PTYS 214 – Spring 2011

1. Announcements PTYS 214 – Spring 2011 • Homework #5available for download at the class website • DUE Thursday, Feb. 24 • Reminder: Extra Credit Presentations(up to 10pts) • Deadline: Thursday, Mar. 3(must have selected a paper) • Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/ • Useful Reading: class website  “Reading Material” http://www.global-greenhouse-warming.com/climate-feedback.html http://en.wikipedia.org/wiki/Carbonate-silicate_cycle http://www.vanderbilt.edu/AnS/physics/astrocourses/AST101/readings/water_on_venus.html http://www.astronomynotes.com/solarsys/s9.htm

2. Homework #4 • Total Students: 26 • Class Average: 7.0 • Low: 2 • High: 10 Homework are worth 30% of the grade

3. Some recent interesting articles in Nature A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214, by G.L. Bean et al. – Nature, vol. 468, p. 669-672, 2010 Telescopic observations of exoplanet GJ 1214 (6.5 times the mass of Earth) suggest the presence of an atmosphere that could be dominated by water vapor or hydrogen A closely packed system of low-mass, low-density planets transiting Kepler-11, by J. J. Lissauer et al. – Nature, vol. 470, p. 53-58, 2011 Reports the latest discovery by Kepler of a system of 6 planets all orbiting very close to a Sun-like star

4. Multiple Feedback Systems Odd numbers of negative couplings: Overall negative (stable) loop Even number of negative couplings: Overall positive (unstable) loop

5. Climate Feedbacks: 1. Water Vapor Feedback Atmospheric H2O (+) Ts (+) (+) (+) Greenhouse Effect (+) × (+) × (+) = (+)

6. Climate Feedbacks: 2. Snow and Ice Albedo Feedback (-) Snow and Ice Cover Ts (+) (+) (-) Planetary Albedo (-) × (+) × (-) = (+)

7. Climate Feedback: 3. The IR Flux/Temperature Feedback (+) Outgoing IR flux Ts (-) (-) (+) × (-) = (-) Short-term climate stabilization

8. In typical glaciations ice stops growing because of the IR Flux/Temperature Feedback

9. The Carbonate-Silicate Cycle Requires plate tectonics! H2O + CO2 >300°C Weathering CaSiO3 + CO2 CaCO3 + SiO2 Metamorphosis CaCO3 + SiO2  CaSiO3 + CO2 Overall: CaSiO3 + CO2 CaCO3 + SiO2

10. Climate Feedback: 4. The Carbonate/Silicate Cycle Feedback Atmospheric H2O + Rainfall + + Silicate weathering rate Ts + (-) - + Greenhouse effect Atmospheric CO2 + (+) × (-) × (+) × (+) × (+) = (-)

11. The Carbonate-Silicate Cycle Needs water in atmosphere and plate tectonics H2O + CO2 Long-term climate stabilization

12. Climate Feebacks Affect the Habitability of a Planet

13. The Inner Edge of the HZ • The limiting factor for the inner boundary of the Habitable Zone is the ability of the planet to avoid a runaway greenhouse effect • Theoretical models predict that a planet with characteristics similar to the Earth would not have stable liquid water at a distance of ~0.84 AU from the Sun, but it may extend even farther out than that…

14. Moist Greenhouse • If a planet is at 0.95 AU it gets about 10% higher solar flux than the Earth • Increase in Solar flux leads to increase in surface temperature more water vapor in the atmosphere  even higher surface temperatures (water vapor feedback) • Eventually all atmosphere becomes rich in water vapor  H2O is broken up by UV in the upper atmosphere  effective hydrogen escape to space  permanent loss of water Runaway Greenhouse!

15. Hydrogen Escape and Permanent Loss of Water Ineffective H-escape (little H2O) Effective H-escape (much H2O) UV UV Space H2O + h  H+ + OH- H2O + h  H++ OH- Upper Atmosphere (Stratosphere to Mesosphere) H2O-poor H2O-rich Lower Atmosphere (Troposphere) H2O-rich H2O-ultrarich <0.95 AU Earth

16. The fate of Venus D=0.72 AU Runaway (or moist) greenhouse and a permanent loss of water probably happened on Venus Evidence: Venus has a very high Deuterium/Hydrogen ratio (~120 times higher than Earth’s and any other body in the Solar System!) suggesting huge hydrogen loss

17. The D/H ratio • Deuterium is a stable isotope of Hydrogen: H: 1 proton in nucleus D: 1 proton + 1 neutron in nucleus • About 1 in 10,000 atoms of Hydrogen is D, and 1 in 5,000 molecules of water is HDO • The lighter H is more likely to escape from a planetary atmosphere than D A high D/H ratio indicates preferential loss of H On Venus, the D/H ratio suggests a loss of 99.9% of the waterVenus originally had

18. The Fate of Venus With no water to dissolve it, CO2 accumulated in the atmosphere, further increasing the greenhouse effect Current atmosphere of Venus is ~ 90 times more massive than Earth’s and almost entirely CO2 Earth will eventually follow the fate of Venus!

19. The Outer Edge of the HZ • The outer edge of the Habitable Zone is the distance from the Sun at which even a strong greenhouse effect would not allow liquid water on the planetary surface • The carbonate-silicate cycle can help in extending the outer edge of the Habitable Zone by accumulating more CO2 in the atmosphere and partially offsetting the low solar luminosity

20. Limit of the CO2 Greenhouse • With a low Solar constant, a high atmospheric CO2 abundance is required to keep the planet warm • Theoretical models predict that for planets farther than 1.7 AU, no matter how high the CO2 abundance would be in the atmosphere, the temperature would not exceed the freezing point of water …but it get worse… at low temperatures CO2 may condense out!

21. CO2 Condensation • At high atmospheric CO2 abundance and low temperatures carbon dioxide can start to condense (like water condenses into liquid droplets and/or ice crystals) • CO2 clouds increase the planet’s albedo (less solar radiation is absorbed by the planet) End Result: The planet cannot build CO2 in the atmosphere if its distance from the Sun is more than 1.4 AU 1 atm 1 atm 1 atm

22. D=1.52 AU The Fate of Mars Today Mars is on the margin of the Habitable Zone Problems: • being a small planet Mars cooled relatively fast, and it does not have as much internal energy as Earth • Mars cannot sustain a Carbonate-Silicate cycle feedback (no plate tectonics) and efficiently outgas CO2 • the low Martian gravity and the lack of a magnetic field allow H to escape efficiently from its atmosphere Liquid water is not stable on the surface of Mars

23. Was it always that way for Mars?

24. Nanedi Vallis (from Mars Global Surveyor) River channel ~3 km

25. Grand Canyon required several millions of years to form The same should be true for Nanedi Vallis

26. Conditions for habitability (stability of liquid water on the surface) vary over geologic time

27. Solar Luminosity in Time Solar luminosity increases with time  Boundaries of the Habitable Zone are changing with time How? Byr B.P.= billion years before present

28. Continuous Habitable Zone Region in which a planet may reside and maintain liquid water throughout most of a star’s life CHZ VI = HZ, start (e.g., 4 byr B.P.) = HZ, today = CHZ Why is it important?

29. Stellar Habitable Zone The boundaries of the HZ depend on the class of the star How? Habitable Zone Mass of star relative to Sun Star Luminosity Radius of orbit relative to Earth

30. Assume a planet is within the Habitable Zone Does it mean that for sure it would have liquid water on its surface?

31. Additional conditions for liquid water on a planetary surface • Planet should get enough water during its formation or shortly after • Planet should be massive enough to retain water • Planet should have enough internal heat to maintain plate tectonics Even if all of the above is true a water-rich planet can be affected by extreme climate changes

32. Environmental Extremes on a Habitable Planet • Just because a planet is in the habitable zone does not mean that it is habitable always! • The environment can cause tremendous stresses on a potential biosphere • Climate extremes, such as snowball glaciations and episodes of mass extinctions occurred several times on Earth

33. Earth’s Climate Earth's climate has changed throughout its history, from glacial periods (or "ice ages") where ice covered significant portions of the Earth to interglacial periods where ice retreated to the poles or melted entirely Ice Age ~530 Myr ~300 Myr ~145 Myr