Lecture 35. Habitable Zones.

1. Lecture 35. Habitable Zones. reading: Chapters 9, 10

2. Goldilocks and the Solar System Venus too hot now/not habitable geologic age of the surface: ~500 Ma could have been habitable in the past before runaway greenhouse Earth liquid water for most or all of geologic history has always been habitable carbonate-silicate cycle stabilizes the climate with its negative feedback loop. Mars too cold for liquid water today geologic evidence of liquid water in the past could have been habitable in the past

3. Runaway Greenhouse Effect Occurs if the T reaches the boiling point of water. Oceans turned into water vapor. Water vapor is a greenhouse gas, causing additional warming. This causes the oceans to evaporate even faster. This is a positive feedback loop. Soon, all the water will be in the atmosphere, which will be very hot. Hot enough (several hundred degrees) to vaporize carbonate rock. This would turn carbonate rock back into CO2. The runaway greenhouse is a permanent state - no known way to escape it.

4. Concept of the Habitable Zone (HZ) Focuses on the presence of liquid water. The HZ is the zone in which temperatures allow for liquid water to exist on the surface. (note: this implies Europa is not in the habitable zone - Europa is an exception to the definition of the habitable zone) Key: distance to the Sun and presence of an atmosphere and magnetic field. Moon: in the Sun’s habitable zone, but lacks an atmosphere. Is the Sun’s habitable zone moving in or out with time?

5. Venus Venus and Earth likely started out with the same amount of volatiles. Evidence of volcanoes/outgassing/active planet on Venus. But Venus is very dry and hot today. Where did all the water go? May have had early oceans. As Sun got brighter, more water went into the atmosphere. 1. Photochemical reactions break water into hydrogen and oxygen. Hydrogen is easily lost to space. Oxygen reacts with other gases in the atmosphere and with rocks on the surface. 2. Water reacts with SO2 to form sulfuric acid.

6. What Controls Surface Habitability? Distance from the Sun Venus 0.7 AU Earth 1.0 AU Mars 1.5 AU Distance from the Sun determines how much solar radiation the planet receives. Solar radiation drops by 1/r2 - This means that if the distance (radius, r) from the Sun is doubled, the amount of solar radiation is 1/22, or 1/4). Solar radiation is important for the greenhouse effect.

7. What Controls Surface Habitability? Planetary Size radius, relative to Earth Venus 0.95 Earth 1 Mars 0.53 Smaller planets: Lose internal heat rapidly, outgassing ceases. Can’t replace volatiles that are lost to space or to chemical reactions. Larger planets: Greater internal heat, internal heat is retained over time. Continued outgassing helps to retain the atmosphere.

8. Plate Tectonics Helps to recycle volatiles, trap volatiles in the mantle so that they aren’t lost to space.

9. What Controls Surface Habitability? Atmospheric Loss Processes Mars has also lost a significant part of its atmosphere. a) lack of magnetic field solar wind particles strip away the atmosphere b) low level of volcanism Early Mars: thicker atmosphere stronger greenhouse effect

10. Inner Boundary of the HZ Determined by the ability to avoid a runaway greenhouse. Inner boundary of the HZ lies between Venus and the Earth If we move Earth to 0.82 AU - Earth would have a runaway greenhouse If we move Earth to 0.95 AU - Earth would have a moist greenhouse where more water is entering the atmosphere where it can be lost to space.

11. Outer Boundary of the HZ Distance from the Sun where a strong greenhouse effect does not allow the planet to stay warm enough to keep water from freezing. Limiting factor determining this boundary: where CO2 condenses into CO2 rain or CO2 ice. For a large planet with a thick atmosphere, might be ~1.7 AU. For a smaller planet with a thinner atmosphere, might be ~1.4 AU (just inside the orbit of Mars).

12. The HZ of the Solar System Optimistic boundaries 0.84 AU - 1.7 AU Conservative boundaries 0.95 AU - 1.4 AU Venus 0.7 AU Earth 1.0 AU Mars 1.5 AU The evolving HZ: as the Sun becomes brighter, the HZ moves outward with time.

13. The Continuously Habitable Zone (CHZ) Region of the solar system that has been habitable at all times since the end of heavy bombardment. . optimistic estimate 0.84 - 1.5 AU Earth and Mars conservative estimate: 0.95-1.2 AU Earth only

14. Habitability Outside the HZ Possible liquid water oceans around Europa and Ganymede. Could be subsurface liquid groundwater on Mars. So, if you have internal heat sources, this expands and complicates the definition of the HZ or the CHZ. Could also have other liquids (methane, ethane). The HZ is a generalization.

15. Star Types Cooler stars: smaller burn slowly have long lifetimes have narrower habitable zones Hotter stars: larger burn quickly have short lifetimes lifetimes may be too short to evolve life Brown dwarfs: not large enough to sustain fusion like a star have no HZ - too cool to heat a planet may have larger planets with moons that are tidally heated

16. What Stars Are “Good” For Life?

17. What Stars Are “Good” For Life? Type O: Very short lifetimes accretion takes 10’s of millions of years Type B: Short lifetimes long enough to form a planet, but Sun dies before heavy bombardment ends Types A and F: 3% of stars lifetimes 1-2 Ga hotter than our Sun, HZ is further out emit more uv light (breaks down and reacts with organic compounds) Type G: 7% of stars

18. What Stars Are “Good” For Life? Types K and M: 90% of stars long lifetimes 20-600 Ga dimmer stars habitable zones much closer in frequent bursts of intense light and radiation K-type stars: 0.25 solar luminosity HZ at 0.5 AU M-type stars: 0.01 solar luminosity HZ 0.1-0.2 AU (inside Mercury’s orbit) size of the HZ is thinner

19. Habitable Zones Around Other Stars

20. Lecture 36. Galactic Habitable Zones. reading: Chapter 10