Recap
• N = N* fs fp nh fl fi fc L/T
– N* =
Recap
• N = N* fs fp nh fl fi fc L/T
– N* = 4 x 1011
– fs
Recap
• N = N* fs fp nh fl fi fc L/T
– N* = 4 x 1011
– fs ~ 0.2
– fp = ?
– Nh = ?
– fl =
Recap
• N = N* fs fp nh fl fi fc L/T
– N* = 4 x 1011
– fs ~ 0.2
– fp = ?
– Nh = ?
– fl = 1
The Terrestrial Worlds
The Earth
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
Characteristics of Earth
Today
Surface:
• 70% water
• 30% land
Atmosphere:
•77% N2
•21% O2
•0.1% H2O
•+Ar, CO2, Ne
The Dynamic Earth
Molten Core  planetary magnetic field
- this protects us from solar and galactic cosmic rays
Plate tectonics  refreshes atmosphere
Oceans  sequester CO2
Subduction  removes CO2 to mantle
The Terrestrial Atmosphere
Equilibrium temperature: 245K
Mean temperature: 287K
Atmospheric O2 is chemically unstable
The Moon
A Double Planet?
As seen
from Mars!
Luna
1/81 M
1/4 R
: 3.3 gm/cm3
No atmosphere
No tectonics
Tidally-locked
Receding from
Earth
Stabilizes Earth
Mercury
Venus
Venus
Venus
A twin of the Earth?
•0.81 M
•0.95 R
• = 5.25 gm/cm3
Equilibrium temperature: 35C
Surface temperature: 470C
Surface pressure: 90 bars
CO2 atmosphere, H2SO4 clouds
No plate tectonics
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
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
Mars
Martian Characteristics
Small: mass = 0.1M
radius = 0.5 R
density = 4.0 gm/cm3
Cold: T ~ -50C
Thin atmosphere:
0.007 bars
95% CO2
An Earthlike Mars?
•Ice clouds
•Polar caps
•Evidence of running water
•Large volcanoes
A Habitable Mars?
Nh
Planets in the Solar Habitable Zone:
Venus
Potentially Habitable (at some time):
Venus, Earth, Mars
Nh > 1