Download ppt

Terrestrial atmospheres
Overview
• Most of the planets, and three large moons (Io, Titan and Triton),
have atmospheres
Mars
•
•
•
•
Very thin
Mostly CO2
Some N2, Ar
Winds, dust storms
Venus
•
•
•
•
Very thick
Mostly CO2
Some N2
Sulfuric acid clouds
Earth
•
•
•
•
Mostly N2, O2
Some H20, Ar
Only 0.03% CO2.
Water clouds
Secondary atmospheres
• Can calculate how many volatiles had to be added to the atmosphere to get
present surface conditions
 Not just current atmosphere content, but also the oceans and CO2 locked up
in rocks and shells.
• Percent (by weight) added to atmosphere by
volcanic outgassing
Gas
Deep
eruptions
Continental
geysers
Required
amount
H2O
57.8
99.4
92.8
CO2
23.5
0.33
5.1
Cl2
0.1
0.12
1.7
N2
5.7
0.05
0.24
S2
12.6
0.03
0.13
Others
<1
<1
<1
Atmospheric compositions
• Comparison of
total volatile
content on
Venus, Earth
and Mars shows
better
agreement.
Volatile
Venus
Earth
Mars
H2O
Atmosphere
60
3
0.02
Oceans/caps
0
250,000
5000?
Crust
160,000? 30,000
10,000?
Total
160,000? 280,000
15,000?
Atmosphere
100,000
0.4
50
Oceans/caps
0
0
10
Crust
0
100,000
>900?
Total
100,000
100,000
>1000?
2,000
2,000
300
4
11
0.5
CO2
Table 11.2: mass
fraction of
volatiles (x109).
N2
Atmosphere
40Ar
Atmosphere
Physical Structure
Use the equation of hydrostatic equilibrium to determine how the
pressure and density change with altitude, in an isothermal
atmosphere. You may neglect the change in gravitational force
with altitude.
dP
GM
  2    g
dr
r
Physical Structure
• Pressure decreases with increasing altitude
• Atmospheres are
compressible, so density
decreases with altitude
• Compare with the pressure
structure of the oceans,
where the density remains
approximately constant.
Physical Structure
• Surface temperatures and pressures are very different for the
three terrestrial planets
 But the pressure scale heights are similar
Venus Earth Mars
Tequil (K)
238
263
222
Tsurf (K)
733
288
215
Psurf (bar)
92
1.013
0.0056
surf (kg/m3) 65
1.2
0.017
H(km)
8.5
18
16
Thermal structure
• The thermal structure of the terrestrial atmospheres are similar
• There is at least one temperature minimum
 Caused by heat being trapped by cloud layers
• Temperature gradient depends on heat transport
 Radiation – depends on the opacity of the atmosphere
 Convection
 Conduction – important near the surface
Earth:
• water concentrated near
surface
• CO2 locked in rocks, shells
• Leads to oxygen-rich
atmosphere
Chemical Structure
Mars
• Water and CO2 clouds
Venus
• Dominated by sulfur,
CO2
• Clouds of sulfuric acid
at 48-58 km
Atmospheres and Water
• Lots of evidence that liquid water existed on Mars’ surface
 Liquid water requires higher temperatures and pressures
 Must have been a much denser atmosphere at one point
• Venus: too hot for liquid
water to exist
 Water evaporates, H
and O dissociate
 H is lost, and O forms
CO2, sulfuric acid.
Present-day
Mars
Opacity
• Consider a completely transparent atmosphere




No radiation (sunlight) is absorbed
Optical sunlight hits the ground and heats it up
Earth reradiates this energy in the infrared
No effect on atmosphere
max T  0.290 cm K
L  4R 2Te4
Opacity
• Earth’s atmosphere is not transparent
 Ozone (high altitude) strongly absorbs UV radiation
 Water and CO2 (lower altitude) strongly absorb infrared radiation
• Upper atmosphere
heated by incoming
solar radiation
• Outgoing radiation
from ground heats
lower atmosphere
Earth transparency
Earth’s atmosphere is opaque at infrared wavelengths
infrared
optical
Greenhouse effect
• Optical radiation strikes the Earth and heats it up
• Infrared radiation is absorbed by lower atmosphere and reradiated in all
directions
 Including back to the ground, heating it further
• Surface and lower atmosphere heat up until the amount of IR radiation
escaping the atmosphere is equal to the amount of solar radiation coming
in
Greenhouse Effect
Assume a simple model of the greenhouse effect, where a cloud
layer (with same surface area as the Earth) is completely
transparent to optical radiation, and completely opaque to
infrared radiation. Calculate by how much the surface
temperature increases when the cloud layer is present.
Solar heating
Heat radiated
away
Heat returned
to surface
Clouds heated from below
Break
Convection
Solar heating alone would cause global circulation, as hot air rises and
cool air sinks
Rotation and the Coriolis force
Recall: in a rotating reference frame, objects do not move in a
straight line.
Applet
Rotation and Coriolis force
Winds
• Coriolis force splits the Hadley cells in each hemisphere into three
cells.
Diversion: Australian toilets
• Does the Coriolis force influence the direction in which a vortex
forms?
The acceleration due to the
Coriolis force is:
 

a  2  v
where  is the angular velocity
of rotation, and v is the
velocity of the moving particle
Upper atmospheres
Upper atmospheres
• The thermosphere is heated by energetic photons from the Sun
• Low density means light atoms are able to float to the top
Escape of atmospheres
• Particles with velocity greater than the escape velocity will leave
the atmosphere (if the density is low enough that they will not
collide with another atom)
• What is the average velocity of a particle at temperature T? How
high a T do you need for a particle with this velocity to escape?
vesc
 2GM 


 R 
1/ 2
Escape of atmospheres
• Particles with velocity greater than the escape velocity will leave
the atmosphere (if the density is low enough that they will not
collide with another atom)
• At a given temperature, particles of a given mass have a
Maxwellian velocity distribution, with a long tail to high velocities:
• At T~2000 K (top
of thermosphere),
how much faster
than the average
velocity must a
hydrogen atom be
moving to escape?
• How about for a
Neon atom?
Venus
• CO2 rich atmosphere
 Leads to a strong greenhouse effect, at high surface temperatures
• Clouds made mostly of sulfuric acid
• Motion of upper atmosphere
is due to convection, as a
result of the strong
temperature gradient.
 Almost no wind or weather
at the surface, due to the
slow rotation of Venus
Mars
• Weather dominated by dust storms
 Very small dust particles (1 micron diameter) can be carried by
strong winds (>180 km/h)
Titan
• N2 rich atmosphere
• Little greenhouse effect, cold surface temperatures
• Smog-colour from interactions of solar radiation and methane in
atmosphere
Next Lecture
The Giant Planets