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행성 체에 생명이 서식할 조건 : II S.S.Hong SNU, 05-09-18 Revised on 06-09-28 REFERENCES : Gilmour, I. 2004, in An Introduction to ASTROBIOLOGY, Chap 2 Habitable World, pp. 43-84. Ulmschneider, P. 2003, in INTELLIGENT LIFE IN THE UNIVERSE, Chap 4 Planets Suitable for Life, pp. 51-78. Kasting, J.F., Whitmire, D.P., and Reynolds, R.T. 1993, Habitable Zones around Main Sequence Stars, Icarus 101, 108. S.S.Hong ; SNU 05-09-18 PLANETS SUITABLE FOR LIFE 1. HABITABLE ZONES Solar Habitable Zone ; Habitable Zones around Other Stars 2. PLANETARY MASS, RADIUS AND TEMPERATURE Evaporation of the Atmosphere 3. MAIN SEQUENCE LIFE TIME 4. TIDAL EFFECTS ON PLANETS 5. CONTINUOUSLY HABITABLE ZONES 6. INSTABILITIES OF THE PLANETARY ATMOSPHERE Greenhouse; Carbon Silicate Cycle ; Irreversible Glaciation 7. AXIS VARIATIONS OF THE PLANETS 8. BIOGENIC EFFECTS ON PLANETARY ATMOSPHERE 9. THE NUMBER OF HABITABLE PLANETS The Drake Formula ; NHP 10. DISCUSSION S.S.Hong ; SNU 05-09-18 6. INSTABILITIES OF PLANETARY ATMOSPHERE 1) GREENHOUSE EFFECT COMETS & VOLCANIC OUTGASSING OF UPPER MANTLE N2, CO2, CH4, H2O THE GREENHOUSE GAS REDUCES COOLING RATE OF THE ATMOSPHERE BY TRAPPING IR RADIATION, WHICH IN TURN RAISES THE TEMPERATURE INSIDE. The 100 m should be changed to 10 m . S.S.Hong ; SNU 05-09-18 2) THE CARBON-SILCATE CYCLE CO2 gas dissolves into H2O water to form carbonic acid H2CO3 . H2CO3 weathers silicate rocks CaSiO3 and form calcium carbonate CaCO3 (chalk), which in turn precipitates onto ocean floor as a solid. IN THIS WAY CO2 CAN BE REMOVED FROM THE ATMOSPHERE IN 4 X 108 YRS . CO2 GETS RE-SUPPLIED TO THE ATMOSPHERE BY VOLCANIC ACTIVITY. AN IMPORTANCE OF THE PLATE TECTONICS ! + BIOLOGICAL CYCLE Photo-synthesis removes CO2 gas from atmosphere; while respiration and burning return the gas back to the atmosphere. A large amount of CO2 gas is locked up within clams, oysters, and shells too. S.S.Hong ; SNU 05-09-18 The CARBON-SILCATE cycle = Thermostat Together with the BIOLOGICAL cycle the CARBON-SILICATE one regulates the amount of atmospheric CO2 and thus determines magnitude of greenhouse effect. The cycle depends on TEMPERATURE, since WEATHERING becomes inefficient at low temperature. A long SPELL of COLD increases ice coverage on Earth. Lack of liquid water on Earth tends to stop weathering, hence it would increase atmospheric CO2 , which in turn raises up the temperature. THE CARBON-SILICATE CYCLE ACTS LIKE A THERMOSTAT ! 3) THE MOIST GREENHOUSE EFFECT The CARBON-SILICATE cycle rests on the AVAILABILITY of LIQUID water. OUTGASSING under MOIST EVIRONMENTS : Injection of CO2 gas into atmosphere warming of the atmosphere via greenhouse acceleration of H2O evaporation cut off the carbonate formation process RUNAWAY HEATING OF ATMOSPHERE ! S.S.Hong ; SNU 05-09-18 Venus had water early in the solar system history : MOIST GREENHOUSE. Subsequent UV PHOTOLYSIS splits H2O into O and H2 ; the lightest H2 gas quickly evaporates from the atmosphere. Moist Greenhouse starts at 340K (67OC), which corresponds to 0.95 AU for the present solar luminosity. REVISION OF THE CONTINUOUSLY HABITABLE ZONE 0.95 ~ 1.13 AU Mankind has only another billion years before the Earth becomes uninhabitable. 4) IRREVERSIBLE GLACIATION albedo ocean 0.04 rocks 0.15 cloud 0.52 ice 0.70 White Catastrophe ! S.S.Hong ; SNU 05-09-18 Irreversible Glaciation imposes another constraint upon the outer boundary of the Habitable Zone. Actual point of the irreversible glaciation depends on the roles of many warming agents: warming by carbon-silicate cycle reaches a limit when CO2 cloud forms, since it has high albedo. This places position of the Irreversible Glaciation at 1.67AU today. But, the plate tectonics has an ultimate power. PLATE TECTONICS INTERNAL HEAT CONTENT PLANET MASS ‘Giant Impact’ SSH ADDS GIANT IMPACT TO THE CHAIN, BECAUSE VENUS LACKS ACTIVITIES OF THE PLATE TECTONICS. Mars is a victim of the White Catastrophe. • small mass, low heat content, quick cooling, early death of volcanism, no further supply of CO2 , set-in of the glaciation. • Where is then the ice gone? Small mass, evaporation, UV photolysis, H2 lost to space, O in oxidizing undifferentiated Fe to Fe2O3, the remaining ice buried in the permafrost layer. S.S.Hong ; SNU 05-09-18 7. AXIS VARIATIONS OF THE PLANETS SPIN – ORBIT RESONANCE rotational bulge of Earth Sun spin - precession Earth orbit planets + Moon orbit - precession Spin-Orbit Resonance changes inclination angle up to 60o for Mars; 85o for Earth without Moon. Nearby Moon actually stabilizes the spin-orbit resonance. Consequences of the Equatorial glaciation and very warm pole may not be fatal for the development of life. But frequent change in climate should be fatal. S.S.Hong ; SNU 05-09-18 S.S.Hong ; SNU 05-09-18 8. BIOGENIC EVOLUTION OF ATMOSPHERES LIFE AFFECTS LATER HABITABLITY OF A PLANET. Photosynthetic-bacteria generated O2 is used up in the first 2 billion years in oxidizing the iron in the sea water, and later enriched the atmosphere and then destroyed CH4 , which is a greenhouse gas. L(t) for Sun, volcanism, greenhouse, carbon-silicate cycle, gas reactions, H2O photolysis, and subsequent escape of H2 Quenching of greenhouse action at -2 x109 yrs Huronia Glaciation at about the same time, 2.3 billion yrs ago. Continuing increase in T L(t) Hart, H.M. 1978, Icarus 33, 23 The Evolution of the Atmosphere of the Earth S.S.Hong ; SNU 05-09-18 Banded Iron Formations BIFS in Western Australia Mt. Tom Price iron ore mine, Hamersley Ranges 300 km in diameter finely banded dark brown, ironrich layers alternating with lighter iron-poor layers the layers range in thickness less than a mm to about a cm Iron-rich bands contain the highly insoluable iron oxides: hematite Fe2O3, limonite Fe2O3· 3H2O, magnetite Fe3O4. Chert, a precipitated silica, occupies the iron-poor bands. appearance of free oxygen oxidation state of the Earth’s ocean and atmosphere in the Archaean Era S.S.Hong ; SNU 05-09-18 Iron was actually consumed by bacteria flourished near thermal vents. Bacteria drifted away in vast colonies into shallow water where they died, depositing organic-rich material. After a while the organic material would have been recycled, leaving iron behind in highly in-soluable oxide form. IN SUMMARY Habitable Zone Glaciation Major Cause and Epoch______ 0.71 ~ 1.42 AU 1.67 AU present solar luminosity 0.56 ~ 1.13 AU 1.35 AU onset of H-ignition, CO2 build-up 0.95 ~ 1.13 AU Moist Greenhouse 0.95 ~ 1.01 AU Snowball Earth / precession Quenching of GH Gas The CHZ extends over 0.06 AU from 0.95 to 1.01 AU. The expectancy period of the Earth life is less than one billion years ! How many planets can we expect to have in such a narrow CHZ around the right kind of central star ? S.S.Hong ; SNU 05-09-18 9. NUMBER OF HABITABLE PLANETS 1) DRAKE’s FORMULA N = NHP fL fI fC L/LS, where NHP = NS fP nE N = number of communicating extraterrestrial civilizations NHP = number of habitable planets in the Galaxy NS -- number of stars in the Galaxy that are suitable for life fP -- fraction of such stars that have planets nE -- number of Earth-like planets orbiting in HZ for each star fL fI fC = fraction of planets on which life develops, evolves into intelligent being, and engages in radio communication L = life time of communicating societies LS = time-span over which Earth-like planets and their parent stars have formed in the Galaxy L/LS = probability of detecting a communicating society S.S.Hong ; SNU 05-09-18 2) BEHIND THE DRAKE’s FORMULA Project OZMA by Frank Drake (1960) 1950s G. Cocconi, P. Morrison, I.S. Shklovskii possibility of interstellar radio communication 1960 Green Bank 25m Telescope 6 hours/ day monitoring of 2 G stars, Ceti & Eri 1961 Green Bank Conference organized by F. Drake Parameterize one’s own ignorance into factors ! N = NHP fL fI fC L / LS NHP = NS fP nE astronomy fL fI fC L biochemistry, evolution, engineering, civilization LS astronomy S.S.Hong ; SNU 05-09-18 3) ESTIMATES OF THE NUMBER, NHP Total number of stars in the Galaxy 6.4 x 1010 M_sun / 0.4 M_sun 1.6 x1011 stars G type 0.1 ; Metal Rich Pop I 0.6 ; Exclusion of Multi Sys 0.3 1.6 x1011 stars x 0.1 x 0.6 x 0.3 2.9 x 109 stars An Adequate Supply of Water and Carbon limit to the inner part of cluster 0.5 ; hence, NS 1.4 x 109 stars Accretion Disks Around the Ns Stars Solar Nebula ; fP 1.0 nE = [ 0.06 AU / 5.2 AU ] x 4 terrestrial planets x 0.06 = 0.003 migration-safe factor NHP = 4 x 106 stars in the Galaxy, i.e. one in every 40,000 stars ! Trilling, D.E. et al. 1998, ApJ 500, 428 : Orbital Evolution and Migration of Giant Planets: Modeling Exrasolar Planets Del Popolo, A., Gambera, M., Ercan, N. 2001, MNRAS 325, 1402 : Migration of Giant Planets in Planetesimal Discs S.S.Hong ; SNU 05-09-18 4) COMPARISON OF THE NHP ESTIMATES A wide range of uncertainty suggests a tremendous difficulty involved in making an estimation of NHP. The number 4 x 106 still seems to be an optimistic estimate, if giant Moon is necessary to make Earth suitable for life. The same number could well be a pessimistic estimate, if migration of Jovian planets is not so common a phenomenon in reality. Recent progress in theory of planet formation and discovery of exoplanets has refined fuzzy parameters sharper and sharper. S.S.Hong ; SNU 05-09-18 10. DISCUSSION Better understanding is required on the long range evolution of planet atmosphere, gas dynamics and thermodynamics of accretion disk, and migration of proto-Jovian planets. Being biased with terrestrial life, we are in a danger of blindly exercising human chauvinism. Even so, Earth-like planets turn out to be very rare, one in every 40,000. As human beings, we are an extremely lucky product of the cosmic evolution over the last 14 billion years. The TPF Project of NASA is expected to pin-down some of the major uncertainties involved in the NHP estimation. Mother Earth is a Beautiful Place! In recent years we are witnessing enormous progress in IT, whose ‘muscle’ may be combined with that of brain science. If transhumans are populous in exo-planetary systems, Drake’s formula may lose its predicting power. S.S.Hong ; SNU 05-09-18 SNU, 05-06-01 PLANETS SUITABLE FOR LIFE SNU, 05-06-01 THE CONCLUSION Mother Earth is a Beautiful Place ! Underneath the beauty resides turmoil of the plate tectonics, which turns out to be the motor driving the evolution of life. As long as competing factors of the turmoil find a balance, our mother Earth reaches another vantage point towards the OMEGA. 그래서 나는 천국의 고요보다 지옥의 소란을 택하겠습니다. 불행한 천국인 미국보다, 행복한 지옥인 한국을, 그리곤 가서 얘기하겠습니다: 아름다운 이 세상 소풍 끝내는 날, / 가서, 아름다웠더라고 말하리라 …