Search for Life in the Universe

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Search for Life in the Universe

• Chapter 10
• Nature Evolution of Habitability

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Outline

• Concept of Habitable Zone
• Venus
• Climate Regulation
• Greenhouse Warming
• Water on Venus
• Runaway Greenhouse Effect
• Suns Habitable Zone
• Surface Habitability
• Habitable Zone Today
• Evolving Habitable Zone
• Habitability Outside the Zone
• Future of Life on Earth
• End of Habitability on Earth
• Death of the Sun
• Survival

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Concept of Habitable Zone

• Surface habitability
• Solar System we may find underground
  habitability by traveling to the site
• Extrasolar habitability travel unlikely and
  distant observations (imagery spectroscopy) can
  only detect surface habitability
• Extraterrestrial intelligence surface
  habitability
• Surface liquid water key factor
• Habitability in the Solar System
• Habitability today Venus, Earth and Mars so
  similar, yet conditions so different
• How does habitability evolve?
• Stability of habitability

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Climate Regulation

• Comparison Venus, Earth and Mars
• Mars water would freeze almost anywhere
• Earth well
• Venus water would boil everywhere
• Greenhouse warming
• All planets frozen w/o greenhouse effect
• Little effect on Mars weak atmosphere
• Venus and Earth similar planets, yet vastly
  different greenhouse effect
• CO2
• Venus and Earth same amount of CO2
• Earth CO2 cycle ? CO2 locked in oceans and rocks
• Venus no oceans ? no CO2 cycle

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Greenhouse Warming
Difference No Greenhouse Temp. Average Surface Temp. Planet
513?C ?43?C 470?C Venus
32?C ?17?C 15?C Earth
5?C ?55?C ?50?C Mars
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Water on Venus

• Any water
• Surface ice or water would boil
• Atmospheric water vapor not seen
• Total water lt10-4 of quantity on Earth
• Never any water?
• Most planetessimals forming Venus and Earth had
  little ice
• Water from planetessimals or comets originating
  farther away
• But collisions with those objects similar for
  Venus and Earth
• Water lost to space?
• Volcanic activity plenty of outgassing of water
  to the atmosphere
• Water lost to space UV H2O ? H2 (lost) O2
  (to surface)
• Evidence
• Deuterium (2H) 135 times more abundant on Venus
  than Earth
• Lower limit several meters of global ocean, lt1
  of Earth water

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Runaway Greenhouse Effect

• Why didnt Earth lose its water?
• Water locked up in the ocean, little exposed to
  UV in the upper atmosphere
• Ozone extra protection, but not there in the
  early Earth
• If we moved Earth to Venus?
• Average global temperature 15?C ? 45?C ? more
  evaporation ? water-induced greenhouse effect ?
  higher temperature ?
• Runaway greenhouse effect heating continues
  until all the oceans evaporate ? no CO2 cycle ?
  all CO2 outgassed
• Venus when the Sun was less luminous
• Sun originally 30 dimmer ? conditions at Venus
  similar to those on Earth today ? stable oceans
• As Sun warms up ? runaway greenhouse effect
• Evidence lost because of volcanic repaving of
  the surface

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Surface Habitability

• Distance from Sun
• Gross effect Mercury much too hot, outer planets
  much too cold
• Subtle effect runaway greenhouse effect on Venus
• Planetary size
• Gross effect Moon cannot hold atmosphere
• Subtle effect plate tectonics depends on size,
  details not well understood
• Atmospheric processes
• Venus major part of runaway greenhouse effect
• Mars loss of atmosphere due to lack of magnetic
  field and low level of volcanism

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Venus, Earth Mars
Planet Distance from Sun 106 km Radius km Distance from Sun AU Radius Earth Radii
Venus 108 6050 km 0.72 0.95
Earth 150 6380 km 1 1
Mars 228 3400 km 1.52 0.53
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Habitable Zone Today

• Inner boundary
• Somewhere between Venus (0.72 AU) and Earth (1
  AU)
• Optimistic model, 0.84 AU runaway greenhouse
  effect
• Pessimistic model, 0.95 AU moist runaway
  greenhouse effect (water vapor circulating higher
  in the atmosphere)
• Outer boundary
• Where the atmosphere of an Earth-size planet has
  enough greenhouse effect
• Optimistic model, 1.7 AU (cf., Mars 1.52 AU)
  enough greenhouse effect
• Pessimistic model, 1.4 AU middle atmosphere too
  cold ? CO2 snow ? CO2 loss from atmosphere ? less
  greenhouse effect
• Habitable zone
• There is a habitable zone around the Earth
• Exact limits are model-based and uncertain

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Evolving Habitable Zone

• Dependence on solar luminosity
• Sun less luminous in the past ? habitable zone
  moves in
• Sun more luminous in the future ? habitable zone
  moves out
• Stellar evolution
• H to He ? fewer particles at the core ? less
  pressure ? squeezing by layers above ?
  temperature rise ? luminosity rise
• Quantitative stellar structure and evolution is
  well modeled
• Checked against observations of stars of all
  masses and ages
• Evolving habitability
• Habitable until now optimistic 0.73?1.5 AU,
  pessimistic 0.85?1.15 AU
• Habitable also until the death of the Sun
  optimistic 1.3?1.5 AU, pessimistic at most
  another 2.5 byr

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Habitability Outside the Zone

• Life just under the surface e.g., Mars with
  possible life a few hundred meters under the
  surface
• Life deep underground e.g., Europa, Ganymede,
  Callisto
• Liquid other than water e.g., Titan
• Tidal heating Energy source is a planet, not the
  star ? any distance from star
• Brown dwarfs
• Mass lt 0.08 MSun 80 MJupiter
• May be very common
• Tidal heating can be significant
• Internal heat hydrogen atmosphere enough for
  liquid water on Earth ? any distance from star

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End of Habitability on Earth

• Dont lose sleep over it
• Several hundred myr to several byr to go
• Pessimistic estimate
• Runaway moist greenhouse effect in lt 1 byr
• Is model correct? E.g., what is effect of
  clouds?
• Optimistic estimate
• Regular runaway greenhouse effect in 3?4 byr
• Sunshade
• Build a huge sunshade
• Use the solar energy
• Emigration
• How?
• To where?

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Death of the Sun

• Red giant star
• 100 times larger (engulfs Venus)
• Surface temperature on Earth 700?C
• Underground life will not survive
• Planetary nebula
• Outer part of the Sun (0.4 MSun) expelled into
  the interstellar medium (ISM)
• White dwarf
• Remaining core (0.6 MSun) collapses to a white
  dwarf
• Radius Earth radius
• Density 106 g/cm3
• Held up by degeneracy pressure, does not need
  thermal pressure
• Slowly losing energy over many byr ? stellar death

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Survival

• Emigration requires travel
• Other life elsewhere
• Of course, thats what we are looking for
• Ultimately all stars die and recycled ISM is lost
• Radiating black holes
• Massive stars (gt 25 MSun) form black holes, maybe
  with only part of their masses
• Black holes radiate 1012 kg 10?18 MSun radiate
  themselves away over current age of the universe
  10 byr
• Radiation timescale ? mass, hopeless for stellar
  mass or higher
• Death of the Universe
• Universe expands forever
• No recollapse
• No other source of energy