Announcement • MT2: questions/answers/grades are now posted • My mistake and apologies: Three questions (22, 37 & 50) relied on material in Ch.
Download ReportTranscript Announcement • MT2: questions/answers/grades are now posted • My mistake and apologies: Three questions (22, 37 & 50) relied on material in Ch.
Announcement • MT2: questions/answers/grades are now posted • My mistake and apologies: Three questions (22, 37 & 50) relied on material in Ch. 9. They will be withdrawn and the exams regraded. 11/7/2015 AST 248, Fall 2005 1 Search for Life in the Universe Chapter 9 Nature & Evolution of Habitability 11/7/2015 AST 248, Fall 2005 2 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 11/7/2015 AST 248, Fall 2005 3 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 11/7/2015 AST 248, Fall 2005 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 11/7/2015 AST 248, Fall 2005 5 Greenhouse Warming “No Greenhou Difference se” Temp. Planet Average Surface Temp. Venus 470C 43C 513C Earth 15C 17C 32C Mars 50C 55C 5C 11/7/2015 AST 248, Fall 2005 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 11/7/2015 AST 248, Fall 2005 7 11/7/2015 AST 248, Fall 2005 8 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 11/7/2015 AST 248, Fall 2005 9 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 11/7/2015 AST 248, Fall 2005 10 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 11/7/2015 AST 248, Fall 2005 11 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 11/7/2015 AST 248, Fall 2005 12 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 11/7/2015 AST 248, Fall 2005 13 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 11/7/2015 AST 248, Fall 2005 14 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? 11/7/2015 AST 248, Fall 2005 15 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 11/7/2015 AST 248, Fall 2005 16 11/7/2015 AST 248, Fall 2005 17 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 11/7/2015 AST 248, Fall 2005 18 Student Evaluation • Please provide an anonymous student evaluation on the forms circulating now. • Please use a #2 pencil on the university form. Fill the bubbles properly. Do not put check marks or crosses. • Instructor: Yahil • Instructor #: 28011 11/7/2015 AST 248, Fall 2005 19