PTYS 214 Spring 2018 Announcements Midterm 3 next Thursday! Midterms 4 and 5 more spread out 1
Previously Geothermal Energy Radioactive Decay Accretional Energy Heat of Differentiation Why Water? Phase Diagrams 2
Major advantages of water 1. Ingredients (H and O) are abundant in the universe 2. A wide (and high) range of temperatures over which it remains liquid (major advantage) 3. Water ice floats, whereas the other substances sink when frozen 4. Water is a polar molecule (hydrogen bond!) Water can dissolve some substances (salts) but cannot dissolve membranes 3
Phase Diagrams Temperature and Pressure specify the phase of any substance 1 2 3 Conditions (1) solid phase Conditions (2) liquid phase Conditions (3) gas phase We can make a liquid boil by either: a) increasing temperature (at constant pressure) or b) decreasing pressure (at constant temperature) 4
Habitable Zone A circumstellar habitable zone (HZ) is defined as a region around any star where a planetary body can maintain liquid water on its surface Under the present Earth s atmospheric pressure (1 atm ~ 105 Pa) water is stable if the temperature is 273K < T < 373K On a planetary surface temperature (T) is key assuming the planet has some atmosphere! 5
How far from a star should an Earth-like planet be to maintain liquid water on its surface? Solar energy from hydrogen fusion Electromagnetic Radiation (waves) Temperature? 6
How do we determine a Planet s surface temperature? We must examine the Planet s Planetary Energy Budget: absorbed energy = emitted energy 7
Incoming Energy How much solar energy gets to the Earth? Total amount of energy per unit time (i.e., power) reaching the Earth is given by the amount of radiative flux hitting (e.g., W/m2) an area corresponding to the disk of the Earth: πre2 W/m2 m2 W Pin =S0 ( 2 πr E ) re Ein πre2 8
How much solar energy is absorbed by the Earth s surface? Some energy is reflected away 9
Albedo Fraction of incident sunlight that is reflected Range: 0 1 (no reflection) (100% reflection) Typical Surface Albedos: Sand 0.2-0.4 Forest 0.08-0.18 Green grass 0.25 Ocean 0.03-0.06 Fresh Snow 0.8-0.9 Average Earth s albedo: A = 0.30 10
Absorbed Power The amount of absorbed power is given by the amount of incident power minus the amount of reflected power: Pabs = Pin Prefl or Pabs = Pin APin = Pin(1-A) 11
How do we determine the Energy emitted by the Earth? 12
How do we determine the Energy emitted by the Earth? We can use Stefan-Boltzmann law to calculate the flux radiating from the Earth F=σT 4 F = flux of energy (W/m2) T = temperature (K) = 5.67 x 10-8 W/m2K4 (constant) 13
Total energy emitted by the Earth Start from Stefan-Boltzmann s law: F = T4 [W/m2] This is a flux, power per unit area, not total power We must multiply the flux by an area (area of the Earth s surface) Pemm = σt4 aearth 14
Energy Balance Over time, the power absorbed by the Earth should be equal to the power emitted (Pout) by the Earth Otherwise, the Earth s temperature would steadily rise (or fall) Pemm Pabs 15
Energy Balance 2 Pabs =(1 A )S 0 πr E Pemm =σt 4 ( 4 πr 2E ) Pemm Pabs re 16
Energy Balance: Pabs = Pemm 2 4 ( 1-A ) S 0 πr E =σt ( (1-A )S 0 =4σT 4 2 4πr E ) Pemm Pabs 17
Energy Balance: Pabs = Pout ( 1-A ) S 0 T = 4σ 4 Pout Pin 18
Earth Surface Temperature (1 A )S 0 4 T em = 4σ ==> Emission Temperature So = 1370 W/m2 A = 0.3 = 5.67 x 10-8 W/m2/K4 4 T em = 2 (1-0.3) 1370W/m -8 2 4 4 5.67 10 W/m / K 19
Earth s Average Surface Temperature 4 9 4 T em =4.23 10 K 4 9 T em = 4.23 10 K We expect an average surface temperature (emission temperature) of: T em =255K ~ -18 C or 0 F 20
Is the Earth s surface at 255K? 21
Earth Surface Temperature The average observed temperature at the Earth s surface is: Tobs = 288K (or +15oC, +59oF) Difference between observed and expected temperatures: T = Tobs Tem = 288K 255K T = + 33K = 33 C = 59.4 F What did we do wrong? 22
We must consider the interaction of atmospheric gases with the incoming and outgoing radiation Greenhouse Effect 23
How does a greenhouse work? Reduces heat loss primarily by inhibiting the upward air motion (convection) Solar energy is used more effectively: Same solar input higher temperatures 24
Atmospheric Greenhouse Effect Thermal radiation (infrared) from the surface is absorbed and re-emitted by some gases in the atmosphere Some of the re-emitted radiation radiates back to the planet 25
Composition of the Atmosphere Air is composed of a mixture of gases: Gas Concentration (%) N2 O2 Ar H2O CO2 greenhouse gases CH4 N2O O3 78 99.9% 21 Non-greenhouse gases 0.9 variable 0.037 (=370 ppm) 1.7 0.3 1.0 -- 0.01 (stratosphere -- surface) 26
Molecules with an uneven distribution of electrons are especially good absorbers and emitters These molecules are said to be dipoles (+) Water Electron-poor region: Partial positive charge H O (-) Electron-rich region: Partial negative charge H (+) oxygen is more electronegative than hydrogen 27
Greenhouse Gases O C O carbon dioxide O H water H H H C H -O O+ ozone O H methane 28
Non-greenhouse Gases N N nitrogen O O oxygen 29
Non-greenhouse Gases N N nitrogen O O oxygen Electron cloud is distributed equally over the atoms in the molecule (Technically speaking, greenhouse gases have (or can be made to have) a dipole moment whereas N2 and O2 don t/can't) 30
Greenhouse gases and radiation Molecules of greenhouse gases absorb energy from infrared radiation The energy increases the movement of the molecules, including vibration and rotation The molecules gain kinetic energy that may then be transmitted to other molecules such as oxygen and nitrogen and cause a general heating of the atmosphere 31
CO2 Vibration modes Which are IR active? 32
CO2 Vibration modes 15 μm 4.2 μm Which are IR active? 33
H2O Vibration Absorption wavelengths: 2.7 m 6.2 m 2.6 m 34
Solar Spectrum at Earth s Surface Greenhouse gases absorb IR radiation at specific wavelengths CO2 35
Homework See web page for suggested reading Homework #10 available shortly on the web site 36