* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Search for Life in the Universe
Survey
Document related concepts
Formation and evolution of the Solar System wikipedia , lookup
Life on Mars wikipedia , lookup
Interplanetary contamination wikipedia , lookup
Transit of Venus wikipedia , lookup
Astronomical unit wikipedia , lookup
Observations and explorations of Venus wikipedia , lookup
Extraterrestrial atmosphere wikipedia , lookup
Astronomy on Mars wikipedia , lookup
Geocentric model wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Rare Earth hypothesis wikipedia , lookup
Circumstellar habitable zone wikipedia , lookup
Dialogue Concerning the Two Chief World Systems wikipedia , lookup
Astrobiology wikipedia , lookup
Comparative planetary science wikipedia , lookup
Transcript
Search for Life in the Universe Chapter 10 Nature & Evolution of Habitability 5/22/2017 AST 248, Spring 2007 1 Outline • Concept of Habitable Zone • Venus – – – – Climate Regulation Greenhouse Warming Water on Venus Runaway Greenhouse Effect • Sun’s 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 5/22/2017 AST 248, Spring 2007 2 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 5/22/2017 AST 248, Spring 2007 3 5/22/2017 AST 248, Spring 2007 4 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 5/22/2017 AST 248, Spring 2007 5 Greenhouse Warming “No Greenhou Difference se” Temp. Planet Average Surface Temp. Venus 470C 43C 513C Earth 15C 17C 32C Mars 50C 55C 5C 5/22/2017 AST 248, Spring 2007 6 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 5/22/2017 AST 248, Spring 2007 7 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: 15C 45C 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 5/22/2017 AST 248, Spring 2007 8 5/22/2017 AST 248, Spring 2007 9 5/22/2017 AST 248, Spring 2007 10 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 5/22/2017 AST 248, Spring 2007 11 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 5/22/2017 AST 248, Spring 2007 12 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 5/22/2017 AST 248, Spring 2007 13 5/22/2017 AST 248, Spring 2007 14 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 0.731.5 AU, pessimistic 0.851.15 AU – Habitable also until the death of the Sun: optimistic 1.31.5 AU, pessimistic at most another 2.5 byr 5/22/2017 AST 248, Spring 2007 15 5/22/2017 AST 248, Spring 2007 16 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 5/22/2017 AST 248, Spring 2007 17 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 34 byr • Sunshade – Build a huge sunshade – Use the solar energy • Emigration – How? – To where? 5/22/2017 AST 248, Spring 2007 18 Death of the Sun • Red giant star – 100 times larger (engulfs Venus) – Surface temperature on Earth 700C – 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 5/22/2017 AST 248, Spring 2007 19 5/22/2017 AST 248, Spring 2007 20 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 = 1018 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 5/22/2017 AST 248, Spring 2007 21