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# T 4

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```Aula 10
Sigam a ГЃgua + Atmosfera
Habitable Zone
вЂў A circumstellar habitable zone (HZ) is defined as
encompassing the range of distances from a star
for which liquid water can exist on a planetary
surface.
вЂў Under the present EarthвЂ™s atmospheric pressure (1
atm = 101325 Pa) water
is stable if temperature is
273K < T < 373K (0-100В°C)
вЂў Planetary surface
temperature (T) is the key
вЂў No nГ­vel do mar: 1 atmosfera = 101 325 Pa = 101.325 kPa =
1.01325 bar
na superfГ­cie
Phase Diagrams
Condensation
Vapor
Liquid
Evaporation
At some point Condensation = Evaporation вЂ“ liquid and vapor phases
are in Equilibrium вЂ“ saturation curve
T вЂ“ triple point of a substance is the temperature and pressure at which three
phases (gas, liquid, and solid) of that substance may coexist in
thermodynamic equilibrium
C вЂ“ critical point вЂ“ liquid phase cease to exist
1. Conjunto de condiГ§Гµes (1) вЂ“ fase sГіlida
2. Conjunto de condiГ§Гµes (2) вЂ“ fase lГ­quida
3. Conjunto de condiГ§Гµes (3) вЂ“ fase gasosa
Pode-se fazer um lГ­quido ferver ou
aumentando sua temperatura ou
diminuindo sao pressГЈo
Example: Earth-Sun
The EarthвЂ™s temperature (about 300K) is maintained
by the energy radiating from the Sun.
6,000 K
300 K
Planetary Energy Balance
вЂў We can estimate average planetary
temperature using the Energy Balance
approach
Ein = Eout
Ein
How much solar energy gets to the Earth?
Assuming solar radiation covers the area of a circle
defined by the radius of the Earth (re)
Ein = So (W/m2) x 4пЃ° re2 (m2) / 4
Ein = So x пЃ° re2 (W)
Ein
re
Ein
How much solar energy gets to the EarthвЂ™s surface?
**Some energy is reflected away**
пѓћ Essa fraГ§ГЈo = Albedo (A)
Ein = So x пЃ° re2 x (1-A)
Sample albedos on Earth
Surface
Typical Albedo
Fresh asphalt
0.04
Conifer forest
(Summer)
0.08, 0.09 to 0.15
Worn asphalt
0.12
Trees
0.15 to 0.18
Bare soil
0.17
Green grass
0.25
Desert sand
0.40
New concrete
0.55
Fresh snow
0.80вЂ“0.90
Ocean Ice
0.5вЂ“0.7
Albedos of planets
Mercury - 0.11
Venus - 0.65
Earth - 0.37
Mars - 0.15
Jupiter - 0.52
Saturn - 0.47
Uranus - 0.51
Neptune - 0.41
Pluto - 0.3
Eout
Energy Balance:
The amount of energy delivered to the Earth is
equal to the energy lost from the Earth.
Otherwise, the EarthвЂ™s temperature would
continually rise (or fall).
Eout
пѓ† Stefan-Boltzmann law
F = пЃі T4
F = flux of energy (W/m2)
T = temperature (K)
пЃі = 5.67 x 10-8 W/m2K4 (a constant)
Energy Balance:
Ein = Eout
Ein = So пЃ° re2 (1-A)
Eout = пЃі T4(4 пЃ° re2)
Eout
Ein
Energy Balance:
Ein = Eout
So (1-A) = пЃі T4 (x4)
T4 = [So (1-A)] / 4пЃі
Eout
Ein
EarthвЂ™s average temperature
T4 = So(1-A)
4пЃі
For Earth:
So = 1370 W/m2
A = 0.3
пЃі = 5.67 x 10-8 W/m2K4
(oC) = (K) - 273
(oC x 1.8) + 32 = oF
EarthвЂ™s average temperature
T4 = So(1-A)
4пЃі
For Earth:
So = 1370 W/m2
A = 0.3
пЃі = 5.67 x 10-8
T4 =
(1370 W/m2)(1-0.3)
4 (5.67 x 10-8 W/m2K4)
T4 = 4.23 x 109 (K4)
T = 255 K
Earth expected Temperature:
Texp = 255 K
(oC) = (K) - 273
SoвЂ¦.
Texp = (255 - 273) = -18 oC
Is the EarthвЂ™s surface really -18 oC?
NO. The actual temperature is warmer!
The observed temperature (Tobs) is 15 oC, or
The difference between observed and expected
temperatures (пЃ„T):
пЃ„T = Tobs вЂ“ Texp
пѓњ пЃ„T = 15 - (-18)
пЃ„T = + 33 oC = 33 K
We call this warming the greenhouse effect, and
is due to absorption of energy by gases in the
atmosphere.
Atmospheric Greenhouse Effect
Incoming Solar
Outgoing IR
Greenhouse
gases (CO2)
N2, O2
EarthвЂ™s Surface
Original Greenhouse
вЂў Precludes heat loss by inhibiting the
upward air motion
вЂў Solar energy is used more effectively.
Same solar input вЂ“ higher
temperatures.
Warming results from interactions of gases in the
atmosphere with incoming and outgoing radiation.
To evaluate how this happens, we will focus on the
composition of the EarthвЂ™s atmosphere.
Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas
N2
O2
Ar
H2O
CO2
greenhouse
gasesCH4
N2O
O3
concentration (%)
78
21
0.9
variable
0.037
370 ppm
1.7
0.3
1.0 to 0.01
(stratosphere-surface)
O
C
O
c a r b o n d io x id e
Greenhouse Gases
O
H
H
w a te r
H
H
C
O
H
-
H
m e th a n e
+
O
O
ozone
Non-greenhouse Gases
N2
O2
N п‚є N
O = O
The energy increases the movement of the
molecules.
The molecules rotate and vibrate.
stretching
bending
Vibration
Non-greenhouse Gases
N п‚є N
O = O
Non-greenhouse gases have symmetry!
(Technically speaking, greenhouse gases
have a dipole moment whereas N2 and O2
donвЂ™t)
(в€’)
O
H
H
(+)
вЂў Oxygen has an unfilled outer shell
of electrons (6 out of 8), so it wants
to attract additional electrons. It gets
them from the hydrogen atoms.
Molecules with an uneven distribution of
electrons are especially good absorbers and
emitters.
These molecules are called dipoles.
Water
Electron-poor region:
Partial positive charge
H
O
H
Electron-rich region:
Partial negative charge
oxygen is more
electronegative
than hydrogen
Absorption wavelength is a characteristic of each molecule
Thermal IR Spectrum for Earth
H2O pure rotation
H2O vibration/rotation
CO2 (15 пЃ­m)
(6.3 пЃ­m)
O3 (9.6 пЃ­m)
Ref.: K.-N. Liou, Radiation and Cloud Physics Processes in
the Atmosphere (1992)
Non-Greenhouse Gases
вЂў The molecules/atoms that constitute the bulk of
the atmosphere: O2, N2 and Ar; do not interact
вЂў While the oxygen and nitrogen molecules can
vibrate, because of their symmetry these
vibrations do not create any transient charge
separation.
вЂў Without such a transient dipole moment, they
can neither absorb nor emit infrared radiation.
Atmospheric Greenhouse Effect
(AGE)
вЂў AGE increases surface temperature by
returning a part of the outgoing radiation
back to the surface
вЂў The magnitude of the greenhouse effect
is dependent on the abundance of
greenhouse gases (CO2, H2O etc.)
Clouds
вЂў Just as greenhouse gases, clouds also
affect the planetary surface temperature
(albedo)
вЂў Clouds are droplets of liquid water or ice
crystals
вЂў Cumulus clouds вЂ“ puffy, white clouds
вЂў Stratus clouds вЂ“ grey, low-level clouds
вЂў Cirrus clouds вЂ“ high, wispy clouds
Cumulus cloud
Cirrus cloud
Climatic Effects of Clouds
вЂў Clouds reflect sunlight (cooling)
вЂў Clouds absorb and re-emit outgoing IR
вЂў Low thick clouds (stratus clouds) tend to
cool the surface
вЂў High, thin clouds (cirrus clouds) tend to
warm the surface
Back to the HZ
вЂў LetвЂ™s assume that a planet has EarthвЂ™s
atmospheric greenhouse warming (33 K)
and EarthвЂ™s cloud coverage (net planetary
albedo ~ 0.3)
вЂў Where would be the boundaries of the HZ
for such planet?
вЂў Recall that the Solar flux: S = L/(4пЃ°R2)
вЂў We can substitute formula for the Solar
flux by planetary energy balance equation:
вЂў S Г—(1-A) = пЃіГ—T4 Г—4
L/(4пЃ°R2)Г— (1-A) = пЃіГ—T4 Г—4
L п‚ґ (1 пЂ­ A)
пЂЅR
4
16 п‚ґ пЃ° п‚ґ пЃі п‚ґ T
(R = distance from star)
Global surface temperature (Ts)
вЂў
Global surface temperature (Ts) depends
on three main factors:
a) Solar flux
b) Albedo (on Earth mostly clouds)
c) Greenhouse Effect (CO2, H2O , CH4, O3
etc.)
вЂў We can calculate Te from the вЂњEnergy
greenhouse warming:
Ts = Te + в€†Tg
вЂў But! The amount of the atmospheric
greenhouse warming (в€†Tg) and the
planetary albedo can change as a function
of surface temperature (Ts) through
different feedbacks in the climate system.
Climate System and Feedbacks
вЂў We can think about climate system as a
number of components (atmosphere,
ocean, land, ice cover, vegetation etc.)
which constantly interact with each other.
вЂў There are two ways components can
interact вЂ“ positive and negative couplings
Systems Notation
= system component
= positive coupling
= negative coupling
Positive Coupling
CarвЂ™s gas pedal
CarвЂ™s speed
Amount of food
eaten
Body weight
вЂў A change in one component leads to a change of the same
Negative Coupling
CarвЂ™s break
system
CarвЂ™s speed
Exercise
Body weight
вЂў A change in one component leads to a change of the opposite
Positive Coupling
Atmospheric
CO2
Greenhouse
effect
вЂў An increase in atmospheric CO2 causes
a corresponding increase in the greenhouse
effect, and thus in EarthвЂ™s surface temperature
вЂў Conversely, a decrease in atmospheric CO2
causes a decrease in the greenhouse effect
Negative Coupling
EarthвЂ™s albedo
(reflectivity)
EarthвЂ™s
surface
temperature
вЂў An increase in EarthвЂ™s albedo causes a
corresponding decrease in the EarthвЂ™s surface
temperature by reflecting more sunlight back to
space
вЂў Or, a decrease in albedo causes an increase in
surface temperature
Feedbacks
вЂў In nature component A affects component
B but component B also affects
component A. Such a вЂњtwo-wayвЂќ
interaction is called a feedback loop.
A
вЂў Loops can be stable or unstable.
B
Climate Feedbacks
Water Vapor Feedback
Snow and Ice Albedo Feedback
The IR Flux/Temperature
Feedback
Short-term climate stabilization
The Carbonate-Silicate Cycle
(metamorphism)
Long-term climate stabilization
вЂў CaSiO3 + CO2 пѓ CaCO3 + SiO2
(weathering)
вЂў CaCO3 + SiO2 пѓ CaSiO3 + CO2
(metamorphosis)
Negative Feedback Loops
The carbonate-silicate cycle feedback
Rainfall
Silicate
weathering
rate
Surface
temperature
(в€’)
Greenhouse
effect
Atmospheric
CO2
The inner edge of the HZ
вЂў The limiting factor for the inner boundary
of the HZ must be the ability of the planet
to avoid a runaway greenhouse effect.
вЂў Theoretical models predict that an Earthlike planet would convert all its ocean into
the water vapor at ~0.84 AU
вЂў However it is likely that a planet will lose
water before that.
Moist Greenhouse
вЂў If a planet is at 0.95 AU it gets about 10%
higher solar flux than the Earth.
вЂў Increase in Solar flux leads to increase in
surface temperature пѓ more water vapor
in the atmosphere пѓ even higher
temperatures
вЂў Eventually all atmosphere becomes rich in
water vapor пѓ effective hydrogen escape
to space пѓ permanent loss of water
hпЃ®
Ineffective
H escape
Space
Effective
H escape
hпЃ®
H2O + hпЃ® пѓ H + OH
H2O + hпЃ® пѓ H + OH
Upper Atmosphere
(Stratosphere, Mesosphere)
H2O-poor
H2O-rich
H2O-rich
Lower Atmosphere
(Troposphere)
H2O-ultrarich
Venus fate
вЂў Runaway (or moist) greenhouse and the
permanent loss of water could have
happened on Venus
вЂў Venus has very high D/H (~120 times
higher than EarthвЂ™s) ratio suggesting huge
hydrogen loss
вЂў Without water CO2 would accumulate in
the atmosphere and the climate would
become extremely hot вЂ“ present Venus is
~ 90 times more massive than EarthвЂ™s and
almost entirely CO2.
вЂў Eventually Earth will follow the fate of
Venus
The outer edge of the HZ
вЂў The outer edge of the HZ is the distance
from the Sun at which even a strong
greenhouse effect would not allow liquid
water on the planetary surface.
вЂў Carbonate-silicate cycle can help to
extend the outer edge of the HZ by
accumulating more CO2 and partially
offsetting low solar luminosity.
Limit from CO2 greenhouse
вЂў At low Solar luminosities high CO2 abundance
would be required to keep the planet warm.
вЂў But at high CO2 abundance Atm does not
produce as much net warming because it also
вЂў Theoretical models predict that no matter how
high CO2 abundance would be in the
atmosphere, the temperature would not exceed
the freezing point of water if a planet is further
than 1.7 A.U.
Limit from CO2 condensation
вЂў At high CO2 abundance and low
temperatures carbon dioxide can start to
condense out (like water condense into
rain and snow)
вЂў Atmosphere would not be able to build
CO2 if a planet is further than 1.4 A.U.
Fate of Mars
вЂў Mars is on the margin of the HZ at the
present
вЂў But! Mars is a small planet and cooled
relatively fast
вЂў Mars cannot outgas CO2 and sustain
Carbonate-Silicate feedback.
вЂў Also hydrogen can escape effectively due
to the low Martian gravity and lack of
magnetic field.
River channel
Nanedi Vallis
(from Mars Global Surveyor)
~3 km
Why the Sun gets brighter with
time
вЂў
вЂў
вЂў
вЂў
вЂў
H fuses to form He in the core
Core becomes denser
Core contracts and heats up
Fusion reactions proceed faster
More energy is produced пѓћ more
energy needs to be emitted
Solar Luminosity versus Time
See The Earth System, ed. 2, Fig. 1-12
Continuous Habitable Zone (CHZ)
вЂў A region, in which a planet may reside and
maintain liquid water throughout most of a
starвЂ™s life.
```
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