The Habitable Zone

1. The Habitable Zone The habitable zone in a solar system is the range of locations around the parent star where life based on liquid water could exist!

2. HZ dependent upon: • mass and age of parent star • planet’s mass, atmosphere and distance from its parent star

3. What is a Habitable Planet? Not too big A habitable planet is: • Not too small • Not too hot or toocold • … and it has beer!

4. Why is the Earth Habitable? • Solid surface - useful for concentrating chemicals & reactions • Not located in a “bad” neighborhood - low supernova rate, no nearby gamma ray bursters or “death rays” • Sun had plenty of heavy elements to make terrestrial planets from (not Pop II star) • Relatively low major impact rate - major “killers” only every 100 Myr - planet-sterilizers less frequent • Large Moon stabilizes rotation axis to prevent some huge changes in climate • Location, location, location!

5. 1970’s — Michael Hart Re-posed Fermi’s Question: “Where IS Everyone?” • Are the conditions for life common or rare? • How does one decide the issue? • What is necessary for a terrestrial planet to have suitable conditions for the emergence of life as we understand it? • Hart decided to model why it is that the Earth is currently capable of sustaining life!

6. Earth — the Goldilocks Planet How much closer or farther from the Sun could the Earth be and still be habitable?

7. THINGS THAT AFFECT TEMPERATURE • Need temperature just right for liquid water on planet’s surface Temperature of star Distance from the star Shape of planet’s orbit: circular or elliptical Planet’s atmosphere: greenhouse gases • These define Habitable Zone (HZ)

8. Question Perhaps the most critical factor that affects the temperature of any planet is ________. where the planet is located in the Milky Way whether or not it has a store of fossil fuel whether or not it rotates clock-wise or counter clock-wise how far the planet is from its parent Sun whether or not its inhabitants drive oversized SUV’s “The Man in the Green Hat” Gin

9. 1000 100 10 1 0.1 0.01 Flux from Sun: FS =  TS4 Sun Earth Flux from Earth: FE =  TE4  (m) Hotter objects emit more energy per square meter F than colder objects How much energy do objects of temperature T emit? F =  T4

13. F =  T4

14. The intensity of light diminishes like the inverse square of the distance from source Light passing thru 1 square Now passes thru 4 squares Then thru 9 squares • Why? Same amount of light within cone, but spreads out over an area that increases as the square of the distance

15. Light Intensity on Terrestrial Planets • Mercury’s average distance from Sun is 0.39 AU Average intensity 1/(0.39)2 = 6.6 x Earth’s • Venus’ average distance from Sun is 0.72 AU Average intensity 1/(0.72)2= 1.93 x Earth’s • Mars’ average distance from Sun is 1.52 AU Average intensity 1/(1.52)2 = 0.43 x Earth’s • But how does this translate into temperature?

16. Calculate the Surface Temperature of the Earth 6,000 K 300 K

17. Some Basic Information: Area of a circle =  r2 Surface area of a sphere = 4  r2 r

18. Energy Balance: The amount of energy delivered to the Earth is equal to the energy lost from the Earth. Otherwise, the Earth’s temperature would continually rise (or fall).

19. Energy Balance: Incoming energy = Outgoing energy Ein = Eout Eout Ein Earth

20. How Much Solar Energy Reaches Earth?

21. How Much Solar Energy Reaches Earth? As energy moves away from the sun, it is spread over a greater and greater area.

22. How Much Solar Energy Reaches Earth? As energy moves away from the sun, it is spread over a greater and greater area. Intensity decreases as Inverse Square

23. L = Luminosity of Sun = 3.9 x 1026 Watts So = Intensity of Light striking Earth So = L / area of sphere = L/(4  d2) So is the solar constant for Earth=1370 W/m2 d So is determined by the distance between Earth and the Sun (d) and the Sun’s luminosity.

24. Each planet has its own solar constant… … energy / m2 striking the planet

25. How Much Solar Energy Reaches Earth? Solar radiation would pass through the area of a circle defined by the radius of the Earth (re) … A =  re2 … if the Earth weren’t there! Ein re …but the Earth is there…and blocks its passage!

26. How Much Solar Energy Reaches Earth? Solar radiation would pass through the area of a circle defined by the radius of the Earth (re) … but the Earth is there … and blocks its passage! Ein = (L/4 d2) • ( re2 ) Ein re

27. How Much Solar Energy Reaches Earth? BUT THIS IS NOT QUITE CORRECT! **Some energy is reflected away** Ein = (L/4 d2) • ( re2 ) Ein re

28. How Much Solar Energy Reaches Earth? Albedo (a) = % reflected energy a = 0.39 today Ein = (L/4 d2) • ( re2 ) • (1 – a) re Ein

29. Energy Balance: Ein = Eout Incoming energy = Outgoing energy Ein = (L/4 d2) • ( re2 ) • (1 – a) How much energy does the Earth emit … what is Eout? Eout Ein Earth

30. Emitted Energy Depends on Blackbody Radiation Flux F and Earth’s Surface Area Eout = F • (surface area of Earth) F =  T4 Area = 4  re2 Eout = ( T4) • (4  re2) 300 K

31. Eout Energy Balance: Ein= Eout Ein = (L/4 d2) • ( re2 ) • (1 – A) = ( T4) • (4  re2) = Eout T4 = (L/4d2) (1 – A) / (16  ) T = 1/ (16  )1/4 • (L(1 – A)1/4 d -1/2 Ein

32. Eout Final Result for Earth’s Temperature: a = 0.39 Tp = 250 K oh no! Ein

33. Planet “Equilibrium” Temperatures Planet d(AU) a Predicted T Observed T –––––––––––––––––––––––––––––––––––––––––––––––––––- Mercury 0.39 0.056 440 100-620 Venus 0.72 0.76 230 750 uniform Earth 1.00 0.39 250 288 global mean Mars 1.52 0.16 220 213 global mean Jupiter 5.2 0.51 104 160 (cloud tops) Saturn 9.5 0.61 81 90(cloud tops) Question: Why are most planets hotter than this? Jupiter & Saturn - internal heat source emit more than they absorb! Venus & Earth - ????? Greenhouse Effect!

34. Question A planet’s temperature reaches equilibrium when _______. people quit burning fossil fuels the solar energy absorbed by the planet is balanced by the total energy that it emits back into space all life on Earth goes extinct the Sun has burned all of the hydrogen in its core another snowball Earth episode occurs

36. Planetary Temperature — Recalculated With greenhouse effect - need additional term:   = 1 means no greenhouse effect. Otherwise  < 1. For Earth .… ε = 0.53 Tp = 288 K For Venus … ε = 0.01 Tp = 725 K

37. Temperature in a Solar System Stars get hotter as they age, so yellow zone moves out. Planet rotation smooths out temperature. Tidal lock = same face to sun (sort-of) Temperature drops as distance from star increases. Yellow zone = liquid water at planet’s surface.

38. The Inner Edge of the HZ • The limiting factor for the inner boundary of the HZ must be the ability of the planet to avoid a runaway greenhouseeffect. • Theoretical models by James Kasting predict that an Earth-like planet would convert all its ocean into the water vapor ~0.84 AU. • However it is likely that a planet will lose water at somewhat greater distances than that!

39. Moist Greenhouse Effect At 0.95 AU, Solar Intensity is 10% greater … • Higher surface temperature • More H2O vapor in atmosphere • Even higher temperatures • More CO2 in atmosphere • Even higher temperatures • H2O broken apart by UV • Hydrogen escapes into space • Permanent loss of water

40. Venus’ Fate Venus has very high D/H ratio (~120 times higher than Earth’s) suggesting huge hydrogen loss … why? • H2O is lighter than ‘HDO’ and H more easily lost than D from atmosphere. • The implication is clear … Venus once had water but lost it all because of the onset of a runaway (or moist) greenhouse effect.

41. Without water, CO2 accumulated in the Venusian atmosphere and the planet grew increasingly hotter • Venus current atmosphere is ~ 90 times more massive than Earth’s and almost entirely CO2 • Eventually Earth will follow the fate of Venus!

42. If the Earth were moved to where Venus is today, _______. Question the oceans would evaporate, blocking light from the Sun and causing global temperatures to fall carbon dioxide would be released from the oceans leading to higher temperatures but liquid water could still exist on the surface the oceans would evaporate slightly producing a slightly warmer, more humid planet dinosaurs would evolve again and take over the planet the oceans would evaporate and CO2 would build up in the atmosphere triggering a runaway (or moist) greenhouse causing the temperature to rise as high as the one that exists on Venus today

43. The Outer Edge of the HZ • 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. • A CO2 cycle can extend the outer edge of the HZ somewhat by helping a planet maintain a rich CO2 atmosphere … partially offsetting low solar luminosity.

44. Limit From CO2 Greenhouse • At low solar luminosities, high CO2 abundance required to keep planet warm. • But high CO2 abundance does not produce as much net warming because it also scatters solar radiation. • Theoretical models predict that no matter how high CO2 abundance is in atmosphere, the temperature would not exceed the freezing point of water if a planet is further than 1.7 A.U.

45. Limit From CO2 Condensation … A problem at high CO2 abundances and low temperatures • CO2 can start to condense out (like water condenses into rain and snow) • Atmosphere would not be able to build CO2 if a planet is further than 1.4 A.U.

46. Fate of Mars • Mars lies just outside of the HZ at the present. • Mars is too small. It cooled too fast. • Mars has no plate tectonics. • Mars can no longer outgas CO2 • Therefore, Mars has no CO2 cycle. • Any hydrogen quickly escapes due to the low Martian gravity and lack of magnetic field.

47. Stellar habitable zones

48. Question The habitable zone is the area where temperatures on a planet are reasonable. terrestrial planets can form around a star. terrestrial planets could have liquid water on their surfaces. liquid water can condense into rain in the atmosphere. Sun-like stars can exist in the Milky Way Galaxy.

49. Solar Luminosity Versus Time The Sun is getting brighter!

50. Fusionreactions proceed faster • More energy is produced • More energy is emitted Sun gets brighter!