Aula 10 Sigam a ГЃgua + Atmosfera Habitable Zone вЂў A circumstellar habitable zone (HZ) is defined as encompassing the range of distances from a star for which liquid water can exist on a planetary surface. вЂў Under the present EarthвЂ™s atmospheric pressure (1 atm = 101325 Pa) water is stable if temperature is 273K < T < 373K (0-100В°C) вЂў Planetary surface temperature (T) is the key PressГЈo AtmosfГ©rica вЂў No nГvel do mar: 1 atmosfera = 101 325 Pa = 101.325 kPa = 1.01325 bar вЂў A pressГЈo Г© devida ao impacto das molГ©culas na superfГcie Phase Diagrams Condensation Vapor Liquid Evaporation At some point Condensation = Evaporation вЂ“ liquid and vapor phases are in Equilibrium вЂ“ saturation curve T вЂ“ triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium C вЂ“ critical point вЂ“ liquid phase cease to exist 1. Conjunto de condiГ§Гµes (1) вЂ“ fase sГіlida 2. Conjunto de condiГ§Гµes (2) вЂ“ fase lГquida 3. Conjunto de condiГ§Гµes (3) вЂ“ fase gasosa Pode-se fazer um lГquido ferver ou aumentando sua temperatura ou diminuindo sao pressГЈo Example: Earth-Sun The EarthвЂ™s temperature (about 300K) is maintained by the energy radiating from the Sun. 6,000 K 300 K Planetary Energy Balance вЂў We can estimate average planetary temperature using the Energy Balance approach Ein = Eout Ein How much solar energy gets to the Earth? Assuming solar radiation covers the area of a circle defined by the radius of the Earth (re) Ein = So (W/m2) x 4пЃ° re2 (m2) / 4 Ein = So x пЃ° re2 (W) Ein re Ein How much solar energy gets to the EarthвЂ™s surface? **Some energy is reflected away** пѓћ Essa fraГ§ГЈo = Albedo (A) Ein = So x пЃ° re2 x (1-A) Sample albedos on Earth Surface Typical Albedo Fresh asphalt 0.04 Conifer forest (Summer) 0.08, 0.09 to 0.15 Worn asphalt 0.12 Trees 0.15 to 0.18 Bare soil 0.17 Green grass 0.25 Desert sand 0.40 New concrete 0.55 Fresh snow 0.80вЂ“0.90 Ocean Ice 0.5вЂ“0.7 Albedos of planets Mercury - 0.11 Venus - 0.65 Earth - 0.37 Mars - 0.15 Jupiter - 0.52 Saturn - 0.47 Uranus - 0.51 Neptune - 0.41 Pluto - 0.3 Eout Energy Balance: The amount of energy delivered to the Earth is equal to the energy lost from the Earth. Otherwise, the EarthвЂ™s temperature would continually rise (or fall). Eout пѓ† Stefan-Boltzmann law F = пЃі T4 F = flux of energy (W/m2) T = temperature (K) пЃі = 5.67 x 10-8 W/m2K4 (a constant) Energy Balance: Ein = Eout Ein = So пЃ° re2 (1-A) Eout = пЃі T4(4 пЃ° re2) Eout Ein Energy Balance: Ein = Eout So (1-A) = пЃі T4 (x4) T4 = [So (1-A)] / 4пЃі Eout Ein EarthвЂ™s average temperature T4 = So(1-A) 4пЃі For Earth: So = 1370 W/m2 A = 0.3 пЃі = 5.67 x 10-8 W/m2K4 (oC) = (K) - 273 (oC x 1.8) + 32 = oF EarthвЂ™s average temperature T4 = So(1-A) 4пЃі For Earth: So = 1370 W/m2 A = 0.3 пЃі = 5.67 x 10-8 T4 = (1370 W/m2)(1-0.3) 4 (5.67 x 10-8 W/m2K4) T4 = 4.23 x 109 (K4) T = 255 K Earth expected Temperature: Texp = 255 K (oC) = (K) - 273 SoвЂ¦. Texp = (255 - 273) = -18 oC (which is about 0 oF) Is the EarthвЂ™s surface really -18 oC? NO. The actual temperature is warmer! The observed temperature (Tobs) is 15 oC, or about 59 oF. The difference between observed and expected temperatures (пЃ„T): пЃ„T = Tobs вЂ“ Texp пѓњ пЃ„T = 15 - (-18) пЃ„T = + 33 oC = 33 K We call this warming the greenhouse effect, and is due to absorption of energy by gases in the atmosphere. Atmospheric Greenhouse Effect Incoming Solar radiation Outgoing IR radiation Greenhouse gases (CO2) N2, O2 EarthвЂ™s Surface Original Greenhouse вЂў Precludes heat loss by inhibiting the upward air motion вЂў Solar energy is used more effectively. Same solar input вЂ“ higher temperatures. Warming results from interactions of gases in the atmosphere with incoming and outgoing radiation. To evaluate how this happens, we will focus on the composition of the EarthвЂ™s atmosphere. Composition of the Atmosphere Air is composed of a mixture of gases: Gas N2 O2 Ar H2O CO2 greenhouse gasesCH4 N2O O3 concentration (%) 78 21 0.9 variable 0.037 370 ppm 1.7 0.3 1.0 to 0.01 (stratosphere-surface) O C O c a r b o n d io x id e Greenhouse Gases O H H w a te r H H C O H - H m e th a n e + O O ozone Non-greenhouse Gases N2 O2 N п‚є N O = O Molecules absorb energy from radiation. The energy increases the movement of the molecules. The molecules rotate and vibrate. stretching bending Vibration Non-greenhouse Gases N п‚є N O = O Non-greenhouse gases have symmetry! (Technically speaking, greenhouse gases have a dipole moment whereas N2 and O2 donвЂ™t) (в€’) O H H (+) вЂў Oxygen has an unfilled outer shell of electrons (6 out of 8), so it wants to attract additional electrons. It gets them from the hydrogen atoms. Molecules with an uneven distribution of electrons are especially good absorbers and emitters. These molecules are called dipoles. Water Electron-poor region: Partial positive charge H O H Electron-rich region: Partial negative charge oxygen is more electronegative than hydrogen Absorption wavelength is a characteristic of each molecule Thermal IR Spectrum for Earth H2O pure rotation H2O vibration/rotation CO2 (15 пЃm) (6.3 пЃm) O3 (9.6 пЃm) Ref.: K.-N. Liou, Radiation and Cloud Physics Processes in the Atmosphere (1992) Non-Greenhouse Gases вЂў The molecules/atoms that constitute the bulk of the atmosphere: O2, N2 and Ar; do not interact with infrared radiation significantly. вЂў While the oxygen and nitrogen molecules can vibrate, because of their symmetry these vibrations do not create any transient charge separation. вЂў Without such a transient dipole moment, they can neither absorb nor emit infrared radiation. Atmospheric Greenhouse Effect (AGE) вЂў AGE increases surface temperature by returning a part of the outgoing radiation back to the surface вЂў The magnitude of the greenhouse effect is dependent on the abundance of greenhouse gases (CO2, H2O etc.) Clouds вЂў Just as greenhouse gases, clouds also affect the planetary surface temperature (albedo) вЂў Clouds are droplets of liquid water or ice crystals вЂў Cumulus clouds вЂ“ puffy, white clouds вЂў Stratus clouds вЂ“ grey, low-level clouds вЂў Cirrus clouds вЂ“ high, wispy clouds Cumulus cloud Cirrus cloud Climatic Effects of Clouds вЂў Clouds reflect sunlight (cooling) вЂў Clouds absorb and re-emit outgoing IR radiation (warming) вЂў Low thick clouds (stratus clouds) tend to cool the surface вЂў High, thin clouds (cirrus clouds) tend to warm the surface Back to the HZ вЂў LetвЂ™s assume that a planet has EarthвЂ™s atmospheric greenhouse warming (33 K) and EarthвЂ™s cloud coverage (net planetary albedo ~ 0.3) вЂў Where would be the boundaries of the HZ for such planet? вЂў Recall that the Solar flux: S = L/(4пЃ°R2) вЂў We can substitute formula for the Solar flux by planetary energy balance equation: вЂў S Г—(1-A) = пЃіГ—T4 Г—4 L/(4пЃ°R2)Г— (1-A) = пЃіГ—T4 Г—4 L п‚ґ (1 пЂ A) пЂЅR 4 16 п‚ґ пЃ° п‚ґ пЃі п‚ґ T (R = distance from star) Global surface temperature (Ts) вЂў Global surface temperature (Ts) depends on three main factors: a) Solar flux b) Albedo (on Earth mostly clouds) c) Greenhouse Effect (CO2, H2O , CH4, O3 etc.) вЂў We can calculate Te from the вЂњEnergy balance equationвЂќ and add the greenhouse warming: Ts = Te + в€†Tg вЂў But! The amount of the atmospheric greenhouse warming (в€†Tg) and the planetary albedo can change as a function of surface temperature (Ts) through different feedbacks in the climate system. Climate System and Feedbacks вЂў We can think about climate system as a number of components (atmosphere, ocean, land, ice cover, vegetation etc.) which constantly interact with each other. вЂў There are two ways components can interact вЂ“ positive and negative couplings Systems Notation = system component = positive coupling = negative coupling Positive Coupling CarвЂ™s gas pedal CarвЂ™s speed Amount of food eaten Body weight вЂў A change in one component leads to a change of the same direction in the linked component Negative Coupling CarвЂ™s break system CarвЂ™s speed Exercise Body weight вЂў A change in one component leads to a change of the opposite direction in the linked component Positive Coupling Atmospheric CO2 Greenhouse effect вЂў An increase in atmospheric CO2 causes a corresponding increase in the greenhouse effect, and thus in EarthвЂ™s surface temperature вЂў Conversely, a decrease in atmospheric CO2 causes a decrease in the greenhouse effect Negative Coupling EarthвЂ™s albedo (reflectivity) EarthвЂ™s surface temperature вЂў An increase in EarthвЂ™s albedo causes a corresponding decrease in the EarthвЂ™s surface temperature by reflecting more sunlight back to space вЂў Or, a decrease in albedo causes an increase in surface temperature Feedbacks вЂў In nature component A affects component B but component B also affects component A. Such a вЂњtwo-wayвЂќ interaction is called a feedback loop. A вЂў Loops can be stable or unstable. B Climate Feedbacks Water Vapor Feedback Snow and Ice Albedo Feedback The IR Flux/Temperature Feedback Short-term climate stabilization The Carbonate-Silicate Cycle (metamorphism) Long-term climate stabilization вЂў CaSiO3 + CO2 пѓ CaCO3 + SiO2 (weathering) вЂў CaCO3 + SiO2 пѓ CaSiO3 + CO2 (metamorphosis) Negative Feedback Loops The carbonate-silicate cycle feedback Rainfall Silicate weathering rate Surface temperature (в€’) Greenhouse effect Atmospheric CO2 The inner edge of the HZ вЂў The limiting factor for the inner boundary of the HZ must be the ability of the planet to avoid a runaway greenhouse effect. вЂў Theoretical models predict that an Earthlike planet would convert all its ocean into the water vapor at ~0.84 AU вЂў However it is likely that a planet will lose water before that. Moist Greenhouse вЂў If a planet is at 0.95 AU it gets about 10% higher solar flux than the Earth. вЂў Increase in Solar flux leads to increase in surface temperature пѓ more water vapor in the atmosphere пѓ even higher temperatures вЂў Eventually all atmosphere becomes rich in water vapor пѓ effective hydrogen escape to space пѓ permanent loss of water hпЃ® Ineffective H escape Space Effective H escape hпЃ® H2O + hпЃ® пѓ H + OH H2O + hпЃ® пѓ H + OH Upper Atmosphere (Stratosphere, Mesosphere) H2O-poor H2O-rich H2O-rich Lower Atmosphere (Troposphere) H2O-ultrarich Venus fate вЂў Runaway (or moist) greenhouse and the permanent loss of water could have happened on Venus вЂў Venus has very high D/H (~120 times higher than EarthвЂ™s) ratio suggesting huge hydrogen loss вЂў Without water CO2 would accumulate in the atmosphere and the climate would become extremely hot вЂ“ present Venus is ~ 90 times more massive than EarthвЂ™s and almost entirely CO2. вЂў Eventually Earth will follow the fate of Venus The outer edge of the HZ вЂў The outer edge of the HZ is the distance from the Sun at which even a strong greenhouse effect would not allow liquid water on the planetary surface. вЂў Carbonate-silicate cycle can help to extend the outer edge of the HZ by accumulating more CO2 and partially offsetting low solar luminosity. Limit from CO2 greenhouse вЂў At low Solar luminosities high CO2 abundance would be required to keep the planet warm. вЂў But at high CO2 abundance Atm does not produce as much net warming because it also scatter solar radiation. вЂў Theoretical models predict that no matter how high CO2 abundance would be in the atmosphere, the temperature would not exceed the freezing point of water if a planet is further than 1.7 A.U. Limit from CO2 condensation вЂў At high CO2 abundance and low temperatures carbon dioxide can start to condense out (like water condense into rain and snow) вЂў Atmosphere would not be able to build CO2 if a planet is further than 1.4 A.U. Fate of Mars вЂў Mars is on the margin of the HZ at the present вЂў But! Mars is a small planet and cooled relatively fast вЂў Mars cannot outgas CO2 and sustain Carbonate-Silicate feedback. вЂў Also hydrogen can escape effectively due to the low Martian gravity and lack of magnetic field. River channel Nanedi Vallis (from Mars Global Surveyor) ~3 km Why the Sun gets brighter with time вЂў вЂў вЂў вЂў вЂў H fuses to form He in the core Core becomes denser Core contracts and heats up Fusion reactions proceed faster More energy is produced пѓћ more energy needs to be emitted Solar Luminosity versus Time See The Earth System, ed. 2, Fig. 1-12 Continuous Habitable Zone (CHZ) вЂў A region, in which a planet may reside and maintain liquid water throughout most of a starвЂ™s life.