Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 18 : Habitable Zone II

Ty Robinson

Questions of the Day

• How have we constrained the locations of the inner and outer edges of the Habitable Zone?

• How does the presence of clouds affect the limits of the Habitable Zone?

• What is the Continuously Habitable Zone and how do we determine its location?

HZ Plot

The HZ around the Sun

Deuterium and Venus

Hydrogen

Hydrogen DeuteriumVenus then

Venus now

Water on Venus

Limits of HZ around Sun

• Water on Venus– 0.75 AU

Inner Edge Outer Edge

Water on Mars

Mars then

Mars now

Limits of HZ around Sun

• Water on Venus– 0.75 AU

• Water on Mars– 1.77 AU

Inner Edge Outer Edge

The HZ around the Sun I

separation = 1au

separation

surf

ace

tem

pera

ture

hot

cold

small large

The HZ around the Sun II

separation

surf

ace

tem

pera

ture

hot

cold

small large

separation

surf

ace

tem

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ture

hot

cold

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The HZ around the Sun III

The HZ around the Sun IV

separation

surf

ace

tem

pera

ture

hot

cold

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CO2 Clouds

QuestionWhy would forming CO2 clouds spell the end of a world’s habitability?

clouds increase a planet’s albedo, so less radiation is absorbed

clouds decrease a planet’s albedo, so less radiation is absorbed

clouds do not have a very strong greenhouse effect

clouds emit thermal radiations to space

Cloud Uncertainty

}

}

providewarming

providecooling

Limits of HZ around Sun

• Water on Venus– 0.75 AU

• Water on Mars– 1.77 AU

• CO2 Clouds– 1.37 AU

Inner Edge Outer Edge

Maximum Greenhouse

CO2

CO2 Amountsu

rfac

e te

mpe

ratu

re

hot

cold

small large

maximum greenhouse

Question

Why doesn’t Earth simply get warmerand warmer as you add CO2?

Rayleigh scattering

Limits of HZ around Sun

• Water on Venus– 0.75 AU

• Water on Mars– 1.77 AU

• CO2 Clouds– 1.37 AU

• Maximum greenhouse– 1.67 AU

Inner Edge Outer Edge

CO2 Amount

surf

ace

tem

pera

ture

hot

cold

small large

The HZ around the Sun V

separation = 1au

separation

surf

ace

tem

pera

ture

hot

cold

small large

separation

surf

ace

tem

pera

ture

hot

cold

small large

The HZ around the Sun VI

separation

surf

ace

tem

pera

ture

hot

cold

small large

The HZ around the Sun VII

separation

surf

ace

tem

pera

ture

hot

cold

small large

The HZ around the Sun VIII

Water Critical Point

bar

s2

21

Runaway Greenhouse

surface temperatureincreases

more waterevaporates

greenhouse effectstrengthens

Limits of HZ around Sun

• Water on Venus– 0.75 AU

• Critical point– 0.84 AU

• Water on Mars– 1.77 AU

• CO2 Clouds– 1.37 AU

• Maximum greenhouse– 1.67 AU

Inner Edge Outer Edge

Moist Greenhouse

~1 Gyr

cool

warm

Limits of HZ around Sun

• Water on Venus– 0.75 AU

• Critical point– 0.84 AU

• Moist Greenhouse– 0.95 AU

• Water on Mars– 1.77 AU

• CO2 Clouds– 1.37 AU

• Maximum greenhouse– 1.67 AU

Inner Edge Outer Edge

HZ Plot

Solar Luminosity Changes with Time

Continuously Habitable Zone

Continuously Habitable Zone = The range of distances arounda star where an Earth-like planet could remain habitable for

some large portion of the star’s lifetime

= HZ

CHZ II

= HZ

CHZ III

= HZ

CHZ IV

= HZ

CHZ V

= HZ, start

CHZ VI

= HZ, start = HZ = CHZ

CHZ VII

= HZ, start = HZ = CHZ

The CHZ around the Sun

= CHZ

Questions of the Day

• How have we constrained the locations of the inner and outer edges of the Habitable Zone?

• How does the presence of clouds affect the limits of the Habitable Zone?

• What is the Continuously Habitable Zone and how do we determine its location?