Follow the Water Part 3

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Follow the Water (Part 3)

• Lecture 11
• Limits of the Habitable Zone. Climate Feedbacks.

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From Last Lecture

• Habitable zone (HZ) is defined as the range of
  distances from a star for which liquid water can
  exist on a planetary surface.
• Under the present Earths atmospheric pressure (1
  atm 101325 Pa) water is stable if the surface
  temperature is 273 K (freezing) lt Ts lt 373 K
  (boiling)

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Boundaries of the HZ(without climate feedbacks)

• Inner boundary 0.56 AU
• Outer boundary 1.13 AU

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Global surface temperature (Ts)

• Global surface temperature (Ts) depends on three
  main factors
• Solar flux
• Albedo (on Earth mostly clouds)
• Greenhouse Effect (CO2, H2O , CH4, O3 etc.)
• We can calculate Te (effective temperature) or
  Trad (radiative temperature) from the Energy
  balance equation and add the greenhouse warming
• Ts Te ?Tg

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• But! The amount of the atmospheric greenhouse
  warming (?Tg) and the planetary albedo can change
  as a function of surface temperature (Ts) through
  different feedbacks in the climate system.

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Climate System

• We can think about climate system as a number of
  components (atmosphere, ocean, land, ice cover,
  vegetation etc.) which constantly interact with
  each other.
• There are two ways components can interact
  positive and negative couplings

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Systems Notation
system component
positive coupling
negative coupling
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Positive Coupling
Cars speed
Cars gas pedal
Body weight
Amount of food eaten

• A change in one component leads to a change of
  the same
• direction in the linked component

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Negative Coupling
Cars speed
Cars break system
Body weight
Exercise

• A change in one component leads to a change of
  the opposite
• direction in the linked component

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Positive Coupling
Atmospheric CO2
Greenhouse effect

• An increase in atmospheric CO2 causes
• a corresponding increase in the greenhouse
• effect, and thus in Earths surface
  temperature
• Conversely, a decrease in atmospheric CO2
• causes a decrease in the greenhouse effect

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Negative Coupling
Earths albedo (reflectivity)
Earths surface temperature

• An increase in Earths albedo causes a
• corresponding decrease in the Earths surface
• temperature by reflecting more sunlight back to
• space
• Or, a decrease in albedo causes an increase in
• surface temperature

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Feedbacks

• In nature component A affects component B but
  component B also affects component A. Such a
  two-way interaction is called a feedback loop.
• Loops can be stable or unstable.

B
A
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NOT A Harmonious Family
positive coupling
parents anger
childrens noise
positive coupling
street noise
A positive feedback loop Unstable system which
changes further following a perturbation
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A Harmonious Family
positive coupling
parents anger
childrens noise
negative coupling
street noise
A negative feedback loop Stable system which
resists change following a perturbation
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• Negative feedback loops have an odd number of
  negative couplings within the loop.

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Climate Feedbacks
Water Vapor Feedback
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Snow and Ice Albedo Feedback
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The IR Flux/Temperature Feedback
Short-term climate stabilization
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In a typical glaciation ice stops growing
because of the IR Flux/Temperature Feedback
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The Carbonate-Silicate Cycle
(metamorphism)
Long-term climate stabilization
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• CaSiO3 CO2 ? CaCO3 SiO2 (weathering)
• CaCO3 SiO2 ? CaSiO3 CO2 (metamorphosis)

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Negative Feedback Loops
The carbonate-silicate cycle feedback
Rainfall
Surface temperature
Silicate weathering rate
(-)
Atmospheric CO2
Greenhouse effect
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The inner edge of the HZ

• The limiting factor for the inner boundary of the
  HZ must be the ability of the planet to avoid a
  runaway greenhouse effect.
• Theoretical models predict that an Earth-like
  planet would convert all its ocean into the water
  vapor 0.84 AU
• However it is likely that a planet will lose
  water before that.

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Moist Greenhouse

• If a planet is at 0.95 AU it gets about 10
  higher solar flux than the Earth.
• Increase in Solar flux leads to increase in
  surface temperature ? more water vapor in the
  atmosphere ? even higher temperatures
• Eventually all atmosphere becomes rich in water
  vapor ? effective hydrogen escape to space ?
  permanent loss of water

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Effective H escape
Space
h?
h?
Ineffective H escape
H2O h? ? H OH
H2O h? ? H OH
Upper Atmosphere (Stratosphere, Mesosphere)
H2O-poor
H2O-rich
H2O-rich
Lower Atmosphere (Troposphere)
H2O-ultrarich
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Venus fate

• Runaway (or moist) greenhouse and the permanent
  loss of water could have happened on Venus
• Venus has very high D/H (120 times higher than
  Earths) ratio suggesting huge hydrogen loss

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• Without water CO2 would accumulate in the
  atmosphere and the climate would become
  extremely hot present Venus is 90 times more
  massive than Earths and almost entirely CO2.
• Eventually Earth will follow the fate of Venus

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The outer edge of the HZ

• 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.
• Carbonate-silicate cycle can help to extend the
  outer edge of the HZ by accumulating more CO2 and
  partially offsetting low solar luminosity.

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Limit from CO2 greenhouse

• At low Solar luminosities high CO2 abundance
  would be required to keep the planet warm.
• But at high CO2 abundance does not produce as
  much net warming because it also scatter solar
  radiation.
• 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.

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Limit from CO2 condensation

• At high CO2 abundance and low temperatures carbon
  dioxide can start to condense out (like water
  condense into rain and snow)
• Atmosphere would not be able to build CO2 if a
  planet is further than 1.4 A.U.

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Fate of Mars

• Mars is on the margin of the HZ at the present
• But! Mars is a small planet and cooled relatively
  fast
• Mars cannot outgas CO2 and sustain
  Carbonate-Silicate feedback.
• Also hydrogen can escape effectively due to the
  low martian gravity and lack of magnetic field.
  

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River channel
Nanedi Vallis (from Mars Global Surveyor)
3 km
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• Grand Canyon required several millions
• of years to form
• The same should be true for Nanedi Vallis

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Solar Luminosity versus Time
Solar luminosity is changing with time gt
Boundaries of the HZ are changing with time. How?
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Continuous Habitable Zone (CHZ)

• A region, in which a planet may reside and
  maintain liquid water throughout most of a stars
  life.

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HZ boundaries depend on the class of the
star. How?
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Summary Habitable Zone

• The boundaries of the habitable zone depend on
  the stellar luminosity, planetary albedo,
  atmospheric greenhouse effect
• Atmospheric greenhouse and planetary albedo can
  change through climate feedbacks
• Stellar luminosity changes through time gt HZ
  boundaries change through time

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• Suppose a planet is within the HZ.
• Does it mean that such planet would have to have
  liquid water on its surface?

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Additional conditions for liquid water on the
planetary surface

• Planet should get enough water during its
  formation or shortly after
• Planet should be massive enough to retain water
• Planet should have enough internal heat to
  maintain plate tectonics
• Even if all of the above is true a water-rich
  planet can get into Snowball glaciations

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Low Latitude Glaciations

• Paleomagnetic data indicate low-latitude
  glaciation at 0.63 (Marinoan), 0.75 (Sturtian)
  and 2.3 billions years ago (Huronian).