Planetary Atmospheres
• Greenhouse Effect
90 atmospheres!
Planetary Atmospheres
• Greenhouse Effect
240
watts/m2
in
Small fraction
escapes into
space
240
watts/m2
out
50% reflected by
clouds and surface
CO2
H20
Planetary Atmospheres
• Greenhouse Effect
Energy In
100 energy units
100% gate
Energy Out
200 energy units
stored
in equilibrium
100 energy units
50% gate
Planetary Atmospheres
• Greenhouse Effect
Energy In
100 energy units
100% gate
Energy Out
10, 000 energy units
stored
in equilibrium
100 energy units
1% gate
Planetary Atmospheres
• Greenhouse Effect
ConcepTest!
Frosts are most likely to happen on clear nights rather than
cloudy nights. This is because
A)
B)
C)
D)
Cloudy nights lead to falling snow
Clouds are not transparent to optical light
More infrared light can be radiated through clear skies
Snow and frost reflect rather than absorb visible light
Planetary Evolution
• Essential Idea
• Evolution of both geology and atmospheres
driven by energy flow from planetary cores
Planetary Evolution
• Essential Idea
• Recall
• Amount of residual thermal energy stored in planet
depends on planet volume (amount of material)
• Rate of energy loss depends on planet surface area
Lifetime of geological activity
Residual thermal energy
=
Rate of energy loss
• Small planets go dormant sooner
4 3
R planet
 3 2
 R planet
4 R planet
Planetary Evolution
• Comparative Planetology - Earth and Mars
Mars
Dormant volcanos
Earth
Active volcanos
Planetary Evolution
• Comparative Planetology - Earth and Mars
Mars
Dormant volcanos
Water in past
Earth
Active volcanos
Water at present
Planetary Evolution
• Comparative Planetology - Earth and Mars
Mars
Dormant volcanos
Water in past
Thin CO2 atmosphere
(0.01 atmos)
Earth
Active volcanos
Water at present
N2/O2 atmosphere
(1 atmos)
Note: Liquid water would not survive on Mars today it would boil due to low atmospheric pressure.
So if liquid water in past, must also have been
substantial atmosphere and greenhouse effect in past
Active volcanoes could have provided such an atmosphere
Planetary Evolution
• Comparative Planetology - Earth
Ozone O3
washed out with H20 rain
Plate tectonics is critical to Earth atmosphere
Planetary Evolution
• Comparative Planetology - Earth
Ozone O3
washed out with H20 rain
Life is critical to oxygen in Earth atmosphere
ConcepTest!
If plate tectonics on the Earth were to end, you would expect the
temperature of the Earth’s surface to
A) Increase
B) Change little or not at all
C) Decrease
Planetary Evolution
• Comparative Planetology - Mars at 1 billion yr
(end of geologic activity)
UV
O,C
1.
2.
3.
4.
5.
H2O rain washes CO2 into surface - permanently
Greenhouse effect reduced
Colder temperatures lead to enhanced rain, less gaseous CO2, less greenhouse
(“Runaway Icehouse Effect”)
Most CO2 in carbonate rocks, most H2O in permafrost
UV sunlight breaks up molecules and provides energy of escape
Planetary Evolution
• Comparative Planetology - Mars at 4.5 billion yr
Recent liquid water?
Planetary Evolution
• Comparative Planetology - Earth and Venus
Venus
Active volcanos
Earth
Active volcanos
Planetary Evolution
• Comparative Planetology - Earth and Venus
Venus
Active volcanos
Vesc = 10.3 km/sec
Earth
Active volcanos
Vesc = 11.2 km/sec
Planetary Evolution
• Comparative Planetology - Earth and Venus
Venus
Active volcanos
Vesc = 10.3 km/sec
CO2 atmosphere
(90 atmos)
Earth
Active volcanos
Vesc = 11.2 km/sec
N2/O2 atmosphere
(1 atmos)
Planetary Evolution
• Comparative Planetology - Earth and Venus
Venus
Active volcanos
Vesc = 10.3 km/sec
CO2 atmosphere
(90 atmos)
Dry (10-4 Earth H20)
Earth
Active volcanos
Vesc = 11.2 km/sec
N2/O2 atmosphere
(1 atmos)
Water at present
Note: Based on measurements of deuterium (“heavy water”)
Venus had substantially more water in the past
Consistent with outgassing from volcanos and large
escape velocity
Planetary Atmospheres
• Comparative Planetology - Venus
• Suppose Venus were “Earth-like” (liquid water) and …
Temperature
Light from Sun
or CO2, H2O
CO2, H2O
from rocks
from oceans
“Runaway Greenhouse Effect”
Planetary Atmospheres
• Runaway Greenhouse Effect
• once RGE begins …
Escape
RGE
UV
H20 in
+ from
atmos Sun
Irreversible!!
H2
O2
Surface
Compounds
Planetary Atmospheres
• Comparative Planetology - Venus at 4.5 billion yr
Planetary Evolution - Summary
Earth
77% N2, 21% O2
15 oC
1 atmosphere
Mars
Venus
Mercury
Trace of
He, Na, O
95% CO2
-50 oC
0.01 atmospheres
96% CO2
470 oC
90 atmospheres
Moon Trace of He, Na, Ar
Planetary Evolution - Summary
• Geological Evolution
• Mercury, Moon - smallest radii => dormant, cratered
• Mars - active for 109 yr, now dormant (shield volcanoes)
• Venus - current volcanos (shield volcanoes)
• Earth - convection => plate tectonics
• Atmospheric Evolution
• Moon, Mercury - low escape velocity, hot, atmos escapes
• Venus
RGE, massive CO2 atmos
• Earth
CO2, H2O , N2
N2, O2
• Mars
Runaway Icehouse,
thin CO2 atmos