet?

As seen from Mars! Luna

1/81 M⊕ 1/4 R⊕ r: 3.3 g/cm3 No atmosphere No tectonics

Tidally-locked

Receding from Earth

Stabilizes Earth Highlands: • 83% of area • Light colored • Cratered • Anorthosites (silicate rocks)

Maria: • 17% of area • Darker • Fewer craters • Basalts (lava) Lunar History

• Pre-Nectarian ( on Earth) • Nectarian 3.8-4 Gya – Maria form • 3.2- 3.8 Gya – Imbrium basin; Mare Orientale formed • 3.2 – 1.1 Gya – Crater rays obliterated • Copernican <1.1 Gya There is no Dark Side of the Moon Backside of the Moon 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 Crater Shapes 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 Lunar Water

• Radar suggests ice in permanently shadowed craters near poles. • Confirmed with LCROSS mission • Estimated mass: 6 x 1012 kg 3 – That’s about 6 km of H2O • Lake Erie: 450 km3 • Cayuga Lake: 9.4 km3 • Lake Ronkonkoma: 0.02 km3 • Lunar rocks are anhydrous Mercury

Distance from Sun: 0.39 au Orbital Period: 88 days Eccentricity: 0.21 Rotation Period: 58.6 days (3 days = 2 )

Radius: 0.38 R⊕ Mass: 0.06 M⊕ Density: 5.4 g/cm3 Albedo: 0.12 Moons: 0 Mercury

Old, cratered surface

No maria

Lower crater density than Luna Inferring Planetary History from its surface Gravity Anomalies

Red: high gravity

Centered on Caloris Basin Caloris Basin

Impact basin

1500 km diameter color- enhanced Water Ice on Mercury Habitability of 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 R⊕ Mass: 0.82 M⊕ Density: 5.3 g/cm3 Albedo: 0.75

Moons: 0 Venus

Radar image: PVO Earth’s Twin? Venus A twin of the Earth?

• 0.81 M⊕ • 0.95 R⊕ • r = 5.25 g/cm3

Equilibrium temperature: 234K (293K for a=0.39) Surface temperature: 470C Surface pressure: 90 bars CO2 atmosphere with H2SO4 clouds No continental plates

Venera 13

Maat Mons

11 km high Lava Flows Crater Counts

~1000 craters 3-280 km

Little erosion

Resurfaced ~ 300-600 Mya Tectonic Activity

• More volcanoes than Earth – 167 volcanoes over 100 km wide • Less subduction • Ongoing vulcanism:

– Variable SO2 concentrations – Lightning. Associated with volcanic ash? Phosphine (PH3) on Venus?

• Announced September 2020 • Discovered in sub-mm spectra • In dis-equilibrium in acidic atmosphere – Of biological origin? – Of geological origin? • Requires explosive vulcanism 3- • P + H2SO4 efficiently forms PH3

• More likely SO2 – common in Venusian atmosphere A Habitable Venus? A Habitable Venus?

No Venus: What Went Wrong?

The CO2 cycle Venus: What Went Wrong?

The temperature cycle Venus: What Went Wrong? Venus and Earth have similar masses

Venus and Earth probably outgassed similar amounts of CO2 Venus has no magnetic field

Venus is closer to the Sun, and has twice the insolation (offset by higher albedo)

A runaway greenhouse: • H2O evaporates; re-enforces greenhouse • H2O dissociates from UV radiation; removed by solar wind • CO2 can’t dissolve in oceans • carbonate rocks do not form

• H2O evaporates from crust • plate tectonics slows • carbon is not sequestered in mantle Venus: What Went Wrong?

Was a massive resurfacing event 700 Mya to blame?

Like the Siberian Traps, but bigger? (cf., next week)

Reference: https://phys.org/news/2019-09-venus-habitable.html Mars

Distance from Sun: 1.52 au Orbital Period: 1.88 years Eccentricity: 0.09 Rotation Period: 1.03 days

Radius: 0.53 R⊕ Mass: 0.11 M⊕ Density: 3.9 g/cm3 Albedo: 0.16

Moons: 2 Martian Characteristics

Small: mass = 0.1R⊕ radius = 0.5 R⊕ density = 4.0 g/cm3

Cold: ~ -50C (220K)

Thin atmosphere: 0.007 bars 95% CO2 No magnetic field An Earth-like Mars? •Ice clouds •Polar caps •Evidence of running water •Large volcanoes A Habitable Mars? The Terrestrial Planets Nh

Planets in the Solar Habitable Zone: Earth

Potentially Habitable (at some time): Venus, Earth, Mars

Nh > 1 The Terrestrial Worlds