PTYS 214 – Spring 2011

Homework #5 available 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

Announcements

Homework #4

Total Students: 26

Class Average: 7.0

Low: 2

High: 10

Homework are worth 30% of the grade

1 2 3 4 5 6 7 8 9 10 110

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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

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

(+) × (+) × (+) = (+)

(+)

(+)

Ts

Atmospheric H2O

GreenhouseEffect

(+)

(+)

Climate Feedbacks:

2. Snow and Ice Albedo Feedback

(-) × (+) × (-) = (+)

(-)

(-)

Ts

Snow and Ice Cover

Planetary Albedo

(+)

(+)

Climate Feedback: 3. The IR Flux/Temperature Feedback

Short-term climate stabilization

(+) × (-) = (-)

(+)

(-)

Ts Outgoing

IR flux(-)

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

The Carbonate-Silicate Cycle

H2O + CO2

>300°C

Overall: CaSiO3 + CO2 CaCO3 + SiO2

WeatheringWeatheringCaSiOCaSiO33 + CO + CO22 CaCO CaCO33 + SiO + SiO22

MetamorphosisMetamorphosisCaCOCaCO33 + SiO + SiO22 CaSiO CaSiO33 + CO + CO22

Requires plate tectonics!

Climate Feedback: 4. The Carbonate/Silicate Cycle Feedback

(-)

Ts

Rainfall

Silicateweathering

rate

AtmosphericCO2

Greenhouseeffect

++

+

+

+ -

(+) × (-) × (+) × (+) × (+) = (-)

AtmosphericH2O

+

The Carbonate-Silicate Cycle

Long-term climate stabilization

Needs water in Needs water in atmosphere andatmosphere andplate tectonicsplate tectonicsH2O + CO2

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 H2O is broken up by UV in the upper atmosphere effective hydrogen escape to space permanent loss of water

Runaway Greenhouse!

H2O + h H+ + OH-

H2O-rich

H2O-poor H2O-rich

Upper Atmosphere(Stratosphere to

Mesosphere)

Lower Atmosphere(Troposphere) H2O-ultrarich

Space

H2O + h H+ + OH-

UV UV EffectiveH-escape

(much H2O)

IneffectiveH-escape(little H2O)

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 nucleusD: 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, 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!

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 CO2 in the atmosphere and partially offsetting the low solar luminosity

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!

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 atm1 atm1 atm

The Fate of MarsToday 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 CO2

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?

Stellar Habitable ZoneThe 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