Transcript Search for Life in the Universe

Search for Life in the Universe
Chapter 10
Nature & Evolution of Habitability
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Outline
• Concept of Habitable Zone
• Venus
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Climate Regulation
Greenhouse Warming
Water on Venus
Runaway Greenhouse Effect
• Sun’s Habitable Zone
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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
“No
Greenhou Difference
se” Temp.
Planet
Average
Surface
Temp.
Venus
470C
43C
513C
Earth
15C
17C
32C
Mars
50C
55C
5C
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Water on Venus
• Any water
– Surface ice or water: would boil
– Atmospheric water vapor: not seen
– Total water: <10-4 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, <1% of Earth water
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Runaway Greenhouse Effect
• Why didn’t 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
Venus
Distance Radius Distance
from Sun
[km]
from Sun
[106 km]
[AU]
108
6050 km
0.72
Radius
[Earth
Radii]
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 < 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
• Don’t lose sleep over it:
– Several hundred myr to several byr to go
• Pessimistic estimate:
– Runaway moist greenhouse effect in < 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
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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, that’s what we are looking for
– Ultimately all stars die and recycled ISM is lost
• Radiating black holes
– Massive stars (> 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
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