PTYS 214 – Spring 2019

Midterm #3 graded!

Moon Observing continues

I will be away Thursday!

• Grad student Amanda Staderm1333ann will lecture!

• No office hours Thursday!

Extra Credit! LPL evening lecture: 10/16; 7:00pm; this room!

Take notes and get them stamped!

Announcements

1

Midterm #3

Total Students: 16

Class Average: 72

Low: 0

High: 99

If you have questions see one of us!

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Energy balance –> Emission temperature

Greenhouse effect

Incoming vs. outgoing spectra

Habitable Zone

Assume T for liquid water, solve for D

~0.56 to 1.1 AU for Earth-like assumptions

Previously

3

But location of HZ depends on intrinsic properties of the planet!

Positive Coupling

gas pedal speed

A change in one component leads to a change of the same direction in the linked component

‏(+)

Negative Coupling

brakepedal

speed‏(-)

A change in one component leads to a change of the opposite direction in the linked component

Coupling of System Components

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Feedbacks

In reality, component A affects component B but component B also affects component A

This “two-way” interaction is called a feedback loop

Loops can be stable or unstable

BA

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Unstable Loops

Number of Births

World Population

Positive feedback loop:An unstable system that changes further following a perturbation

positive coupling

positive coupling

‏(+)

‏(+)

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negative coupling

positive coupling

Negative feedback loop:A stable system that resists change following a perturbation

Stable Loops

‏(-)

‏(+)

Number of Predators

Number ofPrey

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Odd numbers of negative couplings:

Overall negative (stable) loop

Even number of negative couplings:

Overall positive (unstable) loop

Multiple Feedback Systems

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Climate System We can think about climate system as a number of

components (atmosphere, ocean, land, ice cover, vegetation, etc.) that continually interact with one another

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Climate Feedbacks: 1. The IR Flux/Temperature Feedback

Ts Outgoing

IR flux

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Climate Feedbacks: 1. The IR Flux/Temperature Feedback

‏(+)

Ts Outgoing

IR flux

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Climate Feedbacks: 1. The IR Flux/Temperature Feedback

‏(+)

‏(-)

Ts Outgoing

IR flux

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Climate Feedbacks: 1. The IR Flux/Temperature Feedback

Short-term climate stabilization

‏(-) = ‏(-) × ‏(+)

‏(+)

‏(-)

Ts Outgoing

IR flux(-)‏

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Climate Feedbacks:2. Water Vapor Feedback

‏(+) = ‏(+) × ‏(+) × ‏(+)

‏(+)

‏(+)

Ts

Atmospheric H2O

GreenhouseEffect

‏(+)

‏(+)

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Runaway greenhouse effect!

Climate Feedbacks: 3. Ice / Albedo Feedback

‏(+) = ‏(-) × ‏(+) × ‏(-)

‏(-)

‏(-)

Ts

Snow and Ice Cover

Planetary Albedo

‏(+)

‏(+)

15Runaway snowball!

Climate Feedbacks: 4. The Carbonate/Silicate Feedback

Ts‏(?)

Rainfall

Silicateweathering

rate

AtmosphericCO2

Greenhouseeffect

‏(?)‏(?)

‏(?)

‏(?) ‏(?)

AtmosphericH2O

‏(?)

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Climate Feedbacks: 4. The Carbonate/Silicate Feedback

Ts‏(-)

Rainfall

Silicateweathering

rate

AtmosphericCO2

Greenhouseeffect

‏(+)‏(+)

‏(+)

‏(+) ‏(-)

‏(-) = ‏(+) × ‏(+) × ‏(+) × ‏(-) × ‏(+) × ‏(+)

AtmosphericH2O

‏(+)

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Carbonate-Silicate Feedback CO

2 dissolves in water (rain) to form carbonic acid (H

2CO

3)

H2CO

3 helps weather silicate rocks (CaSiO

3)

Increased CO2 leads to increased rainfall, which ultimately

reduces CO2 levels.

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Regulates the amount of atmospheric CO2!

What happens to the CO2?

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What happens to the CO2?

CaCO3 and SiO

2 precipitated when oceans become

saturated

Subduction leads to metamorphism; silicate rocks formed and CO

2 eventually released through volcanoes.

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The Carbonate-Silicate Cycle

→ Long-term climate stabilization

How long?How long?

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H2O + CO2

>300°CWeatheringWeatheringCaSiOCaSiO33 + CO + CO22→→ CaCO CaCO33 + SiO + SiO22

MetamorphosisMetamorphosisCaCOCaCO33 + SiO + SiO22 →→ CaSiO CaSiO33 + CO + CO22

Requires plate tectonics!

The Carbonate-Silicate Cycle

→ Long-term climate stabilization

Hundreds of Hundreds of Millions of Millions of

yearsyears

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H2O + CO2

>300°CWeatheringWeatheringCaSiOCaSiO33 + CO + CO22→→ CaCO CaCO33 + SiO + SiO22

MetamorphosisMetamorphosisCaCOCaCO33 + SiO + SiO22 →→ CaSiO CaSiO33 + CO + CO22

Requires plate tectonics!

Climate Feedbacks Affect the Habitability of a Planet

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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…

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H2O + h H+ + OH-

Photolysis of Water in the Upper Atmosphere:

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Sunlight (UV)

Ultraviolet photons (which are less prevalent in the lower atmosphere) break apart water molecules

Equipartition:

In thermal equilibrium, energy is distributed equally among all molecules

H+ moves much faster than H2O!! Why?

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

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Runaway Greenhouse If a planet is at 0.95 AU it gets about 10% higher solar flux

than the Earth

Greater Solar flux leads to increase in surface temperature more water vapor in the atmosphere even higher surface temperatures

(water vapor feedback)

Eventually upper 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

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Runaway Greenhouse If a planet is at 0.95 AU it gets about 10% higher solar flux

than the Earth

Greater Solar flux leads to increase in surface temperature more water vapor in the atmosphere even higher surface temperatures

(water vapor feedback)

Eventually upper 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

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Eventually the oceans boil away!

The fate of Venus

Runaway greenhouse and a permanent loss of water probably happened on Venus

What’s the evidence?

Hint: Venus' atmosphere has a very high Deuterium / Hydrogen ratio (~120 times higher than Earth’s and any other body in the Solar System!)

D=0.72 AU

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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 high D/H ratio suggests a loss of 99.9% of its original water

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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

The Fate of Venus

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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 a planetary surface

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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 further than 1.7 AU, no matter how high the CO2 abundance, the temperature would not exceed the freezing point of water

…but it get worse…

at low temperatures CO2 may condense out!

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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)

CO2 cannot provide a strong greenhouse effect if its distance from the Sun is more than about 1.4 AU

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The Fate of MarsToday Mars is on the margin of

the Habitable Zone

Problems:1.being a small planet Mars cooled relatively fast, so it

does not have as much internal energy as Earth==> no plate tectonics and no magnetic field

2.Mars cannot sustain a Carbonate-Silicate cycle (no plate tectonics, cool interior) to efficiently outgas CO2

3.The low Martian gravity allowed H to escape efficiently from its atmosphere, thermally and due to solar wind

D=1.52 AU

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The Fate of MarsToday Mars is on the margin of

the Habitable Zone

Problems:1.being a small planet Mars cooled relatively fast, so it

does not have as much internal energy as Earth==> no plate tectonics and no magnetic field

2.Mars cannot sustain a Carbonate-Silicate cycle (no plate tectonics, cool interior) to efficiently outgas CO2

3.The low Martian gravity allowed H to escape efficiently from its atmosphere, thermally and due to solar wind

Liquid water is not stable on the surface of Mars!

D=1.52 AU

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Was it always that way for Mars?

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Nanedi Vallis(from Mars Global Surveyor)

~3 km

River channel

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The same should be true for Nanedi Vallis

Grand Canyon required several millions of years to form

39

Ancient Oceans and Tsunami

40Rodriguez et al. (2016)

How could Mars have ever been warm enough for liquid water?

41

How could Mars have ever been warm enough for liquid water?

CO2 is not the only greenhouse gas!

CH4 and H

2 my have played a role!

42Woodsworth et al. (2017)

Solar Luminosity over Time

Solar luminosity increases with time

Boundaries of the Habitable Zone are changing with time

How?

Byr B.P.= billion years before present43

Stellar Habitable ZoneThe boundaries of the HZ depend on the class of the star

How?

44

= 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?

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Assume a planet is within the Habitable Zone

Does it mean that for sure it would have liquid water on its surface?

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Additional conditions for liquid water on a planetary surface

1. Should get enough water during its formation or shortly after

2. Should be massive enough to retain water

3. Should have enough internal heat to maintain plate tectonics

4. Should have some UV protection (e.g. O2, O

3)

Even if all of the above is true a water-rich planet can be affected by extreme climate changes

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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

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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 49

Homework #11 available shortly on the web site

Homework

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