Lecture 35' Habitable Zones'

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Lecture 35. Habitable Zones.
reading Chapters 9, 10
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Goldilocks and the Solar System
Venus too hot now/not habitable geologic age of
the surface 500 Ma could have been habitable in
the past before runaway greenhouse Earth liquid
water for most or all of geologic history has
always been habitable carbonate-silicate cycle
stabilizes the climate with its
negative feedback loop. Mars too cold for
liquid water today geologic evidence of liquid
water in the past could have been habitable in
the past
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Runaway Greenhouse Effect
Occurs if the T reaches the boiling point of
water. Oceans turned into water vapor. Water
vapor is a greenhouse gas, causing additional
warming. This causes the oceans to evaporate even
faster. This is a positive feedback loop. Soon,
all the water will be in the atmosphere, which
will be very hot. Hot enough (several hundred
degrees) to vaporize carbonate rock. This would
turn carbonate rock back into CO2. The runaway
greenhouse is a permanent state - no known
way to escape it.
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Concept of the Habitable Zone (HZ)
Focuses on the presence of liquid water. The HZ
is the zone in which temperatures allow for
liquid water to exist on the surface. (note
this implies Europa is not in the habitable zone
- Europa is an exception to the definition of the
habitable zone) Key distance to the Sun and
presence of an atmosphere and magnetic
field. Moon in the Suns habitable zone, but
lacks an atmosphere. Is the Suns habitable
zone moving in or out with time?
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Venus
Venus and Earth likely started out with the same
amount of volatiles. Evidence of
volcanoes/outgassing/active planet on Venus. But
Venus is very dry and hot today. Where did all
the water go? May have had early oceans. As Sun
got brighter, more water went into the
atmosphere. 1. Photochemical reactions break
water into hydrogen and oxygen. Hydrogen is
easily lost to space. Oxygen reacts with other
gases in the atmosphere and with rocks on the
surface. 2. Water reacts with SO2 to form
sulfuric acid.
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What Controls Surface Habitability?

• Distance from the Sun
• Venus 0.7 AU
• Earth 1.0 AU
• Mars 1.5 AU
• Distance from the Sun determines how much solar
  radiation
• the planet receives.
• Solar radiation drops by 1/r2 - This means that
  if the distance (radius, r)
• from the Sun is doubled, the amount of solar
  radiation is 1/22, or 1/4).
• Solar radiation is important for the greenhouse
  effect.

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What Controls Surface Habitability?

• Planetary Size
• radius, relative to Earth
• Venus 0.95
• Earth 1
• Mars 0.53
• Smaller planets
• Lose internal heat rapidly, outgassing ceases.
• Cant replace volatiles that are lost to space or
  to chemical reactions.
• Larger planets
• Greater internal heat, internal heat is retained
  over time.
• Continued outgassing helps to retain the
  atmosphere.

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Plate Tectonics
Helps to recycle volatiles, trap volatiles in the
mantle so that they arent lost to space.
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What Controls Surface Habitability?

• Atmospheric Loss Processes
• Mars has also lost a significant part of its
  atmosphere.
• a) lack of magnetic field
• solar wind particles strip away the atmosphere
• b) low level of volcanism
• Early Mars
• thicker atmosphere
• stronger greenhouse effect

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Inner Boundary of the HZ
Determined by the ability to avoid a runaway
greenhouse. Inner boundary of the HZ lies
between Venus and the Earth If we move Earth to
0.82 AU - Earth would have a runaway greenhouse
If we move Earth to 0.95 AU - Earth would have a
moist greenhouse where more water is entering
the atmosphere where it can be lost to space.
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Outer Boundary of the HZ
Distance from the Sun where a strong greenhouse
effect does not allow the planet to stay warm
enough to keep water from freezing. Limiting
factor determining this boundary where
CO2 condenses into CO2 rain or CO2 ice. For a
large planet with a thick atmosphere, might be
1.7 AU. For a smaller planet with a thinner
atmosphere, might be 1.4 AU (just inside the
orbit of Mars).
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The HZ of the Solar System

• Optimistic boundaries 0.84 AU - 1.7 AU
• Conservative boundaries 0.95 AU - 1.4 AU
• Venus 0.7 AU
• Earth 1.0 AU
• Mars 1.5 AU
• The evolving HZ as the Sun becomes brighter, the
  HZ moves
• outward with time.

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The Continuously Habitable Zone (CHZ)
Region of the solar system that has been
habitable at all times since the end of heavy
bombardment. .
optimistic estimate 0.84 - 1.5 AU Earth and
Mars conservative estimate 0.95-1.2 AU Earth
only
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Habitability Outside the HZ
Possible liquid water oceans around Europa and
Ganymede. Could be subsurface liquid groundwater
on Mars. So, if you have internal heat sources,
this expands and complicates the definition of
the HZ or the CHZ. Could also have other liquids
(methane, ethane). The HZ is a generalization.
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Star Types
Cooler stars smaller burn slowly have long
lifetimes have narrower habitable zones Hotter
stars larger burn quickly have short
lifetimes lifetimes may be too short to evolve
life Brown dwarfs not large enough to sustain
fusion like a star have no HZ - too cool to heat
a planet may have larger planets with moons that
are tidally heated
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What Stars Are Good For Life?
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What Stars Are Good For Life?
Type O Very short lifetimes accretion takes 10s
of millions of years Type B Short
lifetimes long enough to form a planet, but Sun
dies before heavy bombardment ends Types A and
F 3 of stars lifetimes 1-2 Ga hotter than our
Sun, HZ is further out emit more uv light (breaks
down and reacts with organic compounds) Type
G 7 of stars
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What Stars Are Good For Life?
Types K and M 90 of stars long lifetimes 20-600
Ga dimmer stars habitable zones much closer
in frequent bursts of intense light and
radiation K-type stars 0.25 solar luminosity HZ
at 0.5 AU M-type stars 0.01 solar luminosity HZ
0.1-0.2 AU (inside Mercurys orbit) size of the
HZ is thinner
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Habitable Zones Around Other Stars
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Lecture 36. Galactic Habitable Zones.
reading Chapter 10