PLATO - 5. Planetary atmospheres

Transcription

1 PLATO - 5 Planetary atmospheres 1

2 Mercury Smallest planet! 0.38 Earth radii! Earth masses! 0.39 AU orbit (eccentric)! 350K surface temperature (ranges from 100K-700K)! Slow 59 day rotation (2/3 orbital period)! No atmosphere 2

3 Surface Dominated by impact craters Evidence of geological activity:! Some craters are flooded with lava! Scarp cliff: Crustal fracture Large Caloris Basin :! Relatively smooth Plateau! Result of major impact Scarp cuts across craters 3

4 Interior Very high density! Mostly iron core Rotation irregularities:! Interior is partly molten Partly liquid Iron-Nickel core Rock (Silicates)! Suggests presence of some Sulfur (see Mars) 4

5 The Moon Nearest celestial object! 0.27 Earth radii Apollo 11! Earth masses (only 60% density of Mercury)! Phase locked rotation near! 220 K surface temperature (average; from 70K - 390K)! No atmosphere! Water ice in polar craters? far 5

6 Maria Lunar Highlands Lunar Maria light: calcium & aluminum oxide dark: basalt 6

7 Formation of Maria Late massive impacts! Flooded Lunar low-lands! Erased low-lying craters! Left dark basalt surface Few impact craters in Maria! Flood must have happened ~3.5 billion years ago.! Ideal site for lunar landings 7

8 Lunar Geology Rilles:! Cracking of cooling surface! Lava flows No sign of current geological activity! Moon has cooled! Similar to Mercury, moon is geologically dormant 8

9 Internal Structure Determined from seismographs left by Apollo missions! Asymmetric interior! Earth-facing side thinner! Easier to flood Maria only on Earthfacing side! Note the small Iron core Crust ~150 km thick Mantle (poor in iron) Crust ~65 km thick To Earth Maria (on Earth-facing side) 9

10 Lunar Formation Composition:! Surface rock very similar to Earth! Much smaller iron content Impact ejection theory! Earth hit by Mars-size object at ~ 4.5 billion years! Obliterated crust Moon formed from crustal debris in orbit around Earth 10

11 Comparative Planetology 11

12 Why do Earth and Venus show geological activity? Liquid Cores Terrestrial planets formed from colliding solid planetesimals! Lots of kinetic energy! Turned into thermal energy = lots of heat All Planets interiors were initially liquid 12

13 Heat Retention Planets are born hot But all things cool by radiation How long does it take a planet to cool? t cool = E thermal L = 3 4πR3 2kT n 3 4πR 2 σt 4 = nkr σt 3 R! Bigger planets take longer to cool! This explains why Earth and Venus are most active today 13

14 Radioactive Heating Recall:! Rock contains small amounts of radioactive material, such as 238 U Each radioactive decay releases:! Nuclear decay products " fast particles with lots of kinetic energy = thermal energy " photons = electromagnetic energy The energy goes into heat! Planets stay hot longer 14

15 Comparative Planetology Now we understand... 15

16 Atmospheres How can we understand the differences in the atmospheres of terrestrial planets? 16

17 Atmospheres Psurface ~ 90 atm. Psurface = 1 atm. Psurface = 0.01 atm. Why are these atmospheres so different? Where do they come from? 17

18 Earth s Atmosphere Mass:! 10-6 of total Earth mass Composition! 78.8% Nitrogen (N2)! 20.9% Oxygen (O2) Mesosphere! 0.9% Argon! 0.04% Carbon Dioxide! 0.002% Neon! % Helium 18

19 Hydrostatic Equilibrium Stratification:! Denser at lower altitudes! Higher pressure at lower altitudes Why is this?! Gravity pulls down! Pressure pushes in all directions! But: Lower pressure at higher altitudes, so downward force from above weaker than upward force from below! Net pressure force = gravity (equilibrium) 19

20 Evaporation Remember:! Thermal velocity is only an average! Some particles move faster, some slower! Fastest particles can escape!! All planets slowly leak particles into space! Rule of thumb:! You can retain an atmosphere over the age of the solar system if the escape velocity is more than ten times the thermal velocity 20

21 Evaporation The escape & thermal velocities of different planets:! Mercury! vesc=4.3 km/s!! vth=0.7 km/s! Venus!! vesc=10.3 km/s! vth=0.7 km/s! Earth!! vesc=11.2 km/s! vth=0.48 km/s! Mars!! vesc=5.0 km/s!! vth=0.35 km/s! Moon!! vesc=2.4 km/s!! vth=0.5 km/s Explains why Mercury and Moon have no atmosphere But: does it explain the presence of Earth s & Venus atmosphere? 21

22 Secondary Atmospheres Where did Earth s atmosphere came from? A) Volcanism B) Comets C) Plants 22

23 Secondary Atmospheres Terrestrial planets:! Not massive enough to accrete gas from the solar nebula! They only accreted solids Atmosphere on terrestrial planets must have formed from solids! This is called a secondary atmosphere! Material for secondary atmosphere comes from " Volcanism (CO2) " Comets (H2O) 23

24 Atmospheres Psurface ~ 90 atm. Psurface = 1 atm. Psurface = 0.01 atm. Earth: Mostly N2, some O2 Mars & Venus: Mostly CO2 and a bit of N2 24

25 Comparative Planetology Start from the assumption that Earth, Mars, and Venus started out roughly similar! How can we understand their different atmospheres today? Critical ingredient:! Water " CO2 cycle " Fosters life (photosynthesis) " Mild green house gas 25

26 Earth s CO2 cycle Why the difference in CO2 concentration?! Rain washes out CO2! Calcium Carbonate deposits in ocean and on land as rock! Photosynthesis turns residual CO2 to O2 and carbohydrates! Tectonic and volcanic activity returns some CO2 Ozone layer absorbs UV light 26

27 Venus vs. Earth Venus is geologically similar to Earth Why is Venus atmosphere so different from Earth s?! CO2 concentration: " Amount of Carbon locked away on Earth comparable to CO2 in Venus atmosphere! Earth once had as much CO2 as Venus Good job, plants and rain! Thanks!! Why did this not happen on Venus?! No liquid water on Venus - but why? 27

28 Venus vs. Earth Why no liquid water on Venus? Here s a theory: 1. Venus closer to Sun started out hotter 2. Temperature so hot that most water is evaporated 3. Water vapor rises above any Ozone layer 4. Sun s UV radiation breaks apart H2O molecules 5. Hydrogen H2 will escape into space 6. Residual O2 will combine with Carbon to make CO2 But why the high pressure and temperature today?! Before we figure that out, we need to talk about clouds... 28

29 Albedo Planet atmospheres transmit some light, reflect the rest.! Fraction of light reflected by an atmosphere is called its albedo Pincoming A = P reflected P incoming P reflected Ptransmitted 29

30 Albedo Planet atmospheres transmit some light, reflect the rest.! Fraction of light reflected by an atmosphere is called its albedo The higher the albedo of a planet! the less Sun light reaches the ground.! the brighter the planet appears in the sky (Venus is bright!) Material Albedo A Snow 0.8 Ice 0.6 soil 0.2 grass 0.25 ocean 0.1 Planet Albedo A Mercury 0.1 Venus 0.75 Earth 0.3 Mars 0.15 Moon

31 Thermal Equilibrium Before talking about the greenhouse effect, let s discuss thermal equilibrium! Equilibrium: Incoming power = outgoing power Psolar Solar power (1-A) x Psolar Incoming power A x Psolar Reflected power (1-A) x Psolar Outgoing power Clouds/surface Planet 31

32 Thermal Equilibrium Before talking about the greenhouse effect, let s discuss thermal equilibrium! Equilibrium: Incoming power = outgoing power Solve for the average surface temperature: T = 1/4 (1 A) L 16πσD 2 L1/4 (1 A)1/4 D1/2! Planets at larger distances to Sun are colder! Planets around dim stars are colder! Planets with high albedo are colder - but what about Venus? 32

33 Blackbody Radiation Every object with non-zero temperature emits light Blackbodies are special emitters:! They are in equilibrium with the radiation inside them! The radiation they emit is of a certain type - we call it thermal or blackbody radiation.! The hotter a blackbody, the more radiation it emits. 33

34 Blackbody Radiation The blackbody spectrum! Has a peak at λ max = nm T! This is called the Wien Law! So: Hotter things are bluer, colder things redder! max 15,000 K star Intensity (relative) the Sun (5,800 K) 3,000 K star human (310 K) wavelength (nm) 34

35 Blackbody Radiation The blackbody spectrum! Has a peak at λ max = nm T! This is called the Wien Law! So: Hotter things are bluer, colder things redder cool burner hot burner 35

36 Blackbody Radiation The higher the temperature, the more radiation A blackbody of temperature T, the flux F at the surface of the object (say, a star) is F BB = σ SB T 4! Stefan-Boltzmann constant: " SB =5.67x10-8 Wm -2 K -4 If the surface area of the object is A, the power is L BB = A σ SB T 4 36

37 The Greenhouse Effect 37

38 Intensity The Greenhouse Effect How does a greenhouse work?! Glass is a filter: Transmits visible light but blocks infrared light " Radiation from the Sun is in the visible " Radiation from the ground up is in the infrared! Heat is trapped! Energy gets in, but can t get out. incoming Sun light high enough Temp. glass ground higher Temp. Blackbody spectrum with greenhouse greenhouse gas filter low Temperature Wavelength Blackbody spectrum without greenhouse 38

39 The Greenhouse Effect How does a greenhouse work?! Glass is a filter: Transmits visible light but blocks infrared light " Radiation from the Sun is in the visible " Radiation from the ground up is in the infrared! Heat is trapped! Energy gets in, but can t get out.! Temperature rises until as much energy escapes as is transmitted from the Sun (equilibrium: in = out) " Higher temperature means more flux (Wien law) " Higher temperature means shorter wavelength (Stephan- Boltzman law), less filtering 39

40 The Greenhouse Effect Some gases act like glass:! They are transparent to visible light but block infrared light! Examples: Visible radiation " CO2 (comletely transparent in visible) " Water vapor " Methane " Ozone! The concentration of any of these gases in Earth s atmosphere is small, so Earth is not a good greenhouse. 40

41 Venus Venus is perfect illustration of the greenhouse effect! Very large CO2 concentration due to lack of liquid water! Temperature now so high that liquid water evaporates " Exacerbates greenhouse effect " H2O destroyed by Sun s UV radiation " Thus, no water, no O2 41

42 Runaway Greenhouse Effect If CO2 content of Earth s atmosphere rises drastically 1. Temperature goes up because of greenhouse effect 2. Ocean water evaporates more rapidly 3. Water is a greenhouse gas, so temperature rises more 4. Once water vapor reaches upper atmosphere: " UV light dissociates (destroys) water into H2 and O2 " H2 escapes into space and is lost " No H2O, so CO2 can no longer be washed out of atmosphere! This runaway greenhouse effect is irreversible 42

43 Comparative Planetology Comparison: Planets Semi major axis (AU) Albedo Surface temp. without greenhouse Actual surface temp. Greenhouse warming Venus K 750K 523K Earth K 287K 33K Mars K 221K 5K Why does Mars have such a weak greenhouse effect?! Mars had liquid water in the past Must have had moderate green house effect, like Earth! But: Tectonic activity is over, so no more CO2 release 43

44 Planets: Greenhouse Effect Comparison: Solar wind Why does Mars have such a weak greenhouse effect?! Mars had liquid water in the past Must have had moderate green house effect, like Earth! But: Tectonic activity is over, so no more CO2 release " Ice-house: Freeze-out of Carbon, Mars lost atmosphere 44

45 Planets: Greenhouse Effect Comparison: Solar wind So Mars might have been more habitable if it were big enough:! Bigger planets stay hotter! Hotter planets maintain tectonics/geological activity! That replenishes CO2 in the atmosphere 45

46 The Jovian Planets Cassini image of Jupiter, Io, and Io s shadow (NASA/JPL) 46