Dust Storms
Mars and Earth
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Nuclear Fusion - A Digression
Thermonuclear Hydrogen Fusion
4 H => He
Why “Thermonuclear”?
Bringing two positive charges together
Requires high velocity = high temperature
Hydrogen
Deuterium
Positron
Neutrino
Helium 3
Helium 4
Nuclear Fusion - A Digression
Thermonuclear Hydrogen Fusion
D + T => He + n
Nuclear Fusion - A Digression
Thermonuclear Hydrogen Fusion
D + T => He + n
Nuclear Fusion - A Digression
Thermonuclear Hydrogen Fusion
D + T => He + n
Nuclear Fusion - A Digression
Thermonuclear Hydrogen Fusion
D + T => He + n
Advantages
•
•
•
•
Fuel source - deuterium, tritium from water
- tritium also from reactors
No risk of nuclear accident
No air pollution
Minimal radioactivity?
The “Big Idea” of Solar System Evolution
The Second Law of Thermodynamics
Heat can never pass spontaneously from a colder
to a hotter body.
As a result of this fact,
natural processes that involve energy transfer
must have one direction,
and all natural processes are
irreversible.
Profound Consequence:
Equilibrium requires an Energy Source
Planetary Atmospheres
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 Atmospheres
• Retention of Atmosphere
• Atmospheric particles (atoms, molecules, dust)
respond to gravity in same way as any body
Atmosphere will escape
if velocities of atmospheric particles
(THERMAL VELOCITY)
are greater than
ESCAPE VELOCITY of planet
Planetary Atmospheres
• Escape Velocity - “a threshold velocity”
• Escape Velocity - Minimum speed at which two objects will
not be held together by their mutual gravity
2GM
Vescape 
R
M
R
Planetary Atmospheres
• Thermal Velocities of Atmospheric Particles
Values for speeds are just for example - depends on temperature
Planetary Atmospheres
• Peak Thermal Velocity
• Note Vthermal increases with higher T
• Note Vthermal decreases with higher mparticle
2kT
Vthermal 
m particle
mparticle = mass of atom or molecule
T = temperature in oK
k = Boltzmann constant = 1.4 x 10-23 joules/K
Planetary Atmospheres
• Peak Thermal Velocity
Same mass particles
Low temperature
Medium temperature
High temperature
0
Velocity of particles
Planetary Atmospheres
• Peak Thermal Velocity
Same temperature
High mass particles (e.g., CO2)
Medium mass particles (e.g., H2O)
Low mass particles (e.g., H)
0
Velocity of particles
Planetary Atmospheres
• Peak Thermal Velocity
Planetary Atmospheres
• Retention of Atmosphere
Rule of Thumb - if Vesc > 5 x Vthermal, then retain an atmosphere
Surface
Escape
Tension
Velocity
of drop of Water
Escape
Velocity
Low temperature
Medium temperature
High temperature
Escape
Evaporation
from
Escape
fromPlanet
Planet
0
Velocity of particles
Planetary Atmospheres
• Retention of Atmosphere
• Example - consider Earth and Moon - T ≈ 300 K
H = 1.7 x 10-27 kg
O = 2.7 x 10-26 kg
For H,
For H2O,


H2O = 3.0 x 10-26 kg
Vthermal =
2 x 1.4x10 -23 x 300
 2300 m/sec = 2.3 km/sec
-27
1.7 x 10
Vthermal =
2 x 1.4x10 -23 x 300
 500 m/sec = 0.5 km/sec
-26
3.0 x 10
Planetary Atmospheres
• Retention of Atmosphere
• Example - consider Earth and Moon - T ≈ 300 K
Vescape, Earth = 11.2 km/sec
Vescape, Moon = 2.4 km/sec
No H, retains H20
No H, no H20
For H,
Vthermal = 2.3 km/sec
For H2O,
Vthermal = 0.5 km/sec
ConcepTest!
The escape velocity of Mercury is 4.3 km/sec. The escape velocity
of Mars is 5.0 km/sec. Mercury does not have a substantial
atmosphere; Mars has an atmosphere. This is primarily because
A)
B)
C)
D)
Mars is further from the Sun so any gases are much colder
Mars has a 20% higher escape velocity
CO2 has more mass on Mars than on Mercury
Mars is a dynamically active planet with active volcanoes
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 equlibrium
100 energy units
50% gate
Planetary Atmospheres
• Greenhouse Effect
Energy In
100 energy units
100% gate
Energy Out
10, 000 energy units
stored
in equlibrium
100 energy units
1% gate
Planetary Atmospheres
• Greenhouse Effect