PTYS 214 – Spring 2011  Homework #5 available for download at the class websi te DUE Thursday, Feb. 24  Reminder: Extra Credit Presentations (up to 10pts) Deadline: Thursday, Mar. 3 (must have selected a paper)  Class website:  Useful Reading: class website  “Reading Material” _venus.html Announcements

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

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 , 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 , 2011 Reports the latest discovery by Kepler of a system of 6 planets all orbiting very close to a Sun-like star

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

Climate Feedbacks: 1. Water Vapor Feedback (+) × (+) × (+) = (+) (+) TsTs Atmospheric H 2 O Greenhouse Effect (+)

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

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

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

The Carbonate-Silicate Cycle H 2 O + CO 2 >300°C Overall: CaSiO 3 + CO 2  CaCO 3 + SiO 2 Weathering CaSiO 3 + CO 2  CaCO 3 + SiO 2 Metamorphosis Metamorphosis CaCO 3 + SiO 2  CaSiO 3 + CO 2 Requires plate tectonics!

Climate Feedback: 4. The Carbonate/Silicate Cycle Feedback (-)(-) TsTs Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect (+) × (-) × (+) × (+) × (+) = (-) Atmospheric H 2 O +

The Carbonate-Silicate Cycle Long-term climate stabilization Needs water in atmosphere and plate tectonics H 2 O + CO 2

Climate Feebacks Affect the Habitability of a Planet

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…

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  H 2 O is broken up by UV in the upper atmosphere  effective hydrogen escape to space  permanent loss of water Runaway Greenhouse!

H 2 O + h  H + + OH - H 2 O-rich H 2 O-poorH 2 O-rich Upper Atmosphere (Stratosphere to Mesosphere) Lower Atmosphere (Troposphere) H 2 O-ultrarich Space H 2 O + h  H + + OH - UV Effective H-escape (much H 2 O) Ineffective H-escape (little H 2 O) Hydrogen Escape and Permanent Loss of Water Earth <0.95 AU

The fate of Venus 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 D=0.72 AU

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 water Venus originally had

With no water to dissolve it, CO 2 accumulated in the atmosphere, further increasing the greenhouse effect Current atmosphere of Venus is ~ 90 times more massive than Earth’s and almost entirely CO 2 Earth will eventually follow the fate of Venus! The Fate of Venus

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 CO 2 in the atmosphere and partially offsetting the low solar luminosity

Limit of the CO 2 Greenhouse  With a low Solar constant, a high atmospheric CO 2 abundance is required to keep the planet warm  Theoretical models predict that for planets farther than 1.7 AU, no matter how high the CO 2 abundance would be in the atmosphere, the temperature would not exceed the freezing point of water …but it get worse… at low temperatures CO 2 may condense out!

CO 2 Condensation  At high atmospheric CO 2 abundance and low temperatures carbon dioxide can start to condense (like water condenses into liquid droplets and/or ice crystals)  CO 2 clouds increase the planet’s albedo (less solar radiation is absorbed by the planet) End Result: The planet cannot build CO 2 in the atmosphere if its distance from the Sun is more than 1.4 AU 1 atm

The Fate of Mars Today Mars is on the margin of the Habitable Zone Problems: 1.being a small planet Mars cooled relatively fast, and it does not have as much internal energy as Earth 2.Mars cannot sustain a Carbonate-Silicate cycle feedback (no plate tectonics) and efficiently outgas CO 2 3.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 D=1.52 AU

Was it always that way for Mars?

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

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

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

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

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

Radius of orbit relative to Earth Habitable Zone Stellar Habitable Zone The boundaries of the HZ depend on the class of the star How?

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

Additional conditions for liquid water on a planetary surface 1.Planet should get enough water during its formation or shortly after 2.Planet should be massive enough to retain water 3.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

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

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